In aqueous solution, more than of glucose molecules, exist in cyclic isomeric form. This conversion to a ring form arises from the open-chain form by a nucleophilic addition reaction between the aldehyde group at and the hydroxyl group at or , yielding a hemiacetal group , at . The reaction between and creates a molecule with a six-membered ring, called pyranose. The position of the hydroxyl on the C-1 determines the alpha or the beta form of the pyranose. In general, -D-glucopyranose may also be called as -D-glucose.