Reactions in Tirapazamine Induced by the Attachment of Low-Energy Electrons: Dissociation Versus Roaming of OH

Tirapazamine (TPZ) has been tested in clinical trials on radio-chemotherapy due to its potential highly selective toxicity towards hypoxic tumor cells. It was suggested that either the hydroxyl radical or benzotriazinyl radical may form as bioactive radical after the initial reduction of TPZ in solution. In the present work, we studied low-energy electron attachment to TPZ in the gas phase and investigated the decomposition of the formed TPZ⁻ anion by mass spectrometry. We observed the formation of the (TPZ–OH)⁻ anion accompanied by the dissociation of the hydroxyl radical as by far the most abundant reaction pathway upon attachment of a low-energy electron. Quantum chemical calculations suggest that NH₂ pyramidalization is the key reaction coordinate for the reaction dynamics upon electron attachment. We propose an OH roaming mechanism for other reaction channels observed, in competition with the OH dissociation.