Peripheral charge effects on the kinetics of Zn(II)-porphyrin system

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Abstract

Kinetics of the reaction between Zn(II) ion and the peripherally positively charged tetra-(p-trimethylammoniumphenyl)porphyrin (H2TAPP) were investigated at 30°C and 0.5°F ionic strength (NaNO3). The reaction was found to be first order in Zn(II) ion and porphyrin concentration. The specific rate constant for the reaction, if the reacting species were [Zn(OH)+], is 210±30M −1 sec−1 in comparison with 1.50±0.10 M−1 sec−1 determined for the reaction of Zn2+. Both these rates are faster than the corresponding rates for the reaction of a negatively charged tetra-(p-sulfophenyl)porphyrin (H2TPPS). But the demetallation reaction of Zn(II)TAPP is about eight times slower than that of Zn(II)TPPS. No unusual anion dependence of the reaction is observed. Isotope effect and activation parameters of these reactions are presented.

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