Oxidation of tryptophan and N-methylindole by N3.cntdot., Br2.-, and (SCN)2.- radicals in light- and heavy-water solutions: a pulse radiolysis study

Abstract

Reactions of N{sub 3}{sup {sm bullet}}, Br{sub 2}{sup {sm bullet}{minus}}, and (SCN){sub 2}{sup {sm bullet}{minus}} with tryptophan (TRPH) and N-methylindole (NMI) have been investigated in H{sub 2}O and D{sub 2}O solutions. The main transients produced were the TRP{sup {sm bullet}} radical and the TRPH{sup {sm bullet}+} radical cation from TRPH and the NMI{sup {sm bullet}+} radical cation from NMI. Their extinction coefficients ({epsilon}) as well as the rate constants for their formation and decay were determined in the pH range 3-10. The pK of the TRPH{sup {sm bullet}+} radical cation was confirmed to be 4.2 {plus minus} 0.1. These rate constants were within experimental error, independent of pH, and the differences between the rate constants in H{sub 2}O and D{sub 2}O were small. On the basis of the results, it was concluded that of the two possible reactions - electron or hydrogen atom transfer - the data are more consistent with electron transfer in all cases. However, secondary acid-base reactions have an important effect on the overall redox equilibria, especially with N{sub 3}{sup {sm bullet}}. Experimental data on these effects have also been obtained.