Synthesis and photophysical properties of zinc myoglobin appending an ethidium ion as a DNA intercalator.

In order to elucidate the intramolecular photoinduced electron-transfer or energy-transfer mechanisms of the zinc myoglobin (ZnMb) dyad and to construct a photoreaction system within a Mb-DNA complex, we newly prepared ZnMb appending an ethidium ion (Et(+)). The steady-state fluorescence of ZnMb-Et(+) at 600 nm and its lifetime (2.2 ns) indicate that the excited singlet state of (1)(ZnMb)* is not quenched by the Et(+) moiety, whereas the lifetime of the excited triplet state of (3)(ZnMb)*-Et(+) was shorter (tau = 4.3 ms) than those of ZnMb and the intermolecular (ZnMb + ethidium) system. Upon photoirradiation of Et(+), fluorescence studies indicated the intramolecular quenching reactions from the excited singlet state, (1)(Et(+))*, to ZnMb, the process of which is likely the photoinduced energy-transfer reaction via a through-space mechanism. We also demonstrate the photophysical and spectroscopic properties of ZnMb-Et(+) in the presence of calf thymus (CT) DNA. The changes in the absorption and fluorescence spectra of ZnMb-Et(+) on the addition of CT-DNA up to 15 equiv were very small, indicating that there are no major changes in the heme pocket. However, we observed a longer lifetime of (3)(ZnMb)*-Et(+) in the presence of CT-DNA (tau = 5.3 ms) by single flash photolysis. This was induced by noncovalent interactions between Et(+) and CT-DNA, followed by a conformational change of Et(+) at the surface of ZnMb, where the donor-acceptor distance was probably elongated by CT-DNA. The synthetic manipulation at the Mb surface, by using a DNA intercalator coupled with photoinduced reaction, may provide a sensitive transient signal for DNA and valuable information to construct new Mb-DNA complex.