OXIDATION OF TRYPTOPHAN AND N-METHYLINDOLE BY N3., BR2-, AND (SCN)2- RADICALS IN LIGHT-WATER AND HEAVY-WATER SOLUTIONS - A PULSE-RADIOLYSIS STUDY

Reactions of N3., Br2.-, and (SCN)2.- with tryptophan (TRPH) and N-methylindole (NMI) have been investigated in H2O and D2O solutions. The main transients produced were the TRP. radical and the TRPH.+ radical cation from TRPH and the NMI.+ 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.+ radical cation was confirmed to be 4.2 +/- 0.1. These rate constants were within experimental error, independent of pH, and the differences between the rate constants in H2O and D2O 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 N3.. Experimental data on these effects have also been obtained.