Pulse-radiolysis study of the effect of pH on the one-electron reduction potentials of lumichrome derivatives

By equilibrating one-electron reduced species of lumichrome, 1-methyl-, 3-methyl- and dimethyl-lumichrome with electron acceptors of known reduction potential using pulse-radiolysis techniques, it has been possible to measure E1 values of these lumichrome derivatives over the pH ranges 2 to 4 and 8.6 to 10.7. From the existing pK values for dissociation of protons from the cationic, neutral and anionic forms of the free radicals, it was possible to fit the experimental E1 values to calculated E1 values over the entire pH range for three of the derivatives. For 1-methyl-lumichrome, however, it proved necessary to invoke the existence of tautomeric forms of the neutral radical to obtain an acceptable fit of the data. Interpolation of all the data treated in this way yields E71 values of -530 +- 7, -509 +- 7, -535 +- 7 and -502 +- 7 mV for dimethyl-lumichrome, 1-methyl-lumichrome, 3-methyl-lumichrome and lumichrome, respectively. (author)