Hexose

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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
   |              |             |               |
  HC-OH          HC-OH       HO-CH           HO-CH
   |              |             |               |
  HC-OH          HC-OH         HC-OH           HC-OH
   |              |             |               |
  HC-OH          HC-OH         HC-OH           HC-OH
   |              |             |               |
   CH2OH          CH2OH         CH2OH           CH2OH 
 D-Allose       D-Altrose     D-Glucose       D-Mannose
   CH=O         CH=O            CH=O            CH=O
   |            |               |               |
  HC-OH      HO-CH             HC-OH         HO-CH
   |            |               |               |
  HC-OH        HC-OH         HO-CH           HO-CH
   |            |               |               |
HO-CH        HO-CH           HO-CH           HO-CH
   |            |               |               |
  HC-OH        HC-OH           HC-OH           HC-OH
   |            |               |               |
   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.[1]

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

Ketohexoses

Structure formula of fructose

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 
  |            |             |            |
  C=O          C=O           C=O          C=O
  |            |             |            |
 HC-OH      HO-CH           HC-OH      HO-CH
  |            |             |            |
 HC-OH        HC-OH       HO-CH        HO-CH
  |            |             |            |
 HC-OH        HC-OH         HC-OH        HC-OH
  |            |             |            |
  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 or 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.

Other Information

It is a weak acid and a weak electrolyte.[citation needed]

References

  1. US patent 4966845, Stack; Robert J., "Microbial production of L-altrose", issued 1990-10-30, assigned to Government of the United States of America, Secretary of Agriculture 

See also


be-x-old:Гексозы da:Hexose de:Hexosen eo:Heksozo it:Esoso nl:Hexose sv:Hexos uk:Гексози


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