chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

, a hexose is a

monosaccharide Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. They are usually colorless, water- so ...

(simple sugar) with six

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

atoms. The chemical formula for all hexoses is C₆H₁₂O₆, and their

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions. The open-chain form of a hexose, which usually is favored in solutions, has the general structure H–(CHOH)_''n''−1–C(=O)–(CHOH)_4−''n''–H, where ''n'' is 1, 2, or 3. Namely, five of the carbons have one

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydrox ...

functional group (–OH) each, connected by a

single bond In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons. That is, the atoms share one pair of electrons where the bond forms. Therefore, a single bond is a type of covalent bond. When shared, each of ...

, and one has an oxo group (=O), forming a

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

group (C=O). The remaining bonds of the carbon atoms are satisfied by seven

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

atoms. The carbons are commonly numbered 1 to 6 starting at the end closest to the carbonyl. Hexoses are extremely important in

biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

, both as isolated molecules (such as

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

and

fructose Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorb ...

) and as building blocks of other compounds such as

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...

, and

glycoside In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...

s. Hexoses can form dihexose (like

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refine ...

) by a condensation reaction that makes 1,6- glycosidic bond. When the carbonyl is in position 1, forming an formyl group (–CH=O), the sugar is called an aldohexose, a special case of

aldose An aldose is a monosaccharide (a simple sugar) with a carbon backbone chain with a carbonyl group on the endmost carbon atom, making it an aldehyde, and hydroxyl groups connected to all the other carbon atoms. Aldoses can be distinguished from ...

. Otherwise, if the carbonyl position is 2 or 3, the sugar is a derivative of a

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...

, and is called a ketohexose, a special case of ketose; specifically, an ''n''-ketohexose. However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize; so the term "ketohexose" usually means 2-ketohexose. In the linear form, there are 16 aldohexoses and eight 2-ketohexoses, stereoisomers that differ in the spatial position of the hydroxyl groups. These species occur in pairs of optical isomers. Each pair has a conventional name (like "glucose" or "fructose"), and the two members are labeled "-" or "-", 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 Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

of the isomers. In general, only one of the two enantiomers occurs naturally (for example, -glucose) and can be

metabolized Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

by animals or fermented by

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...

s. The term "hexose" sometimes is assumed to include deoxyhexoses, such as fucose and

rhamnose Rhamnose (Rha, Rham) is a naturally occurring deoxy sugar. It can be classified as either a methyl-pentose or a 6-deoxy- hexose. Rhamnose predominantly occurs in nature in its L-form as L-rhamnose (6-deoxy-L-mannose). This is unusual, since most ...

: compounds with general formula 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 In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl gro ...

derivative with structure H–C(=O)–(CHOH)₅–H. The most important example is

. In linear form, an aldohexose has four chiral centres, which give 16 possible aldohexose stereoisomers (2⁴),comprising 8 pairs of

enantiomer In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical anti ...

s. The linear forms of the eight -aldohexoses, in the Fischer projection, are File:DAllose Fischer.svg ,

Allose Allose is an aldohexose sugar. It is a rare monosaccharide that occurs as a 6-O-cinnamyl glycoside in the leaves of the African shrub ''Protea rubropilosa''. Extracts from the fresh-water alga ''Ochromas malhamensis'' contain this sugar but of unk ...

000

File:DAltrose Fischer.svg ,

Altrose Altrose is an aldohexose sugar. D-Altrose is an unnatural monosaccharide. It is soluble in water and practically insoluble in methanol. However, L-altrose has been isolated from strains of the bacterium ''Butyrivibrio fibrisolvens''. Altrose is a ...

001

File:DGlucose Fischer.svg ,

Glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

010

File:Mannose.svg ,

Mannose Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylat ...

011

File:DGulose Fischer.svg ,

Gulose Gulose is an aldohexose sugar. It is a monosaccharide that is very rare in nature, but has been found in archaea, bacteria and eukaryotes. It also exists as a syrup with a sweet taste. It is soluble in water and slightly soluble in methanol. Ne ...

100

File:DIdose Fischer.svg ,

- Idose
101

File:DGalactose Fischer.svg ,

- Galactose
110

File:DTalose Fischer.svg ,

- Talose
111

Of these -isomers, all except -altrose occur in living organisms, but only three are common: -glucose, -galactose, and -mannose. The -isomers are generally absent in living organisms; however, -altrose has been isolated from strains of the bacterium '' Butyrivibrio fibrisolvens''. When drawn in this order, the Fischer projections of the -aldohexoses can be identified with the 3-digit

binary number A binary number is a number expressed in the base-2 numeral system or binary numeral system, a method of mathematical expression which uses only two symbols: typically "0" ( zero) and "1" (one). The base-2 numeral system is a positional notati ...

s from 0 to 7, namely 000, 001, 010, 011, 100, 101, 110, 111. The three bits, 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 Emil Fischer is said to have devised the following

mnemonic A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding. Mnemonics make use of elaborative encoding, retrieval cues, and image ...

device for remembering the order given above, which corresponds to the configurations about the chiral centers when ordered as 3-bit binary strings: :All altruists gladly make gum in gallon tanks. referring to allose, altrose, glucose, mannose, gulose, idose, galactose, talose. The Fischer diagrams of the eight -aldohexoses are the mirror images of the corresponding -isomers; with all hydroxyls reversed, including the one on carbon 5.

Ketohexoses

A ketohexose is a

-containing hexose. The important ketohexoses are the 2-ketohexoses, and the most important 2-ketose is

. 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 (2³), comprising four pairs of enantiomers. The four -isomers are: File:DPsicose Fischer.svg ,

Psicose D-Psicose (C6H12O6), also known as D-allulose, or simply allulose, is a low-calorie epimer of the monosaccharide sugar fructose, used by some major commercial food and beverage manufacturers as a low-calorie sweetener. First identified in wheat ...

File:D-Fructose.svg ,

Fructose Fructose, or fruit sugar, is a ketonic simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorb ...

File:DSorbose Fischer.svg ,

Sorbose Sorbose is a ketose belonging to the group of sugars known as monosaccharides. It has a sweetness that is equivalent to sucrose (table sugar). The commercial production of vitamin C (ascorbic acid Vitamin C (also known as ascorbic acid ...

File:DTagatose Fischer.svg ,

- Tagatose

The corresponding forms have the hydroxyls on carbons 3, 4,and 5 reversed. Below are depiction of the eight isomers in an alternative style: File:Psicose.png ,

File:D-fructose CASCC.png ,

File:D-sorbose.png ,

File:Tagatose.png ,

- Tagatose

File:L-psicose.png ,

File:L-fructose.png ,

File:Sorbose.png ,

File:L-tagatose.png ,

- Tagatose

3-Ketohexoses

In theory, the ketohexoses include also the 3-ketohexoses, which have the carbonyl in position 3; namely H–(CHOH)₂–C(=O)–(CHOH)₃–H. However, these compounds are not known to occur in nature, and are difficult to synthesize. In 1897, an unfermentable product obtained by treatment of fructose with bases, in particular

lead(II) hydroxide Lead(II) hydroxide, Pb(OH)2, is a hydroxide of lead, with lead in oxidation state +2. In 1964 it was believed that such a simple compound did not exist, as lead basic carbonate (2PbCO3·Pb(OH)2) or lead(II) oxide (PbO) was encountered where lead ...

, was given the name ''

glutose In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily conver ...

'', a

portmanteau A portmanteau word, or portmanteau (, ) is a blend of words However, subsequent studies showed that the substance was a mixture of various other compounds.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. 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. The unequivocal synthesis and isolation of a 3-ketohexose, ''xylo''-3-hexulose, through a rather complex route, was first reported in 1961 by G. Yuen and J. Sugihara.George U. Yuen and James M. Sugihara (1961): "". ''Journal of Organic Chemistry'', volume 26, issue 5, pages 1598-1601.

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 internal rearrangement between the carbonyl group and one of the hydroxyl groups. The reaction turns the =O group into a hydroxyl, and the hydroxyl into an ether bridge (–O–) between the two carbon atoms, thus creating a ring with one

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

atom and four or five carbons. If the cycle has five carbon atoms (six atoms in total), the closed form is called a

pyranose Pyranose is a collective term for saccharides that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. There may be other carbons external to the ring. The name derives from its similarit ...

, after the 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 Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...

.Robert Thornton Morrison and Robert Neilson Boyd (1998): ''Organic Chemistry'', 6th edition. 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 A hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol to an aldehyde or a ketone, although the latter are sometimes called hemi ...

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 A hemiacetal or a hemiketal has the general formula R1R2C(OH)OR, where R1 or R2 is hydrogen or an organic substituent. They generally result from the addition of an alcohol to an aldehyde or a ketone, although the latter are sometimes called hemike ...

with a five-membered ring. The closure turns the carboxyl carbon into a 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". 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, -glucose forms an α crystal that has

specific rotation In chemistry, specific rotation ( �'') is a property of a chiral chemical compound. It is defined as the change in orientation of monochromatic plane-polarized light, per unit distance–concentration product, as the light passes through a sampl ...

of +112° and melting point of 146 °C, as well as a β crystal that has specific rotation of +19° and melting point of 150 °C. The linear form does not crystallize, and exists only in small amounts in water solutions, where it is in equilibrium with the closed forms. 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 Mutarotation is the change in the '' optical rotation'' because of the change in the equilibrium between two anomers, when the corresponding stereocenters interconvert. Cyclic sugars show mutarotation as α and β anomeric forms interconvert. The ...

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. 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''_a of 10.28, will only deprotonate at high pH, so are marginally less stable than

aldohexose In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert ...

s in solution.

Natural occurrence and uses

The aldohexose that is most important in biochemistry is -

, which is the main "fuel" for metabolism in many living organisms. The 2-ketohexoses

psicose D-Psicose (C6H12O6), also known as D-allulose, or simply allulose, is a low-calorie epimer of the monosaccharide sugar fructose, used by some major commercial food and beverage manufacturers as a low-calorie sweetener. First identified in wheat ...

and tagatose occur naturally as the -isomers, whereas

sorbose Sorbose is a ketose belonging to the group of sugars known as monosaccharides. It has a sweetness that is equivalent to sucrose (table sugar). The commercial production of vitamin C (ascorbic acid Vitamin C (also known as ascorbic acid ...

occurs naturally as the -isomer. -Sorbose is commonly used in the commercial synthesis of ascorbic acid. -Tagatose is a rare natural ketohexose that is found in small quantities in food. -

is responsible for the sweet taste of many fruits, and is a building block of

, the common sugar.

Deoxyhexoses

The term "hexose" may sometimes be used to include the deoxyaldohexoses, which have one or more

s (–OH) replaced by

atoms (–H). 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 (6-deoxy--galactose) * -Rhamnose (6-deoxy--mannose) * -Quinovose (6-deoxy--glucose), found as part of the

sulfolipid Sulfolipids are a class of lipids which possess a sulfur-containing functional group. An abundant sulfolipid is sulfoquinovosyl diacylglycerol, which is composed of a glycoside of sulfoquinovose and diacylglycerol. In plants, sulfoquinovosyl ...

sulfoquinovosyl diacylglycerol Sulfoquinovosyl diacylglycerols, abbreviated SQDG, are a class of sulfur-containing but phosphorus-free lipids (sulfolipids) found in many photosynthetic organisms. Discovery, structure and chemical properties In 1959 A. A. Benson and cowork ...

(SQDG) * -Pneumose (6-deoxy--talose)

References

External links

* * {{Carbohydrates Monosaccharides

Classification

Aldohexoses

Ketohexoses

3-Ketohexoses

Cyclic forms

Chemical properties

Natural occurrence and uses

Deoxyhexoses

See also

References

External links