SOLVENT EFFECT ON THE STABILITY OF ISOMALTOSE CONFORMERS

The conformational equilibria of the low‐energy conformers of isomaltose have been studied in different solvents. The structure of each individual conformer was refined from the 18 distinct low‐energy regions determined from potential energy function. Molecular geometry optimization was carried out using the quantum chemical method of perturbative configuration interaction with localized orbitals or PCILO and yielded to 15 distinct minima. Carbon‐proton vicinal coupling constants 3JC‐H have been calculated based on the finite perturbation theory formulation with the intermediate neglect of differential overlap for these minima. For evaluation of the influence of the environment on the conformational structure of isomaltose, the population of the low‐energy conformers was estimated in four solvents by a method in which the total energy is divided into the energy of the isolated molecule and the solvation energy. The calculated abundances of conformers depend strongly on the solvents. Based on the determined abundance of conformers, thermodynamically averaged torsion angles and coupling constants 3JC‐H describing the “average” conformation about glycosidic linkage have been calculated. Results obtained are compared with available experimental data from solution and solid state. Copyright © 1990 John Wiley & Sons, Inc.