Monosaccharide

Monosaccharides 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 (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.

Cyclic structure

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.

α-D-Glucopyranose

β-D-Glucopyranose.

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.

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