Zinc's affect on proton transfer between imidazole and acetate predicted by ab initio calculations

We have recently reported proton dissociation energies of common zinc coord inates, in the presence and absence of the zinc divalent cation, that were obtained from large basis set density functional theory calculations, which suggest a possibility of imidazolate as a zinc coordinate in proteins. To investigate this possibility in further detail, we have searched for potent ial proton accepters for the zinc-coordinated imidazole group, as potential proton donors in proteins, through a survey of zinc protein crystal struct ures and ab initio calculations. Herein, we report the result of our survey of the Protein Data Bank and the ab initio calculations using the B3LYP/6- 311+G(d,p) and MP2/6-311+G(d,p) methods. The results reveal that the imidaz ole-acetate dyad is energetically less stable than the imidazolate-acetic a cid dyad when the imidazole and imidazolate are coordinating to the zinc di valent cation, and vice versa in the absence of zinc. The results suggest c hat the carboxylate group of Asp(Glu) at the second coordination shell of a zinc complex in proteins is not a hydrogen bond acceptor but rather a prot on acceptor for the zinc-coordinated imidazole.