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Kinetics and activation parameters of the reaction of organoarsenic(V) compounds with glutathione

Kretzschmar, J.; Brendler, E.; Wagler, J.; Schmidt, A.-C.

In this work the kinetics of the reaction of glutathione (GSH) with different organoarsenic(V) compounds is investigated. This includes phenylarsonic acid (PAA), 4-hydroxy-3-nitrophenylarsonic acid (HNPAA), p-aminophenylarsonic acid (p-APAA) and o-aminophenylarsonic acid (o-APAA) as well as monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA). The reaction progress was monitored in real time by 1H-NMR, which allowed the determination of rate coefficients and half-lives as well as activation energies, enthalpies and entropies of activation and, eventually, Gibbs free energies of activation. The reaction consists of two steps: redox reaction and conjugation. In all investigated systems, the conjugation is fast compared to the redox reaction, which therefore is rate determining. All investigated phenylarsonic acids are subject to the same rate law, showing overall reaction orders of 3 and half-life values between 47.7 ± 0.2 and 71.0 ± 3.6 min, corresponding to reaction rates between 1330 ± 10 L2 mol-2 min-1 and 850 ± 5 L2 mol-2 min-1, respectively. The methylated compounds react slower, showing half-lives of 76.6 ± 0.4 and 444 ± 10 min for DMAA and MMAA, respectively. The obtained enthalpies of activation range from 20 to 36 (± 2) kJ mol-1 and the entropies of activation are within −154 and −97 (± 7) J mol-1 K-1. The results reveal a correlation of the toxicity of the arsenic compound and the reaction rate with GSH. This may pave the way for the estimation of the toxicity of such compounds by simple kinetic studies.

Keywords: glutathione; roxarsone; phenylarsonic acid; arsanilic acid; arsenic detoxification; kinetics; NMR

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Publ.-Id: 19836