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Radiation Chemical Studies of the Sensitizer Diamide

D. W. Whillans and P. Neta
Radiation Research
Vol. 64, No. 3 (Dec., 1975), pp. 416-430
DOI: 10.2307/3574232
Stable URL: http://www.jstor.org/stable/3574232
Page Count: 15
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Radiation Chemical Studies of the Sensitizer Diamide
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Abstract

A detailed study of the radiation chemistry of the radiosensitizer Diamide (diazenedi-carboxylic acid bis-dimethylamide) using 60 Co irradiation, pulse radiolysis, pulse conductivity, and ESR techniques is presented. Reduction of Diamide by $e{}_{{\rm aq}}{}^{-}$ ($k=2.8\times 10^{10}\ M^{-1}\ {\rm sec}^{-1}$) is followed by rapid protonation of the radical anion by H+ at a diffusion-controlled rate, or by H2 PO4- at a rate of $7\times 10^{7}\ M^{-1}\ {\rm sec}^{-1}$. 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 $\text{thymine}\cdot ^{-}\ (k=5.0\times 10^{9}\ M^{-1}\ {\rm sec}^{-1})$, from ${\rm PNAP}\cdot ^{-}\ (k=1.6\times 10^{7}\ M^{-1}\ {\rm sec}^{-1})$, and from $({\rm CH}_{3})_{2}\dot{{\rm C}}{\rm OH}$ and $\dot{{\rm C}}{\rm O}{}_{2}{}^{-}\ (k\simeq 2.5\times 10^{9}\ M^{-1}\ {\rm sec}^{-1})$. The protonated radical reacts with oxygen at a rate of about $10^{8}\ M^{-1}\ {\rm sec}^{-1}$, but not by electron transfer. The protonated radical decays by a second-order reaction $(2k=4\times 10^{8}\ M^{-1}\ {\rm sec}^{-1})$ 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 $7\times 10^{9}\ M^{-1}\ {\rm sec}^{-1}$ 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 $4.2\times 10^{9}\ M^{-1}\ {\rm sec}^{-1}$ independent of pH from 4 to 8.

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