AlphaDGlucopyranose

Alpha-D-glucopyranose is the alpha anomer of D-glucose in its six-membered ring form, known as glucopyranose. In solution glucose cyclizes by reaction of the aldehyde group at C1 with the hydroxyl group on C5 to give a hemiacetal, producing a pyranose ring in which the anomeric carbon is C1. In the alpha anomer, the hydroxyl group on C1 is oriented on the opposite side of the ring from the CH2OH substituent at C5 (trans relationship). The ring is sometimes depicted in Haworth or chair representations, with the alpha form distinguished by the down orientation of the C1 OH in the Haworth view.

In aqueous solutions, alpha-D-glucopyranose exists in equilibrium with beta-D-glucopyranose through mutarotation. The open-chain form briefly reopens to allow interconversion between stereochemistry at C1, and the ring closes again in either the alpha or beta configuration. At room temperature in water, the equilibrium mixture is typically about 36% alpha and 64% beta.

Biologically and chemically, alpha-D-glucopyranose is a fundamental building block. It is the monomeric unit of starch and glycogen, where glucose units are linked primarily by α(1→4) and branched α(1→6) glycosidic bonds. In contrast, cellulose is formed from β-D-glucopyranose units linked by β(1→4) bonds. The formula is C6H12O6 (molar mass approximately 180.16 g/mol), and as a reducing sugar, alpha-D-glucopyranose can exist in a free aldehyde form under open-chain conditions.