Sensitization of Peroxynitrite Chemiluminescence by the Triplet Carbonyl Sensitizer Coumarin-525. Effect of CO2

Abstract— The flash of spontaneous chemiluminescence (CL) that reflects the formation of electronically excited intermediates during the decay of peroxynitrite (ONOO) to nitrate was investigated. The half-decay time of the CL flash (0.5 ± 0.1 s) was in agreement with the half-life of peroxynitrite obtained in stopped-flow experiments. The spontaneous CL intensity was linearly dependent on peroxynitrite concentration. The yield of spontaneous CL from peroxynitrite decay, 2 × 10^-9 photons/peroxynitrite at pH 9.5, was strongly enhanced by a sensitizer of triplet carbonyl CL, coumarin-525 (C-525). The maximal yield of sensitized CL was calculated to be 3 × 10^-6 photons/ peroxynitrite molecule for infinite concentration of C-525. The dependence of both spontaneous and sensitized CL on pH has a maximum at about pH 9.5. Bubbling with CO₂ or addition of NaHCO₃ considerably enhanced the flash of CL, and it is concluded that the reaction of the peroxynitrite anion with CO₂ is a major pathway leading to the formation of an electronically excited intermediate of peroxynitrite.