| Abstract
| - Laser-photolysis studies of nitrosyl metalloporphyrins (MP), (NO)FeIIHem (Hem = protoporphyrin IX), (NO)CoIIOEP (OEP = octaethylporphyrin), and (NO)MnIITPP (TPP = tetraphenylporphyrin), in aqueous ionicmicellar solutions were carried out. The nitrosyl porphyrins in the micellar solutions readily photodissociateNO, leaving the MP in micelles: the quantum yields are 0.15 (±0.1) for (NO)FeIIHem, 0.55 (±0.05) for(NO)CoIIOEP, and 0.21 (±0.02) for (NO)MnIITPP. The MP thus produced recombine with NO to regeneratethe parent nitrosyl porphyrins. The decay of MP in the absence of the excess NO follows second-order kineticswith the rate constant kMNO(second) (MNO = FeNO, CoNO, and MnNO). In the presence of excess NO, thedecay of MP follows pseudo-first-order kinetics with the rate constant kM (M = Fe, Co, and Mn). The valueof kM was measured as a function of [NO]. For FeIIHem and CoIIOEP, the plots of kCo vs [NO] and kFe vs[NO] gave straight lines. The slopes of the lines obtained with FeIIHem and CoIIOEP afford the bimolecularrate constants kFeNO(pseudo) and kCoNO(pseudo), respectively. It is found that kFeNO(pseudo) > kFeNO(second)and kCoNO(pseudo) > kCoNO(second). The differences between kMNO(pseudo) and kMNO(second) observed forFeIIHem and CoIIOEP are interpreted by assuming that (1) NO molecules in the micellar solutions are dissolvedin both micelles and the aqueous phase and (2) NO molecules trapped in micelles hardly react with MPbecause of the electrostatic repulsion between ionic micelles. In the case of MnIITPP, the pseudo-first-orderrate constant, kMn, is found to asymptotically increase with an increase in [NO] to a limiting value. Thereaction mechanisms for the nitrosylation of MP in micellar solutions are discussed in detail on the basis ofthe kinetic studies.
|
