Conformational study of two diasteroisomers of vinylcatechin dimers in a methanol solution

摘要

Red wine possess anthocyanins (red colored pigments) and flavan-3-ol compounds (associated to the flavor). In an acidic environment, these compounds have higher reactivity due to the acidic character of the phenolic groups. Vinylcatechin adducts play a crucial role as intermediates in the formation mechanism of some anthocyanin-derived pigments (e.g., pyranoanthocyanins) during wine aging. They have gained increasing attention because of the large number of compounds detected in wines and their unusual spectroscopic properties. However, vinylcatechins are very unstable molecules in acidic solutions and their occurrence in wines has never been detected, probably due to their high reactivity. Using computational methods, we have characterized and identified the different conformations of two stable diasteroisomers (9S,11R)- and (9R,11S)-vinylcatechin dimers, which were obtained in model wine solutions from acid-catalyzed dimerization of vinylcatechin. To identify the different conformations for both isomers, first potential energy surface calculation around the most relevant dihedral angles of each isomer were performed using the density functional theory (DFT) approach with the B3LYP hybrid density functional. Molecular Dynamics (MD) simulations starting from the previous minima were subsequently performed to investigate the possible conformations acquired by the two different compounds in a methanol solution. Six and five different conformations were obtained during the MD simulations for (9S,11R) and (9R,11S) isomers, respectively. Ten nanoseconds of MD simulations were enough to obtain the most favored conformations of these molecules. DFT calculations were used for the optimizations of all structures in vacuum as well as in a methanol environment. Gibbs free energy values were calculated for all structures, and these values show that similar quantities of both (9S,11R) and (9R,11S) diasteroisomers are obtained in a methanol solution, and that their formation is thermodynamically favored.