Role of azide concentration in pulse radiolysis studies of oxidation: 3,4-dihydroxyphenylalanine

The radiolysis of aqueous solutions of 3,4-dihydroxyphenylalanine (dopa) using N˙3 as a one-electron oxidant leads to the formation of dopasemiquinone and, successively, dopaquinone and dopachrome. It is now shown that under pH conditions where significant protonation of N–3 to HN3 occurs, dopachrome formation is suppressed, probably due predominantly to addition of HN3 to dopaquinone. The possibility of such nucleophilic reactions occurring needs to be considered in studies of quinone intermediates generated using N–3 as oxidant.