Reactions of Oxymyoglobin with NO, NO2, and NO2- under Argon and in Air

Publisher Website
Google Scholar
Abstract
Oxymyoglobin under argon reacts with NO2- and NO2 (N2O4) to produce metmyoglobin in a spectrally clean process with clear isosbestic points. In both cases, the reactant is NO2-. The second-order rate constant for NO2- or N2O4 is the same: d(Mb+)/dt = k(MbO2)(NO2-) where k = 0.21 ± 0.02 L mol-1 s-1. The reaction of MbO2 with NO under argon is a complex process and entails the generation of Mb+ and OONO- (peroxynitrite) in the first step. The latter (λmax, 302 nm) was poorly resolved from more intense protein absorbtion in the 300-nm region. However, at pH 9, the change in absorbance corresponded exactly to a quantitative production of the OONO- ion. Hydroxyl radicals from it were trapped with ethylene-1,2-13C. The initial step is followed in sequence by the rapid formation of MbNO+. The iron(III)−nitrosyl adduct hydrolyzes slowly to MbII and NO2- (k = 8.0 ± 0.8 × 10-5 s-1). MbII then rapidly associates with NO, and MbNO is the final product of this reaction. Oxymyoglobin is inert to NO3-. In contrast to the results under argon, in air the reactions of MbO2 with NO2-, NO, and NO2 (N2O4) all proceed in the same autocatalytic fashion with kave (for the autocatalytic rates) ≅ 9 ± 5 L mol-1 s-1. Nitrite is the initial reactant in all cases. Isosbestic points are not observed in the visible spectrum, and additional porphyrin iron-ligated species are intermediates. Based upon work with iron porphyrins [J. Org. Chem. (1996) 61, 6388−6395 and J. Am. Chem. Soc. (1996) 118, 3894−3895], it is proposed that ozone may be an intermediate in the autocatalysis.