The objective is to continue to contribute to an understanding of steric and electronic effects on the ligation of dioxygen, carbon monoxide, and other small molecules to metalloporphyrin systems that are usefully congruent models for the heme centers in hemoprotein. The synthesis of biomimetic porphyrins is a very active field of research. In the past decade considerable data have accumulated on the equilibria involved in the binding of 02, CO, and bases to many of these elaborated porphyrin systems that model the heme centers in hemoproteins. These data display tantalizing trends that are exceedingly difficult to interpret since detailed structural information is generally lacking. But such trends bear directly on structure-function relationships in biological systems. Most of the explanations that have been offered in studies of model systems are derived from model building; the actual structures, few as they are, often present surprises not anticipated by the model builders, as the porphyrin core is remarkably flexible. The structure-function relationships proposed for the biological systems are invariably based on imprecise structural data. To understand ligation in model systems and in the hemoproteins we must be able to correlate binding effects with structure. We propose to synthesize model systems, to obtain their structures by single-crystal X-ray diffraction techniques, including diffraction from the intense photon beam at the Advanced Photon Source, and to obtain binding and kinetic data by spectroscopic techniques. In addition to enabling direct comparisons to be made with similar measurements on the protein systems, these well characterized models will also enable the calibration of new spectroscopic techniques that have potentially important applications to the proteins. Thus the results of this research bear directly on a central problem in hemoprotein chemistry, namely structure-function relationships.
