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

The reconstituted zinc - myoglobin (ZnMb) dyads, ZnMb-[M-II(edta)], have been prepared by incorporating a zinc-porphyrin (ZnP) cofactor modified with ethylenediaminetetraacetic acid (H(4)edta) 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 [Cu-II(edta)] moiety through intramolecular photoinduced electrontransfer (ET) reaction. The rate constant for the intramolecular quenching ET (k(q)) at 25 degrees C was successfully obtained as k(q) = 5.1 x 10(9) s(-1). In the case of Co2+, Ni2+, and Mn2+, intersystem crossing by paramagnetic effect was mainly considered between (1)(ZnP)* and the [M-II(edta)] complex. For the ZnMb-[M-II(edta)] systems, the intramolecular ET reaction between the excited singlet state of (I)(ZnMb)* and the [Cu-II(edta)] moieties provided the slower quenching rate constant, k(q) = 2.1 x 10(8) s(-1), compared with that of the ZnP(py)(edta) one. Kinetic studies also presented the efficient fluorescence quenching of the (1)(ZnMb)*-[Co-II(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.