Estudo teórico para obtenção dos confôrmeros mais estáveis em solução aquosa para D-manose.

Abstract

Experimental results obtained for monosaccharides are always an average of the several conformations of the system. A theoretical study is necessary to obtain the most stable individual conformations. The goal of this work is to select the most stable conformers of D-mannose in aqueous solution at room temperature, and validate them by comparing the experimental value of specific rotation with those obtained from the selected conformations. We started from two initial conformations ( anomer and  anomer), geometrically optimized in vacuum using B3LYP/6-31+G*. Starting from the respective anomeric initial geometries, 1458 possible conformations (729  + 729 ) were obtained assuming that staggered conformations of the hydrogen atoms of hydroxyl groups with regard to the C-OH bond are energetically favored (3 (C1) x 3 (C2) x 3 (C3) x 3 (C4) x 3 (C5) x 3 (C6) = 729). Single-point calculations in vacuum were performed for all 1458 conformational possibilities using B3LYP/6-31+G *. 97 conformational possibilities were selected (those which had relative energy values lower than 10 kcal/mol, from the energy value of the initial geometry, taken as reference), and their geometry optimized in vacuum from B3LYP/6-31+G* calculations, which reduced the original set from 97 to 64 conformers. Geometry optimization and vibrational frequency calculations were performed and used to calculate the specific rotation for the system. The anomeric ratio value (:) obtained was 50:50. The result of rotameric abundance approached to the experimental data only for the gt rotamer. Two conformations presented a peculiar behavior when solvated: the relative abundance of _336661 conformation showed a drastic increase, while the conformation _111166 has decreased very much. Analyzing the solvation process, it was found that when we use the value of Gibbs free energy in solution without computing the entropy, there are changes in the values of the Boltzmann population, which can reproduce the experimental value of anomeric ratio (:), suggesting that the entropy for the system perhaps is not properly computed from the harmonic approximation. The specific rotation value also is closer to the experimental data when the new population values are used.