PORPHYRIN SYNTHETIC METHODS FOR BIOORGANIC APPLICATIONS

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Our long-term objectives are to develop synthetic methods for creating bioorganic molecules which can be used to provide insight into the relation between molecular structure and biological function. We have developed a room temperature biomimetic synthesis of tetraphenyl-porphyrin in 60% yield. We will expand the scope of this methodology to prepare porphyrins which have previously been unobtainable. This includes the class of meso-alkylporphyrins, some of which should be extremely soluble in lipid media. Amphipathic porphyrins can then be prepared for bilayer studies. Then we will exploit this new methodology in the high yield synthesis of cofacial dimers containing porphyrins and chlorins. We plan an extensive study of the electronic interactions resulting from the juxtaposition of rings. To model the light harvesting apparatus of photosynthesis, we will design and synthesize porphyrins bearing bulky substituents which in the crystalline state will hold the porphyrins at fixed distances apart. Fluorescence measurements in applied electric fields should provide data to distinguish the mechanisms of energy transfer. These new synthetic porphyrins should be of considerable value in extending our understanding of the properties of heme and chlorophyll in biological systems. A more global problem in bioorganic chemistry is the synthesis of model systems having well-defined 3-dimensional structures. We propose to create modular model systems of the photosynthetic process which contain a donor and acceptor covalently linked in a stacked geometry. The spacer groups separating the donor and acceptor are composed of p-aminobenzoic subunits, which are coupled together in a stepwise manner analogous to peptide and DNA synthesis. Three porphyrin-quinone compounds of 5, 10, and 15 A separation will be prepared which have 1, 2, and 3 subunits in their spacer groups. The photochemical characterization of this family of molecules will provide a systematic study of the effect of distances on electron transfer reactions. This modular approach should have broad applicability in the bioorganic field.