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Nomenclature

Different projections of α-D-glucopyranose. 1 = Fischer projection with C-1 at the top the anomeric centre. C-5 is the anomeric reference atom. Fig. 4 = absolute configurations.

Two anomers are designated alpha (α) or beta (β), according to the configurational relationship between the anomeric centre and the anomeric reference atom. The anomeric centre in hemiacetals is the anomeric carbon C-1, which is attached to the hemiacetal oxygen (in the ring) and in hemiketals it is carbon C-2, attached to the hemiketal oxygen. In aldohexoses and smaller carbohydrates the anomeric reference atom is the stereocenter that is farthest from anomeric carbon in the ring (the configurational atom, defining the sugar as D or L).

If the absolute configurations of the anomeric carbon and the reference atom are compared, then both are the same (R,R or S,S) in the β anomer and different (R,S or S,R) in the α anomer.

Originally, the alpha and beta terminology was based on the relative positioning of the major ring constituents. For D-hexoses, viewed in Haworth projection, the C6 carbon is drawn above the ring plane. When the anomeric constituent was above the ring (nominally "the same"), the conformation was called beta. When the anomeric constituent was below the ring (nominally "different") the conformation was alpha. For L-hexoses, with C6 drawn below the ring, these definitions would then, necessarily, be reversed. Use of alpha and beta based on the old definition still finds its way into scientific literature and other references.