Monosaccharide

Monosaccharide

Overview
Monosaccharides are the most basic units of biologically important carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s. They are the simplest form of sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 and are usually colorless
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...

, water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

-soluble, crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

line solids. Some monosaccharides have a sweet taste
Sweetness
Sweetness is one of the five basic tastes and is almost universally regarded as a pleasurable experience. Foods rich in simple carbohydrates such as sugar are those most commonly associated with sweetness, although there are other natural and artificial compounds that are sweet at much lower...

. Examples of monosaccharides include glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 (dextrose), fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...

 (levulose), galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....

, xylose
Xylose
Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...

 and ribose
Ribose
Ribose is an organic compound with the formula C5H10O5; specifically, a monosaccharide with linear form H––4–H, which has all the hydroxyl groups on the same side in the Fischer projection....

. Monosaccharides are the building blocks of disaccharide
Disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...

s such as sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

 and polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

s (such as cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 and starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

). Further, each carbon atom that supports a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 group (except for the first and last) is chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

, giving rise to a number of isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

ic forms all with the same chemical formula.
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Encyclopedia
Monosaccharides are the most basic units of biologically important carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s. They are the simplest form of sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 and are usually colorless
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...

, water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

-soluble, crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

line solids. Some monosaccharides have a sweet taste
Sweetness
Sweetness is one of the five basic tastes and is almost universally regarded as a pleasurable experience. Foods rich in simple carbohydrates such as sugar are those most commonly associated with sweetness, although there are other natural and artificial compounds that are sweet at much lower...

. Examples of monosaccharides include glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 (dextrose), fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...

 (levulose), galactose
Galactose
Galactose , sometimes abbreviated Gal, is a type of sugar that is less sweet than glucose. It is a C-4 epimer of glucose....

, xylose
Xylose
Xylose is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional group. It is the precursor to hemicellulose, one of the main constituents of biomass...

 and ribose
Ribose
Ribose is an organic compound with the formula C5H10O5; specifically, a monosaccharide with linear form H––4–H, which has all the hydroxyl groups on the same side in the Fischer projection....

. Monosaccharides are the building blocks of disaccharide
Disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...

s such as sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

 and polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

s (such as cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 and starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

). Further, each carbon atom that supports a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 group (except for the first and last) is chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

, giving rise to a number of isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

ic forms all with the same chemical formula. For instance, galactose and glucose are both aldohexose
Aldohexose
An aldohexose is a hexose with an aldehyde group on one end.The aldohexoses have four chiral centres for a total of 16 possible aldohexose stereoisomers . Of these, only three commonly occur in nature: D-glucose, D-galactose, and D-mannose...

s, but have different chemical and physical properties.

Structure and nomenclature


With few exceptions (e.g., deoxyribose
Deoxyribose
Deoxyribose, more, precisely 2-deoxyribose, is a monosaccharide with idealized formula H---3-H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of an oxygen atom...

), monosaccharides have the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 Cx(H2O)y, where x is at least 3. Monosaccharides can be classified by the number x of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atoms they contain: diose
Diose
A diose is a monosaccharide containing two carbon atoms. Because the general chemical formula of an unmodified monosaccharide is n, where n is three or greater, it does not meet the formal definition of a monosaccharide. However, since it does fit the formula n, it is sometimes thought of as the...

 (2) triose
Triose
A triose is a monosaccharide, or simple sugar, containing three carbon atoms. There are only three possible trioses: L-Glyceraldehyde and D-Glyceraldehyde, both aldotrioses because the carbonyl group is at the end of the chain, and dihydroxyacetone, a ketotriose because the carbonyl group is in...

 (3) tetrose
Tetrose
A tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde functional group in position 1 or a ketone functional group in position 2 ....

 (4), pentose
Pentose
A pentose is a monosaccharide with five carbon atoms. Pentoses are organized into two groups. Aldopentoses have an aldehyde functional group at position 1...

 (5), hexose
Hexose
In organic chemistry, a hexose is a monosaccharide with six carbon atoms, having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2....

 (6), heptose
Heptose
A heptose is a monosaccharide with seven carbon atoms.They have either an aldehyde functional group in position 1 or a ketone functional group in position 2 ....

 (7), and so on.

The most important monosaccharide, glucose, is a hexose. Examples of heptoses include the ketoses mannoheptulose
Mannoheptulose
Mannoheptulose is a hexokinase inhibitor. It is a heptose, a monosaccharide with seven carbon atoms. By blocking the enzyme hexokinase, it prevents glucose phosphorylation. As a result, the breakdown of glucose is inhibited. It is found as D-mannoheptulose in avocados....

 and sedoheptulose
Sedoheptulose
Sedoheptulose or D-altro-heptulose is a ketoheptose — a monosaccharide with seven carbon atoms and a ketone functional group. It is one of the few heptoses found in nature....

. Monosaccharides with eight or more carbons are rarely observed as they are quite unstable.

Linear-chain monosaccharides


Simple monosaccharides have a linear and unbranched carbon skeleton with one carbonyl
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....

 (C=O) functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

, and one hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 (OH) group on each of the remaining carbon atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s. Therefore, the molecular structure of a simple monosaccharide can be written as H(CHOH)n(C=O)(CHOH)mH, where n+1+m = x; so that its elemental formula is CxH2xOx.

By convention, the carbon atoms are numbered from 1 to x along the backbone, starting from the end that is closest to the C=O group.

If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with an formyl group H(C=O)-, and is technically an aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

. In that case, the compound is termed an aldose
Aldose
An aldose is a monosaccharide that contains only one aldehyde group per molecule. The chemical formula takes the form Cnn. The simplest possible aldose is the diose glycolaldehyde, which only contains two carbon atoms....

. Otherwise, the molecule has a keto
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

 group, a carbonyl -(C=O)- between two carbons; then it is formally a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

, and is termed a ketose
Ketose
A ketose is a sugar containing one ketone group per molecule.With 3 carbon atoms, dihydroxyacetone is the simplest of all ketoses and is the only one having no optical activity. Ketoses can isomerize into an aldose when the carbonyl group is located at the end of the molecule...

. Ketoses of biological interest usually have the carbonyl at position 2.

The various classifications above can be combined, resulting in names like "aldohexose" and "ketotriose".

A more general nomenclature for open chain monosaccharides combines a Greek prefix to indicate the number of carbons (tri-, tetr-, pent-, hex-, etc.), with the suffixes '-ose' for aldoses and '-ulose' for ketoses. In the latter case, if the carbonyl is not at position 2, its position is indicated by a numeric infix. So, for example, H(C=O)(CHOH)4H is pentose, H(CHOH)(C=O)(CHOH)3H is pentulose, and H(CHOH)2(C=O)(CHOH)2H is pent-3-ulose.

Open-chain stereoisomers


Two monosaccharides with equivalent molecular graph
Molecular graph
In chemical graph theory and in mathematical chemistry, a molecular graph or chemical graph is a representation of the structural formula of a chemical compound in terms of graph theory. A chemical graph is a labeled graph whose vertices correspond to the atoms of the compound and edges correspond...

s (same chain length and same carbonyl position) may still be distinct stereoisomers, whose molecules differ in the three-dimensional arrangement of the bonds
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 of certain atoms. This happens only if the molecule contains a stereogenic center, specifically a carbon atom that is chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 (connected to four distinct molecular sub-structures). Those four bonds can have any of two configurations in space distinguished by their handedness
Handedness
Handedness is a human attribute defined by unequal distribution of fine motor skills between the left and right hands. An individual who is more dexterous with the right hand is called right-handed and one who is more skilled with the left is said to be left-handed...

. In a simple open-chain monosaccharide, every carbon is chiral except the first and the last atoms of the chain, and (in ketoses) the carbon with the keto group.

For example, the triketose H(CHOH)(C=O)(CHOH)H (glycerone, dihydroxyacetone
Dihydroxyacetone
Dihydroxyacetone , or DHA, also known as glycerone, is a simple carbohydrate with formula .DHA is primarily used as an ingredient in sunless tanning products. It is often derived from plant sources such as sugar beets and sugar cane, and by the fermentation of glycerin.-Chemistry:DHA is a...

) has no stereogenic center, and therefore exists as a single stereoisomer. The other triose, the aldose H(C=O)(CHOH)2H (glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

), has one chiral carbon — the central one, number 2 — which is bonded to groups -H, -OH, -C(OH)H2, and -(C=O)H. Therefore, it exists as two stereoisomers whose molecules are mirror images of each other (like a left and a right glove). Monosaccharides with four or more carbons may contain multiple chiral carbons, so they typically have more than two stereoisomers. The number of distinct stereoisomers with the same diagram is bounded by 2c, where c is the number of chiral carbons.

The Fischer projection
Fischer projection
The Fischer projection, devised by Hermann Emil Fischer in 1891, is a two-dimensional representation of a three-dimensional organic molecule by projection. Fischer projections were originally proposed for the depiction of carbohydrates and used by chemists, particularly in organic chemistry and...

 is a systematic way of drawing the skeletal formula
Skeletal formula
The skeletal formula of an organic compound is a shorthand representation of its molecular structure, developed by the organic chemist, Friedrich August Kekulé von Stradonitz. Skeletal formulae are ubiquitous in organic chemistry, because they are relatively quick and simple to draw. Carbon and...

 of an acyclic monosaccharide so that the handedness of each chiral carbon is well specified. Each stereoisomer of a simple open-chain monosaccharide can be identified by the positions (right or left) in the Fischer diagram of the chiral hydroxyls (the hydroxyls attached to the chiral carbons).

Most stereoisomers are themselves chiral (distinct from their mirror images). In the Fischer projection, two mirror-image isomers differ by having the positions of all chiral hydroxyls reversed right-to-left. Mirror-image isomers are chemically identical in non-chiral environments, but usually have very different biochemical properties and occurrences in nature.

While most stereoisomers can be arranged in pairs of mirror-image forms, there are some non-chiral stereoisomers that are identical to their mirror images, in spite of having chiral centers. This happens whenever the molecular graph is symmetrical, as in the 3-ketopentoses H(CHOH)2(CO)(CHOH)2H, and the two halves are mirror images of each other. In that case, mirroring is equivalent to a half-turn rotation. For this reason, there are only three distinct 3-ketopentose stereoisomers, even though the molecule has two chiral carbons.

Distinct stereoisomers that are not mirror-images of each other usually have different chamical properties, even in non-chiral environments. Therefore, each mirror pair and each non-chiral stereoisomer may be given a specific monosaccharide name
Monosaccharide nomenclature
Monosaccharide nomenclature is a set of conventions used in chemistry to name the compounds known as monosaccharides or "simple sugars" — the basic structural units of carbohydrates, which cannot be hydrolysed into simpler units....

. For example, there are 16 distinct aldohexose stereoisomers, but the name "glucose" means a specific pair of mirror-image aldohexoses. In the Fischer projection, one of the two glucose isomers has the hydroxyl at left on C3, and at right on C4 and C5; while the other isomer has the reversed pattern. These specific monosaccharide names have conventional three-letter abbreviations, like 'Glu' for glucose and 'Thr' for threose
Threose
Threose is a four-carbon monosaccharide or carbohydrate with molecular formula C4H8O4. It has a terminal aldehyde group rather than a ketone in its linear chain, and so is considered part of the aldose family of monosaccharides...

.

Generally, a monosaccharide with N asymmetrical carbons has 2^N stereoisomers.
The number of open chain stereoisomers for an Aldose monosaccharide, is larger by 1 than that of a Ketose monosaccharide of the same length.

Every Ketose will have 2^(n-3) stereoisomers where n>2 is the number of carbons.
Every Aldose will have 2^(n-2) stereoisomers where n>2 is the number of carbons.

The D/L nomenclature


Like many chiral molecules, the two stereoisomers of glyceraldehyde will gradually rotate the polarization direction of linearly polarized light as it passes through it, even in solution. The two stereoisomers are identified with the prefixes 'D-' and 'L-', according to the sense of rotation: D-glyceraldehyde is dextrorotary (rotates the polarization axis clockwise), while L-glyceraldehyde is levorotary (rotates it counterclockwise).


The 'D-' and 'L-' prefixes are also used with other monosaccharides, to distinguish two particular stereoisomers that are mirror-images of each other. For this purpose, one considers the chiral carbon that is furthest removed from the C=O group. Its four bonds must connect to -H, -OH, -C(OH)H, and the rest of the molecule. If the molecule can be rotated in space so that the directions of those four groups match those of the analog groups in D-glyceraldehyde's C2, then the isomer receives the 'D-' prefix. Otherwise, it receives the 'L-' prefix.

In the Fischer projection, the 'D-' and 'L-' prefixes specifies the configuration at the carbon atom that is second from bottom: 'D-' if the hydroxyl is on the right side, and 'L-' if it is on the left side.

Note that the 'D-' and 'L-' prefixes do not indicate the direction of rotation of polarized light, which is a combined effect of the arrangement at all chiral centers. However, the two isomers will always rotate the light in opposite directions, at the same rate.

Cyclic isomers


A monosaccharide often switches from the acyclic (open-chain) form to a cyclic
Cyclic compound
In chemistry, a cyclic compound is a compound in which a series of atoms is connected to form a loop or ring.While the vast majority of cyclic compounds are organic, a few inorganic substances form cyclic compounds as well, including sulfur, silanes, phosphanes, phosphoric acid, and triboric acid. ...

 form, through a nucleophilic addition
Nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a π bond is removed by the creation of two new covalent bonds by the addition of a nucleophile....

 reaction between the carbonyl group and one of the hydroxyls of the same molecule. The reaction creates a ring of carbon atoms closed by one bridging oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 atom. The resulting molecule has an hemiacetal
Hemiacetal
Hemiacetals and hemiketals are compounds that are derived from aldehydes and ketones respectively. The Greek word hèmi means half...

 or hemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form.

In these cyclic forms, the ring usually has 5 or 6 atoms. These forms are called furanose
Furanose
A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom...

s and pyranose
Pyranose
Pyranose is a collective term for carbohydrates that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds...

s, respectively — by analogy with furan
Furan
Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. The class of compounds containing such rings are also referred to as furans....

 and pyran
Pyran
In chemistry, a pyran, or oxine, is a six-membered heterocyclic, non-aromatic ring, consisting of five carbon atoms and one oxygen atom and containing two double bonds. The molecular formula is C5H6O. There are two isomers of pyran that differ by the location of the double bonds...

, the simplest compounds with the same carbon-oxygen ring (although they lack the double bonds of these two molecules). For example, the aldohexose glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 may form a hemiacetal linkage between the hydroxyl on carbon 1 and the oxygen on carbon 4, yielding a molecule with a 5-membered ring, called glucofuranose. The same reaction can take place between carbons 1 and 5 to form a molecule with a 6-membered ring, called glucopyranose. Cyclic forms with a 7-atom ring (the same of oxepane
Oxepane
Oxepane is a heterocyclic chemical compound with the formula .Oxepane can be polymerized by cationic initiators such as to form a crystalline solid with a melting point around 56–58 °C....

), rarely encountered, are called septanoses.


For many monosaccharides (including glucose), the cyclic forms predominate, in the solid state and in solutions, and therefore the same name commonly is used for the open- and closed-chain isomers. Thus, for example, the term "glucose" may signify glucofuranose, glucopyranose, the open-chain form, or a mixture of the three.

Cyclization creates a new stereogenic center at the carbonyl-bearing carbon The -OH group that replaces the carbonyl's oxygen may end up in two distinct positions relative to the ring's midplane. Thus each open-chain monosaccharide yields two cyclic isomers (anomer
Anomer
In carbohydrate chemistry, an anomer is a special type of epimer. It is one of two stereoisomers of a cyclic saccharide that differs only in its configuration at the hemiacetal or hemiketal carbon, also called the anomeric carbon. Anomerization is the process of conversion of one anomer to the other...

s), denoted by the prefixes 'α-' and 'β-'. The molecule can change between these two forms by a process called mutarotation
Mutarotation
Mutarotation is the change in the optical rotation that occurs by epimerization...

, that consists in a reversal of the ring-forming reaction followed by another ring formation.

Haworth projection


The three-dimensional structure of a monosaccharides in cyclic form is usually represented by its Haworth projection
Haworth projection
A Haworth projection is a common way of representing the cyclic structure of monosaccharides with a simple three-dimensional perspective.The Haworth projection was named after the English chemist Sir Norman Haworth....

. In this diagram, the α-isomer has the OH- of the anomeric carbon below the plane of the carbon atoms, and the β-isomer has the OH- of the anomeric carbon above the plane. Pyranoses typically adopt a chair conformation, similar to cyclohexane
Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon...

. In this conformation, the α-isomer has the OH- of the anomeric carbon in an axial position, whereas the β-isomer has the OH- of the anomeric carbon in equatorial position.






Derivatives


A large number of biologically important modified monosaccharides exist:
  • Amino sugar
    Amino sugar
    In chemistry, an amino sugar contains an amine group in place of a hydroxyl group. Derivatives of amine containing sugars, such as N-acetylglucosamine and sialic acid, while not formally containing an amine, are also considered amino sugars....

    s such as:
    • Galactosamine
      Galactosamine
      Galactosamine is a hexosamine derived from galactose with the molecular formula C6H13NO5. This amino sugar is a constituent of some glycoprotein hormones such as follicle-stimulating hormone and luteinizing hormone...

    • Glucosamine
      Glucosamine
      Glucosamine is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of the polysaccharides chitosan and chitin, which compose the exoskeletons of crustaceans and other arthropods, cell walls in fungi and...

    • Sialic acid
      Sialic acid
      Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone. It is also the name for the most common member of this group, N-acetylneuraminic acid...

    • N-Acetylglucosamine
      N-Acetylglucosamine
      N-Acetylglucosamine is a monosaccharide derivative of glucose. It is an amide between glucosamine and acetic acid...

  • Sulfosugars such as:
    • Sulfoquinovose
      Sulfoquinovose
      Sulfoquinovose, also known as 6-sulfoquinovose and 6-deoxy-6-sulfo-D-glucopyranose is a monosaccharide sugar that is found as a building block in the sulfolipid sulfoquinovosyl diacylglycerol . Sulfoquinovose is a sulfonic acid derivative of glucose, the sulfonic acid group is introduced into the...


See also

  • Pyranose
    Pyranose
    Pyranose is a collective term for carbohydrates that have a chemical structure that includes a six-membered ring consisting of five carbon atoms and one oxygen atom. The name derives from its similarity to the oxygen heterocycle pyran, but the pyranose ring does not have double bonds...

  • Furanose
    Furanose
    A furanose is a collective term for carbohydrates that have a chemical structure that includes a five-membered ring system consisting of four carbon atoms and one oxygen atom...

  • Glucose
    Glucose
    Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

  • Monosaccharide nomenclature
    Monosaccharide nomenclature
    Monosaccharide nomenclature is a set of conventions used in chemistry to name the compounds known as monosaccharides or "simple sugars" — the basic structural units of carbohydrates, which cannot be hydrolysed into simpler units....

  • Disaccharides
  • Oligosaccharide
    Oligosaccharide
    An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...

  • Polysaccharides
  • Sugar acid
  • Sugar alcohol
    Sugar alcohol
    A sugar alcohol is a hydrogenated form of carbohydrate, whose carbonyl group has been reduced to a primary or secondary hydroxyl group . Sugar alcohols have the general formula Hn+1H, whereas sugars have HnHCO...

  • Reducing sugar
    Reducing sugar
    A reducing sugar is any sugar that either has an aldehyde group or is capable of forming one in solution through isomerisation. This functional group allows the sugar to act as a reducing agent, for example in the Tollens' test or Benedict's test.-Chemistry:...


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