Free energy surfaces for the α(1→4)-glycosidic linkage:: Implications for polysaccharide solution structure and dynamics

We present a potential of mean force surface for rotation about phi and psi dihedral angles of the alpha(1 -> 4)-glycosidic linkage in the maltose disaccharide (4-O-alpha-D-glucopyranoSyl-D-glucopyranose) in aqueous solution. Comparison of the vacuum and solution free energy surfaces for maltose shows the principal effects of water to be an increase in the rotational freedom of the alpha(1 -> 4) linkage brought about by lowering the energy barrier for syn to anti conformational changes as well as expansion of the range of low-energy phi,psi conformations. This free energy analysis thus provides a thermodynamic and conformational rationale for the effects of water on alpha(1 -> 4)-linked polysaccharides and carbohydrate glasses.