Hexose


Hexose

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.

Aldohexoses

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:

CH=O CH=O CH=O CH=O
| |
HC-OH HO-CH HC-OH HO-CH
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HC-OH HC-OH HO-CH HO-CH
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HC-OH HC-OH HC-OH HC-OH
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HC-OH HC-OH HC-OH HC-OH
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CH2OH CH2OH CH2OH CH2OH D-Allose D-Altrose D-Glucose D-Mannose

CH=O CH=O CH=O CH=O
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HC-OH HO-CH HC-OH HO-CH
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HC-OH HC-OH HO-CH HO-CH
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HO-CH HO-CH HO-CH HO-CH
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HC-OH HC-OH HC-OH HC-OH
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CH2OH CH2OH CH2OH CH2OH D-Gulose D-Idose D-Galactose D-Talose

Of these D isomers, all except altrose are naturally occurring. L-altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens. [Ref patent
country=US |number=4966845 |title=Microbial production of L-altrose |invent1=Stack; Robert J. |status= patent |gdate=1990-10-30
assign1=Government of the United States of America, Secretary of Agriculture
class=
]

A mnemonic (attributed to Louis Fieser) often employed to remember the eight aldohexoses is "all altruists gladly make gum in gallon tanks".

Cyclic Hemiacetals

It has been known since 1926 that 6-carbon aldose sugars form cyclic hemiacetals.cite book|author=Morrison, Robert Thornton and Boyd, Robert Neilson|title=Organic Chemistry|, 2nd ed.|publisher=Allyn and Bacon Library of Congress catalog 66-25695] 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.

Ketohexoses

The ketohexoses have 3 chiral centres and therefore eight possible stereoisomers (23). Of these, only the four D-isomers are known to occur naturally: CH2OH CH2OH CH2OH CH2OH
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C=O C=O C=O C=O
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HC-OH HO-CH HC-OH HO-CH
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HC-OH HC-OH HO-CH HO-CH
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HC-OH HC-OH HC-OH HC-OH
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CH2OH CH2OH CH2OH CH2OH D-psicose D-fructose D-sorbose D-tagatose

Only the naturally occurring hexoses are capable of being fermented by yeasts.

Mutarotation

The aldehyde and ketone functional groups in these carbohydrates react with neighbouring hydroxyl functional groups to form intramolecular hemiacetals and hemiketals, respectively. The resulting ring structure is related to pyran, and is termed a pyranose. The ring spontaneously opens and closes, allowing rotation to occur about the bond between the carbonyl group and the neighbouring carbon atom, yielding two distinct configurations (α and β). This process is termed mutarotation. Hexose sugars can form dihexose sugars with a condensation reaction to form a 1,6-glycosidic bond.

References

ee also

* Triose
* Tetrose
* Pentose
* Heptose
* Glucose


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Look at other dictionaries:

  • hexose — [ ɛgzoz ] n. m. • 1890; de hex(a) et 1. ose ♦ Biochim. Sucre non hydrolysable possédant dans sa molécule six atomes de carbone (ex. galactose, glucose, fructose). ● hexose nom masculin Nom générique des oses C6H12O6, dérivant de l hexane.… …   Encyclopédie Universelle

  • Hexose — Hex ose, n. [Hexa + ose.] (Chem.) Any member of a group of sugars containing six carbon atoms in the molecule. Some are widely distributed in nature, esp. in ripe fruits. [Webster 1913 Suppl.] …   The Collaborative International Dictionary of English

  • hexose — hexose. См. гексоза. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • hexose — [hek′sōs΄] n. [ HEX(A) + OSE1] any monosaccharide containing six carbon atoms in each molecule, as dextrose or fructose …   English World dictionary

  • hexose — A monosaccharide containing six carbon atoms in the molecule (C6H12O6); d glucose is the principal h. in nature. * * * hex·ose hek .sōs, .sōz n any monosaccharide (as glucose) containing six carbon atoms in the molecule * * * n. a simple sugar… …   Medical dictionary

  • hexose — n. a simple sugar with six carbon atoms. Hexose sugars are the sugars most frequently found in food. The most important hexose is glucose …   The new mediacal dictionary

  • Hexose — Les hexoses sont des oses (monosaccharides) qui comportent 6 atomes de carbone. Ils ont tous la même formule brute C6H12O6. Ils possèdent tous un groupement carbonyle : soit une fonction aldéhyde en position 1 (aldohexoses) soit une fonction …   Wikipédia en Français

  • hexose — noun A sugar or saccharide containing six carbon atoms. Glucose is a common hexose …   Wiktionary

  • hexose — heksozė statusas T sritis chemija apibrėžtis Monosacharidas, kurio pagrindinė grandinė susideda iš šešių C atomų. atitikmenys: angl. hexose rus. гексоза …   Chemijos terminų aiškinamasis žodynas

  • hexose — noun Etymology: International Scientific Vocabulary Date: 1890 a monosaccharide (as glucose) containing six carbon atoms in a molecule …   New Collegiate Dictionary


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