Molecular dynamics and NMR study of the α(1→4) and α(1→6) glycosidic linkages:: Maltose and isomaltose

We report the results of molecular dynamics (MD) simulations compared with NMR relaxation experiments for maltose and isomaltose. The (Phi,Psi) adiabatic map for maltose shows a single principal energy well, while the (Phi,Psi,Omega) map of isomaltose reveals multiple low energy minima separated by significant barriers (9 kcal/mol) in some cases. The greater accessible conformational space of the alpha (1 -->6) linkage appears to make it more flexible as compared with the alpha (1 -->4) linkage, especially in the presence of water. Correlation times for glycosidic dihedral angle fluctuations are significantly shorter in the case of isomaltose. While the generalized order parameters calculated from the simulations do not show a large difference in the spatial restriction of the motion, they are nonetheless generally lower for isomaltose. The time scales of the overall rotational motion (tau (M)) and the local molecular motion (tau (e)) are similar for both maltose and isomaltose. This makes reliable estimates of order parameters from experimental relaxation data (using the model-free formalism) unfeasible. We were, however, able to show that T-1 relaxation times calculated from the MD data agree well with the experimental values. As a further measure of solution flexibility, three-dimensional water distributions were calculated about each disaccharide. These demonstrate that the more rigid maltose solute causes the water to adopt a more localized structure about it. Because of its extended structure, isomaltose appears to make a greater number of hydrogen bonds to water.