Elucidation of phenol-Cu interaction mechanisms by potentiometry, ESR, UV absorption spectroscopy and molecular simulations

In order to determine the dominant properties, which control the interactions of monomeric hydrosoluble phenols with Cu2+ in soil solutions, four different techniques were used. CuSO4 was added to ortho-dihydroxy-phenol (ODP) solutions under controlled temperature, pH, and ionic strength conditions and Cu2+-ODP interactions were studied by alkaline potentiometric titrations, electron spin resonance and UV absorption. Molecular dynamic simulations were also performed for conformational studies. From the titration curves, the shifts in isotropic A and g values, the evolution of the UV absorption band at 400 nm and the calculation of the potential energy of the different complexes, a three-step interaction mechanism was proposed. In the process an inner-sphere complex was formed, Cu(II) is being reduced and an oxydative polymerisation chain reaction initiated. The driving force of the reaction was the energetic stability obtained upon interaction of Cu2+ with the substrate. The ideal conformation was reached at the catechol site and not at carboxylic or hydroxy–methoxy sites. The pKa values of the phenols had little influence on the interaction process but the type and number of interacting sites on the benzene ring influenced the proceeding of progress of the reaction. Increasing the pH of the solution increased the amount of complexed species, but also the number of ligands around the Cu2+ion.