Kinetics Of Demetallation Of A Lead Porphyrin As Influenced By DNA

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Inorganic Chemistry


The kinetics of the demetalation of (tetrakis( 1-methylpyridinium-4-yl)porphine)lead(II) (PbTMpyP) by EDTA was investigated at 25 °C as a function of concentration, pH, and ionic strength. The rate law is first order in both the lead porphyrin and EDTA with the second-order rate constant showing a pH profile consistent with two acid forms for each of the reactants being important over the pH range considered. The dependence of the rate constant on ionic strength leads to the conclusion that the "effective" charge of the metalloporphyrin is nearly +2. DNA also induces the demetalation reaction. Under the conditions of the experiments considered here, the reaction is first order, yet the rate constant for the DNA-assisted demetalation is about 10⁴ larger than that for spontaneous demetalation. Adding both DNA and EDTA to PbTMpyP leads to the formation of the free-base porphyrin (H₂TMpyP) with a rate law that is first order in both [PbTMpyP] and [EDTA] and has an inverse dependence on [DNA]. The effect of added NaCl is more complicated than that observed for EDTA alone, a maximum in the rate appearing at μ ~ 0.3 M. These results can be interpreted as indicating that the reaction of the multivalent anion (EDTA) with the porphyrin-nucleic acid complex proceeds primarily via a pathway in which the cationic metalloporphyrin first dissociates from the DNA surface and is then attacked by the anion while free in solution.

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