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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 (derived from the carbonyl carbon) is denoted alpha- or beta- by reference to the centre that determines the absolute configuration. If the substituent off the anomeric centre is on the same face of the ring as that at the configurational (D- or L-) carbon, then it is the beta-anomer. If it is oriented in the oppposite direction it is the alpha-anomer.
In the case of D-hexoses drawn in the 'usual' Haworth projection, alpha- corresponds to the anomeric substituent being on the bottom face, and beta- corresponds to the anomeric substituent being on the top face.
References
- Carbohydrates: The essential molecules of life, R.V. Stick, S.J. Williams, Elsevier, 2009, 474 pages.