monosaccharide

Monosaccharides (from Greek monos: single, sacchar: sugar) are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose (levulose), 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.

Structure

With few exceptions (e.g., deoxyribose), monosaccharides have the chemical formula C_x(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.

Fischer projections

Not all of the following monosaccharides are found in nature—some have been synthesized:

Aldoses

Aldotriose	D-Glyceraldehyde
Aldotetroses	D-Erythrose				D-Threose
Aldopentoses	D-Ribose		D-Arabinose		D-Xylose		D-Lyxose
Aldohexoses	D-Allose	D-Altrose	D-Glucose	D-Mannose	D-Gulose	D-Idose	D-Galactose	D-Talose

Ketoses

Ketotriose	Dihydroxyacetone
ketotetrose	D-Erythrulose
ketopentoses	D-Ribulose		D-Xylulose
ketohexoses	D-Psicose	D-Fructose	D-Sorbose	D-Tagatose

Monosaccharides with even longer chains are known, such as the ketoheptoses, mannoheptulose and sedoheptulose.

Cyclic structure

α-D-Glucopyranose (Haworth projection)

β-D-Glucopyranose (Haworth projection)

Most monosaccharides will cyclize in aqueous solution, 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 carbon₁ and oxygen₅ to form a 6-membered ring called a pyranoside. The same reaction can take place between carbon₁ and oxygen₄ to form a 5-membered furanoside. In general, pyranosides are more stable and are the major form of the monosaccharide observed in solution. Since cyclization forms a new stereogenic center at carbon₁, two anomers can be 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 structure of monosaccharides is the Haworth projection. In a Haworth projection, 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. Monosaccharides typically adopt a chair conformation, similar to cyclohexane. 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.

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

Monosaccharides are classified the type of carbonyl group they contain:

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

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 monosaccharide 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.

Derivatives

A large number of biologically important modified monosaccharides exist:

• Amino sugars such as:
  • Galactosamine
  • Glucosamine
  • Sialic acid
  • N-Acetylglucosamine
• Sulfosugars such as:
  • Sulfoquinovose

See also

Look up monosaccharide in Wiktionary, the free dictionary.

• Pyranose
• Furanose
• Glucose
• Monosaccharide nomenclature
• Disaccharides
• Oligosaccharide
• Polysaccharides
• Sugar acid
• Sugar alcohol

References

External links

• Nomenclature of Carbohydrates