Pulse Radiolysis and ESR Studies of the Electron-Affinic Properties of Nitroheterocyclic Radiosensitizers

An attempt has been made to correlate some aspects of the radiation chemistry of various nitroheterocyclic compounds as cellular radiosensitizers with their electron-affinities as measured polarographically by their half-wave redox potentials. The nitroheterocyclic compounds accept electrons from a variety of radicals and radical anions formed in simple biological "target" molecules following H, OH, and <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> attack. The resulting nitroheterocyclic radical anions can themselves donate electrons to both oxygen and quinones as electron acceptors. The rate constants and efficiencies of these electron-transfer reactions, many of which are diffusion-controlled, have been determined by pulse radiolysis, and the effect of nitroheterocyclic electron-affinity is discussed. The electron-affinities of the nitroheterocyclic compounds, as measured by the half-wave redox potentials, correlate well with the electron spin densities on the nitro groups of their radical anions, as determined by ESR. The spin density, which decreases linearly with increasing nitroheterocyclic redox potential, is also a useful measure of cellular radiosensitizing effectiveness.