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-π 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-π affinity of K(+) in solution.