Aldohexose

An aldohexose is a monosaccharide with six carbon atoms in which the main carbonyl group is an aldehyde, giving the class of hexose sugars known as aldohexoses. Its molecular formula is C6H12O6. As an aldose, the aldehyde is at C-1, while C-2 through C-5 are stereogenic centers, producing 16 possible stereoisomers. When the four chiral centers are assigned (C2–C5), the isomers divide into two enantiomeric series: eight D- aldohexoses and eight L- aldohexoses. The D-series includes the biologically common glucose, galactose, and mannose, along with idose, allose, altrose, talose, and gulose; the L-series are their mirror images.

In solution, aldohexoses exist in equilibrium between linear and cyclic forms. The predominant cyclic form is the six-membered pyranose ring, giving aldopyranoses such as D-glucopyranose and its diastereomers. Rings can display α or β anomerism at the anomeric carbon (C-1) due to mutarotation.

Biologically, D-glucose is the principal energy source for most organisms. The different stereoisomers have distinct tastes, reactivities, and metabolic paths; epimers at C-2, C-3, or C-4 give differences in enzymatic recognition. Aldohexoses contrast with ketohexoses, such as fructose, which are ketoses rather than aldoses.

Aldohexoses occur in nature in various polysaccharides and glycoconjugates, including starch and glycogen, and serve as building blocks for complex carbohydrates. They are widely studied in carbohydrate chemistry and biochemistry, including stereochemistry, ring-closure equilibria, and mutarotation.