Science Fair Project Encyclopedia
The aldopentoses have three chiral centres ("asymmetric carbon atoms") and so 8 different stereoisomers are possible.
The 4 D-aldopentoses are :-
CH=O CH=O CH=O CH=O | | | | HC-OH HO-CH HC-OH HO-CH | | | | HC-OH HC-OH HO-C HO-CH | | | | HC-OH HC-OH HC-OH HC-OH | | | | CH2OH CH2OH CH2OH CH2OH D-Ribose D-Arabinose D-Xylose D-Lyxose
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 furan, and is termed a furanose. 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.
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