GEMINATE PROCESSES IN THE REACTION OF NITRIC-OXIDE WITH 1-METHYLIMIDAZOLE-IRON(II) PORPHYRIN COMPLEXES - STERIC, SOLVENT POLARITY, AND VISCOSITY EFFECTS

The quantum yields for photolysis of imidazole-iron(II) porphyrin-NO c omplexes have been determined in the following solvents: toluene, aque ous 1-methylimidazole, Nujol, and glycerol. The corresponding yields a re 0.2, 0.077 (30% 1-methylimidazole in water), 0.064, and 0.007. Ster ic effects on NO photolysis were also investigated. The increasingly h indered porphyrins (mesoheme, anthracene-7,7-cyclophane heme, adamanta ne-6,6-cyclophane heme, and pyridine-5,5-cyclophane heme) had, in tolu ene, quantum yields of 0.21, 0.23, 0.29, and 0.49, respectively. Sever al conclusions are drawn from these results. (1) Polar solvents decrea se NO quantum yields. (2) Considering the enormous differences in liga nd association rates, increased steric effects have relatively little influence. This is in accord with previous studies on isocyanides. (3) As is expected from a consideration of cage effects, increased viscos ity reduces quantum yield. The relevance of these findings to heme pro tein reactions is discussed.