Reduction of a water-soluble iron(III) porphyrin and an iron(II) nitrosyl porphyrin by benzophenone ketyl radical

Abstract

Reactions of benzophenone ketyl radicals with the water-soluble iron(III)porphyrin, [Fe^III(tpps)]^3–[tpps =meso-tetrakis(p-sulfonatophenyl)porphyrinate], and a nitric oxide adduct of the iron(II) form of the porphyrin, [Fe^II(tpps)(NO)]^4–, have been studied by laser photolysis in aqueous solutions. Reactions of ferricytochrome c and metmyoglobin with the ketyl radicals were also studied for comparison with the iron(III) porphyrin. Benzophenone ketyl radicals were produced from the reaction of the triplet state of benzophenone with the glucose used as a suitable hydrogen atom donor. The transient absorption spectrum observed after 266 nm laser pulse excitation for the [Fe^III(tpps)]^3– solution containing benzophenone and glucose shows that the iron(III) porphyrin is reduced by the ketyl radical to the iron(II) form. Ferricytochrome c and metmyoglobin were found also to be reduced to the iron(II) form by the ketyl radical. The rate constants for the reductions of [Fe^III(tpps)]^3–, ferricytochrome c and metmyoglobin were determined to be 3.0 × 10⁹, 4.5 × 10⁸ and 2.2 × 10⁸ dm³ mol^–1 s^–1 respectively. Nitric oxide irreversibly reacts with [Fe^II(tpps)]^4– to yield [Fe^II(tpps)(NO)]^4– in an aqueous solution. The transient absorption spectrum observed 500 µs after laser pulsing for the [Fe^II(tpps)(NO)]^4– solution containing benzophenone and glucose indicates that the ketyl radical reacts with [Fe^II(tpps)(NO)]^4– to form a new transient species which displays the spectrum with a peak at 418 nm in the Soret band region. The transient species has been identified as a one-electron adduct of [Fe^II(tpps)(NO)]^4– where the added electron possibly exists on the coordinated NO moiety.