[Study of kinetic parameters of singlet molecular oxygen in aqueous porphyrin solutions. Effect of detergents and the quencher sodium azide].

Abstract

The kinetic parameters of porphyrin-photosensitized formation and deactivation of singlet molecular oxygen (1O2) and their dependence on the concentration of the 1O2 quencher sodium azide were investigated in air-saturated water, ethanol, and aqueous micellar solutions of detergents using time-resolved measurements of oxygen phosphorescence under pulsed laser excitation. The lifetimes of 1O2 formation and deactivation and the rate constants of 1O2 quenching by sodium azide were determined. It was shown that, with no azide in the solutions, the rise in phosphorescence intensity after the laser flash corresponded to the kinetics of energy transfer from the porphyrin triplet molecules to oxygen, while the decay kinetics corresponded to the kinetics of 1O2 deactivation. In the presence of detergent, a considerable increase in the 1O2 lifetime was observed, which is likely due to the localization of 1O2 molecules mostly in lipophilic micelles and not in the water phase. If relatively high azide concentrations were used, the lifetime of the porphyrin triplet state did not change but the 1O2 lifetime decreased to values similar to those in living cells. In this case, the inversion of the phosphorescence kinetic phases was observed. The rise corresponded to 1O2 deactivation, and the decay, to the energy transfer from triplet porphyrin to oxygen. The data suggest that, in living cells, 1O2 molecules are also located mainly in lipophilic structures and the 1O2 lifetime determines the kinetics of the phosphorescence rise after the laser pulse.