A theoretical study of the interactions of water with gallic acid and a PEO/TGG complex

Abstract

PM3 Semi-empirical molecular orbital calculations on the interaction of water and gallic acid (GA) show the hydrophilic part of the solute (GA) to form strong directional bonds to 5 H2O molecules. To form a monolayer around GA, containing trigalloyl glucose both hydrophilic and hydrophobic regions, requires about 43 H2O molecules. PM3, MM and MD calculations on complexes between the hexamer of poly(ethylene oxide) (PEO)6 and cofactor containing trigalloyl glucose (TGG) show that (PEO)6/TGG complexes do not form in aqueous solution, which agrees with the experimental results in pure water, in the absence of salt, obtained with PEO and corilagin, a very similar molecule. It is possible that a (PEO) conformers, with the oxygens outside, are favoured in aqueous solution, because the H2O molecules can more easily hydrogen bond with the PEO ether oxygens. A distinct change in the rate of change of the relative total energy per H2O molecule occurs at 10 H2O molecules: beyond 10 H2O molecules a slow varying plateau is observed to and beyond 120 H2O molecules. This is critical when calculating the enthalpy of association of a solvated molecular complex.