Hexose
In bio-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:
D-Allose
D-Altrose
D-Glucose
D-Mannose
D-Gulose
D-Idose
D-Talose
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 mnemonic for the aldohexoses is "All Altruists Gladly Make Gum in Gallon Tanks": allose, altrose, glucose, mannose, gulose, idose, galactose, talose. When drawn in this order, the Fischer projections of the D-aldohexoses follow a pattern: beginning with allose having all four hydroxyl groups on the right, going through the mnemonic at carbon 2 the hydroxyl groups alternate right-left with each different sugar (allose left, altrose right, glucose left, etc.). At carbon 3, the first two are on the right, the next two are on the left, and so on. At carbon 4, the first four are on the right and the rest are on the left. At carbon 5, all eight D-aldohexoses have the hydroxyl group on the right (because if the hydroxyl group were on the left instead, due to the d/l system naming convention, they would be L-enantiomers of the D versions seen above).
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