(d) : Due to cyclic hemiacetal or cyclic hemiketal structures, all the pentoses and hexoses exist in two stereoisomeric forms i.e. α form in which the OH at C1 in aldoses and C2 in ketoses towards the right and β form in which it towards left. Thus glucose, fructose, ribose, etc., all exist in α and β form. Glucose exists in two forms α-Dglucose and β-D glucose.
α-D(+) glucose ⇋ equilibrium mixture ⇋ β-( D)(+) glucose
As a result of cyclization the 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 of two isomers (anomers). The isomers having – OH group to the left of the C -1 is designated β-Dglucose and other having – OH group on the right as α-Dglucose.
