Self-assembling nanostructures of water-soluble fullerene [60]-chlorin e6 dyads: Synthesis, photophysical properties, and photodynamic activity

The synthesis of water-soluble dyads PFD-Chl(1), PFD-Chl(Zn) and PFD-Chl(2) based on a polyanionic fullerene [60] derivative PFD covalently linked to chlorin e 6 was described. Photophysical properties, generation of superoxide and singlet oxygen in water and liposomes, and photodynamic activity on HeLa cancer cells of obtained dyads were analyzed. The significant influence of linker length between fullerene[60] core and dye, presence of metal (Zn) atom in chlorin moiety on photophysical properties, and photodynamic activity of the dyads was demonstrated. All dyads exhibit quenching of chlorin fluorescence by 7-120 times due to the electron transfer from the dye to fullerene and they formed nanoassociates in aqueous solutions with an effective radius in the range of 30-500 nm due to their pronounced amphiphilic properties. Such nanoassociates were able to effectively interact with membranes of liposomes, which led to a sharp increase of fluorescence signal. This effect could be applied for the design of various fullerene-based nanoscale switch-off systems. The dyad PFD-Chl(1) - with the shortest linker and metal-free dye - had the superoxide generation efficiency 5.6 and 3.4 times higher compared to free chlorin Ce6 in water and liposomes, respectively, while its singlet oxygen generation was reduced. The phototoxicity of this dyad in HeLa cells was almost equal to that of the native chlorin, despite the reduction of singlet oxygen generation efficiency. Upon the incorporation of zinc atom into a chlorin moiety of this dyad, photodynamic activity and phototoxicity of PFD-Chl(Zn) dyad became negligible. The PFD-Chl(2) dyad - with the longest linker and metal-free chlorin - had about two times lower photodynamic activity and phototoxicity compared to Ce6 due to weak interaction between fullerene core and the dye. A significant increase of the type I pathway efficiency, demonstrated in the PFD-Chl(1) dyad, could be applied for the creation of photosensitizers, highly effective against hypoxic tumors. The demonstrated approach opens up vast opportunities for a directional design of highly efficient fullerene-based water-soluble photosensitizers for photodynamic therapy.