Photophysical and DNA-Binding Properties of Cytochrome c Modified with a Platinum(II) Complex

Cytochrome c (cyt c) derivatives modified with a platinum(II) complex at the lysine residue, cyt c(III)-[Pt(bpy)(dapap)]₁ {bpy = 2,2′-bipyridine, and dapap = 3-(2,3-diaminopropionylamino)propionic acid}, have been prepared. The modified residues are Lys8, Lys13, Lys55, Lys60, Lys73, and Lys88. In the case of the cyt c(III)-[Pt(bpy)(dapap)]₁ dyad, the photoexcited singlet state of ¹([Pt(bpy)(dapap)]₁)* was quenched by the heme Fe(III) moiety through the intramolecular photoinduced energy-transfer reaction via a through-space mechanism. Next, in the presence of calf thymus (CT)-DNA, the DNA-responsive fluorescence properties of cyt c(III)-[Pt(bpy)(dapap)]₁ isomers were investigated. The order of the obtained binding constants between the cyt c(III)-[Pt(bpy)(dapap)]₁ isomer and CT-DNA in an aqueous solution suggested that the electrostatic interaction is one of the important factors to stabilize the cyt c-DNA complex. Finally, we discussed the rotational motion of the [Pt(bpy)(dapap)]²⁺ moiety at the surface of cyt c by fluorescence anisotropy measurement. The increase in the anisotropy parameter, r, for each cyt c isomer clearly revealed that the noncovalent recognition of the [Pt(bpy)(dapap)]²⁺ moiety by CT-DNA is an essential event in the formation of the cyt c-DNA complex and generation of DNA-sensitive fluorescence signals.