KINETICS AND MECHANISM OF THERMAL-OXIDATION AND PHOTOOXIDATION OF NITROSYLMYOGLOBIN IN AQUEOUS-SOLUTION

In aqueous solution, the rate of oxidation of chromatographically purified nitrosylmyoglobin (MbNO), synthesized from equine metmyoglobin (MMb), is strongly dependent on oxygen partial pressure and temperature but shows little dependence on pH, ionic strength, and solution viscosity. At an oxygen partial pressure of 0.2 atm and above, the rate approaches a limiting value with an observed first-order constant at 25-degrees-C of k(obs) = (2.32 +/- 0.09) x 10(-4) s-1 (DELTAH double dagger = 110.1 +/- 0.4 kJ mol-1, DELTAS double dagger = 55 +/- 3 J mol-1 K-1). The oxidation products are MMb and nitrate, as identified spectrophotometrically and by means of a nitrate electrode, respectively, and the rate of MbNO oxidation and the rate of nitrate formation are very similar; a reaction mechanism is suggested in which dissolved oxygen forms a complex with MbNO (K(ass) = 5 +/- 3 atm-1) prior to intermolecular rate-determining electron transfer with a high activation barrier. The rate of exchange of nitric oxide in MbNO with solution nitric oxide, as determined by N-15 isotopic labeling, is fast (greater than 3.4 x 10(-7) mol L-1 s-1 in a NO-saturated aqueous 3.4 x 10(-5) M MbNO solution at 0-degrees-C), excluding a common rate-determining step for ligand substitution and oxidation of MbNO. The reaction quantum yield for photooxidation of MbNO to MMb in air-saturated aqueous solution at 5-degrees-C (i) is only moderately dependent on the wavelength of irradiation and ranges from 6.9 x 10(-3) (254 nm) to 4.3 X 10(-4) mol einstein-1 (546 nm), (ii) is proportional to the partial oxygen pressure, and (iii) increases with increasing solution viscosity (glycerol/water mixtures). A bimolecular reaction between an electronically excited MbNO and oxygen with a transition state with a partly dissociated nitric oxide molecule is suggested.