A Photochemical Study of Photo-Induced Electron Transfer from DNAs to a Cationic Phthalocyanine Derivative

Water-soluble cationic gallium(III)-Pc complex (GaPc) is capable of photogenerating ROSs but does not exhibit photocytotoxicity in vivo. GaPc binds selectively, through a π-π stacking interaction, to the 5'-terminal G-quartet of a G-quadruplex DNA. The photo-excited state of GaPc of the complex is effectively quenched through electron transfer (ET) from the ground state of DNA guanine (G) bases to the photo-excited state of GaPc (ET_(G-GaPc)). Hence the loss of the photocytotoxicity of GaPc in vivo is most likely to be due to the effective quenching of its photo-excited state through ET_(G-GaPc). In this study, we investigated the photochemical properties of GaPc in the presence of duplex DNAs formed from a series of sequences to elucidate the nature of ET_(G-GaPc). We found that ET_(G-GaPc) is allowed in electrostatic complexes between GaPc and G-containing duplex DNAs and that the rate of ET_(G-GaPc) (k_ET(G-GaPc)) can be reasonably interpreted in terms of the distance between Pc moiety of GaPc and DNA G base in the complex. We also found that the quantum yields of singlet oxygen (¹O₂) generation (Φ_Δs) determined for the GaPc-duplex DNA complexes were similar to the value reported for free GaPc (Fujishiro R, Sonoyama H, Ide Y, et al (2019) J Inorg Biochem 192:7-16), indicating that ET_(G-GaPc) in the complex is rather limited. These results clearly demonstrated that photocytotoxicity of GaPc is crucially affected by ET_(G-GaPc). Thus elucidation of interaction of a photosensitizer with biomolecules, i.e., an initial process in PDT, would be helpful to understand its subsequent photochemical processes.