In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions. The open-chain form of a hexose, which usually is favored in solutions, has the general structure H−(CHOH)n−1−C(=O)−(CHOH)6−n−H, where n is 1, 2, 3, 4, 5. Namely, five of the carbons have one hydroxyl functional group (−OH) each, connected by a single bond, and one has an oxo group (=O), forming a carbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by seven hydrogen atoms. The carbons are commonly numbered 1 to 6 starting at the end closest to the carbonyl. Hexoses are extremely important in biochemistry, both as isolated molecules (such as glucose and fructose) and as building blocks of other compounds such as starch, cellulose, and glycosides. Hexoses can form dihexose (like sucrose) by a condensation reaction that makes 1,6-glycosidic bond. When the carbonyl is in position 1, forming a formyl group (−CH=O), the sugar is called an aldohexose, a special case of aldose. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a ketone, and is called a ketohexose, a special case of ketose; specifically, an n-ketohexose. However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize; so the term "ketohexose" usually means 2-ketohexose. In the linear form, there are 16 aldohexoses and eight 2-ketohexoses, stereoisomers that differ in the spatial position of the hydroxyl groups. These species occur in pairs of optical isomers. Each pair has a conventional name (like "glucose" or "fructose"), and the two members are labeled "D-" or "L-", depending on whether the hydroxyl in position 5, in the Fischer projection of the molecule, is to the right or to the left of the axis, respectively. These labels are independent of the optical activity of the isomers. In general, only one of the two enantiomers occurs naturally (for example, D-glucose) and can be metabolized by animals or fermented by yeasts. The term "hexose" sometimes is assumed to include deoxyhexoses, such as fucose and rhamnose: compounds with general formula C6H12O6−y that can be described as derived from hexoses by replacement of one or more hydroxyl groups with hydrogen atoms.
Classification
Aldohexoses The aldohexoses are a subclass of the hexoses which, in the linear form, have the carbonyl at carbon 1, forming an aldehyde derivative with structure H−C(=O)−(CHOH)5−H. The most important example is glucose. In linear form, an aldohexose has four chiral centres, which give 16 possible aldohexose stereoisomers (24), comprising 8 pairs of enantiomers. The linear forms of the eight D-aldohexoses, in the Fischer projection, are
Of these D-isomers, all except D-altrose occur in living organisms, but only three are common: D-glucose, D-galactose, and D-mannose. The L-isomers are generally absent in living organisms; however, L-altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens. When drawn in this order, the Fischer projections of the D-aldohexoses can be identified with the 3-digit binary numbers from 0 to 7, namely 000, 001, 010, 011, 100, 101, 110, 111. The three bits, from left to right, indicate the position of the hydroxyls on carbons 4, 3, and 2, respectively: to the right if the bit value is 0, and to the left if the value is 1. The chemist Emil Fischer is said to have devised the following mnemonic device for remembering the order given above, which corresponds to the configurations about the chiral centers when ordered as 3-bit binary strings:
All altruists gladly make gum in gallon tanks. referring to allose, altrose, glucose, mannose, gulose, idose, galactose, talose. The Fischer diagrams of the eight L-aldohexoses are the mirror images of the corresponding D-isomers; with all hydroxyls reversed, including the one on carbon 5.
Ketohexoses A ketohexose is a ketone-containing hexose. The important ketohexoses are the 2-ketohexoses, and the most important 2-ketose is fructose. Besides the 2-ketoses, there are only the 3-Ketoses, and they do not exist in nature, although at least one 3-ketohexose has been synthesized, with great difficulty. In the linear form, the 2-ketohexoses have three chiral centers and therefore eight possible stereoisomers (23), comprising four pairs of enantiomers. The four D-isomers are:
The corresponding L forms have the hydroxyls on carbons 3, 4, and 5 reversed. Below are depiction of the eight isomers in an alternative style:
3-Ketohexoses In theory, the ketohexoses include also the 3-ketohexoses, which have the carbonyl in position 3; namely H−(CHOH)2−C(=O)−(CHOH)3−H. However, these compounds are not known to occur in nature, and are difficult to synthesize. In 1897, an unfermentable product obtained by treatment of fructose with bases, in particular lead(II) hydroxide, was given the name glutose, a portmanteau of glucose and fructose, and was claimed to be a 3-ketohexose. However, subsequent studies showed that the substance was a mixture of various other compounds. The unequivocal synthesis and isolation of a 3-ketohexose, xylo-3-hexulose, through a rather complex route, was first reported in 1961 by George U. Yuen and James M. Sugihara.
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