Intramolecular quenching of porphyrin fluorescence by a covalently linked ferrocene in DNA scaffolding

Intramolecular quenching of fluorescence from a cationic porphyrin by a covalently attached ferrocene in both solution and DNA is reported. Two ferrocenyl porphyrins have been prepared, tris(4-N-methylpyridiniumyl), mono(phenyl-OCH(2)CH(2)ferrocene)porphyrin, P3Fc, and cis-bis(4-N-methylpyridiniumyl), bis(phenyl-OCH(2)CH(2)ferrocene)porphyrin. Binding studies for P3Fc indicate that intercalation of the porphyrin moiety into DNA occurs at low ionic strength (10 mM NaCl); outside binding of the complex is favored at increased ionic strength (100 mM NaCl). The outside binding of P3Fc at high ionic strength is attributed to the hydrophobicity of the molecule, which causes it to undergo salt-induced stacking in aqueous solution. The photophysical properties of P3Fc are examined in MeOH, in phosphate buffer, and in the presence of double-stranded DNA. Efficient quenching of the photoexcited singlet state of the porphyrin by electron transfer from the appended ferrocene is observed in MeOH and buffer solutions with average rate constants of greater than or equal to 1 x 10(10) and 9 x 10(9) s(-1), respectively. When P3Fc is intercalated into DNA, the average rate of photoinduced intramolecular electron transfer is not appreciably reduced (7 x 10(9) s(-1)), suggesting that electronic coupling between the D-A pair is strong even under conditions where close contact is restricted. Assuming that for this donor-linker-acceptor complex, in which the porphyrin and ferrocene are separated by an -OCH(2)CH(2)- spacer, intercalation of the porphyrin into DNA does not significantly reduce the electronic coupling between the donor and acceptor, the observed subnanosecond electron-transfer rates in and out of DNA show that changes in redox potentials or reorganization energy either compensate one another or have negligible kinetic consequences.