Inchemistry, ahexose is amonosaccharide (simple sugar) with sixcarbon atoms.[1][2] The chemical formula for all hexoses isC6H12O6, and theirmolecular weight is 180.156 g/mol.[3]
Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions.[4] The open-chain form of a hexose, which usually is favored in solutions, has the general structureH−(CHOH)n−1−C(=O)−(CHOH)6−n−H, wheren is 1, 2, 3, 4, 5. Namely, five of the carbons have onehydroxyl functional group (−OH) each, connected by asingle bond, and one has an oxo group (=O), forming acarbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by sevenhydrogen atoms. The carbons are commonly numbered 1 to 6 starting at the end closest to the carbonyl.
Hexoses are extremely important inbiochemistry, both as isolated molecules (such asglucose andfructose) and as building blocks of other compounds such asstarch,cellulose, andglycosides. Hexoses can formdihexose (likesucrose) by a condensation reaction that makes 1,6-glycosidic bond.
When the carbonyl is in position 1, forming aformyl group (−CH=O), the sugar is called analdohexose, a special case ofaldose. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of aketone, and is called aketohexose, a special case ofketose; specifically, ann-ketohexose.[1][2] However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize;[5] 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 ofoptical 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 theFischer projection of the molecule, is to the right or to the left of the axis, respectively. These labels are independent of theoptical activity of the isomers. In general, only one of the two enantiomers occurs naturally (for example,D-glucose) and can bemetabolized by animals orfermented byyeasts.
The term "hexose" sometimes is assumed to includedeoxyhexoses, such asfucose andrhamnose: compounds with general formulaC6H12O6−y that can be described as derived from hexoses by replacement of one or more hydroxyl groups with hydrogen atoms.
The aldohexoses are a subclass of the hexoses which, in the linear form, have the carbonyl at carbon 1, forming analdehyde derivative with structureH−C(=O)−(CHOH)5−H.[1][2] The most important example isglucose.
In linear form, an aldohexose has fourchiral centres, which give 16 possible aldohexosestereoisomers (24), comprising 8 pairs ofenantiomers. The linear forms of the eightD-aldohexoses, in theFischer projection, are
Of theseD-isomers, all exceptD-altrose occur in living organisms, but only three are common:D-glucose,D-galactose, andD-mannose. TheL-isomers are generally absent in living organisms; however,L-altrose has been isolated from strains of the bacteriumButyrivibrio fibrisolvens.[6]
When drawn in this order, the Fischer projections of theD-aldohexoses can be identified with the 3-digitbinary numbers from 0 to 7, namely 000, 001, 010, 011, 100, 101, 110, 111. The threebits, 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 chemistEmil Fischer is said[citation needed] to have devised the followingmnemonic device for remembering the order given above, which corresponds to the configurations about the chiral centers when ordered as 3-bit binary strings:
referring toallose,altrose,glucose,mannose,gulose,idose,galactose,talose.
The Fischer diagrams of the eightL-aldohexoses are the mirror images of the correspondingD-isomers; with all hydroxyls reversed, including the one on carbon 5.
A ketohexose is aketone-containing hexose.[1][2][7] The important ketohexoses are the 2-ketohexoses, and the most important 2-ketose isfructose.
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 fourD-isomers are:
![D-Psicose[8]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.m.wikipedia.org%2fwiki%2fFile%3aDPsicose_Fischer.svg)
![D-Fructose[9]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.m.wikipedia.org%2fwiki%2fFile%3aD-Fructose.svg)
![D-Sorbose[10]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.m.wikipedia.org%2fwiki%2fFile%3aDSorbose_Fischer.svg)
![D-Tagatose[11]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.m.wikipedia.org%2fwiki%2fFile%3aDTagatose_Fischer.svg)
The correspondingL forms have the hydroxyls on carbons 3, 4, and 5 reversed. Below aredepiction of the eight isomers in an alternative style:
In theory, the ketohexoses include also the 3-ketohexoses, which have the carbonyl in position 3; namelyH−(CHOH)2−C(=O)−(CHOH)3−H. However, these compounds are not known to occur in nature, and are difficult to synthesize.[5]
In 1897, anunfermentable product obtained by treatment of fructose withbases, in particularlead(II) hydroxide, was given the nameglutose, aportmanteau ofglucose andfructose, and was claimed to be a 3-ketohexose.[12][13] However, subsequent studies showed that the substance was a mixture of various other compounds.[13][14]
The unequivocal synthesis and isolation of a 3-ketohexose,xylo-3-hexulose, through a rather complex route, was first reported in 1961 byGeorge U. Yuen andJames M. Sugihara.[5]
Like most monosaccharides with five or more carbons, each aldohexose or 2-ketohexose also exists in one or more cyclic (closed-chain) forms, derived from the open-chain form by aninternal rearrangement between the carbonyl group and one of the hydroxyl groups.
The reaction turns the=O group into a hydroxyl, and the hydroxyl into anether bridge (−O−) between the two carbon atoms, thus creating a ring with oneoxygen atom and four or five carbons.
If the cycle has five carbon atoms (six atoms in total), the closed form is called apyranose, after thecyclic ethertetrahydropyran, that has the same ring. If the cycle has four carbon atoms (five in total), the form is calledfuranose after the compoundtetrahydrofuran.[4] The conventional numbering of the carbons in the closed form is the same as in the open-chain form.
If the sugar is an aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating ahemiacetal with five- or six-membered ring, respectively. If the sugar is a 2-ketohexose, it can only involve the hydroxyl in carbon 5, and will create ahemiketal with a five-membered ring.
The closure turns the carboxyl carbon into achiral center, which may have either of two configurations, depending on the position of the new hydroxyl. Therefore, each hexose in linear form can produce two distinct closed forms, identified by prefixes "α" and "β".
It has been known since 1926 that hexoses in the crystalline solid state assume the cyclic form. The "α" and "β" forms, which are not enantiomers, will usually crystallize separately as distinct species. For example,D-glucose forms an α crystal that hasspecific rotation of +112° and melting point of 146 °C, as well as a β crystal that has specific rotation of +19° and melting point of 150 °C.[4]
The linear form does not crystallize, and exists only in small amounts in water solutions, where it is in equilibrium with the closed forms.[4] Nevertheless, it plays an essential role as the intermediate stage between those closed forms.
In particular, the "α" and "β" forms can convert to into each other by returning to the open-chain form and then closing in the opposite configuration. This process is calledmutarotation.
Although all hexoses have similar structures and share some general properties, each enantiomer pair has its own chemistry. Fructose is soluble in water, alcohol, and ether.[9] The two enantiomers of each pair generally have vastly different biological properties.
2-Ketohexoses are stable over a wide pH range, and with a primary pKa of 10.28, will only deprotonate at high pH, so are marginally less stable thanaldohexoses in solution.
The aldohexose that is most important in biochemistry isD-glucose, which is the main "fuel" for metabolism in many living organisms.
The 2-ketohexosespsicose,fructose andtagatose occur naturally as theD-isomers, whereassorbose occurs naturally as theL-isomer.
D-Sorbose is commonly used in the commercial synthesis ofascorbic acid.[10]D-Tagatose is a rare natural ketohexose that is found in small quantities in food.[11]D-Fructose is responsible for the sweet taste of many fruits, and is a building block ofsucrose, the common sugar.
The term "hexose" may sometimes be used to include the deoxyhexoses, which have one or morehydroxyls (−OH) replaced byhydrogen atoms (−H). It is named as the parent hexose, with the prefix "x-deoxy-", thex indicating the carbon with the affected hydroxyl. Some examples of biological interest are
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