Radiation Chemical Studies of the Sensitizer Diamide

A detailed study of the radiation chemistry of the radiosensitizer Diamide (diazenedi-carboxylic acid bis-dimethylamide) using⁶⁰ Co irradiation, pulse radiolysis, pulse conductivity, and ESR techniques is presented. Reduction of Diamide by <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> (<tex-math>$k=2.8\times 10^{10}\ M^{-1}\ {\rm sec}^{-1}$</tex-math>) is followed by rapid protonation of the radical anion by H⁺ at a diffusion-controlled rate, or by H₂ PO₄^- at a rate of <tex-math>$7\times 10^{7}\ M^{-1}\ {\rm sec}^{-1}$</tex-math>. The protonation is confirmed by pulse conductivity experiments and supported by ESR observation of the dicarboxyhydrazyl radical. The radical anion is also formed by electron transfer from <tex-math>$\text{thymine}\cdot ^{-}\ (k=5.0\times 10^{9}\ M^{-1}\ {\rm sec}^{-1})$</tex-math>, from <tex-math>${\rm PNAP}\cdot ^{-}\ (k=1.6\times 10^{7}\ M^{-1}\ {\rm sec}^{-1})$</tex-math>, and from <tex-math>$({\rm CH}_{3})_{2}\dot{{\rm C}}{\rm OH}$</tex-math> and <tex-math>$\dot{{\rm C}}{\rm O}{}_{2}{}^{-}\ (k\simeq 2.5\times 10^{9}\ M^{-1}\ {\rm sec}^{-1})$</tex-math>. The protonated radical reacts with oxygen at a rate of about <tex-math>$10^{8}\ M^{-1}\ {\rm sec}^{-1}$</tex-math>, but not by electron transfer. The protonated radical decays by a second-order reaction <tex-math>$(2k=4\times 10^{8}\ M^{-1}\ {\rm sec}^{-1})$</tex-math> with G(-Diamide) = 3.1 in the formate system, suggesting a disproportionation reaction.⁶⁰ Co irradiation of N₂ O saturated solutions of Diamide leads to a shift in λ _max and an increase in absorption before subsequent destruction. Optical studies using pulse radiolysis show a threefold increase in transient absorption at 400 nm between pH 6 and 9, and the relative spectra at these pH's are dissimilar, suggesting the presence of a radical with pK ≃ 7.5. Pulse conductivity experiments show that about 50% of the radicals are dissociated at pH 9.8. It is suggested that OH radicals react by both addition to the N=N bond and abstraction from the methyl groups at approximately equal rates and that the OH adduct dissociates with pK ≃ 7.5. The rate of signal formation at 400 nm is also pH dependent. A rate constant of <tex-math>$7\times 10^{9}\ M^{-1}\ {\rm sec}^{-1}$</tex-math> was observed at pH 2, 2.3, 2.8, 4, 4.8, 6.5, and 6.8 and a lower value is measured at higher pH. Indirect measurement of the rate of OH attack by phenylalanine competition gives a value of <tex-math>$4.2\times 10^{9}\ M^{-1}\ {\rm sec}^{-1}$</tex-math> independent of pH from 4 to 8.