Effect of carbon dioxide on autoxidation and photooxidation of nitrosylmyoglobin

Autoxidation and photooxidation of nitrosylmyoglobin, MbFe(II)NO, the pigment of nitrite-cured meat, was investigated in aqueous solution saturated with gas mixtures containing varying levels of CO2 (0%, 20% and 90%) in the presence of either 1.5 or 10% O2 and balanced with N2 in order to mimic modified atmosphere (MA) packaging of meat products. Quantum yields for photooxidation at 1.5% O2 for monochromatic UV-light (366 nm) were slightly higher than for visible light (436 nm) in agreement with previous findings, while the quantum yields showed no dependence on the CO2 levels. Autoxidation of MbFe(II)NO was evaluated by global analysis of spectral data applying a kinetic model with two consecutive reaction steps, and the second rate constant was significantly reduced at the highest CO2 level investigated, while the rate constant of the initial reaction step was found independent of the CO2 levels. The varying dependence observed for autoxidation and photooxidation of MbFe(II)NO in relation to CO2 level confirms the differing reaction mechanisms for the two types of MbFe(II)NO degradation. Photooxidation of nitrite-cured meat products packed in MA is accordingly expected to be independent of the presence of varying CO2 levels, while thermal oxidation (autoxidation) of MbFe(II)NO is reduced at elevated levels of CO2, which may be of some relevance during product storage and retail display.