One-electron reduction and oxidation studies of the radiation sensitizer gadolinium(III) texaphyrin (PCI-0120) and other water soluble metallotexaphyrins

The radiation sensitizer gadolinium(III) texaphyrin 2 (XYTRIN; PCI-0120; Gd-Tex(2+)) and several other water soluble metallotexaphyrin complexes were prepared and studied using pulse radiolysis. All of the metallotexaphyrins were found to react with solvated electrons and hydroxyl radicals, yielding the corresponding one-electron reduced and oxidized metallotexaphyrins, respectively. The rates of the reduction processes range from 3.7 x 10(10) to 6.8 x 10(10) M-1 s(-1) (+/-10%), while those involving oxidation range from 2.5 x 10(9) to 7.4 x 10(9) M-1 s(-1) (+/-10%). The spectral characteristics of the transformed metallotexaphyrins produced by these reactions, e.g., a broad absorption band with a lambda(max) centered around 830 nm, are consistent with ligand-centered redox processes. Reaction of the metallotexaphyrins with solvated electrons affords species which exhibit metal dependent behavior. In the absence of hydroxyl radicals, the decay of the reduced metallotexaphyrins produced by reaction with electrons involves an initial protonation event followed by either a dimerization process or a disproportionation step. These latter transformations are followed by a second protonation event.