Photodissociation of Nitric Oxide from Nitrosyl Metalloporphyrins in Micellar Solutions

Abstract

Laser-photolysis studies of nitrosyl metalloporphyrins (MP), (NO)FeIIHem (Hem = protoporphyrin IX), (NO)CoIIOEP (OEP = octaethylporphyrin), and (NO)MnIITPP (TPP = tetraphenylporphyrin), in aqueous ionic micellar solutions were carried out. The nitrosyl porphyrins in the micellar solutions readily photodissociate NO, 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 regenerate the parent nitrosyl porphyrins. The decay of MP in the absence of the excess NO follows second-order kinetics with the rate constant kMNO(second) (MNO = FeNO, CoNO, and MnNO). In the presence of excess NO, the decay of MP follows pseudo-first-order kinetics with the rate constant kM (M = Fe, Co, and Mn). The value of 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 bimolecular rate 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 for FeIIHem and CoIIOEP are interpreted by assuming that (1) NO molecules in the micellar solutions are dissolved in both micelles and the aqueous phase and (2) NO molecules trapped in micelles hardly react with MP because of the electrostatic repulsion between ionic micelles. In the case of MnIITPP, the pseudo-first-order rate constant, kMn, is found to asymptotically increase with an increase in [NO] to a limiting value. The reaction mechanisms for the nitrosylation of MP in micellar solutions are discussed in detail on the basis of the kinetic studies.