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Anomeric centre (alpha and beta)
The anomeric centre of a sugar is a stereocentre created from the intramolecular formation of an acetal (or ketal) of a sugar hydroxyl group and an aldehyde (or ketone) group. The two stereoisomers formed from the two possible stereochemistries at the anomeric centre are called anomers. These are diastereoisomers of one another.
The configuration at the anomeric centre (where the carbonyl carbon is located in the open chain form: the hemiacetal centre) is denoted alpha- or beta- by reference to the centre that determines the absolute configuration. Thus, if the anomeric centre in its ring closed (hemiacetal) form has the same configuration as the centre that determines absolute configuration, it is the a-anomer. If it has the opposite configuration it is the b-anomer.
The terms alpha and beta are NOT defined by whether the anomeric hydroxyl is axial or equatorial. That just happens to be the case for the most common sugars, the D-hexoses.
References
- Carbohydrates: The essential molecules of life, R.V. Stick, S.J. Williams, Elsevier, 2009, 474 pages.