We plan to use the synthetic model approach to probe the mechanisms by which nature controls and modifies the chemistries of the hemes and chlorophylls. We will concentrate on two areas of strong current interest, the photosynthetic reaction centers and cytochrome P-450. A new method is proposed for the preparation of covalently stacked porphyrin derivatives which will serve as accurate models for reaction centers. Using a newly synthesized symmetrical porphyrin precursor we will prepare dimers, trimers and oligomers of varying geometries, compositions and metallation states. Detailed physical and spectroscopic analysis of these molecules will provide insights into the geometrical arrangement of chlorophylls in photosynthesis. We are also proposing to synthesize models of the chlorophyll dimer - pheophytin donor-acceptor complex in order to probe the factors which control forward and reverse electron transfer in the first photosynthetic charge separation step. Our second research theme involves a study of the role of the substrate-binding site and distal protein groups on the active site of the important monooxygenase enzyme, cytochrome P-450. We have prepared a series of molecular receptors which contain a synthetic substrate binding site in close proximity to a porphyrin. These novel species should accurately model the enzyme-substrate complex and allow us to study the molecular mechanism in detail. Various distal groups will be incorporated into the molecular receptor and their influence of heme chemistry will be monitored. The receptors also show considerable promise as catalysts for alkane hydroxylation and alkene epoxidation. The enzyme-like properties of substrate selectivity and reaction regiospecificity should in principle be controlled by careful design of the binding site.
