Stopped-flow-optical Absorption and -electron Spin Resonance Studies on Short-lived Quercetin Semiquinone Radical Produced by Redox Reactions with O-2(-center dot) Radical in DMSO

The stopped-flow-optical absorption and -ESR spectra of a quercetin (QueH(2)) derived semiquinone anion radical (Que(-center dot), half-life time similar to 31 ms) were detected by mixing the O-2(-center dot) radical and QueH(2) in dimethylsulfoxide (DMSO). Based on the timedependent optical absorption and ESR measurements, the second-order decay rate of the Que(-center dot) radical yielding charge- transfer dimer complex (Que(2)) was determined to be 2 x 10(6) M-1.s(-1).