Photodissociation of nitric oxide from nitrosyl metalloporphyrins in micellar solutions

Laser-photolysis studies of nitrosyl metalloporphyrins (MP), (NO)Fe(II)Hem (Hem protoporphyrin IX), (NO)(CoOEP)-O-II (OEP = octaethylporphyrin), and (NO)(MnTPP)-T-II (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 an 0.15 (+/-0.1) for (NO)Fe(II)Hem, 0.55 (+/-0.05) for (NO)(CoOEP)-O-II, and 0.21 (+/-0.02) for (NO)(MnTPP)-T-II. 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 k(MNO)(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 k(M) (M = Fe, Co, and Mn). The value of k(M) was measured as a function of [NO]. For Fe(II)Hem and (CoOEP)-O-II, the plots of k(Co) vs [NO] and k(Fe) vs [NO] gave straight lines. The slopes of the lines obtained with Fe(II)Hem and (CoOEP)-O-II afford the bimolecular rate constants k(FeNo)(pseudo) and k(CoNo)(pseudo), respectively. It is found that k(FeNo)(pseudo) > k(FeNO)(second) and k(CoNo)(pseudo) > k(CoNo)(second). The differences between k(MNO)(pseudo) and k(MNO)(sccond) observed for Fe(II)Hem and (CoOEP)-O-II an 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 (MnTPP)-T-II, the pseudo-first-order rate constant, k(Mn), 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.