First principles study of alkali-tyrosine complexes: Alkali solvation and redox properties

The oxidation of alkali-tyrosinate to radical alkali-tyrosine in aqueous solution is studied using ab initio Car-Parrinello molecular dynamics (CPMD). The aim is to investigate the cation-pi interactions between alkali cations M and the aromatic ring of tyrosine, in gas phase and in aqueous solution, using the influence of the cation M on the reaction M+(Tyr(-)) -> M+(Tyr*) + e(-) as a probe. To this end, we calculate the redox potential and the reorganization free energy using a CPMD-based method derived from the Marcus theory of electron transfer. We discuss the redox properties of Tyr, Na+(Tyr), and K+(Tyr), in reduced and oxidized states, by analyzing selected interatomic distances, coordination numbers, and charge populations. Our results confirm the known inversion in the relative stabilities of Na+(Tyr) and K+(Tyr) in going from gas phase to solution and point to a stronger cation-pi affinity of K+ in solution.