User:Minihaa/Impact of Lipidation

1.1 Peptides

Fairly general. May not be used?

Peptides are small assemblies of amino acid monomers linked by covalent peptide bonds. There are 20 natural amino acids which make up the building blocks for various peptides and proteins. Most amino acids with the exception of proline consist of an alpha carbon atom to which a hydrogen atom, an amino group, a carboxyl group, and a side chain R group are attached. The only variable group on an amino acid is the R group, and it is this that is responsible for the different properties of the various amino acids.

The peptide bond is formed by the linking of two amino acids, with the elimination of water via a condensation reaction between the neighbouring –NH₂ and –COOH groups, resulting in a dipeptide.

The peptide bond has a double bond character due to delocalisation of the lone pair of electrons on the nitrogen atom, which means that no rotation is possible around the bond. This allows the peptide to have either a cis or trans conformation. The delocalisation of electrons results in the peptide bond having resonance, meaning it is relatively unreactive under physiological conditions which is important in biological systems.

Multiple amino acid residues bound by peptide bonds are known as polypeptides, and these are what make up the backbone structures of proteins. Proteins are classified as having around 50 or more amino acid monomers in the polypeptide chain. There are 20 different types of amino acid that can be combined to form a protein, and it is the sequence of amino acids that determines the structure and function of a protein.

There are four categories of peptide structure; primary, secondary, tertiary and quaternary. The primary structure of a peptide refers to the amino acid sequence from the N-terminus to the C-terminus. The secondary structure is when the peptide is held into a structure, such as an alpha helix or a beta sheet. The secondary structures are held together by hydrogen bonds. Tertiary structure is the final three dimensional shape, and it is held together by interactions between the R groups. These interactions can be ionic, hydrogen bonding, van der Waals forces and also sulphur bridges.

1.1.2 Peptide self-assembly

Could be used in Self-assembling peptide but this text is fairly general. May not be used?

Supramolecular self-assembly is the ability of molecules to spontaneously form ordered nanostructures via non-covalent interactions such as van der Waals, electrostatic, hydrogen bonding, and stacking interactions. Self-assembly is important in many biological systems such as the self-assembly of lipid membranes, the DNA double helix, and folded proteins. Peptides can self-assemble into a wide range of different nanostructures depending on the hydrophile/lipophile balance of molecules, the number and sequence of amino acids, as well as the interactions between the peptide units. Hydrogen bonding between backbones of peptides plays an important role in self-assembly and can cause the peptide monomers to pack longitudinally into β-sheets, a common secondary structure arising from peptide self-assembly, and they either have anti-parallel or parallel conformations (Figure 1.3). Peptides with a β-sheet secondary structure often form fibril structures that can subsequently cross-link to form hydrogels, allowing them to act as slow releasing systems in drug delivery. On the other hand they can also be detrimental by forming amyloid fibrils, heavily associated with neurodegenerative diseases, such as Alzheimer’s disease. Another common secondary structure formed by self-assembly is the α-helix (Figure 1.4), characterised by a single spiral chain of amino acids stabilised by hydrogen bonding between N-H and C=0 groups. An example of an α-helical compound is the iron and oxygen binding protein, myoglobin, found in most mammals. Peptides that typically form α-helices exhibit amino acids of similar properties every 3-4 residues, giving rise to the structural repeat of 3.6 residues per α-helical turn.

As a result of self-assembly being so important in nature, the use of self-assembling peptides for use in therapeutics has gained increasing attention. This is due to them being versatile molecules with tuneable functionality, and high biocompatibility and biodegradability. A polypeptide for example can consist of a targeting sequence, a self- assembly domain, and a biologically active peptide. This is extremely attractive from an industrial point of view as it prevents the need for complex synthetic chemical reactions. Consequently, peptides provide a unique platform for the design of nanomaterials with controllable structural features.

Environmental factors such as pH and temperature play a key role in peptide self- assembly. A simple change of environment allows for fine tuning of peptide self- assembly for a variety of applications. A study by Stevens et al. discovered that is was possible to control the assembly of coiled-coil based gold nanoparticles using a leucine zipper-like peptide, with changes in pH and temperature. It was found that the nanoparticles aggregated when the pH was lowered to 4.5, and disassembly occurred as the temperature was increased due to thermal unfolding of the α-helices. In another study by Thanh et al. the aggregation of an endogenous opioid peptide neurotransmitter derivative, with the sequence YVIFL, as a function of pH was investigated. Aggregates were obtained throughout the pH range 2-11, but with different morphologies ranging from amorphous structures at low pH to fibrils at increased pH. The prevailing driving forces behind aggregation are salt bridges, hydrogen-bonds, hydrophobicity, and net charge contributions from ionizable side chains, with charged residues causing interruptions in contiguous stretches of hydrophobic sequences, as well as causing electrostatic repulsion between units. With this is mind, the propensity of the YVIFL peptide to aggregate into amorphous structures at low pH is more clearly understood.

1.2 Peptide amphiphiles

Most self-assembling molecules are amphiphilic, meaning they have both hydrophobic and hydrophilic character. Peptide amphiphiles are a class of molecules comprised of either hydrophobic and hydrophilic peptide sequences, or a hydrophilic peptide with an attached hydrophobic group, which is usually an alkyl chain. The structure of a peptide amphiphiles has four key domains. Firstly there is a hydrophobic section, typically an alkyl chain. Secondly there is the peptide sequence which forms intermolecular hydrogen bonding. Thirdly there is a section of charged amino acid residues to enhance the solubility of the peptide in water. The final structural feature allows the peptide to interact with biomolecules, cells, or proteins, and this is often through epitopes (part of antigens recognised by the immune system).

As with other amphiphilic molecules, above a critical aggregation concentration peptide amphiphiles associate through non-covalent interactions to form ordered assemblies of different sizes, from nanometres to microns. Molecules that contain both polar and non-polar elements minimise unfavourable interactions with the aqueous environment via aggregation, which allows the hydrophilic moieties to be exposed to the aqueous environment, and the hydrophobic moieties to be protected. When aggregation occurs, a variety of assemblies can be formed depending on many parameters such as concentration, pH, temperature and geometry. The assemblies formed range from micelles to bilayer structures, such as vesicles, as well as fibrils and gels.

Micelles consist of a hydrophobic inner core surrounded by a hydrophilic outer shell that is exposed to a solvent, and their structures can be spheres, disks or wormlike assemblies. Micelles form spontaneously when the concentration is above a critical micelle concentration and temperature. Amphiphiles with an intermediate level of hydrophobicity prefer to assemble into bilayer vesicles. Vesicles are spherical, hollow, lamellar structures that surround an aqueous core. The hydrophobic moiety faces inwards and forms the inner section of the bilayer, and the hydrophilic moiety is exposed to the aqueous environment on the inner and outer surface. Micelle structures have a hydrophobic interior and hydrophilic exterior.

There is normally a distinct relationship between the amphiphilic character of a peptide and its function in that the amphiphilic character determines the self-assembly properties, and in turn this is what gives the peptide its functionality. The level of amphiphilicity can vary significantly in peptides and proteins; as such they can display regions that are either hydrophobic or hydrophilic in nature. An example of this is the cylindrical structure of an α-helix, as it could contain a section of hydrophobic residues along one face of the cylinder and a hydrophilic section of residues on the opposite face of the cylinder. For β-sheet structures, the peptide chain can be composed of alternating hydrophilic and hydrophobic residues, so that the side chains of the residues are displayed on opposite faces of the sheet. In the cell membrane peptides fold into helices and sheets to allow the non-polar residues to interact with the membrane interior, and to allow the polar residues to be exposed to the aqueous environment. This self-assembly allows the peptides to further optimise their interaction with the surroundings.

Peptide amphiphiles are very useful in biomedical applications, and can be utilised to act as therapeutic agents to treat diseases by transporting drugs across membranes to specific sites. They can then be metabolised into lipids and amino acids, which are then easily removed in the kidneys. This occurs by the hydrophobic tail being able to cross the cell membrane, allowing the peptide epitope to target a specific cell by a ligand- receptor complex. Other applications of peptide amphiphiles are use in antimicrobials, skincare and cosmetics, and also gene delivery to name a few.

1.3 Lipidated Peptides

The text would be well suited to be used in Proteolipid.

Lipidated peptides are a type of peptide amphiphile that incorporate one or more alkyl/lipid chains, attached to a peptide head group. As with peptide amphiphiles, they self-assemble depending on the hydrophilic/hydrophobic balance, as well interactions between the peptide units, which is dependent on the charge of the amino acid residues. Lipidated peptides combine the structural features of amphiphilic surfactants with the functions of bioactive peptides, and they are known to assemble into a variety of nanostructures.

Examples

Examples include ghrelin (a peptide hormone associated with feeding), and bacterial antibiotics that aren’t synthesised in the ribosome. Further examples include those produced by the Bacillus subtilis family which are composed of a cyclic structure made up of 7-10 amino acids, and a β-hydroxy fatty acid chain of varying length ranging from 13-19 carbon atoms. These can be divided into three families depending on the structure of the cyclic peptide sequence: surfactins, iturins, and fengycins. Lipidated peptides produced by Bacillus strains have many useful bio-activities such as anti-bacterial, anti- viral, anti-fungal, and anti-tumour properties, making them very attractive for use in a wide range of industries. As the name implies, surfactins are potent biosurfactants (surfactants produced by bacteria, yeast, or fungi), and they have been shown to reduce the surface tension of water from 72 to 27 mN/m at very low concentrations. Furthermore, surfactins are also able to permeabilize lipid membranes, allowing them to have specific antimicrobial and antiviral activities. Since surfactins are biosurfactants, they have diverse functional properties. These include low toxicity, biodegradability and a higher tolerance towards variation of temperature and pH, making them very interesting for use in a wide range of applications. Iturins are pore‐forming lipopeptides with antifungal activity, and this is dependent on the interaction with the cytoplasmic membrane of the target cells. Mycosubtilin is an iturin isoform that can interact with membranes via its sterol alcohol group, to target ergosterol (a compound found in fungi) to give it antifungal properties. Finally, fengycins are another class of biosurfactant produced by Bacillus subtilis, with antifungal activity against filamentous fungi. There are two classes of Fengycins, Fengycin A and Fengycin B, with the two only differing by one amino acid at position 6 in the peptide sequence, with the former having an alanine residue, and the latter having valine.

Daptomycin is another naturally occurring lipidated peptide, produced by the Gram positive bacterium Streptomyces roseoporous. The structure of Daptomycin consists of a decanoyl lipid chain attached to a partially cyclised peptide head group. It has very potent antimicrobial properties and is used as an antibiotic to treat life-threatening conditions caused by Gram positive bacteria including MRSA (methicillin-resistant Staphylococcus aureus) and vancomycin resistant Enterococci. As with the Bacillus subtillis lipidated peptides, the permeation of the cell membrane is what gives it its properties, and the mechanism of action with daptomycin is thought to involve the insertion of the decanoyl chain into the bacterial membrane to cause disruption. This then causes a serious depolarization resulting in the inhibition of various synthesis processes including those of DNA, protein and RNA, leading to apoptosis.

Application

Due to the desirable properties of peptides such as high receptor affinity and bioactivity, and low toxicity, the use of peptides in therapeutics (i. e. as peptide therapeutics) has great potential; shown by a fast growing market with over 100 approved peptide-based drugs. The disadvantages are that peptides have low oral bioavailability and stability. Lipidation as a chemical modification tool in the development of therapeutic agents has proven to be useful in overcoming these issues, with four lipidized peptide drugs currently approved for use in humans, and various others in clinical trials. Two of the approved drugs are long-acting anti-diabetic GLP-1 analogues liraglutide (Victoza®), and insulin detemir (Levemir®). The other two are the antiobiotics daptomycin and polymyxin B.

Lipidated peptides also have applications in other areas, such as use in the cosmetic industry. A commercially available lipidated peptide, Matrixyl, is used in anti-wrinkle creams. Matrixyl is a pentapeptide and has the sequence KTTKS, with an attached palmitoyl lipid chain, that is able to stimulate collagen and fibronectin production in fibroblasts. Several studies have shown promising results of palmitoyl-KTTKS, and topical formulations have been found to significantly reduce fine lines and wrinkles, helping to delay the aging process in the skin. The Hamley group have also carried out investigations of palmitoyl-KTTKS, and found it so self-assemble into nanotapes in the pH range 3-7, in addition to stimulating human dermal and corneal fibroblasts in a concentration dependant manner, suggesting that stimulation occurs above the critical aggregation concentration.

1.4 Peptide modification to increase stability

In "Peptide therapeutics" ergänzen

Peptide Therapeutics are peptides or polypeptides (oligomers or short polymers of amino acids) which are used to for the treatment of diseases. Naturally occurring peptides may serve as hormones, growth factors, neurotransmitters, ion channel ligands, and anti-infectives; peptide therapeutics mimic such functions. Peptide Therapeutics are seen as relatively safe and well-tolerated as peptides can be metabolized by the body.

TradeName	Peptide	Company	MolecularProperties	RelatedReference
Copaxone	Glatiramer	Teva	Four amino acids (L‐glutamicacid, L‐alanine, L‐lysine, and L‐tyrosine) in a defined molar ratio	128-129
Lupron	Leuprolide	Abbott	Synthetic nonapeptide analogue of naturally occurring gonadotropin-releasing hormone (GnRHorLH-RH)	130-131
Vicoza	Liraglutide	Novo	97%homologoustonativehuman GLP-1(7-37)by substituting arginineforlysineat position 34& addition of a fatty acid chain	132
Zoladex	Goserelin	AZ	NaturalLHRH/GnRHdecapeptidewithtwosubstitutionstoinhibit rapiddegradation.	133-134
Sandostation	Octreotide	Novartis	Longeractingsyntheticoctapeptideanalogueofnaturallyoccurringsomatostatin	135-136
Forteo	Teriparatide	Lilly/Amylin	Recombinant form of parathyroid hormone consisting of the first (N-terminus) 34 aminoacids, which is the bioactive portion of the hormone	137-138
Byetta	Exenatide	Lilly/Amylin	Syntheticversionofexendin-4,ahormone foundinthesalivaofthe Gilamonster	139-140
Cubicin	Daptomycin	Cubist	Cycliclipopeptide,consistsof13aminoacids,10ofwhich arearrangedinacyclicfashion, andthreeonanexocyclic tail	141-142
Integrilin	Eptifibatide	Merck	CyclicheptapeptidecomposedwithS-Sbridge, 2unnaturalbuildingblocksandamide	143-144
Angiomax/Angiox	Bivalirudin	Medicines	20-aminoacidpolypeptide	145-146
Fortical	Calcitonin	Upsher-Smith	32amino-acidpolypeptidesimilartocalcitonin	147-149
Somatuline	Lanreotide	Ipsen	Cyclicpeptidethatisalongactinganalogueofsomatostatin.	150-151

Examples

There are many peptide therapeutics on the market at present. Due to their high specificity, potency, and low toxicity, the number is only going to increase.117-119 Table 1.1 shows some of the leading peptide therapeutics. Not all of these peptide therapeutics self-assemble or are formulated as solutions, but the table shows the extent and importance of the use of peptides as drugs. -> page 38 of the thesis in PDF format.

The current highest selling marketed diabetic drug Liraglutide, incorporates a lipid chain to extent plasma circulation and ensures prolonged bioavailability.120-121 Liraglutide is a GLP-1 agonist drug that self-assembles into an alpha-helical structure, and it requires once a day administration.122 Lipid conjugation of a palmitoyl chain to a lysine residue at position 26 of Liraglutide results in an extended half-life (around 13-14 hours) in the blood. This is due to the palmitoyl chain allowing non covalent binding to albumin, which delays proteolytic attack by DPP IV and also rapid renal clearance. Furthermore, the addition of the lipid chain could further prolong half-life by sterically hindering the DPP IV enzyme from degradation.123

Another peptide known to self-assemble is the octapeptide, Lanreotide. This compound is a synthetic analogue of the peptide hormone somatostatin and it is used to treat acromegaly 124 (a condition where the body produced too much growth hormone). In water, Lanreotide self-assembles into monodisperse liquid crystalline nanotubes. The nanotubes are made up of dimers that self-assemble into a 2D crystal, which is held together by lateral chain interactions, and also by antiparallel ß-sheets.124 125

Further insight into how self-assembly and peptide hormones are related has been provided by studies on self-assembling amyloid structures formed by peptide hormones and neuropeptides. Peptide hormones and neuropeptides form dense-cored aggregates that pack into dense-core vesicles (DCVs), which are used to temporarily store peptide messengers in secretory cells.126 When dense-core vesicles are triggered, they release the stored information into the blood or extracellular space,127 resulting in amyloid disassembly, in order for action.126 Therefore for these types of peptides, reversibility of peptide aggregation is essential for their function.

Increasing stability of peptide drugs

Many strategies have been employed to increase the stability of peptide drugs, because although they have so many desirable characteristics, they are short lived in the body as a result of rapid degradation and clearance. With half-lives of some peptides and proteins only being a few minutes, they are very ineffective in drug delivery.43 Mechanisms involved in their clearance include peripheral blood mediated elimination by proteolysis, real and hepatic elimination, and also receptor-mediated endocytosis.44 One of the main reasons for such rapid clearance is molecular weight. Molecules that have a low molecular weight (40-50 kDa) are rapidly cleared by renal filtration via the glomerular filtration barrier (GBM) into the urine. As a result of this, increasing the size of a peptide drug is a good starting point to improve half-life.45

Peptide modifications to extend half-life include PEGylation, glycosylation, cyclization, serum albumin binding, and lipidation. PEGylation is the attachment of polyethylene glycol (PEG) chains to the peptide via covalent bonds, helping to increase molecular weight, and limit enzymatic degradation as a result of steric hindrance caused by adding the PEG. PEGylation offers a number of benefits for pharmaceutical applications such as improved water solubility, high mobility in solution, as well as low toxicity and lowimmunogenicity. This does however depend on the molecular weight of the attached PEG. PEGylation as a method to improve half-life has been successfully demonstrated many times; in one example it was shown that site specific mono-PEGylation of GLP-1 led to a 16-fold increase in plasma half life time in rats. On the other hand, covalently attaching PEG can often lead to loss of biological activity.

Another chemical modification is the attachment of glycosyl (carbohydrate) units to the peptide to help with peptide delivery to target sights. The introduction of carboydrates to peptides can alter the physiological properties, to improve bioavailability. Advantages of this technique include increased metabolic stability, and facilitated transport across cell membranes, although of the most favourable aspects is their ability to promote oral absorption. Peptides have a very low oral availability (less than 1-2%), as a result of insufficient absorption and rapid degradation and clearance, thus making this method an attractive one. N- and O-glycosylation in which carbohydrates are attached to the peptide are naturally occurring, where N-glycosylation occurs through the amine group of an asparagine residue to form an amide bond. O-glycosylation occurs via serine or threonine residues, where the oxygen atom on the side chain binds to the carbohydrate through an ether bond. There is also non-natural glycosylation, known as chemical glycosylation, which involves the attachment of carbohydrate units to different amino acid residues at the N-terminus of the peptide's sequence. A further way of carrying out glycosylation is by using enzymes, known as chemo-enzymatic glycosylation. This method is used for complex chemical synthesis. Chemical and chemo-enzymatic methods can be used for the synthesis of glycopeptides and glycoproteins.

Cyclization can also be used as a method to decrease proteolytic degradation and prolong half-life, to make the peptide conformation more rigid to hinder enzymatic cleavage. This method can however lead to loss of biological function due to the reduced flexibility making the peptide inactive. For example, side chain to side chain cyclization between asparagine (position 8) and lysine (position 12), of a growth regulating factor (GRF) analogue was found to increase the half-life from 17 minutes to more than 2 hours.

Another way to extend half-life do is to bind serum albumin to the peptide. Human serum albumin is the most abundant plasma protein with a molecular weight of 66.4 kDa, and it is involved in many essential bodily functions to maintain homeostasis. As a result, albumin binding would significantly increase the molecular weight of the peptide, restricting it from being filtered into the urine by the GBM. Serum albumin has an extraordinary long half-life of 2-4 weeks which is much longer than other plasma proteins, due to it binding to the neonatal Fc receptor (FcRn). Fc receptors are proteins found on the surface of certain cells that help to protect the functions of the immune system, by binding to the Fc region of antibodies, which attach to pathogens and destroy them. This mechanism of the neonatal FcRn involves albumin binding to the FcRn in an acidic pH environment to divert it from degradation in the lysosomal compartment of the cell, and redirecting it to the plasma membrane, where it is released back into the blood plasma due to neutral pH.

As mentioned above, lipidation is a further technique to use when improving peptide stability and half-life. Attaching a lipid chain to the peptide head group has been found to inhibit proteolytic attack due to the lipid chain non-covalently interacting with serum albumin to increase the molecular weight, thus reducing renal filtration. Studies on a lipidated analogue of insulin, detemir, revealed a prolonged action as a result of its affinity for human serum albumin. As well as this, lipidation has been shown to enhance the interaction of peptides with cell membranes, allowing them to be up taken into the cell more readily compared to the peptide lacking the lipid moiety. There are three types of lipidation, and they differ based on the bond formation methods between the lipid and the peptide: amidation, esterification (S- or O-) and S-bond (ether or disulphide) formation. Amidation and O-esterification form strong covalent bonds that are irreversible, whereas the other two methods are weak and reversible covalent bonds. The method used, as well as the alkyl/lipid chain, position of lipidation, and the spacer used, all have significant impacts on physiochemical properties and bioactivity. The level of lipophilicity can be significantly modulated by lipidation, and since lipophilicity is detrimental for the absorption, distribution, metabolism, and excretion of drugs, it provides a way of fine tuning peptides for use in therapeutics.

A study on lipidation and PEGylation on the GLP-1 peptide was carried out and the results showed that lipidation had no significant effect on peptide activity in vitro, whereas PEGylation did, especially when the PEG is attached to internal amino acids of the peptide e.g. positions 20 and 21. The reduction in activity from PEGylation compared to lipidation is due to the loss of receptor affinity, and it is suggested that this is because of its increased molecular weight which causes steric hindrance.

1.5 Hydrogels

Copied in Hydrogel.

Einleitung oder Definition (Quelle schon in Artikel): Hydrogels are three dimensional supramolecular assemblies formed by cross-linking of polymer networks, that are able to absorb large amounts of water.65

Mischmasch (Quelle wertvoll und schon in Artikel?): Peptides can form hydrogels by non-covalent interactions of self-assembled structures to establish well- ordered scaffolds, and these can be very useful in biotechnological and medical applications, in particular for use as slow drug delivery systems. Lipidated peptides can also form hydrogels, since the increased hydrophobic interactions from the addition of the hydrophobic chain facilitates its self-assembling ability.66

Classification (Absatz "Chemistry"): Hydrogels can be classified as either chemical or physical gels, with the former having covalent cross-linking bonds, and the latter having non-covalent bonds, as in the case of peptide hydrogels.67 Chemical hydrogels result in strong irreversible gels due to the covalent bonding, and they may also possess harmful properties which makes them unfavourable for medical applications. Physical hydrogels on the other hand have high biocompatibility, aren’t toxic, and are also easily reversible, by simply changing an external stimulus such as pH or temperature; thus they are favourable for use in medical applications.

Mechanism of formation (Absatz "Chemistry"): There are two suggested mechanisms behind physical hydrogel formation, the first one being the gelation of nanofibrous peptide assemblies, usually observed for oligopeptide precursors. The precursors self-assemble into fibers, tapes, tubes, or ribbons that entangle to form non-covalent cross-links. The second mechanism involves non-covalent interactions of cross-linked domains that are separated by water-soluble linkers, and this is usually observed in longer multi-domain structures.68 Tuning of the supramolecular interactions to produce a self-supporting network that does not precipitate, and is also able to immobilize water which is vital for to gel formation. Most oligopeptide hydrogels have a β-sheet structure, and assemble to form fibers, although α-helical peptides have also been reported.69-70 Figure 1.7 shows the typical mechanism of gelation where the oligopeptide precursors self-assemble into fibers that become elongated, and entangle to form cross-linked gels.

Peptides based hydrogels: Peptides based hydrogels possess exceptional biocompatibility and biodegradability qualities, giving rise to their wide use of applications, particularly in biomedicine; as such, their physical properties can be fine-tuned in order to maximise their use. Methods to do this are: modulation of the amino acid sequence, pH, chirality, and increasing the number of aromatic residues. The order of amino acids within the sequence is crucial for gelation, as has been shown many times. In one example, a short peptide sequence Fmoc-Phe-Gly readily formed a hydrogel, whereas Fmoc-Gly-Phe failed to do so as a result of the two adjacent aromatic moieties being moved, hindering the aromatic interactions. Altering the pH can also have similar effects, an example involved the use of the naphthalene (Nap) modified dipeptides Nap-Gly-Ala, and Nap- Ala-Gly, where a drop in pH induced gelation of the former, but led to crystallisation of the latter.74 A controlled pH decrease method using glucono-δ-lactone (GdL), where the GdL is hydrolysed to gluconic acid in water is a recent strategy that has been developed as a way to form homogeneous and reproducible hydrogels. The hydrolysis is slow, which allows for a uniform pH change, and thus resulting in reproducible homogenous gels. In addition to this, the desired pH can be achieved by altering the amount of GdL added. The use of GdL has been used various times for the hydrogelation of Fmoc and Nap-dipeptides. In another direction, Morris et al reported the use of GdL as a ‘molecular trigger’ to predict and control the order of gelation. Chirality also plays an essential role in gel formation, and even changing the chirality of a single amino acid from its natural L-amino acid to its unnatural D-amino acid can significantly impact the gelation properties, with the natural forms not forming gels. Furthermore, aromatic interactions play a key role in hydrogel formation as a result of π- π stacking driving gelation, shown by many studies.

1.6 Peptide hormones

Text is fairly general. May not be used.

Peptide hormones are hormones made up of amino acid chains that primarily have an effect on the endocrine system. Based on the building units, the hormones can be classified as either amino-acid based, or steroid based systems. The former are water soluble due to their composition comprising of amino acids, allowing them to act on the surface of target cells via secondary messengers. This differs from steroid hormones which are lipid soluble, and so can move through the plasma membranes of target cells and act within the nuclei.

The endocrine system is composed of many different glands and it can be divided into two categories: classical and non-classical. In the endocrine system, hormones are secreted into the circulatory system where they are distributed throughout the body, regulating bodily functions. The classical endocrine glands include the, pituitary gland, pancreas, thyroid gland, adrenal cortex and medulla. The primary function of these glands is to manufacture specific hormones. Non-classical endocrine glands include the heart, hypothalamus, kidneys, liver, and the gastrointestinal tract. Many of the classical hormones are controlled by the hypothalamus and pituitary which can also be classified as being an extension of the nervous system.

An imbalance of hormones or an inappropriate bodily response to them indicates a disorder of the endocrine system, with the most common one being diabetes. There are two types of diabetes: type I and type II; the former is due to the pancreas failing to produce enough insulin, and the latter is where cells fail to respond to insulin. Type II diabetes is associated with obesity, and is therefore controllable with lifestyle choices such as diet and exercise.

1.7 Gut-brain interactions

The gut-brain axis allows the two-way communication between the gastrointestinal tract (GI) and both the central (CNS) and enteric nervous system (ENS), allowing the body to link emotional and cognitive processes of the brain with peripheral intestinal functions.

Gut-brain interactions are increasingly recognised as playing an important role in determining overall food intake. Many peptides are synthesised and released from the gastrointestinal tract, and it has been shown that they physiologically influence eating behaviour via gut-brain signalling. Ghrelin is an orexigenic (appetite stimulating) peptide produced in the stomach which acts as a meal initiator. This differs from peptide YY, Pancreatic polypeptide, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and cholecystokinin which are all derived from the intestine and pancreas, and have been shown to produce satiety signals. From this it has been suggested that gut hormones can be manipulated to regulate energy balance, and as a result gut hormone based therapies could be a possible treatment for obesity.

Although the full mechanism of gut-brain interactions are extremely complex, a vital component of the hypothalamic metabolic regulatory circuit is the arcuate nucleus and the vagus nerve. Vagal afferent neurons express two different neurochemical phenotypes that either inhibit or stimulate food intake. These are: neuropeptide Y (NPY) and agouti-related neuropeptide (AgRP), which act to stimulate food intake and pro-opiomelanocortin (POMC) that inhibits feeding. The balance in activity of these neuronal circuits is critical to body weight regulation. As shown in Figure 1.7, many gut hormones have direct access to the hypothalamus, brainstem and vagus nerve. They also have a significant impact on feeding in reaction to calorific intake, which is the case for PYY3-36, via Y2 receptors. Further discussion of PYY3-36 and Y2 receptors is presented in chapter 2.

1.8 Gastrointestinal peptide hormones

1.8 Gastrointestinal peptide hormones

1.8.1 Leptin

Leptin is a hormone made by adipose cells that affects many biological mechanisms including: reproduction, the immune and inflammatory response, haematopoiesis, angiogenesis, bone formation, and wound healing. More interestingly however, Leptin helps to regulate energy balance by inhibiting hunger. This happens via a feedback mechanism where signals are sent to key regulatory centres in the brain to inhibit food intake.

After Leptin is released by adipose tissue into the bloodstream, it crosses the blood brain barrier (BBB) and binds to the hypothalamic leptin receptors. This affects the activity of many hypothalamic neurones, and the expression of various orexigenic (appetite stimulating) and anorexigenic (appetite inhibiting) neuropeptides. Orexigenic peptides include neuropeptide Y (NPY), and anorexigenic peptides include pro-opiomelanocortin (POMC) as mentioned above. It has been suggested that the interaction of both types of these neuropeptides is what provides leptin with its mechanism of action in the hypothalamus to inhibit hunger.

1.8.2 Ghrelin

Ghrelin is a 28 amino acid peptide with an octanoylated serine residue at position 3, and is produced and secreted by cells within the oxyntic glands (acid secreting glands) of the stomach. Peripheral administration of ghrelin has been shown to stimulate food intake and decrease fat utilization. This means it is involved in energy homeostasis, with

the serine residue appearing to give ghrelin these effects. Ghrelin’s function is unique compared to other gut hormones, where it acts to increase food intake rather than decrease it, making it a very important component of weight control. A study on mice that were lacking ghrelin showed evidence of this and were found to be resistant to diet

induced obesity when fed a high-fat diet. This was due to them eating less, and therefore utilizing more stored fat as an energy source.

1.8.3 Cholecystokinin (CCK)

Cholecystokinin (CCK) is an endogenous gut hormone mainly found in the duodenum and jejunum, that exists in several molecular forms with differing numbers of amino acids. Examples include CCK-8 and CCK-54 (the number indicates the number of amino

acid residues). CCK is known to act as a postprandial satiety signal and it acts via two receptors: CCK1 and CCK2. The CCK1 receptor is more important in appetite control. The receptors are located on the peripheral vagal afferent terminals, which transmit signals to part of the brain stem that is associated with appetite, such as the nucleus of the solitary tract.

1.8.4 Oxyntomodulin (OXM)

Oxyntomodulin (OXM) is a 37 amino acid peptide expressed in the central nervous system and the L cells of the intestine and pancreas. OXM seems to mediate its effects via the glucagon-like peptide 1 (GLP-1) receptor as shown in experiments carried out on rat parietal cells. This has been proven by its anorectic actions being blocked when administration of the GLP-1 antagonist was carried out. Intravenous administration of OXM in humans inhibits gastric emptying and gastric acid secretion, which leads to a feeling of satiety. This feeling of satiety can cause a reduction in both food intake and overall body weight, and this is brought about by the suppression of ghrelin.

1.8.5 Glucagon-like peptide 1 (GLP-1)

Glucagon-like peptide 1 (GLP-1) is a 30-amino acid gut derived incretin (decreases blood glucose levels) peptide hormone, meaning that it stimulates insulin secretion in response to eating, resulting in suppressed glucagon secretion. In addition to this, GLP 1 inhibits gastric emptying, and reduces appetite and food intake. GLP-1 is produced in the intestinal epithelial endocrine L-cells in the distal small bowel and colon by differential processing of proglucagon. Proglucagon is a gene expressed in the L-cells and is regulated in the gut and brain. Within minutes of food intake, the plasma levels of GLP-1 rise rapidly. GLP-1 exists in two circulating molecular forms: GLP-1(7-37) and GLP-1(7-36) amide, with GLP-1(7-36) amide representing the majority of circulating active GLP-1 in human plasma. Both forms of GLP-1 are rapidly metabolised and inactivated by the enzyme dipeptidyl peptidase-4 (DPP-4) to GLP-1(9-37), or GLP-1(9- 36)amide following the release from gut L cells. DPP-4 is a widely expressed enzyme that cleaves both forms of GLP-1 at the position 2 alanine of the N-terminal to make them inactive. The expression of DPP-4 in the gut and vascular endothelium explains the short half-life of GLP-1 of just several minutes, because the majority of immunoreactive GLP-1 entering the portal venous circulation has already been inactivated by N-terminal cleavage.

1.8.6 Pancreatic Polypeptide (PP)

Pancreatic polypeptide (PP) is a 36-amino acid peptide belonging to the family containing neuropeptide Y (NPY) and peptide YY (PYY). All of these peptides are members of the PP fold peptide family and they consist of a signal peptide, followed by a 36 amino acid active peptide and a carboxyl-terminal. The PP fold family bind to receptors Y1-Y6, but PP in particular has the highest affinity for the Y4 and Y5 receptors. The Y-receptors belong to the G protein- coupled receptor family, and they mediate a wide variety of physiological effects such as regulation of blood pressure, anxiety, memory retention, hormone release, and food intake.

PP is similar to GLP-1, where it is released into circulation after the ingestion of food, except it is produced in the endocrine F cells, located in the periphery of pancreatic islets, whereas GLP-1 is produced in the L-cells of the GI tract. PP is responsible for a number of regulatory actions, such as the inhibition of pancreatic exocrine secretion, and the modulation of gastric acid secretion, and gastric emptying. The amount of PP released is affected by the digestive state, i.e. release is very low in the fasted state, but is significantly increased throughout all phases of digestion. In addition to this, a decrease in blood glucose levels and insulin induced hypoglycaemia are stimuli for PP secretion in the brain. As a result, it is thought that PP could potentially play a significant role in the regulation of feeding behaviour to control energy homeostasis.

1.8.7 Peptide YY

Peptide YY (PYY) is a gut hormone belonging to the pancreatic polypeptide (PP) fold family, along with PP and neuropeptide Y (NPY) as mentioned above. The PP-fold motif is found throughout this family and relates to the 3D structure. The PP-fold is formed through the incorporation of certain residues which are predominately Pro2, Pro5, Pro8, Gly9, Tyr20 and Tyr27. This PP-fold has been found to protect the peptide against enzymatic attack as well as producing a hydrophobic pocket which is inherently overall energy reducing. In addition to containing the PP-fold motif, PYY and its derivative PYY3- 36 also have a high C-terminal α-helix proportion, suggested to be extremely important for the structural integrity of PYY.

PYY is released by the L-cells of the gastrointestinal tract following food intake, and there are two main endogenous forms: PYY1-36 and PYY3-36. PYY1-36 is rapidly processed by the enzyme DPP4 to the 34-amino acid peptide PYY_3-36. DPP4 hydrolyses PYY and removes the first two amino acids, tyrosine and proline, at the N-terminal, which changes the receptor selectivity. As a result of this, PYY3-36 has a high selectivity for the Y2-receptor, compared to PYY1-36 which has selectivity for the Y1, Y2, and Y5 receptors. It is thought that the Y1 receptor requires both the C-terminus and N-terminus for recognition, binding and then subsequent activation. The Y2 receptor is thought to have a smaller receptor site and also only requires the C-terminus for recognition (figure 1.8).

This could explain the reduced affinity for PYY3-36 on any other Y receptor other than Y2. Other studies replacing the amide bonds with ester bonds also confirm that the end section is important in binding and activation. The Y2 receptors are located in the hippocampus, sympathetic and parasympathetic nerve fibres, intestines, and certain blood vessels, and have been implicated in regulating food intake and gastric emptying. As a result of this, the Y2 receptor is considered a target for the treatment of obesity and type II diabetes.

Figure 1.9. Illustrations of binding sites for Y1 and Y2 with hPYY + hPYY3-36.

1.10 Physical characterisation techniques

1.10.1 Circular dichroism (CD)

Circular dichroism is a spectroscopic technique used to study biological molecules, in particular the secondary structure of peptides and proteins. Since secondary structure is sensitive to its environment, CD is a good way of observing how secondary structure changes with environmental conditions such as temperature and pH. This can then give an estimation of its conformation.

CD is a result of the interaction of polarised light with chiral molecules, and it is defined as the unequal absorption of left and right handed circularly polarised light (Equation 1.1). This occurs because when chiral molecules interact with light, one circularly polarised light state is absorbed to a great extent than the other, to produce a CD signal that is non-zero.152 Depending on which circularly polarised light (left or right) is absorbed more, determines if the CD plot is positive or negative. If the left hand circularly polarised light is absorbed to a greater extent, then the signal will be positive and vice versa. The absorbance change is written as

ΔA(λ) = A(λ)LCPL ‐ A(λ)RCPL (Equation 1.1)

LCPL = left handed circularly polarised light RCPL = right handed circularly polarised light λ = wavelength

CD spectra in the far UV region (below 260 nm), can be used to measure secondary structure, and also predict the percentages of each one present in the molecule. Each

secondary structure has distinctive peaks in the spectrum, for example peptides or proteins containing α-helices have negative bands at 222 nm and 208 nm, and a positive band at 193 nm. Antiparallel β-pleated sheets have negative bands near 216 nm and positive bands at 195 nm. Disordered structures have very low ellipticity above 210 nm and negative bands at around 195 nm.152

1.10.2 Fluorescence spectroscopy

Fluorescence occurs when an electron relaxes back to its ground state from an excited singlet state, where it releases a photon in the process. In most cases, the emitted photon has a longer wavelength than the excitation wavelength, and therefore a lower energy than the absorbed radiation; this is known as the Stokes shift. The Jablonski diagram is used to describe the fluorescence mechanism of most molecules by illustrating the electronic states of molecules and the transitions between them (Figure 1.10). When a molecule becomes excited, it transitions from its ground state, S0, to either a singlet first electronic excited state, S1 or to a second electronic excited state, S2. At each excited energy level, fluorophores have multiple vibrational levels, represented by the multiple lines in each electronic state (Figure 1.10). As the molecule loses energy and returns to its ground state, S0, a fluorescent photon is released.

Figure 1.10. Illustration of the Jablonski diagram to show the mechanism of fluorescence.153

Compounds that fluoresce are known as fluorophores and generally they have one or more aromatic groups in the structure; although non-fluorescent compounds can also fluoresce by the addition of a fluorescent probe containing aromatic structures.

Fluorescence instruments contain three basic components: a light source, a sample holder, and a detector. In fluorescence spectroscopy, a beam is passed through the sample in solution at a wavelength ranging from 180-800 nm, and the light emitted by the sample is measured by the detector. The light can be measured either by the excitation spectrum (light absorbed) or the emission spectrum (light emitted). Fluorescence spectroscopy is used to determine the concentration of a compound in solution based on its fluorescence properties, and the concentration is directly proportional to absorbance, shown by the Beer-Lambert law: A= ε l c, where A is absorbance, ε is the molar extinction coefficient (M-1.cm-1), l is pathlength (cm), and c is concentration (M).

There are several factors that affect the intensity of the fluorescence spectrum, and it depends on the excitation wavelength, the concentration of the sample, the pathlength of the cuvette, and the self-absorption of the sample.

1.10.3 Cryo-TEM and TEM

Transmission electron microscopy (TEM) is a high resolution microscopy technique where a beam of electrons is transmitted through a thin sample specimen (<100 nm thick), to form an image, as a result of the transmission of electrons through regions of the sample with different density. Electrons have a much shorter wavelength than visible light, resulting in a resolution limit of a TEM being around a thousand times smaller than a light microscope.154 The increased resolution allows structures at the nanoscale to be studied, such as bacteria and viruses.

Electrons are emitted by a negatively charged electron gun (cathode), which may be a tungsten filament needle, or a lanthanum hexaboride source. The electrons are accelerated by a series of anodes at a high voltage and under vacuum to reduce the amount of electrons colliding with the air. Typical voltages used range from 60-200 kV depending on the type of sample, but for biological samples a range of 60-100 kV is used to reduce the amount of sample damage.

TEM instruments use a range of electromagnetic lenses which consist of copper coil wires inside iron pole pieces, to create an electric field, and they help to focus the electron beam and magnify the image. There are three main lenses: the condenser lens, the objective lens, and the projector lens. The condenser lens is used to focus the electron beam onto the specimen, the objective lens is used primarily to focus and initially magnify the image. Finally the projector lens is used to further magnify the image and project it onto the imaging device such as a CCD camera.

Apertures are also used in TEM to decrease the beam intensity, and to remove electrons that are scattered to high angles, as a result of spherical or chromatic aberration, or from diffraction.155 The condenser aperture is used to control the fraction of the beam that is allowed to hit the sample, and the objective aperture is used to select which beams in the diffraction pattern contribute to the image, thus producing a diffraction contrast.

The electrons that pass through the sample can be diffracted by crystalline samples, resulting in diffraction patterns that give information about the crystal structure of the sample. Using a TEM in diffraction mode can be used in conjunction with X-ray diffraction to determine crystal structures.

Sample preparation can be quite complex in TEM, especially for biological samples as they often require negative staining to increase the contrast (electron density difference), and reveal the structure. Negative staining involves the surrounding of biomolecules with a thin layer of a heavy metal salt such as uranyl acetate.

Cryogenic transmission electron microscopy (cryo-TEM) uses the same technique as TEM but at cryogenic temperatures. Aqueous samples are plunge frozen in liquid ethane, allowing the structure of biomolecules in solution to be determined without the need for crystallisation.

1.10.4 Small angle X-ray scattering

Small angle X-ray scattering (SAXS) is a small angle scattering technique used to investigate the shape and size of nanostructures from 1-100 nm in a typical set up, with a scattering angle ranging from 0.1-10 o. In a SAXS instrument a monochromatic beam of X-rays is sent through a sample, where some interact and are scattered by the sample to produce a scattering pattern containing information on the structure. By measuring the angle-dependant distribution of the scattered intensity, the average particle structure can be determined.156

There are five main basic components of a SAXS instrument: an X-ray source, a collimation system, a sample holder, a beam stop, and a detection system. The X-ray source irradiates the sample and in lab based systems this is usually a sealed X-ray tube, a microfocus X-ray tube, or a rotating anode. Synchrotron facilities are often favoured though as they can provide a higher flux and can provide X-rays of all wavelengths as a result of the production of Bremsstrahlung, therefore producing a continuous wavelength spectrum. The collimation system is used to narrow the beam, reducing its intensity. There are two types of collimation: point collimation, where the beam is shaped to a small circular or elliptical spot, and line collimation where the beam is confined only in one dimension to give a long but narrow beam. This results in a larger illuminated sample volume and a bigger scattered intensity. The sample holders vary significantly depending on the type and state of sample being measured, and usually they are handmade. The function of the beam stop is to protect the detector from being damaged by the X-ray beam, and the detection system can be wire detectors, CCD detectors, imaging plates, and solid-state detectors.

Scattered radiation can have the same wavelength or a different wavelength than the incident radiation, as a result of scattering being able to occur with or without the loss of energy. Scattered radiation that has a different wavelength is known as inelastic scattering, such as Compton scattering, and scattered radiation with the same wavelength as the incident radiation is known as elastic scattering, such as Rayleigh or Thomson.157 SAXS analyses elastic scattering at small angles.

The interaction of X-rays with the structure produces an interference pattern at the detector, which are either constructive (in phase), or destructive (out of phase), depending on the observational angle 2θ, the orientation, and the distance r of the atoms from each other. In constructive interference, a bright spot is produced at the detector, whereas for destructive interference the radiation waves extinguish each other, producing a dark spot at the detector. The result is a 2D interference pattern, characteristic to the internal structure of the sample to give information on the orientation and distances of atoms relative to one another. The scattering angle (2θ) depends to the wavelength of the applied radiation through Bragg’s Law. To overcome this and become independent from the wavelength the scattering pattern is presented as a function of q (length of the scattering vector), where q = 4π sinθ with units of nm-λ 1. Form factors give information about the shape of particles, but require the particles to be in a dilute system where they can be considered as independent scatters without any interactions. The form factor also assumes that particles are monodisperse, but if they are polydisperse, then an average scattering pattern of all of the different sizes are obtained.158

In more concentrated systems where the particles are densely packed, the distances between the particles become similar to the distances inside the particles. This results in the interference pattern containing contributions from the neighbouring particles. The addition of this interference is multiplied with the form factor of a single particle to become the structure factor. The structure factor therefore contains information about the positions of particles with respect to each other and is calculated using the equation: I(q) = P(q)S(q), where I(q) is the intensity as a function of q, P(q) is the form factor, and S(q) is the structure factor.159 An increase in intensity generally indicates an attractive interaction, suggesting aggregation of particles. When particles aggregate they can align into an ordered arrangement to create a crystalline structure, creating distinct peaks in the scattering intensity profile. These are known as Bragg peaks, and using Bragg’s law it is possible to determine the distance, d, between aligned particles: d = 2π/q.160

1.10.5 Fibre X-ray diffraction

Fiber X-ray diffraction (XRD) is a form of X-ray scattering used to determine the structural information of a molecule, by the production of a fibre pattern from the scattered X-rays. The technique is suitable for investigation of samples that form fibres or stalks, and as such is used to investigate amyloid and related peptides.161-162 This technique is similar to SAXS, except the sample-detector distance is shorter, thus giving diffraction patterns at wider angles. Similar to SAXS, the distance between the regularly spaced atoms in a crystalline structure can be measured by determining the d-spacing using Bragg’s law. In fibre diffraction the sample has cylindrical symmetry around an axis (the fibre axis) and the ideal fibre pattern contains 4-quadrant symmetry, where the fibre axis is called the meridian and the perpendicular direction is called the equator. An example of an orientated and un-orientated fibre diffraction pattern is shown in Figure 1.11 by the Serpell group.163

References

Xiubo Zhao, F. P., Jian R. Lu, Recent development of peptide self-assembly

Progess in Natural Science 2008, 653-660.

Drew, R. C.; Gordy, W., Electron Spin Resonance Studies of Radiation Effects on Polyamino Acids. Radiation Research 1963, 18 (4), 552-579.

Mandal, D.; Nasrolahi Shirazi, A.; Parang, K., Self-assembly of peptides to nanostructures. Org Biomol Chem 2014, 12 (22), 3544-61.

Whitesides, G. M.; Boncheva, M., Beyond molecules: self-assembly of mesoscopic and macroscopic components. P. Natl. Acad. Sci. 2002, 99 (8), 4769-4774.

I.W.Hamley, Lipopetides: from self-assembly to bioactivity. The Royal Society of Chemistry 2015, 8574-8583.

Cui, H. G.; Webber, M. J.; Stupp, S. I., Self-Assembly of Peptide Amphiphiles: From Molecules to Nanostructures to Biomaterials. Biopolymers 2010, 94 (1), 1-18.

Lin, L.; Pinker, R. J.; Kallenbach, N. R., Alpha-helix stability and the native state of myoglobin. Biochem. 1993, 32 (47), 12638-43.

Rad-Malekshahi, M.; Lempsink, L.; Amidi, M.; Hennink, W. E.; Mastrobattista, E., Biomedical Applications of Self-Assembling Peptides. Bioconjugate Chem 2016, 27 (1), 3-18.

Stevens, M. M.; Flynn, N. T.; Wang, C.; Tirrell, D. A.; Langer, R., Coiled‐Coil Peptide‐Based Assembly of Gold Nanoparticles. Adv. Mater. 2004, 16 (11), 915-918.

Do, T. D.; LaPointe, N. E.; Economou, N. J.; Buratto, S. K.; Feinstein, S. C.; Shea, J.- E.; Bowers, M. T., Effects of pH and Charge State on Peptide Assembly: The YVIFL Model System. The Journal of Physical Chemistry B 2013, 117 (37), 10759-10768.

Tu, R. S.; Tirrell, M., Bottom-up design of biomimetic assemblies. Adv. Drug. Deliv. Rev. 2004, 56 (11), 1537-1563.

Dennis W. P. M. Lowik, J. C. M. v. H., Ppetide based amphiphiles. The Royal Society of Chemistry 2004, 33, 234-245.

Giddi HS, A. M., Bellare JR., Self-assembled surfactant nano-structures important in drug delivery: a review. Indian Journal of Experimental Biology 2007, 45, 133-159.

Torchilin, V. P., Micellar nanocarriers: pharmaceutical perspectives.

Pharmaceutical Research 2007, 24 (1), 1-16.

Katarzyna Kita-Tokarczyk, J. G., Thomas Haefele, Wolfgang Meier, Block copolymer vesicles - using concepts from polymer chemistry to mimic biomembranes. Polymer 2005, 11, 3540-3563.

Geourjon, C.; Deléage, G., SOPM: a self-optimized method for protein secondary structure prediction. Protein Engineering, Design and Selection 1994, 7 (2), 157-164.

Monica C. Branco, J. P. S., Self-assembling materials for therapeutic delivery.

Acta Biomateriala 2009, 817-831.

Shah RN, S. N., Lim MMDR, Hsieh C, Nuber G, Stupp SI, Supramolecular design of self-assembling nanofibers for cartilage regeneration. Proceedings of the National Academy of Sciences U.S.A 2010, 107, 3293-3298.

Accardo A, T. D., Mangiapia D, Pedone C, Morelli G, Nanostructures by self- assembly peptide amphiphile as potential selective drug carriers. J. Pept. Sci. 2006, 88, 115-121.

Dehsorkhi, A.; Castelletto, V.; Hamley, I. W., Self-assembling amphiphilic peptides. J Pept Sci 2014, 20 (7), 453-67.

Hamley, I. W., Lipopeptides: from self-assembly to bioactivity. Chem Commun (Camb) 2015, 51 (41), 8574-83.

Zhang, L.; Bulaj, G., Converting Peptides into Drug Leads by Lipidation. Curr. Med. Chem. 2012, 19 (11), 1602-1618.

Zhao, H.; Shao, D.; Jiang, C.; Shi, J.; Li, Q.; Huang, Q.; Rajoka, M. S. R.; Yang, H.; Jin, M., Biological activity of lipopeptides from Bacillus. Applied microbiology and biotechnology 2017, 101 (15), 5951-5960.

Mnif, I.; Ghribi, D., Review lipopeptides biosurfactants: Mean classes and new insights for industrial, biomedical, and environmental applications. Biopolymers 2015, 104 (3), 129-47.

Singh, P.; Cameotra, S. S., Potential applications of microbial surfactants in biomedical sciences. Trends in Biotechnology 2004, 22 (3), 142-146.

Steller, S.; Vollenbroich, D.; Leenders, F.; Stein, T.; Conrad, B.; Hofemeister, J.; Jacques, P.; Thonart, P.; Vater, J., Structural and functional organization of the fengycin synthetase multienzyme system from Bacillus subtilis b213 and A1/3. Chemistry & biology 1999, 6 (1), 31-41.

Ohno, A.; Ano, T.; Shoda, M., Production of a lipopeptide antibiotic, surfactin, by recombinant Bacillus subtilis in solid state fermentation. Biotechnology and Bioengineering 1995, 47 (2), 209-214.

Heerklotz, H.; Seelig, J., Detergent-like action of the antibiotic peptide surfactin on lipid membranes. Biophys J 2001, 81 (3), 1547-1554.

Carrillo, C.; Teruel, J. A.; Aranda, F. J.; Ortiz, A., Molecular mechanism of membrane permeabilization by the peptide antibiotic surfactin. Biochim Biophys Acta 2003, 1611 (1-2), 91-7.

Hamley, I. W.; Dehsorkhi, A.; Jauregi, P.; Seitsonen, J.; Ruokolainen, J.; Coutte, F.; Chataigné, G.; Jacques, P., Self-assembly of three bacterially-derived bioactive lipopeptides. Soft Matter 2013, 9 (40), 9572-9578.

Nasir, M. N.; Besson, F., Interactions of the antifungal mycosubtilin with ergosterol-containing interfacial monolayers. Biochim Biophys Acta 2012, 1818 (5), 1302-8.

Deleu, M.; Paquot, M.; Nylander, T., Effect of fengycin, a lipopeptide produced by Bacillus subtilis, on model biomembranes. Biophys J 2008, 94 (7), 2667-2679.

Meena, K. R.; Kanwar, S. S., Lipopeptides as the Antifungal and Antibacterial Agents: Applications in Food Safety and Therapeutics. BioMed Research International 2015, 2015, 9.

Woodworth, J. R.; Nyhart, E. H., Jr.; Brier, G. L.; Wolny, J. D.; Black, H. R., Single- dose pharmacokinetics and antibacterial activity of daptomycin, a new lipopeptide antibiotic, in healthy volunteers. Antimicrobial agents and chemotherapy 1992, 36 (2), 318-325.

Baltz, R. H.; Miao, V.; Wrigley, S. K., Natural products to drugs: daptomycin and related lipopeptide antibiotics. Nat. Prod. Rep. 2005, 22 (6), 717-741.

Silverman, J. A.; Perlmutter, N. G.; Shapiro, H. M., Correlation of daptomycin bactericidal activity and membrane depolarization in Staphylococcus aureus. Antimicrobial agents and chemotherapy 2003, 47 (8), 2538-2544.

2

2.1 Introduction

Peptide YY (PYY) is a gut hormone released by the L-cells in the gastrointestinal tract following food intake. There are two endogenous forms: PYY1—36, and PYY3-36, with the latter having a high selectivity for the Y2 G-coupled protein receptors, associated with reduced food intake via the vagal inhibitory loop.1 The vagus nerve plays an important role in controlling metabolism, by communicating with the gut via cell-signalling to understand its nutritional status. This allows the stimulation or inhibition of food intake depending on the feeding status. Unfortunately, the sensitivity of this mechanism is reduced when chronic calorie overload occurs, which helps to drive obesity.2 Studies of PYY3-36 in humans and mice have shown it to significantly reduce calorific intake.3-6 This makes PYY3-36 an attractive agent for the treatment of obesity and type II diabetes. One major downfall is its short half-life of around 9 minutes, due to rapid clearance from circulation as a result of low molecular weight.7 Peptide lipidation to overcome this issue would allow the peptide to non-covalently bind to serum albumin, increasing the molecular weight substantially, protecting the whole complex from proteolytic degradation.8

Previous self-assembly studies of PYY3-36 have shown it to adopt a predominant α-helical secondary structure.9 Here, we use a range of spectroscopic, scattering, and microscopy techniques to further investigate the self-assembly of PYY3-36, and how lipidation impacts the self-assembly. Three lipidated derivatives of PYY3-36 and the peptide itself were examined, to determine whether lipidation has an impact on the self-assembly and aggregation, as a result of the increased amphiphilicity caused by lipidation. Palmitoyl chains were covalently attached to the peptides, by substitution of γ-L- glutamoyl(Nα-hexadecanoyl) lysine residues at position 11 (PYY11), 17 (PYY17), or 23 (PYY23) as shown in Figure 2.1. Unlipidated PYY3-36 was characterized using the same techniques, and shown for comparison (Figure 2.2). Characterisation of PYY17 in solution was carried out by PhD student Sam Burholt (University of Reading), but results are included here to allow a full comparison between lipidation points.

2.1.1 Solution studies

Pyrene is a fluorophore that is sensitive to the local hydrophobic environment, 11 and has been successfully used previously to determine the cac of peptides and peptide amphiphiles.12-13 In the presence of micelles and other macromolecular systems, it becomes encapsulated in the interior regions of the aggregates.11 This allows the cac to be determined, because upon aggregation, hydrophobic sites are formed that interact with the fluorescent probes. As a result there is a distinct break in intensity of fluorescence which defines the cac. ThT is selective towards amyloid fibril structures,14-15 and upon binding, there is a distinct increase in fluorescence intensity, similar to pyrene.16 The exact binding mechanism of ThT is not yet fully understood, but it is thought that amyloid fibrils provide a ThT binding site that sterically traps the bound dye, leading to an increase in fluorescence.17

3

3.1 Introduction

The palmitoyl chain that is typically attached to the peptide is able to bind to serum albumin, causing steric hindrance, helping to delay proteolytic attack and renal clearance.2, 3-5

Studies have shown that PYY3-36 has a partially α-helical secondary structure.6 The short peptide fragments studied here are not within the α-helical part of the whole sequence, and they also contain proline residues which are structure breaking residues that disfavour α-helix confirmations.7 This occurs due to the disruption of hydrogen bonding

caused by the amide bond lacking the proton necessary for hydrogen bond stabilisation. Lipidation using palmitoyl (C16, hexadecyl) chains is favourable, as it allows the molecule to fuse with the cell membrane and potentially act as a transducing molecule in vivo.8

3.1.1 C16IKPEAP and C16IKPEAPGE

8-anilinonaphthalene-1-sulfonic acid (ANS) interacts with hydrophobic binding sites to cause an increase in fluorescence and a blue shift of the λmax.9 Pyrene is also sensitive to the hydrophobic environment, and in the presence of micelles and other macromolecular systems, it becomes encapsulated in the interior hydrophobic regions of the aggregates.10 Pyrene has been used a number of times to detect the critical aggregation concentration (cac) of peptides and amphiphilic molecules.5-6

4

4.1 Introduction

The use of short peptides in the treatment of disease is very attractive due to relative ease of synthesis and cost in comparison to larger peptide therapeutics.1 Truncated peptide fragments provide a cost effective way of investigating peptide behaviour, and to study isolated sections in more detail, to fully understand important aspects such as self-assembly, binding affinity, and bioactivity. In one example, Kumar et al. found that a truncated pentapeptide had a higher affinity for a Casitas B-Lineage Lymphoma (Cbl) tyrosine kinase binding domain than the parent peptide.2 Another study by Swedberg et al. discovered that a C-terminal truncation of exendin-4, with removal of the nine C- terminal residues, had little effect on cAMP signalling when compared to the native peptide.3 Exendin-4 is an exogenous GLP-1R agonist with a similar pharmacological profile to GLP-1, and therefore of relevance here since it is a gut hormone, similar to PYY3-36.

In this chapter the self-assembly of another truncated fragment derived from PYY3-36 is investigated. PYY3-36 is known to have a predominant alpha-helical secondary structure in solution, and aggregates into long inter-tangling fibres at its native pH in water.4 The peptide sequence studied here lies within the β-turn and α-helical section, making it very interesting in terms of its self-assembly and aggregation behaviour. The full peptide sequence of PYY3-36 is shown in Figure 4.1a, with the peptide to be studied, EELNRYY, highlighted in red. The effect of palmitoyl lipidation at the N-terminus is also examined in order to determine the impact of increased amphiphilicity on self-assembly and stability. Furthermore, gelation properties are investigated, and cytocompatibility studies are carried out using the HCT-116 cancer cell line. The amino acid sequence of the lipidated and native peptide fragments, along with their chemical structures, are shown in Figure 4.1.