A Thermodynamic “Vocabulary” for Metal Ion Interactions in Biological Systems

Abstract

This Account focuses on metal ion-ligand complexes of biological relevance and measurements of the bond dissociation energies (BDEs) of such species. These complexes yield thermochemistry that begins to provide a thermodynamic "vocabulary" for thinking quantitatively about the strength of interactions in biological systems. The method utilized is threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer. Accurate determination of BDEs requires attention to many details of the experiments and data analysis, as outlined here. Trends in metal ion-ligand BDEs are examined as a function of the metal ion, ligand, and extent of ligation. We elucidate the importance of ion-dipole and ion-induced dipole interactions, chelation, conformation, tautomeric form, steric interactions, and electronic effects such as hybridization and promotion. Interactions of metal ions with nucleobases and amino acids are quantified and the effects of hydration on these values are explored for the amino acid systems. Although data limitations restrict the present discussion to monocations, the trends elucidated here should be relevant to multiply charged metal ions, for which data is forthcoming.