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{{Short description|6-carbon simple sugar}}
{{multiple image
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|caption_align = center
|image1 = DGlucose Fischer.svg
|caption1 = {{sc|D}}-[[Glucose]].
|class1 = skin-invert-image
|image2 = D-Fructose.svg
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|caption2 = {{sc|D}}-[[Fructose]].
|footer = Two important hexoses, in the [[Fischer projection]].
|footer_align = center
}}

In [[chemistry]], a '''hexose''' is a [[monosaccharide]] (simple sugar) with six [[carbon]] atoms.<ref name=lind2007/><ref name=robyt1997/>  The chemical formula for all hexoses is {{chem2|C6H12O6}}, and their [[molecular weight]] is 180.156 g/mol.<ref name=":0">{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/D-Psicose#section=Names-and-Identifiers|title=<small>D</small>-Psicose|last=Pubchem|website=pubchem.ncbi.nlm.nih.gov|language=en|access-date=2018-04-26}}</ref>

Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions.<ref name=morr1998/> The open-chain form of a hexose, which usually is favored in solutions, has the general structure {{chem2|H\s(CHOH)_{''n''−1}\sC(\dO)\s(CHOH)_{6−''n''}\sH}}, where ''n'' is 1, 2, 3, 4, 5.  Namely, five of the carbons have one [[hydroxyl]] functional group ({{chem2|\sOH}}) each, connected by a  [[single bond]], and one has an oxo group ({{chem2|\dO}}), forming a [[carbonyl]] group ({{chem2|C\dO}}). The remaining bonds of the carbon atoms are satisfied by seven [[hydrogen]] atoms.  The carbons are commonly numbered 1 to 6 starting at the end closest to the carbonyl.

Hexoses are extremely important in [[biochemistry]], both as isolated molecules (such as  [[glucose]] and [[fructose]]) and as building blocks of other compounds such as [[starch]], [[cellulose]], and [[glycoside]]s. Hexoses  can form [[disaccharide|dihexose]] (like [[sucrose]]) by a condensation reaction that makes 1,6-[[glycosidic bond]].

When the carbonyl is in position 1, forming a [[formyl group]] ({{chem2|\sCH\dO}}), the sugar is called an '''aldohexose''', a special case of [[aldose]]. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a [[ketone]], and is called a '''ketohexose''', a special case of [[ketose]]; specifically, an '''''n''-ketohexose'''.<ref name=lind2007/><ref name=robyt1997/> However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize;<ref name=yuen1961/> so the term "ketohexose" usually means 2-ketohexose.

In the linear form, there are 16 aldohexoses and eight 2-ketohexoses, [[stereoisomer]]s that differ in the spatial position of the hydroxyl groups. These species occur in pairs of [[optical isomer]]s.  Each pair has a conventional name (like "glucose" or "fructose"), and the two members are labeled "{{sc|D}}-" or "{{sc|L}}-", depending on whether the hydroxyl in position 5, in the [[Fischer projection]] of the molecule, is to the right or to the left of the axis, respectively.  These labels are independent of the [[optical activity]] of the isomers. In general, only one of the two enantiomers occurs naturally (for example, {{sc|D}}-glucose) and can be [[metabolism|metabolized]] by animals or [[fermentation|fermented]] by [[yeast]]s.

The term "hexose" sometimes is assumed to include '''deoxyhexoses''', such as [[fucose]] and [[rhamnose]]: compounds with general formula {{chem2|C6H12O_{6-''y''}|}} that can be described as derived from hexoses by replacement of one or more hydroxyl groups with hydrogen atoms.

==Classification==
===Aldohexoses===
The aldohexoses are a subclass of the hexoses which, in the linear form, have the carbonyl at carbon 1, forming an [[aldehyde]] derivative with structure {{chem2|H\sC(\dO)\s(CHOH)5\sH}}.<ref name=lind2007>{{cite book |title=Essentials of Carbohydrate Chemistry and Biochemistry |author=Thisbe K. Lindhorst |authorlink=Thisbe Lindhorst |publisher=Wiley-VCH |edition=1 |year=2007 |isbn=3-527-31528-4}}</ref><ref name=robyt1997>{{cite book |title=Essentials of Carbohydrate Chemistry |publisher=Springer |author=John F. Robyt |edition=1 |year=1997 |isbn=0-387-94951-8}}</ref>  The most important example is [[glucose]].

In linear form, an aldohexose has four [[chiral centre]]s, which give 16 possible aldohexose [[stereoisomer]]s (2<sup>4</sup>), comprising 8 pairs of [[enantiomer]]s.  The linear forms of the eight {{sc|D}}-aldohexoses, in the [[Fischer projection]], are

<gallery class=skin-invert-image>
File:DAllose Fischer.svg | <div align="center">{{sc|D}}-[[Allose]]<br />000</div>
File:DAltrose Fischer.svg | <div align="center">{{sc|D}}-[[Altrose]]<br />001</div>
File:DGlucose Fischer.svg | <div align="center">{{sc|D}}-[[Glucose]]<br />010</div>
File:Mannose.svg | <div align="center">{{sc|D}}-[[Mannose]]<br />011</div>
File:DGulose Fischer.svg | <div align="center">{{sc|D}}-[[Gulose]]<br />100</div>
File:DIdose Fischer.svg | <div align="center">{{sc|D}}-[[Idose]]<br />101</div>
File:DGalactose Fischer.svg | <div align="center">{{sc|D}}-[[Galactose]]<br />110</div>
File:DTalose Fischer.svg | <div align="center">{{sc|D}}-[[Talose]]<br />111</div>
</gallery>

Of these {{sc|D}}-isomers, all except {{sc|D}}-altrose occur in living organisms, but only three are common: {{sc|D}}-glucose, {{sc|D}}-galactose, and {{sc|D}}-mannose. The {{sc|L}}-isomers are generally absent in living organisms; however, {{sc|L}}-altrose has been isolated from strains of the bacterium ''[[Butyrivibrio fibrisolvens]]''.<ref>{{Ref patent
 |country=US |number=4966845 |title=Microbial production of {{sc|L}}-altrose |invent1=Stack; Robert J. |status= patent |gdate=1990-10-30
|assign1=Government of the United States of America, Secretary of Agriculture
 |class=}}</ref>

When drawn in this order, the Fischer projections of the {{sc|D}}-aldohexoses can be identified with the 3-digit [[binary number]]s from 0 to 7, namely 000, 001, 010, 011, 100, 101, 110, 111.  The three [[bit]]s, from left to right, indicate the position of the hydroxyls on carbons 4, 3, and 2, respectively: to the right if the bit value is 0, and to the left if the value is 1.

The chemist [[Hermann Emil Fischer|Emil Fischer]] is said{{citation needed|date=July 2020}} to have devised the following [[mnemonic]] device for remembering the order given above, which corresponds to the configurations about the chiral centers when ordered as 3-bit binary strings:
:'''All''' '''altr'''uists '''gl'''adly '''ma'''ke '''gu'''m '''i'''n '''gal'''lon '''ta'''nks.
referring to '''all'''ose, '''altr'''ose, '''gl'''ucose, '''ma'''nnose, '''gu'''lose, '''i'''dose, '''gal'''actose, '''ta'''lose.

The Fischer diagrams of the eight {{sc|L}}-aldohexoses are the mirror images of the corresponding {{sc|D}}-isomers; with all hydroxyls reversed, including the one on carbon 5.

===Ketohexoses===
A ketohexose is a [[ketone]]-containing hexose.<ref name=lind2007/><ref name=robyt1997/><ref name=orch1980>{{cite book | title = The vocabulary of organic chemistry | editor = Milton Orchin | publisher = Wiley | year = 1980 | isbn = 978-0-471-04491-8 | url-access = registration | url = https://archive.org/details/vocabularyoforga0000unse }}</ref> 
The important ketohexoses are the 2-ketohexoses, and the most important 2-ketose is [[fructose]].

Besides the 2-ketoses, there are only the 3-Ketoses, and they do not exist in nature, although at least one 3-ketohexose has been synthesized, with great difficulty.

In the linear form, the 2-ketohexoses have three chiral centers and therefore  eight possible stereoisomers (2<sup>3</sup>), comprising four pairs of enantiomers.  The four {{sc|D}}-isomers are:

<gallery class=skin-invert-image>
File:DPsicose Fischer.svg | <div align="center">{{sc|D}}-[[Psicose]]<ref name=pub-D-psicose>{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/D-Psicose#section=Names-and-Identifiers|title={{sc|D}}-Psicose|last=Pubchem|website=pubchem.ncbi.nlm.nih.gov|language=en|access-date=2018-04-26}}</ref></div>
File:D-Fructose.svg | <div align="center">{{sc|D}}-[[Fructose]]<ref name=pub-fructose>{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/5984#section=Top|title=Fructose|last=Pubchem|website=pubchem.ncbi.nlm.nih.gov|language=en|access-date=2018-04-26}}</ref></div>
File:DSorbose Fischer.svg | <div align="center">{{sc|D}}-[[Sorbose]]<ref name=pub-sorbose>{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/107428#section=Top|title=Sorbose, {{sc|D}}-|last=Pubchem|website=pubchem.ncbi.nlm.nih.gov|language=en|access-date=2018-04-26}}</ref></div>
File:DTagatose Fischer.svg | <div align="center">{{sc|D}}-[[Tagatose]]<ref name=pub-tagatose>{{Cite web|url=https://pubchem.ncbi.nlm.nih.gov/compound/92092#section=Top|title=Tagatose|last=Pubchem|website=pubchem.ncbi.nlm.nih.gov|language=en|access-date=2018-04-26}}</ref></div>
</gallery>

The corresponding {{sc|L}} forms have the hydroxyls on carbons 3, 4, and 5 reversed.  Below are
depiction of the eight isomers in an alternative style:

<gallery class=skin-invert-image>
File:Psicose.png | <div align="center">{{sc|D}}-[[Psicose]]</div>
File:D-fructose CASCC.png | <div align="center">{{sc|D}}-[[Fructose]]</div>
File:D-sorbose.png | <div align="center">{{sc|D}}-[[Sorbose]]</div>
File:Tagatose.png | <div align="center">{{sc|D}}-[[Tagatose]]</div>
</gallery>
<gallery class=skin-invert-image>
File:L-psicose.png | <div align="center">{{sc|L}}-[[Psicose]]</div>
File:L-fructose.png | <div align="center">{{sc|L}}-[[Fructose]]</div>
File:Sorbose.png | <div align="center">{{sc|L}}-[[Sorbose]]</div>
File:L-tagatose.png | <div align="center">{{sc|L}}-[[Tagatose]]</div>
</gallery>

==3-Ketohexoses==
In theory, the ketohexoses include also the 3-ketohexoses, which have the carbonyl in position 3; namely {{chem2|H\s(CHOH)2\sC(\dO)\s(CHOH)3\sH}}.  However, these compounds are not known to occur in nature, and are difficult to synthesize.<ref name=yuen1961/>

In 1897, an [[fermentation|unfermentable]] product obtained by treatment of fructose with [[base (chemistry)|bases]], in particular [[lead(II) hydroxide]], was given the name ''glutose'', a [[portmanteau]] of ''glucose'' and ''fructose'', and was claimed to be a 3-ketohexose.<ref name=bruyn1897>C. A. Lobry de Bruyn and W. Alberda van Ekenstein (1897): "Action des alcalis sur les sucres. VI: La glutose et la pseudo‐fructose". ''Recueil des Travaux Chimiques des Pays-Bas et de la Belgique'', volume 16, issue 9, pages 274-281. {{doi|10.1002/recl.18970160903}}</ref><ref name=clark1949/>  However, subsequent studies showed that the substance was a mixture of various other compounds.<ref name=clark1949>George L. Clark, Hung Kao, Louis Sattler, and F. W. Zerban (1949): "Chemical Nature of Glutose". ''Industrial & Engineering Chemistry'', volume 41, issue 3, pages 530-533. {{doi|10.1021/ie50471a020}}</ref><ref name=sera1962>Akira Sera (1962): "Studies on the Chemical Decomposition of Simple Sugars. XIII. Separation of the So-called Glutose (a 3-Ketohexose)". ''Bulletin of the Chemical Society of Japan'', volume 35, issue 12, pages 2031-2033. {{doi|10.1246/bcsj.35.2031}}</ref>

The unequivocal synthesis and isolation of a 3-ketohexose, [[xylo-3-hexulose|''xylo''-3-hexulose]], through a rather complex route, was first reported in 1961 by [[George U. Yuen]] and [[James M. Sugihara]].<ref name=yuen1961>George U. Yuen and James M. Sugihara (1961): "". ''Journal of Organic Chemistry'', volume 26, issue 5, pages 1598-1601. {{doi|10.1021/jo01064a070}}</ref>

==Cyclic forms==
Like most monosaccharides with five or more carbons, each aldohexose or 2-ketohexose also exists in one or more cyclic (closed-chain) forms, derived from the open-chain form by an [[intramolecular reaction|internal rearrangement]] between the carbonyl group and one of the hydroxyl groups.

The reaction turns the {{chem2|\dO}} group into a hydroxyl, and the hydroxyl into an [[ether group|ether bridge]] ({{chem2|\sO\s}}) between the two carbon atoms, thus creating a ring with one [[oxygen]] atom and four or five carbons.

If the cycle has five carbon atoms (six atoms in total), the closed form is called a [[pyranose]], after the [[ether|cyclic ether]] [[tetrahydropyran]], that has the same ring.  If the cycle has four carbon atoms (five in total), the form is called [[furanose]] after the compound [[tetrahydrofuran]].<ref name=morr1998>Robert Thornton Morrison and Robert Neilson Boyd (1998): ''Organic Chemistry'', 6th edition. {{isbn|9780138924645}}</ref>  The conventional numbering of the carbons in the closed form is the same as in the open-chain form.

If the sugar is an aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating a [[hemiacetal]] with five- or six-membered ring, respectively. If the sugar is a 2-ketohexose, it can only involve the hydroxyl in carbon 5, and will create a [[hemiketal]] with a five-membered ring.

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

{{multiple image
|align = center
|total_width = 600
|image1 = Alpha-D-Glucopyranose-with-H.png
|class1 = skin-invert 
|caption1 = α-{{sc|D}}-Glucopyranose.
|image2 = Beta-D-Glucopyranose-with-H.png
|class2 = skin-invert 
|caption2 = β-{{sc|D}}-Glucopyranose.
|image3 = Alpha-D-Fructofuranose-with-H.png
|class3 = skin-invert 
|caption3 = α-{{sc|D}}-Fructofuranose.
|image4 = Beta-D-Fructofuranose-with-H.png
|class4 = skin-invert 
|caption4 = β-{{sc|D}}-Fructofuranose.
|footer = Closed forms of {{sc|D}}-glucose and {{sc|D}}-fructose, in the [[Haworth projection]].
}}

It has been known since 1926 that hexoses in the crystalline solid state assume the cyclic form.  The "α" and "β" forms, which are not enantiomers, will usually crystallize separately as distinct species. For example, {{sc|D}}-glucose forms an α crystal that has [[specific rotation]] of +112° and melting point of 146&nbsp;°C, as well as a β crystal that has specific rotation of +19° and melting point of 150&nbsp;°C.<ref name=morr1998/>

The linear form does not crystallize, and exists only in small amounts in water solutions, where it is in equilibrium with the closed forms.<ref name=morr1998/> Nevertheless, it plays an essential role as the intermediate stage between those closed forms.

In particular, the "α" and "β" forms can convert to into each other by returning to the open-chain form and then closing in the opposite configuration.  This process is called [[mutarotation]].

==Chemical properties==
Although all hexoses have similar structures and share some general properties, each enantiomer pair has its own chemistry.  Fructose is soluble in water, alcohol, and ether.<ref name=pub-fructose/> The two enantiomers of each pair generally have vastly different biological properties.

2-Ketohexoses are stable over a wide pH range, and with a primary p''K''<sub>a</sub> of 10.28, will only deprotonate at high pH, so are marginally less stable than [[aldohexose]]s in solution.

==Natural occurrence and uses==
The aldohexose that is most important in biochemistry is {{sc|D}}-[[glucose]], which is the main "fuel" for metabolism in many living organisms.

The 2-ketohexoses [[psicose]], [[fructose]] and [[tagatose]] occur naturally as the {{sc|D}}-isomers, whereas [[sorbose]] occurs naturally as the {{sc|L}}-isomer.

{{sc|D}}-Sorbose is commonly used in the commercial synthesis of [[Ascorbic acid (molecular aspects)|ascorbic acid.]]<ref name=pub-sorbose /> {{sc|D}}-Tagatose is a rare natural ketohexose that is found in small quantities in food.<ref name=pub-tagatose />  {{sc|D}}-[[Fructose]] is responsible for the sweet taste of many fruits, and is a building block of [[sucrose]], the common sugar.

==Deoxyhexoses==
{{See also|Deoxy sugar}}
The term "hexose" may sometimes be used to include the deoxyhexoses, which have one or more [[hydroxyl group|hydroxyl]]s ({{chem2|\sOH}}) replaced by [[hydrogen]] atoms ({{chem2|\sH}}). It is named as the parent hexose, with the prefix "''x''-deoxy-", the ''x'' indicating the carbon with the affected hydroxyl.  Some examples of biological interest are
* [[Fucose|{{Sma|L}}-Fucose]] (6-deoxy-{{Sma|L}}-galactose)
* [[Rhamnose|{{Sma|L}}-Rhamnose]] (6-deoxy-{{Sma|L}}-mannose)
* [[Quinovose|{{Sma|D}}-Quinovose]] (6-deoxy-{{Sma|D}}-glucose), found as part of the [[sulfolipid]] [[sulfoquinovosyl diacylglycerol]] (SQDG)
* [[Pneumose|{{Sma|L}}-Pneumose]] (6-deoxy-{{Sma|L}}-talose)
* [[Fuculose|{{Sma|L}}-Fuculose]] (6-deoxy-{{Sma|L}}-tagatose)

==See also==
* [[Diose]]
* [[Triose]]
* [[Tetrose]]
* [[Pentose]]
* [[Heptose]]
* [[Octose]]

==References==
{{Reflist}}

==External links==
* {{Commons category-inline|Aldohexoses}}
* {{Commons category-inline|Ketohexoses}}

{{Carbohydrates}}

[[Category:Hexoses| ]]
[[Category:Monosaccharides]]