Pentose

Overview
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 |          |             |            | 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 -Ribose   D -Arabinose   D -Xylose    D -Lyxose

A mnemonic suggested that can be used to remember the four D -aldopentoses is "ribs are extra lean."

The ketopentoses have 2 chiral centres and therefore 4 possible stereoisomers &mdash; ribulose ( L - and D -form) and xylulose ( L - and D -form).

The D -isomers of both are known to occur naturally as is the L -isomer of xylulose: CH2OH       CH2OH |           |        C=O          C=O |           |       HC-OH      HO-CH |           |       HC-OH        HC-OH |           |        CH2OH        CH2OH D -Ribulose   D -Xylulose

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 &mdash; yielding two distinct configurations (&alpha; and &beta;). This process is termed mutarotation.

Ribose is a constituent of RNA, and the related deoxyribose of DNA.