Pentose

Source: Wikipedia, the free encyclopedia.

In

molecular weight is 150.13 g/mol.[2]

Pentoses are very important in

amino acids
.

Like some other monosaccharides, pentoses exist in two forms, open-chain (linear) or closed-chain (cyclic), that easily convert into each other in water solutions.

carbonyl group (C=O). The remaining bonds of the carbon atoms are satisfied by six hydrogen
atoms. Thus the structure of the linear form is H–(CHOH)x–C(=O)–(CHOH)4-x–H, where x is 0, 1, or 2.

The term "pentose" sometimes is assumed to include deoxypentoses, such as deoxyribose: compounds with general formula C
5H
10O
5-y that can be described as derived from pentoses by replacement of one or more hydroxyl groups with hydrogen atoms.

Classification

The

ketopentoses instead have the carbonyl at positions 2 or 3, forming a ketone
derivative with structure H–CHOH–C(=O)–(CHOH)3–H (2-ketopentose) or H–(CHOH)2–C(=O)–(CHOH)2–H (3-ketopentose). The latter is not known to occur in nature and are difficult to synthesize.

In the open form, there are eight aldopentoses and four 2-ketopentoses,

optical activity
).

Aldopentoses

See also: aldose

The aldopentoses have three

stereoisomers
are possible.



D-Arabinose

D-Lyxose

D-Ribose

D-Xylose


L-Arabinose

L-Lyxose

L-Ribose

L-Xylose

Ribose is a constituent of RNA, and the related molecule, deoxyribose, is a constituent of DNA. Phosphorylated pentoses are important products of the pentose phosphate pathway, most importantly ribose 5-phosphate (R5P), which is used in the synthesis of nucleotides and nucleic acids, and erythrose 4-phosphate (E4P), which is used in the synthesis of aromatic amino acids.

Ketopentoses

See also: ketose

The 2-ketopentoses have two chiral centers; therefore, four (22) different stereoisomers are possible. The 3-ketopentoses are rare.



D-Ribulose

D-Xylulose


L-Ribulose

L-Xylulose

Cyclic form

The closed or cyclic form of a pentose is created when the

ether bridge –O– between the two carbons. This intramolecular reaction yields a cyclic molecule, with a ring consisting of one oxygen atom and usually four carbon atoms; the cyclic compounds are then called furanoses, for having the same rings as the cyclic ether tetrahydrofuran.[3]

The closure turns the carboxyl carbon into a chiral center, which may have any of two configurations, depending on the position of the new hydroxyl. Therefore, each linear form can produce two distinct closed forms, identified by prefixes "α" and "β".

Deoxypentoses

See also: Deoxy sugar

The one deoxypentose has two total stereoisomers.



D-Deoxyribose


L-Deoxyribose

Properties

In the cell, pentoses have a higher

metabolic stability than hexoses
.

A polymer composed of pentose sugars is called a pentosan.

Tests for pentoses

The most important tests for pentoses rely on converting the pentose to furfural, which then reacts with a chromophore. In Tollens’ test for pentoses (not to be confused with Tollens' silver-mirror test for reducing sugars), the furfural ring reacts with phloroglucinol to produce a colored compound;[4] in the aniline acetate test with aniline acetate;[5] and in Bial's test, with orcinol.[6] In each of these tests, pentoses react much more strongly and quickly than hexoses.

References

  1. ^ Pentose, Merriam-Webster
  2. ^ "D-Ribose". PubChem compound webpage, accessed on 2010-02-06.
  3. ^ a b Morrison, Robert Thornton; Boyd, Robert Neilson. Organic Chemistry (2nd ed.). Allyn and Bacon. Library of Congress catalog 66-25695
  4. ISSN 0365-9496
    .
  5. ISBN 9781337517140
    .
  6. ISBN 0534408338
    .
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