One-electron reduction of lumichrome and its methyl derivatives in aqueous solution: a pulse radiolysis study

The one-electron reduction by CO2-. and e/sub aq/- of lumichrome and its 1-methyl,3-methyl, and dimethyl derivatives in aqueous solution has been investigated. By studying spectral changes as a function of pH, we have characterized three distinct types of radical anion. These radical anions derived from the dimethyl,1-methyl, and 3-methyl derivatives respectively exhibit differing absorption spectra in the range 280-600 nm. In addition, pK/sub a/ values for protonation of these species have been measured as 10.5-12.0, 10.5, and 7.8 for the species derived from dimethyllumichrome, 1-methyllumichrome, and 3-methyllumichrome, respectively. The neutral radicals all show similar absorption spectra and appear to be characterized by an absorption band at 450 nm (epsilon = 4000 M-1 cm-1). At low pH (2-5), protonation of the neutral species occurs with a pK/sub a/ close to 3.5 for all derivatives. The data obtained from the methyl derivatives of lumichrome is used to interpret the spectral changes which occur upon the one-electron reduction of lumichrome itself. Tentative structures have been assigned to all forms of the free radicals under investigation