Hexose

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DGlucose Fischer.svg
D-Glucose.
D-Fructose.svg
D-Fructose.
Two important hexoses, in the Fischer projection.

In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. [1] [2] The chemical formula for all hexoses is C6H12O6, and their molecular weight is 180.156 g/mol. [3]

Contents

Hexoses exist in two forms, open-chain or cyclic, that easily convert into each other in aqueous solutions. [4] The open-chain form of a hexose, which usually is favored in solutions, has the general structure H−(CHOH)n−1−C(=O)−(CHOH)6−n−H, where n is 1, 2, 3, 4, 5. Namely, five of the carbons have one hydroxyl functional group (−OH) each, connected by a single bond, and one has an oxo group (=O), forming a carbonyl group (C=O). 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 glycosides. Hexoses can form dihexose (like sucrose) 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. 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. [1] [2] However, the 3-ketohexoses have not been observed in nature, and are difficult to synthesize; [5] 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 "D-" or "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, D-glucose) and can be metabolized by animals or fermented by yeasts.

The term "hexose" sometimes is assumed to include deoxyhexoses, such as fucose and rhamnose: compounds with general formula C6H12O6−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 H−C(=O)−(CHOH)5−H. [1] [2] The most important example is glucose.

In linear form, an aldohexose has four chiral centres, which give 16 possible aldohexose stereoisomers (24), comprising 8 pairs of enantiomers. The linear forms of the eight D-aldohexoses, in the Fischer projection, are

Of these D-isomers, all except D-altrose occur in living organisms, but only three are common: D-glucose, D-galactose, and D-mannose. The L-isomers are generally absent in living organisms; however, L-altrose has been isolated from strains of the bacterium Butyrivibrio fibrisolvens . [6]

When drawn in this order, the Fischer projections of the D-aldohexoses can be identified with the 3-digit binary numbers 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[ citation needed ] 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:

Allaltruists gladly make gum in gallon tanks.

referring to allose, altrose, glucose, mannose, gulose, idose, galactose, talose.

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

Ketohexoses

A ketohexose is a ketone-containing hexose. [1] [2] [7] 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 (23), comprising four pairs of enantiomers. The four D-isomers are:

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

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. [5]

In 1897, an unfermentable product obtained by treatment of fructose with 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. [12] [13] However, subsequent studies showed that the substance was a mixture of various other compounds. [13] [14]

The unequivocal synthesis and isolation of a 3-ketohexose, xylo-3-hexulose, through a rather complex route, was first reported in 1961 by George U. Yuen and James M. Sugihara. [5]

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 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 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. [4] 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 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".

Alpha-D-Glucopyranose-with-H.png
α-D-Glucopyranose.
Beta-D-Glucopyranose-with-H.png
β-D-Glucopyranose.
Alpha-D-Fructofuranose-with-H.png
α-D-Fructofuranose.
Beta-D-Fructofuranose-with-H.png
β-D-Fructofuranose.
Closed forms of D-glucose and 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, D-glucose forms an α crystal that has specific rotation 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. [4]

The linear form does not crystallize, and exists only in small amounts in water solutions, where it is in equilibrium with the closed forms. [4] 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. [9] 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 pKa of 10.28, will only deprotonate at high pH, so are marginally less stable than aldohexoses in solution.

Natural occurrence and uses

The aldohexose that is most important in biochemistry is D-glucose, which is the main "fuel" for metabolism in many living organisms.

The 2-ketohexoses psicose, fructose and tagatose occur naturally as the D-isomers, whereas sorbose occurs naturally as the L-isomer.

D-Sorbose is commonly used in the commercial synthesis of ascorbic acid. [10] D-Tagatose is a rare natural ketohexose that is found in small quantities in food. [11] D-Fructose is responsible for the sweet taste of many fruits, and is a building block of sucrose, the common sugar.

Deoxyhexoses

The term "hexose" may sometimes be used to include the deoxyhexoses, which have one or more hydroxyls (−OH) replaced by hydrogen 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

See also

Related Research Articles

<span class="mw-page-title-main">Carbohydrate</span> Organic compound than consists only of carbon, hydrogen, and oxygen

A carbohydrate is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 and thus with the empirical formula Cm(H2O)n, which does not mean the H has covalent bonds with O. However, not all carbohydrates conform to this precise stoichiometric definition, nor are all chemicals that do conform to this definition automatically classified as carbohydrates.

<span class="mw-page-title-main">Glucose</span> Naturally produced monosaccharide

Glucose is a sugar with the molecular formula C6H12O6. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using energy from sunlight. It is used by plants to make cellulose, the most abundant carbohydrate in the world, for use in cell walls, and by all living organisms to make adenosine triphosphate (ATP), which is used by the cell as energy.

Monosaccharides, also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. Chemically, monosaccharides are polyhydroxy aldehydes with the formula H-[CHOH]
n-CHO or polyhydroxy ketones with the formula H-[CHOH]
m-CO-[CHOH]
n-H with three or more carbon atoms.

In chemistry, a pentose is a monosaccharide with five carbon atoms. The chemical formula of many pentoses is C
5H
10O
5, and their molecular weight is 150.13 g/mol.

<span class="mw-page-title-main">Stereoisomerism</span> When molecules have the same atoms and bond structure but differ in 3D orientation

In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.

Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of a hydroxy group. Discovered in 1929 by Phoebus Levene, deoxyribose is most notable for its presence in DNA. Since the pentose sugars arabinose and ribose only differ by the stereochemistry at C2′, 2-deoxyribose and 2-deoxyarabinose are equivalent, although the latter term is rarely used because ribose, not arabinose, is the precursor to deoxyribose.

<span class="mw-page-title-main">Mannose</span> Chemical compound

Mannose is a sugar with the formula HOCH2(CHOH)4CHO. It is one of the monomers 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 glycosylation are associated with mutations in enzymes involved in mannose metabolism.

An aldose is a monosaccharide 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 ketoses, which have the carbonyl group away from the end of the molecule, and are therefore ketones.

In biochemistry, isomerases are a general class of enzymes that convert a molecule from one isomer to another. Isomerases facilitate intramolecular rearrangements in which bonds are broken and formed. The general form of such a reaction is as follows:

<span class="mw-page-title-main">Ketose</span> Monosaccharides with one >C=O group per molecule

In organic chemistry, a ketose is a monosaccharide containing one ketone group per molecule. The simplest ketose is dihydroxyacetone, which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide ketoses are reducing sugars, because they can tautomerize into aldoses via an enediol intermediate, and the resulting aldehyde group can be oxidised, for example in the Tollens' test or Benedict's test. Ketoses that are bound into glycosides, for example in the case of the fructose moiety of sucrose, are nonreducing sugars.

In carbohydrate chemistry, a pair of anomers is a pair of near-identical stereoisomers or diastereomers that differ at only the anomeric carbon, the carbon atom that bears the aldehyde or ketone functional group in the sugar's open-chain form. However, in order for anomers to exist, the sugar must be in its cyclic form, since in open-chain form, the anomeric carbon atom is planar and thus achiral. More formally stated, then, an anomer is an epimer at the hemiacetal/hemiketal carbon atom in a cyclic saccharide. Anomerization is the process of conversion of one anomer to the other. As is typical for stereoisomeric compounds, different anomers have different physical properties, melting points and specific rotations.

<span class="mw-page-title-main">Reducing sugar</span> Sugars that contain free OH group at the anomeric carbon atom

A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reaction, the sugar becomes a carboxylic acid.

Aldaric acids are a group of sugar acids, where the terminal hydroxyl and carbonyl groups of the sugars have been replaced by terminal carboxylic acids, and are characterised by the formula HO2C-(CHOH)n-CO2H. Oxidation of just the aldehyde yields an aldonic acid while oxidation of just the terminal hydroxyl group yields an uronic acid.) Aldaric acids cannot form cyclic hemiacetals like unoxidized sugars, but they can sometimes form lactones.

<span class="mw-page-title-main">Pyranose</span> Class of saccharide compounds structured as a 5-carbon, 1-oxygen ring

In organic chemistry, 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 similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds. A pyranose in which the anomeric −OH at C(l) has been converted into an OR group is called a pyranoside.

The molecular formula C6H12O6 (molar mass: 180.16 g/mol) may refer to:

The Kiliani–Fischer synthesis, named for German chemists Heinrich Kiliani and Emil Fischer, is a method for synthesizing monosaccharides. It proceeds via synthesis and hydrolysis of a cyanohydrin, followed by reduction of the intermediate acid to the aldehyde, thus elongating the carbon chain of an aldose by one carbon atom while preserving stereochemistry on all the previously present chiral carbons. The new chiral carbon is produced with both stereochemistries, so the product of a Kiliani–Fischer synthesis is a mixture of two diastereomeric sugars, called epimers. For example, D-arabinose is converted to a mixture of D-glucose and D-mannose.

In carbohydrate chemistry, the Lobry de Bruyn–Van Ekenstein transformation also known as the Lobry de Bruyn–Alberda van Ekenstein transformation is the base or acid catalyzed transformation of an aldose into the ketose isomer or vice versa, with a tautomeric enediol as reaction intermediate. Ketoses may be transformed into 3-ketoses, etcetera. The enediol is also an intermediate for the epimerization of an aldose or ketose.

<span class="mw-page-title-main">Isomer</span> Chemical compounds with the same molecular formula but different atomic arrangements

In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism refers to the existence or possibility of isomers.

Monosaccharide nomenclature is the naming system of the building blocks of carbohydrates, the monosaccharides, which may be monomers or part of a larger polymer. Monosaccharides are subunits that cannot be further hydrolysed in to simpler units. Depending on the number of carbon atom they are further classified into trioses, tetroses, pentoses, hexoses etc., which is further classified in to aldoses and ketoses depending on the type of functional group present in them.

<span class="mw-page-title-main">Xylose isomerase</span> Class of enzymes

In enzymology, a xylose isomerase is an enzyme that catalyzes the interconversion of D-xylose and D-xylulose. This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases interconverting aldoses and ketoses. The isomerase has now been observed in nearly a hundred species of bacteria. Xylose-isomerases are also commonly called glucose isomerase or fructose isomerases due to their ability to interconvert glucose and fructose. The systematic name of this enzyme class is α-D-xylopyranose aldose-ketose-isomerase. Other names in common use include D-xylose isomerase, D-xylose ketoisomerase, and D-xylose ketol-isomerase.

References

  1. 1 2 3 4 Thisbe K. Lindhorst (2007). Essentials of Carbohydrate Chemistry and Biochemistry (1 ed.). Wiley-VCH. ISBN   3-527-31528-4.
  2. 1 2 3 4 John F. Robyt (1997). Essentials of Carbohydrate Chemistry (1 ed.). Springer. ISBN   0-387-94951-8.
  3. ↑ Pubchem. "D-Psicose". pubchem.ncbi.nlm.nih.gov. Retrieved 2018-04-26.
  4. 1 2 3 4 Robert Thornton Morrison and Robert Neilson Boyd (1998): Organic Chemistry, 6th edition. ISBN   9780138924645
  5. 1 2 3 George U. Yuen and James M. Sugihara (1961): "". Journal of Organic Chemistry, volume 26, issue 5, pages 1598-1601. doi : 10.1021/jo01064a070
  6. ↑ USpatent 4966845,Stack; Robert J.,"Microbial production of L-altrose",issued 1990-10-30, assigned to Government of the United States of America, Secretary of Agriculture
  7. ↑ Milton Orchin, ed. (1980). The vocabulary of organic chemistry . Wiley. ISBN   978-0-471-04491-8.
  8. ↑ Pubchem. "D-Psicose". pubchem.ncbi.nlm.nih.gov. Retrieved 2018-04-26.
  9. 1 2 Pubchem. "Fructose". pubchem.ncbi.nlm.nih.gov. Retrieved 2018-04-26.
  10. 1 2 Pubchem. "Sorbose, D-". pubchem.ncbi.nlm.nih.gov. Retrieved 2018-04-26.
  11. 1 2 Pubchem. "Tagatose". pubchem.ncbi.nlm.nih.gov. Retrieved 2018-04-26.
  12. ↑ 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
  13. 1 2 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
  14. ↑ 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
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