Monosaccharide

Monosaccharides (from Greek monos: single, sacchar: sugar) are the simplest glucose are both aldohexoses, but they have different chemical and physical properties.

Structure

With few exceptions (e.g., atoms.

Cyclic structure

Most monosaccharides form cyclic structures, which predominate in aqueous solution, by forming 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 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: xylose
- Ketopentoses: ribulose and xylulose
Hexoses:
- Aldohexoses: allose, altrose, mannose and talose
- Ketohexoses: fructose, tagatose
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.

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.