RAMAN STUDIES OF PORPHYRINS AND RELATED MACROCYCLES

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The general objectives of the proposed research are to characterize the structural, electronic and magnetic properties of porphyrins, hydroporphyrins and structurally related bio-macrocycles. A variety of methodologies will be used including Resonance Raman (RR), infrared (IR), absorption and electron paramagnetic resonance (EPR) spectroscopies, normal coordinate analysis, and semiempirical quantum force field calculations. The procedures, aims and rationales for the studies are as follows: (1) The RR and IR spectra of a variety of prototypical chlorins, bacteriochlorins and isobacteriochlorins will be characterized in detail. The vibrational data will be analyzed via both conventional normal coordinate and semiempirical quantum force field methods. The semiempirical calculations will be refined and extended to protein prosthetic groups, such as siroheme, heme d1 and cofactor F430, which are not amenable to conventional vibrational analysis procedures because large numbers of isotopomers are not available. The aim of these studies is to provide a foundation for the interpretation of the vibrational data of reduced-pyrrole macrocycles in general. These bench marks are needed (but not generally available) in order to extract structural information from the vibrational spectra of hydroporphyrins in proteins. (2) The RR and IR spectra of pi-cation and pi-anion radicals of various porphyrins and hydroporphyrins will be characterized. Normal coordinate calculations will be performed on these species by using both conventional and semiempirical quantum force field methods. Vibrational and EPR spectra of various metal-reduced hydroporphyrins, such as complexes of Fe(I) and Ni(I), will also be examined. The aim of these studies is to characterize the structural and electronic perturbations induced by the addition or removal of electrons. A quantitative assessment of the effects of oxidation/reduction is needed (but again not generally available) in order to extract structural information from the spectra of oxidized/reduced tetrapyrrolic intermediates in proteins. (3) The RR, IR, EPR and optical spectra of a series of neutral, oxidized and reduced porphyrin and hydroporphyrin dimers and trimers [structure Ln(ring)2, (ring)Ln(ring') and Ln2(ring)3] will be examined. The aim of these studies is to determine the influence of strong pipi interactions on the electronic properties of the complexes and provide a foundation for assessing how such electronic communication might mediate energy and/or electron transfer between tetrapyrrolic prosthethic groups in proteins.