Nature adapts the electronically flexible heme group to fill a wide range of biochemical roles. The primary objective of the proposed research is to provide a quantitative framework for understanding how changes in axial ligation influence the electronic structure, and thereby the chemical reactivity, of the heme. A model of the frontier orbitals of low-spin ferric hemes, developed from single-crystal electron paramagentic resonance data, will be used in the analysis of series of heme proteins. To the extent that this model is successful in reproducing the EPR data for this series on the basis of a small number of comoon parameters it will find many applications in correlating changes in molecular and electronic structure. Near the limit of axial symmetry, spin-orbit interactions dominate crystal field effects in the determination of the spin distribution and electronic energies of low-spin ferric porphyrins. This situation is encountered in heme proteins when, for example, two axial imidazole ligands adopt nearly orthogonal orientations. In this limit, Jahn-Teller effects are quenched, the spin is delocalized, and the product of the three principal g-values becomes negative. Single-crystal EPR investigations of several small-molecule ferric porphyrins will be undertaken to explore electron distributions near this limit. This same series of investigations will be designed to study the effects of changes in sigma donor strength on the metal-ligand pi interactions. Analysis of hyperfine tensors will be used to complement the information obtained from single-crystal g-tensor determination. The magnetically dilute crystals required will be prepared from a series of "metalloporphyrin sponges" recently identified in this laboratory. These materials, which can accomodate a wide range of ligand and solvate molecules, metal atom oxidation states, and metal atom coordination numbers, have a large number of potential applications. As a part of the proposed investigation they will also be used to construct matrices for the study of ligand and solvate dynamics and the study of excitation and electron transfer.