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{{Short description|5-Carbon simple sugar}}
In [[chemistry]], a '''pentose''' is a [[monosaccharide]] (simple sugar) with five [[carbon]] [[atom]]s.<ref>[http://www.merriam-webster.com/dictionary/pentose Pentose], Merriam-Webster</ref>  The [[chemical formula]] of many pentoses is {{chem|C|5|H|10|O|5}}, and their [[molecular weight]] is 150.13 g/mol.<ref name=pubchem-D-ribose>"[https://pubchem.ncbi.nlm.nih.gov/compound/D-Ribose {{sc|D}}-Ribose]".  PubChem compound webpage, accessed on 2010-02-06.</ref>

Pentoses are very important in [[biochemistry]]. [[Ribose]] is a constituent of [[RNA]], and the related molecule, [[deoxyribose]], is a constituent of [[DNA]]. [[Phosphorylation|Phosphorylated]] pentoses are important products of the [[pentose phosphate pathway]], most importantly [[ribose 5-phosphate]] (R5P), which is used in the synthesis of [[nucleotide]]s and [[nucleic acid]]s.

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.<ref name="m_b"/>    The linear form of a pentose, which usually exists only in solutions, has an open-chain backbone of five carbons.  Four of these carbons have one [[hydroxyl]] [[functional group]] (–OH) each, connected by a single [[covalent bond|bond]], and one has an oxygen atom connected by a double bond (=O), forming a [[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)<sub>''x''</sub>–C(=O)–(CHOH)<sub>4-''x''</sub>–H, where ''x'' is 0, 1, or 2.

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

==Classification==
The [[aldopentose]]s are a subclass of the pentoses which, in the linear form, have the carbonyl at carbon 1, forming an [[aldehyde]] derivative with structure H–C(=O)–(CHOH)<sub>4</sub>–H.  The most important example is [[ribose]].  The [[ketopentose]]s instead have the carbonyl at positions 2 or 3, forming a [[ketone]] derivative with structure H–CHOH–C(=O)–(CHOH)<sub>3</sub>–H (2-ketopentose) or H–(CHOH)<sub>2</sub>–C(=O)–(CHOH)<sub>2</sub>–H (3-ketopentose).  The latter is not known to occur in nature and are difficult to synthesize.  <!--The most important example of a 2-ketopentose is [[??]].-->

In the open form, there are eight aldopentoses and four 2-ketopentoses, [[stereoisomer]]s that differ in the spatial position of the hydroxyl groups.  These forms occur in pairs of [[optical isomer]]s, generally labelled "{{sc|D}}" or "{{sc|L}}" by conventional rules (independently of their [[optical activity]]).

===Aldopentoses===
{{See also|aldose}}
The aldopentoses have three [[chiral center]]s; therefore, eight (2<sup>3</sup>) different [[stereoisomer]]s are possible.

{| class="wikitable skin-invert-image"
|-align="center"
| [[File:D-arabinose.png|150px]]<br /><br /><small>D</small>-[[Arabinose]]
| [[File:D-lyxose.png|150px]]<br /><br /><small>D</small>-[[Lyxose]]
| [[File:D-Ribose.png|150px]]<br /><br /><small>D</small>-[[Ribose]]
| [[File:Xylose linear.png|150px]]<br /><br /><small>D</small>-[[Xylose]]
|-align="center"
| [[File:arabinose.png|150px]]<br /><br /><small>L</small>-[[Arabinose]]
| [[File:lyxose.png|150px]]<br /><br /><small>L</small>-[[Lyxose]]
| [[File:L-ribose.png|150px]]<br /><br /><small>L</small>-[[Ribose]]
| [[File:L-xylose.png|150px]]<br /><br /><small>L</small>-[[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 [[nucleotide]]s and nucleic acids, and [[erythrose 4-phosphate]] (E4P), which is used in the synthesis of [[aromatic amino acid]]s.

===Ketopentoses===
{{see also|ketose}}
The 2-ketopentoses have two chiral centers; therefore, four (2<sup>2</sup>) different stereoisomers are possible. The 3-ketopentoses are rare.

{| class="wikitable skin-invert-image"
|-align="center"
| [[File:Ribulose.png|150px]]<br /><br /><small>D</small>-[[Ribulose]]
| [[File:D-xylulose.png|150px]]<br /><br /><small>D</small>-[[Xylulose]]
|-align="center"
| [[File:L-ribulose.png|150px]]<br /><br /><small>L</small>-[[Ribulose]]
| [[File:Xylulose.png|150px]]<br /><br /><small>L</small>-[[Xylulose]]
|}

==Cyclic forms==
The closed or cyclic form of a pentose forms when the [[carbonyl group]] reacts with a [[hydroxyl]] in another carbon, turning the carbonyl into a hydroxyl and creating an [[hemiacetal|ether bridge]] –O– between the two carbons.  This [[intramolecular reaction]] yields a [[cyclic compound|cyclic]] molecule, with a ring consisting of one oxygen atom and usually four carbon atoms; the cyclic compounds are then called [[furanose]]s, for having the same rings as the [[ether|cyclic ether]] [[tetrahydrofuran]].<ref name="m_b">{{cite book|author1=Morrison, Robert Thornton  |author2=Boyd, Robert Neilson|title=Organic Chemistry|publisher=Allyn and Bacon|edition= 2nd}} Library of Congress catalog 66-25695</ref>

The ring closure converts the carbonyl carbon into a [[stereocenter|chiral center]], which may adopt either 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.
{| class="wikitable skin-invert-image"
|-align="center"
| [[File:D-deoxyribose chain.svg|150px]]<br /><br /><small>D</small>-[[Deoxyribose]]
|-align="center"
| [[File:L-Deoxyribose chain.png|150px]]<br /><br /><small>L</small>-[[L-Deoxyribose|Deoxyribose]]
|}

==Properties==
In the cell, pentoses have a higher [[metabolic]] stability than [[hexose]]s.

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 [[Bernhard Tollens|Tollens]]' test for pentoses (not to be confused with [[Tollens' reagent|Tollens' silver-mirror test]] for [[reducing sugar]]s), the [[furfural]] ring reacts with [[phloroglucinol]] to produce a colored compound;<ref>{{Cite journal |last1=Oshima |first1=Kintaro |last2=Tollens |first2=B. |date=May 1901 |title=Ueber Spectral‐Reactionen des Methylfurfurols |url=https://onlinelibrary.wiley.com/doi/10.1002/cber.19010340212 |journal=Berichte der Deutschen Chemischen Gesellschaft |language=en |volume=34 |issue=2 |pages=1425–1426 |doi=10.1002/cber.19010340212 |issn=0365-9496}}</ref> in the [[aniline acetate test]] with aniline acetate;<ref>{{Cite book|url=https://books.google.com/books?id=xfS5DQAAQBAJ&pg=PA358|title=Safety Scale Laboratory Experiments|last1=Seager|first1=Spencer L.|last2=Slabaugh|first2=Michael R.|last3=Hansen|first3=Maren S.|date=2016-12-05|publisher=Cengage Learning|isbn=9781337517140|pages=358|language=en}}</ref> and in [[Bial's test]], with [[orcinol]].<ref>{{Cite book|url=https://books.google.com/books?id=ega5c11VHvkC&pg=PA447|title=Introduction to Organic Laboratory Techniques: A Small Scale Approach|last=Pavia|first=Donald L.|date=2005|publisher=Cengage Learning|isbn=0534408338|pages=447|language=en}}</ref> In each of these tests, pentoses react much more strongly and quickly than hexoses.

==References==
{{reflist}}

{{carbohydrates}}
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[[Category:Pentoses| ]]