Carbohydrate Conformation The Anomeric Effect

Not only does carbohydrate structure affect properties such as chemical reactivity, but the structure and shape of carbohydrates are also major factors in a number of biological processes that depend on interactions between molecules—a phenomenon known as molecular recognition. In this section, we will consider mainly the conformations of carbohydrates in their pyranose forms. We will return to some familiar concepts of chair conformations and axial versus equatorial groups, but you will see that the presence of an oxygen atom in a six-membered ring leads to some surprising consequences. 

an equatorial OH is less crowded and better solvated by water than an axial one 

The first of these is straightforward and alerts us to the fact that the relative energies of two species may be different in solution than in the solid state or the gas phase. Hydrogen bonding to water stabilizes equatorial OH groups better than axial ones. 

The anomeric effect also influences the conformational equilibria in pyranoses with an electronegative atom, usually oxygen or halogen, at C-1. For example, the equilibrium mixture of the P-pyranosyl chloride derived from xylose triacetate contains 98% of the conformer in which chlorine is axial. These two conformations are not interconverted by mutarotation but by chair-chair interconversion. The anomeric effect is sufficiently large so that all four substituents occupy axial positions in the more stable conformer. 

Because five-membered rings are more flexible than six-membered, the anomeric effect is less important in furanose than in pyranose forms. 

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