In organic chemistry, a hexose is a monosaccharide with six carbon atoms, having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2.
The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers (24). The D/L configuration is based on the orientation of the hydroxyl at position 5, and does not refer to the direction of optical activity. The eight D-aldohexoses are:
Of these D-isomers, all except D-altrose are naturally occurring. L-Altrose, however, has been isolated from strains of the bacterium Butyrivibrio fibrisolvens.
A classical jingle to remember the hexoses is "All atruists gladly make gum in gallon tank".
It has been known since 1926 that 6-carbon aldose sugars form cyclic hemiacetals. The diagram below shows the hemiacetal forms for D-glucose and D-mannose.
The numbered carbons in the open-chain forms correspond to the same numbered carbons in the hemiacetal forms. The formation of the hemiacetal causes carbon number 1, which is symmetric in the open-chain form, to become asymmetric in the cyclic version. This means that both glucose and mannose (as well as all the other aldohexoses) each have two cyclic forms. In solution, both of these exist in equilibrium with the open-chain form. The open-chain form, however, does not crystallize. Hence the two cyclic forms become separable when they are crystallized. For example, D-glucose forms an alpha crystal that has specific rotation of +112° and melting point of 146 °C, as well as a beta crystal that has specific rotation of +19° and melting point of 150 °C.
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