Theoretical study of the complexes of tyrosine and tryptophan with biologically important metal cations in aqueous solutions

In this study geometrical parameters and thermodynamical stability of the complexes formed between zwitterionic tyrosine/tryptophan and some biologically relevant monovalent alkali metal cations (Na+, K+) and divalent alkaline earth metal cations (Mg2+, Ca2+) have been determined on the bases of the new and yet unpublished theoretical calculations performed at the DFT(B3LYP-CAM)/6-31+G(d,p) level in the hydrated environment with the use of the polarizable continuum model (PCM). The obtained results of calculations in aqueous solution indicated that the tyrosine–metal cation and tryptophan–metal cation complexes studied adopted salt bridged (SB) structures involving bidentate coordination of the metal cation (Mn+, n = 1, 2) to the carboxylate moiety. In these structures, as inferred from the Mn+⋯O separation range, the metal cation is oriented almost symmetrically to two oxygen atoms of the functional group. It is believed that the theoretical results obtained in this study may be used to gain a better understanding of the interactions between biologically important metal cations and large biological molecules such as proteins.