Monosaccharide

Monosaccharides (from Greek monos: single, sacchar: sugar) are the simplest carbohydrates. They cannot be hydrolyzed into simpler sugars. 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 like sucrose (common sugar) 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 they have different chemical and physical properties.

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Structure

  With few exceptions (e.g., deoxyribose), monosaccharides have the chemical formula (CH₂O)_{n + m} 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.

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 = 2^c. The only carbohydrate without an isomer is dihydroxyacetone or DHA.

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 keto group they contain:

• Aldose, -CHO (aldehyde)
• Ketose, C=O (ketone)

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:
  • Aldopentoses: arabinose, lyxose, ribose and xylose
  • Ketopentoses: ribulose and xylulose
• Hexoses:
  • Aldohexoses: allose, altrose, galactose, glucose, gulose, idose, mannose and talose
  • Ketohexoses: fructose, psicose, sorbose and tagatose
• Heptoses:
  • Keto-heptoses: mannoheptulose, sedoheptulose
• Octoses: octolose, 2-keto-3-deoxy-manno-octonate
• Nonoses: sialose

Reactions

1. Formation of acetals.
2. Formation of hemiacetals and hemiketals.
3. Formation of ketals.

References

See also

• Oligosaccharide
• Sugar acid
• Sugar alcohol