Abstract
The one-electron reduction potential of 3-amino-l, 2, 4-benzotriazine 1, 4-dioxide, tirapazamine (SR 4233) in aqueous solution has been determined by pulse radiol-ysis. Reversible electron transfer was achieved between radiolytically-generated one-electron reduced radicals of tirapazamine (T), and quinones or benzyl viologen as redox standards. The reduction potential Em7(T/T±) was -0.45 ± 0.01 V vs. NHE at pH 7. From the pH dependence of the reduction potential, pKa = 5.6 ± 0.2 was estimated for the tirapazamine radical, a value similar to the pKa determined by other methods.