OXIDATION POTENTIAL OF LUMINOL - IS THE AUTOXIDATION OF SINGLET ORGANIC-MOLECULES AN OUTER-SPHERE ELECTRON-TRANSFER

The one-electron reduction potential of the LH•/LH- couple was determined by pulse radiolysis to be 0.87 ± 0.02 V vs NHE. The rate constant for the electron-transfer reaction between LH• and O2•- (k-6) was measured to be 3 × 109 M-1 s-1 while the corresponding rate between L•- and O2•- (k-7) was estimated to be less than 108 M-1 s-1. In combination with previously measured data the above values enabled us to quantify the kinetics and thermodynamics of luminol autoxidation. The data were also analyzed in terms of the Marcus-Hush theory. For comparison, a similar analysis was made on the dihydroflavin mononucleotide system (FMNH-). It was concluded that the autoxidation of both LH- and FMNH- and probably of most singlet organic anions is initiated by an outer-sphere electron transfer between the anion and 3O2. In contrast, the autoxidation of L•-, FMN•-, and other semiquinone radicals is suggested to involve inner-sphere electron transfer. © 1990 American Chemical Society.