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Metal Ion-Dependent Fluorescent Dynamics of Photoexcited Zinc−Porphyrin and Zinc−Myoglobin Modified with Ethylenediaminetetraacetic Acid

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journal contribution
posted on 04.12.2008, 00:00 by Hiroshi Takashima, Hiroe Kawahara, Miho Kitano, Satomi Shibata, Hiroshi Murakami, Keiichi Tsukahara
The reconstituted zinc−myoglobin (ZnMb) dyads, ZnMb−[MII(edta)], have been prepared by incorporating a zinc−porphyrin (ZnP) cofactor modified with ethylenediaminetetraacetic acid (H4edta) into apo−Mb. In case of the monomeric ZnP(edta) cofactor coordinated by one pyridine molecule, ZnP(py)(edta), a spontaneous 1:1 complex with a transient metal ion was formed in an aqueous solvent, and the photoexcited singlet state of ZnP, 1(ZnP)*, was quenched by the [CuII(edta)] moiety through intramolecular photoinduced electron-transfer (ET) reaction. The rate constant for the intramolecular quenching ET (kq) at 25 °C was successfully obtained as kq = 5.1 × 109 s−1. In the case of Co2+, Ni2+, and Mn2+, intersystem crossing by paramagnetic effect was mainly considered between 1(ZnP)* and the [MII(edta)] complex. For the ZnMb-[MII(edta)] systems, the intramolecular ET reaction between the excited singlet state of 1(ZnMb)* and the [CuII(edta)] moieties provided the slower quenching rate constant, kq = 2.1 × 108 s−1, compared with that of the ZnP(py)(edta) one. Kinetic studies also presented the efficient fluorescence quenching of the 1(ZnMb)*−[CoII(edta)] dyad. Our study clearly demonstrates that wrapping of the ZnP cofactor by the apoprotein matrix and synthetic manipulation at the Mb surface ensure metal ion-sensitive fluorescent dynamics of ZnMb and provides valuable information to elucidate the complicated mechanism of the biological photoinduced ET reactions of hemoproteins.