Photoinduced intramolecular electron-transfer reactions of reconstituted met- and zinc-myoglobins appending acridine and methylacridinium ion as DNA-binders

Three types of reconstituted met- and zinc-myoglobin (metMb and ZnMb) dyads, ZnMbAc(4)Me⁺, ZnMbAc-(6)Me⁺, and metMbAc(6) have been prepared by incorporating chemically modified metalloporphyrin cofactor appending an acridine (Ac) or a methylacridinium ion ([AcMe]⁺) into apo-Mb. In the bimolecular system between ZnMb and [AcMe]⁺, the photoexcited triplet state of ZnMb, ³(ZnMb)*, was successfully quenched by [AcMe]⁺ to form the radical pair of ZnMb cation (ZnMb^•+) and reduced methylacridine ([AcMe]^•), followed by a thermal back ET reaction. The rate constants for the intemolecular quenching ET (k_q) and the back ET reaction (k_b) at 25 °C were successfully obtained as k_q = (8.8 ± 0.4) × 10⁷ M^-1 s^-1 and k_b = (1.2 ± 0.1) × 10⁸ M^-1 s^-1, respectively. On the other hand, in case of the intramolecular photoinduced ET reactions of ZnMbAc(4)Me⁺ and ZnMbAc(6)Me⁺ dyads, the first-order quenching rate constants (k_ET) of ³(ZnMb)* by [AcMe]⁺ moiety were determined to be k_ET = 2.6 × 10³ and 2.5 × 10³ s^-1, respectively. When such ET occurs along the alkyl spacer via through-bond mechanism at the surface of Mb, the obtained k_ET is reasonable to provide decay constant of β(1.0-1.3 Å^-1). Upon photoirradiation of [AcMe] ⁺ moiety, kinetic studies also presented the intramolecular quenching reactions from the excited singlet state, ¹([AcMe] ⁺)*, whose likely process is the photoinduced energy-transfer reaction. For metMbAc(6) dyad, steady-state fluorescence was almost quenched, while the signal around 440 nm gradually appeared in the presence of various concentrations of DNA. Our study implies that synthetic manipulation at the Mb surface, by using an artificial DNA-binder coupled with photoinduced reaction, may provide valuable information to construct new Mb-DNA complex and sensitive fluorescent for DNA.