Cation-π interactions and oxidative effects on Cu⁺ and Cu²⁺ binding to Phe, Tyr, Trp, and His amino acids in the gas phase. Insights from first-principles calculations

The coordination properties of the four natural aromatic amino acids (AAarom = Phe, Tyr, Trp, and His) to Cu+ and Cu 2+ have been exhaustively studied by means of ab initio calculations. For Cu+-Phe, Cu+-Tyr and Cu+-Trp, the two charge solvated tridentate N/O/ring and bidentate N/ring structures, with the metal cation interacting with the π system of the ring, were found to be the lowest ones, relative ΔG298K energies being less than 0.5 kcal/mol. The Cu+-His ground-state structure has the metal cation interacting with the NH2 group and the imidazole N. For these low-lying structures vibrational features are also discussed. Unlike Cu + complexes, the ground-state structure of Cu2+-Phe, Cu2+-Tyr, and Cu2+-Trp does not present cation-π interactions due to the oxidation of the aromatic ring induced by the metal cation. The ground-state structure of Cu2+-His does not present oxidation of the amino acid, the coordination to Cu2+ being tridentate with the oxygen of the carbonyl group, the nitrogen of the amine, and the N of the imidazole. Other less stable isomers, however, show oxidation of His, particularly of the imidazole ring, which can induce spontaneous proton-transfer reactions from the NH of the imidazole to the NH2 of the backbone. Finally, the computed binding energies for Cu+-AA arom and Cu2+-AAarom systems have been computed, the order found for the single charged systems being Cu+-His > Cu +-Trp > Cu+-Tyr > Cu+-Phe, in very good agreement with the experimental data. © 2006 American Chemical Society.