Rate Constants for Reaction of Hydroxyl Radicals with Water-Soluble Porphyrins and Metalloporphyrins.

Abstract

To determine the rate constants for the reaction of hydroxyl radicals with water-soluble porphyrins and their metalloporphyrins, tetrakis(4-sulfophenyl)porphine(TPPS), tetrakis(5-sulfothienyl)porphine (T(5-ST)P), tetrakis(4-N-methylpyridyl)porphine (TMPyP) and tetrakis(4-N-trimethylamino-phenyl)porphine (TTMAP), and Mn(III), Fe(III), Co(III) and Cu(II) complexes of TPPS and TMPyP, the γ-irradiation method, in which the fluorescence of hydroxybenzoates produced through reaction of benzoate with hydroxyl radicals generated by water-radiolysis is measured, were tried. However, since the porphyrins and metalloporphyrins show the characteristic absorption spectra in the visible region, the fluorescence of hydroxybenzoate products was disturbed by the colors. To separate the hydroxybenzoates from the colored solute, the products were extracted into ether under an acidic condition. The hydroxybenzoates were extracted thoroughly into the ether layer from an aqueous layer of less than pH 3 ; the residue was redissolved in phosphate buffer, pH 7.5; the solution was used to determine the rate constants. The rate constants for anionic porphyrins, TPPS, T(5-ST)P, were in the order of 109, and the constants for cationic porphyrins, TMPyP, TTMAP, in the order of 1010. Metal complexes of TPPS and TMPyP showed the rate constants of 6.9 to 11.4×109M-1S-1 and of 3.2 to 19.7×109M-1S-1, respectively.