They have either 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, in the Fischer projection, are
D-Ribose D-Arabinose D-Xylose D-Lyxose
The ketopentoses have 2 chiral centers and, therefore, 4 possible stereoisomers — 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:
D-Ribulose D-Xylulose
The aldehyde and ketone functional groups in these carbohydrates react with neighbouring hydroxyl functional groups to form intramolecular hemiacetals and 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.
A polymer composed of pentose sugars is called a pentosan.
No comments:
Post a Comment