- Hexose
In
organic chemistry , a hexose is amonosaccharide with sixcarbon atoms, having thechemical formula C6H12O6. Hexoses are classified byfunctional group , with aldohexoses having analdehyde at position 1, and ketohexoses having aketone at position 2.Aldohexoses
The
aldohexose s have fourchiral centre s for a total of 16 possible aldohexosestereoisomer s (24). The D/L configuration is based on the orientation of thehydroxyl at position 5, and does not refer to the direction ofoptical activity .The eight D-aldohexoses are: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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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 toLouis 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
hemiacetal s.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
yeast s.Mutarotation
The aldehyde and ketone functional groups in these carbohydrates react with neighbouring
hydroxyl functional groups to formintramolecular hemiacetal s andhemiketal s, respectively. The resulting ring structure is related topyran , and is termed apyranose . The ring spontaneously opens and closes, allowing rotation to occur about the bond between thecarbonyl group and the neighbouring carbon atom, yielding two distinct configurations (α and β). This process is termedmutarotation . 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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