The aim of the proposed research is to quantitatively establish the effect of hydrogen bonding on the kinetics, thermodynamics, and redox properties of metalloporphyrin reactions. The hydrogen-bonding is of two types: to an axially coordinated ligand (distal effect) and from an axially coordinated ligand (proximal effect). Preliminary experiments suggest that distal hydrogen-bonding can greatly influence the kinetics and thermodynamics of ligand dissociation and association when the coordinated ligand is anionic. One of the reactions to be investigated is Fe(porphyrin)X + 2 RIm greater than Fe(porphyrin) (RIm)2+, X-, where X is one of a variety of anionic ligands and RIm is an imidazole. This reaction goes through the unstable monoimidazole intermediate Fe(porphyrin) (RIm)X which is a model for a number of hemoproteins. The influence of hydrogen-bonding from RIm and to X- in Fe(porphyrin)(RIm)X will be thoroughly investigated by rapid reaction techniques (low temperature stopped-flow, NMR) and electrochemical techniques (stationary electrode voltammetry, polarography). Several other metalloporphyrins, principally those of cobalt and ruthenium containing coordinated dioxygen, will also be investigated. The hydrogen-bonding to be studied can be from the solvent, nucleophile, or other external source to the distal ligand or from coordinated imidazole to solvent or other basic species. The elucidation of the importance of hydrogen-bonding on the ligand association and dissociation rates, reaction thermodynamics, and metalloporphyrin redox potentials will provide a basis for understanding the importance of these effects in reactions mediated by hemoproteins such as hemoglobin, myoglobin, catalases, peroxidases, and cytochromes.
