Pentose

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-C         HO-CH
 |           |             |            |
HC-OH       HC-OH         HC-OH        HC-OH
 |           |             |            |
 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

• triose, tetrose, hexose
• monosaccharide, disaccharide