Maltose
Explanation:
Maltose consists of two molecules of glucose that are linked by an α-(1,4′) glycosidic bond. Maltose results from the enzymatic hydrolysis of amylose, a homopolysaccharide (Section 26.9), by the enzyme amylase. Maltose is converted to two molecules of glucose by the enzyme maltase, which hydrolyzes the glycosidic bond. Commercial maltose is produced from starch that has been treated with barley malt.
The monosaccharide unit on the left is the hemiacetal of the α-d-glucopyranosyl unit. It is linked by an α-(1,4′) glycosidic bond to β-d-glucopyranose, the aglycone. The oxygen atom of the glycosidic bond is approximately in the center of the structure, between the two rings. It is projected down, axial, and therefore α. It is linked to C-4 of the aglycone, and so the link is axial–equatorial.
Maltose has a more formal, IUPAC of name: 4-O-(α-d-glucopyranosyl)-β-d-glucopyranose. This rather forbidding name is not quite as bad as it looks. The term in parentheses refers to the glucose unit on the left, which contributes the acetal portion of the glycosidic bond. The term -pyrano- tells us that this part of the structure is a six-membered ring, and the suffix -osyl indicates that the ring is linked to a partner by a glycosidic bond. The prefix 4-O- refers to the position of the oxygen atom on the aglycone, the right-hand ring. The term β-d-glucopyranose describes the aglycone.
Because the aglycone is a hemiacetal, maltose undergoes mutarotation. For the same reason maltose is a reducing sugar. The free aldehyde formed by ring opening can react with Benedict’s solution. The acetal part of the structure is called the “nonreducing end” of the disaccharide. If we do not want to specify the configuration of the aglycone, we use the name 4-O-(α-d-glucopyranosyl)-d-glucopyranose.
