KINETICS AND ENERGETICS OF ONE-ELECTRON-TRANSFER REACTIONS INVOLVING TRYPTOPHAN NEUTRAL AND CATION RADICALS

Reversible one-electron-transfer reactions involving the tryptophan cation and neutral radicals were investigated byh pulse radiolysis. The one-electron-reduction potential fo the neutral tryptophan radical at pH 7 was determined to be E7 = 1.01 +/- 0.03 V by using the bis(1,4,7-triazacyclononane)nickeld (III/II) redox couple with E = 0.95 V as a standard. The value obtained with promethazine (E = 0.98 V) as a standard at pH 6 was E6 = 1.11 V. From these measurements, a mean value E7 = 1.03 V results, in agreement with some earlier determinations (1.05 and 1.08 V) obtained in experiments with inorganic redox standards. The kietics and energetics of the reactions of the tryptophan neutral and cation radicals with selected electron donors were also investigated. The reactivity of the typtophan radical cation was found to be 1-2 orders of magnitude higher than that of the neutral radical. The lower reactivity of the neutral tryptophan radical is explained by the entropy loss due to protonation of the tryptophan anion, resulting from the changes in the solvation shell.