Due to cyclic hemiacetal or cyclic hemiketal structures, all the pentoses and hexoses exist in two stereoisomeric forms i.e., a form in which the OH at C 1 in aldoese and C 2 in ketoses lies towards the right and b form in which it lies towards left. Thus glucose, fructose, ribose etc, all exist in α and β form. Glucose exists in to forms α -D-glucose and β -D glucose.

α -D-(+) equilibrium mixture β -

glucose (+) glucose

As a result of cyclization of anomeric (C–1) becomes asymmetric and the newly formed –OH group may be either on left or on right in Fischer projection thus resulting in the formation to two isomers (anomers). The isomers having –OH group at the left of the C–1 designated β -D glucose and the other having –OH group on the right as α -D-glucose.

and