Monosaccharide Nomenclature

Monosaccharides are the simplest sugars, having the formula (CH2O)n. The smallest molecules usually considered to be monosaccharides are those with n = 3.

Monosaccharides can be categorized according to their value of 'n,' as shown below:

n	Category
3	Triose
4	Tetrose
5	Pentose
6	Hexose
7	Heptose
8	Octose

Monosaccharides can exist as aldehydes or ketones and are called aldoses or ketoses, respectively. For example, (Figure 9.3) shows the structures of glyceraldehyde, an aldo-triose, and dihydroxyacetone, a keto-triose. Glyceraldehyde and dihydroxyacetone have the same atomic composition, but differ only in the position of the hydrogens and double bonds. Moreover, they can interconvert via an enediol intermediate (Figure 9.4). When the structures of molecules are related in these ways, the molecules are called tautomers.

Carbons in a monosaccharide are numbered such that the aldehyde group is carbon number one or the ketone group is carbon number two.

The three dimensional arrangement of atoms around a carbon atom are such that if four different groups are attached to it, they can be arranged in two different ways. Such a carbon is described as chiral or asymmetric. The two molecules with different three-dimensional arrangement are mirror images of each other, and the two different forms are called stereoisomers. For example, D-glyceraldehyde and L-glyceraldehyde (Figure 9.5) are mirror images of each other (stereoisomers) and cannot be superimposed on each other. Such molecules with these properties are called enantiomers. The designation 'D-' or 'L-' is an older nomenclature still used widely in biochemistry. It originally described whether the compound rotated a plane of polarized light to the right (D for dextro) or left (L for left). This is not absolute, however, because it depends on the reference compound chosen.

The R-S nomenclature, which is an absolute naming scheme, is shown in Figure 9.6. It is commonly used by organic chemists, but is rarely used by biochemists since it is difficult to apply to molecules, such as tetroses, pentoses, hexoses, etc., which may have more than one chiral carbon. The predominant monosaccharides found in nature have the 'D' configuration.

Sugars with more than one asymmetric carbon have many possible three dimensional configurations. In general a molecule with m chiral centers will have 2^m stereoisomers. The multiple stereoisomeric forms means that not all stereoisomers will be mirror images of each other. Stereoisomers that are not mirror images of each other are called diastereomers.

Ketose-aldose pairs of sugars frequently are named by inserting the letters 'ul' in the name of the corresponding aldose to derive the name of the ketose. An example is erythrose - erythrulose.

When sugars cyclize, they typically form furanose or pyranose structures (Figure 9.10). These are molecules with five-membered or six-membered rings, respectively. Cyclization creates a carbon with two possible orientations of the hydroxyl around it. We refer to this carbon as the anomeric carbon and the two possible forms as anomers. The two possible configurations of the hydroxyl group are called and , which correspond to the hydroxyl being in the "down" and "up" positions, respectively, in standard projections (see here)

Figure 9.13 shows that a pyranose, such as glucose, has two common conformational isomers, referred to as the "boat" and "chair" form. For glucose (and most sugars), the chair form is more stable because the hydroxyls of carbons 1 and 2 are further removed and thus have less steric interference with carbons 3, 4, and 5.