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Monosaccharides (from Greek monos: single, sacchar: sugar) are the most basic unit of carbohydrates. They consist of one sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose, galactose, xylose and ribose. Monosaccharides are the building blocks of disaccharides such as sucrose and polysaccharides (such as cellulose and starch). Further, each carbon atom that supports a hydroxyl group (except for the first and last) is chiral, giving rise to a number of isomeric forms all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but have different chemical and physical properties.
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Structure
With few exceptions (e.g., deoxyribose), monosaccharides have the chemical formula Cx(H2O)y with the chemical structure H(CHOH)nC=O(CHOH)mH. If n or m is zero, it is an aldehyde and is termed an aldose, otherwise it is a ketone and is termed a ketose. Monosaccharides contain either a ketone or aldehyde functional group, and hydroxyl groups on most or all of the non-carbonyl carbon atoms.
Cyclic structure
Most monosaccharides form cyclic structures, which predominate in aqueous solution, by forming hemiacetals or hemiketals (depending on whether they are aldoses or ketoses) between an alcohol and the carbonyl group of the same sugar. Glucose, for example, readily forms a hemiacetal linkage between its carbon-1 and the hydroxyl group of its carbon-5. Since such a reaction introduces an additional stereogenic center, two anomers are formed (α-isomer and β-isomer) from each distinct straight-chain monosaccharide. The interconversion between these two forms is called mutarotation.1
A common way of representing the cyclic structure of monosaccharides is the Haworth projection. In Haworth projection, the α-isomer has the OH- of the anomeric carbon under the ring structure, and the β-isomer, has the OH- of the anomeric carbon on top of the ring structure. In chair 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.
Monosaccharide nomenclature
Monosaccharides are classified by the number of carbon atoms they contain:
- Triose, 3 carbon atoms
- Tetrose, 4 carbon atoms
- Pentose, 5 carbon atoms
- Hexose, 6 carbon atoms
- Heptose, 7 carbon atoms
- Octose, 8 carbon atoms
- Nonose, 9 carbon atoms
- Decose, 10 carbon atoms
Monosaccharides are classified the type of carbonyl group they contain:
Isomerism
The total number of possible stereoisomers of one compound (n) is dependent on the number of stereogenic centers (c) in the molecule. The upper limit for the number of possible stereoisomers is n = 2c. The only carbohydrate without an isomer is dihydroxyacetone or DHA.
Monosaccharides are classified according to their molecular configuration at the chiral carbon furthest removed from the aldehyde or ketone group. The chirality at this carbon is compared to the chirality of carbon 2 on glyceraldehyde. If it is equivalent to D-glyceraldehyde's C2, the sugar is D; if it is equivalent to L-glyceraldehyde's C2, the sugar is L. Due to the chirality of the sugar molecules, an aqueous solution of a D or L saccharides will rotate light. D-glyceraldehyde causes polarized light to rotate clockwise (dextrorotary); L-glyceraldehyde causes polarized light to rotate counterclockwise (levorotary). Unlike glyceraldehyde, D/L designation on more complex sugars is not associated with their direction of light rotation. Since more complex sugars contain multiple chiral carbons, the direction of light rotation cannot be predicted by the chirality of the carbon that defines D/L nomenclature.
- D, configuration as in D-glyceraldehyde
- L, configuration as in L-glyceraldehyde
All these classifications can be combined, resulting in names like D-aldohexose or ketotriose.
List of monosaccharides
This is a list of some common monosaccharides, not all are found in nature—some have been synthesized:
- Trioses:
- Aldotriose: glyceraldehyde
- Ketotriose: dihydroxyacetone
- Tetroses:
- Aldotetrose: erythrose and threose
- Ketotetrose: erythrulose
- Pentoses:
- Hexoses:
- Heptoses:
- Keto-heptoses: mannoheptulose, sedoheptulose
- Octoses: octolose, 2-keto-3-deoxy-manno-octonate
- Nonoses: sialose
Reactions
- Formation of acetals.
- Formation of hemiacetals and hemiketals.
- Formation of ketals.
See also
References
- ^ Pigman, Ward; Anet, E.F.L.J. (1972). "Chapter 4: Mutarotations and Actions of Acids and Bases", in Pigman and Horton: The Carbohydrates: Chemistry and Biochemistry Vol 1A, 2nd ed., San Diego: Academic Press, 165-194.
External links
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Wikipedia content modification information:
- This page was last modified on 19 November 2008, at 01:06.
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