Aldopentose

Aldopentose
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Aldopentose".

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 — 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 — yielding two distinct configurations (α and β). This process is termed mutarotation.

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

v • d • e Types of Carbohydrates

General:	Aldose \| Ketose \| Pyranose \| Furanose

Geometry	Triose \| Tetrose \| Pentose \| Hexose \| Heptose \| Cyclohexane conformation \| Anomer \| Mutarotation

Small/Large	Glyceraldehyde \| Dihydroxyacetone \| Erythrose \| Threose \| Erythrulose \| Sedoheptulose

Trioses	Ketotriose (Dihydroxyacetone) \| Aldotriose (Glyceraldehyde)

Tetroses	Erythrulose \| Erythrose \| Threose

Pentoses	Arabinose \| Deoxyribose \| Lyxose \| Ribose \| Ribulose \| Xylose \| Xylulose

Hexoses	Glucose \| Galactose \| Mannose \| Gulose \| Idose \| Talose \| Allose \| Altrose \| Fructose \| Sorbose \| Tagatose \| Psicose \| Fucose \| Fuculose \| Rhamnose

Disaccharides	Sucrose \| Lactose \| Trehalose \| Maltose

Polymers	Glycogen \| Starch (Amylose \| Amylopectin) Cellulose \| Chitin \| Stachyose \| Inulin \| Dextrin

Glycosaminoglycans	Heparin \| Chondroitin sulfate \| Hyaluronan \| Heparan sulfate \| Dermatan sulfate \| Keratan sulfate

Aminoglycosides	Kanamycin \| Streptomycin \| Tobramycin \| Neomycin \| Paromomycin \| Apramycin \| Gentamicin \| Netilmicin \| Amikacin

Major families of biochemicals Peptides \| Amino acids \| Nucleic acids \| Carbohydrates \| Nucleotide sugars \| Lipids \| Terpenes \| Carotenoids \| Tetrapyrroles \| Enzyme cofactors \| Steroids \| Flavonoids \| Alkaloids \| Polyketides \| Glycosides

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See also