Conformational Structures of Cyclic Sugars

For crystalline sugars, a single, most-favored conformation is usually assumed. For sugars in solution, an equilibrium of different conformations is obtained in which the most favored conformation is found in the greatest amount. 

Of the infinite conformations that pyranose sugars can have, there are two extremes that resemble chairs. The two chair conformations are usually nonequivalent, such that one is the most favored conformation and the other is the least favored conformation. There are two types of bonds around each of the carbon atoms in the ring. These bonds are either within the plane of the ring and are called equatorial bonds, or they are perpendicular to the plane of the ring and are called axial bonds [19]. 

For the D-hexoses, the Cl conformation (reaction 2.15) is defined as the form in which the primary hydroxyl group (-CH -OH) is equatorial. It is also the form in which C-1 is down and C-4 is up when the orientation of the ring is in the standard position. The Cl conformation is, thus, sometimes called a conformation. The 1C conformation is then the form in which the primary hydroxyl group is perpendicular to the ring, or axial. In this form, the C-1 is up and C-4 is down this is called a C conformation. For the pyranose ring forms of the D-pentoses, the designation of the conformation has to use the positions of C-1 and C-4, since there is no primary hydroxyl group. 

It should be noted that the different conformations, called conformers, all have the same configurations for the hydroxyl groups. Thus, for P-D-glucopyranose, the P-hemiacetal hydroxyl group is up, C-2 hydroxyl group is down, C-3 hydroxyl group is up, and so forth in the Cl, 1C, and boat conformations. 

P-D-Xylopyranose also can assume a Cj conformation with all of the bulky substituents in an equatorial, but it apparently was not favored in the evolutionary process because it has an odd number of carbon atoms and hence would require an asymmetric metabolism, that is metabolism of a two-carbon fragment and a three-carbon fragment, requiring two distinct metabolic pathways. Whereas a hexose such as D-glucose would require only a single pathway for the metabolism of two three-carbon fragments (see Chapter 11 for the pathway). 

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