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 (ρ) was found to be 22.1. The rate constant (k(T)) of the reaction of TEMP with ¹O₂ in H₂O, D₂O and ethanol were found to be 9.0 x 10⁵ M^-1s^-1, 1.5 x 10⁶ M^-1s^-1 and 7.3 x 10⁵ 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 10⁸ M^-1s^-1 and 1.72 x 10⁴ 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.