Determination of sulfite radical (SO.3-) reaction rate constants by means of competition kinetics.

The sulfite radical anion (SO.3-) was found to react rapidly with the flavonoid quercetin (k = 2.5 x 10(8) dm3mol-1s-1) and the carotenoids crocin (k = 1.0 x 10(9) dm3mol-1s-1) and crocetin (k = 1.5 x 10(9) dm3mol-1s-1). The reactions can easily be monitored due to the strong absorptions of the substrates and, in the case of quercetin, the formation of a strongly absorbing transient species. Using these substances, we determined by means of competition kinetics rate constants of SO.3- reactions with nucleic acid components, polyunsaturated fatty acids, and glutathione.