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:

• 

  D-Allose

• 

  D-Altrose

• 

  D-Glucose

• 

  D-Mannose

• 

  D-Gulose

• 

  D-Idose

• 

  D-Galactose

• 

  D-Talose

Of these D-isomers, all except D-altrose are naturally occurring. L-Altrose, however, has been isolated from strains of the bacterium Butyrivibrio fibrisolvens.^[1]

A mnemonic for the aldohexoses is "All Altruists Gladly Make Gum in Gallon Tanks": allose, altrose, glucose, mannose, gulose, idose, galactose, talose. When drawn in this order, the Fischer projections of the D-aldohexoses follow a pattern. Allose has all four hydroxyl groups on the right. At carbon 2, the hydroxyl groups alternate right-left. At carbon 3, the first two are on the right, the next two are on the left, and so on. At carbon 4, the first four are on the right and the rest are on the left. At carbon 5, all eight D-aldohexoses have the hydroxyl group on the right.

Cyclic hemiacetals

It has been known since 1926 that 6-carbon aldose sugars form cyclic hemiacetals.^[2] 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.^[2]

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:

• 

  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

See also

• Triose
• Tetrose
• Pentose
• Heptose
• Glucose