Kinetics of Magnesium Incorporation into Water Soluble Porphyrins

Abstract The kinetics of Mg2+ incorporation into the three water soluble porphyrin isomers of the relatively planar cationic tetrakis(N-Methyl-X-pyridyl)porphyrins (X = 2,3, or 4) were studied from pH 6.5 to 8.9 at 25 °C, I = 2.6. The reactions were first order in both porphyrin and magnesium ion concentrations, and the specific rate constants increased with an increase in pH. The proposed mechanism involves Mg2+ reacting with the differing centrally protonated forms of these porphyrins, in the order Ρ 2-> H-P-" > H2-P. Various other porphyrins were substantially less reactive. In contrast, Mg reacts orders of magnitude faster with the non-planar ß-octabromo-tetra(N-methyl-4-pyridyI) porphyrin, where the reaction proceeds almost exclusively through the Mg2+ + P2- pathway. As compared to other metal ions, the relatively slow incorporation of Mg2+ into porphyrins is in part a consequence of the comparatively slow water exchange rate constant for the aquo Mg2+ ion.