Hexose
|
|
A hexose is a monosaccharide with six carbon atoms.
They either have an aldehyde functional group in position 1 (aldohexoses), or a ketone functional group in position 2 (ketohexoses).
The aldohexoses have four chiral centres ("asymmetric carbon atoms") and so 16 different stereoisomers are possible. Of these, only 3 occur in nature - D-glucose, D-galactose and D-mannose. The D/L configuration is based on the orientation of the hydroxyl radical at position 3, 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 | | | | CH2-OH CH2-OH CH2-OH CH2-OH 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 | | | | CH2-OH CH2-OH CH2-OH CH2-OH D-Gulose D-Idose D-Galactose D-Talose
Mnemotechnic trick : all altruists gladly make gum in gallon tanks.
The ketohexoses have 3 chiral centres and therefore 8 possible stereoisomers. Of these, only the 4 D-isomers are known to occur naturally - D-Fructose, D-Psicose, D-Sorbose and D-Tagatose.
Interestingly, only the naturally occurring hexoses are capable of being fermented by yeasts.
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.