Sugar Ring Structures

When sugars cyclize, they typically form furanose or pyranose structures (Figure 9.10). These are molecules with five-membered or six-membered rings, respectively. When cyclization happens, the carbon which contained the aldehyde or ketone group is typically linked to the oxygen on carbon 4 or 5 of the linear sugar structure. In the process, the aldehyde or ketone group is converted to a hydroxyl and the carbon to which it is attached becomes chiral. Thus, there are two possible orientations of the hydroxyl around the new chiral carbon. 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). Anomers are capable of interconverting between and positions in a process is called mutarotation.

Sugar ring structures can be written in a variety of ways. The most common forms are the Haworth projections (Figure 9.10) and the Fischer projections (see here). Note that both of these planar representations are inaccurate. Figure 9.11 shows a 3D projection of a furanose. Notice that all of the atoms of the ring do not fit into the same plane. Flexing of the atoms in the ring can give rise to rings with slightly different shapes. Note the very different orientations of carbon #3 in Figure 9.11a and 9.11b. These are called conformational isomers because they have the same chemical composition and the same atoms are bonded to each other, but slightly different bond angles give rise to different structural conformations.

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.

See also: Monosaccharide Nomenclature, Figure 9.14, Monosaccharides