EPR studies on the kinetics of quenching singlet oxygen

The specific reaction between singlet oxygen (1O2) and 2,2,6,6-tetra-methyl-piperidine (TEMP) was utilized to investigate the kinetics of singlet oxygen quenching. Rose bengal was used for the generation of singlet oxygen (1O2). The deuterium isotope effect (rho) was found to be 22.1. The rate constant (kT) of the reaction of TEMP with 1O2 in H2O, D2O and ethanol were found to be 9.0 x 10(5) M-1s-1, 1.5 x 10(6) M-1s-1 and 7.3 x 10(5) M-1s-1, respectively. The intercept of the plot of [TEMPO]-1 at different H2O/D2O ratios was found to be constant, indicating the lifetime of 3Sens is independent of solvent deuteration. The quenching of 1O2 by azide and the sensitizer itself were confirmed by the constant intercept on the kinetic plots. Thus, the quenching rate constants for azide anion in ethanol solution (96%) and for ground state rose bengal in 0.05 M phosphate buffer (pH 7.4), were found to be 8.36 x 10(8) M-1s-1 and 1.72 x 10(4) M-1s-1, respectively. These studies not only verify the specificity of the reaction between TEMP and 1O2 but also provide a sensitive and specific assay for 1O2.