The Analysis of Macromolecules

Separation of macromolecules

Size exclusion chromatography (SEC)
- AKA gel filtration chromatography or gel permeation chromatography (GPC)
Stationary phase is a porous material called Sephadex
- Sephadex is cross-linked dextran, a polymer of glucose
Smaller molecules can penetrate every pore of the stationary phase, so are retained longer
Larger molecules cannot penetrate any of the pores, so elute faster
YouTube animation of SEC in action: http://www.youtube.com/watch?v=wjuWb6XpRmQ
It is possible to calculate the approximate molecular weight of a macromolecule using SEC
- Determine the column void volume, V_o (volume a particle sees if it cannot penetrate any pores) by flowing dextran blue (a huge molecule, m.w. 2 MDa) through the column
- Determine elution volumes V_e of several macromolecules of known molecular weight
- Plot a calibration curve: log(m.w.) vs. V_e/V_o
- Measure V_e of the analyte (a macromolecule of unknown m.w.), read off m.w. from calibration curve
Stokes radius

Scanning electron microscopy (SEM)
- Fire high-energy electrons at a sample in a vacuum chamber
- Electron beam rasters across the sample
- Detect backscattered electrons and secondary electrons
- Backscattering is greater for atoms of higher atomic number