A pentose is a monosaccharide with five carbon atoms.
They either have an aldehyde functional group in position 1 (aldopentoses), or a ketone functional group in position 2 (ketopentoses).
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
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HC-OH HO-CH HC-OH HO-CH
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HC-OH HC-OH HO-C HO-CH
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HC-OH HC-OH HC-OH HC-OH
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CH2OH CH2OH CH2OH CH2OH
D-Ribose D-Arabinose D-Xylose D-Lyxose
The ketopentoses have 2 chiral centres and therefore 4 possible stereoisomers - ribulose (L- and D- form) and xylulose (L- and D- form).
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.
Ribose is a constituent of RNA, and the related Deoxyribose of DNA.
See also