Under normal biological conditions monsaccharides cyclize with only small amounts of the straight chain present in solution. When an aldose cyclizes, the hydroxyl group on the second to last carbon undergoes an intramolecular reaction with the carbonyl group of the aldehyde. The product resulting from aldose cyclization is a hemiacetal. The cyclization of glucose is shown below with the group characteristic of a hemiacetal shown in red.
When a ketose cyclizes, the hydroxyl group on the second to last carbon undergoes an intramolecular reaction with the carbonyl group. The product resulting from ketose cyclization is a hemiketal. The cyclization of fructose is shown below with the group characteristic of a hemiketal shown in red.
