The aluminium(III) complex of hypocrellin B as a PDT photosensitizer

Abstract

The complex of hypocrellin B (HB) with aluminium was synthesized to improve the water solubility and red absorption of the parent pigment. The complexation of HB with Al³⁺ resulted in the formation of a complex with 1 : 1 Al(III) : HB stoichiometry (Al³⁺–HB) measured by molar ratio and continuous variation methods, respectively. The structure of this complex has been characterized by UV-Vis, IR, ¹H NMR and elemental analysis data. Compared with HB, the complex possesses excellent solubility in water; in addition, the absorption wavelength of Al³⁺–HB shifts bathochromically into the phototherapeutic window (600–900 nm). The generation of active oxygen species, including the superoxide radical anion (O₂^•−) and singlet oxygen (¹O₂), by Al³⁺–HB photosensitization was observed. The efficiency of O₂^•− generation by Al³⁺–HB was twice as much as that of HB. Under our experimental conditions, the rate of O₂^•− generation by Al³⁺–HB was about 2.3 μM min⁻¹ measured by the cytochrome c reduction method using 578 nm light. The quantum yield of singlet oxygen (¹O₂) was 0.23 in DMSO determined by the 9,10-DPA (diphenylanthracene) photobleaching experiments. These results suggest that the new complex possesses an enhanced type I process but a decreased type II process as compared with hypocrellin B.