Photocytotoxic action of cyanine dye with two chromophores. Formation of superoxide anion radical.

This study investigates how a bis-cyanine dye (BCD) induces light-triggered cytotoxicity in human colorectal cancer HCT116 cells. The photodynamic therapy mechanism was investigated using optical absorption, steady-state and time-resolved fluorescence spectroscopy, confocal microscopy, and flow cytometry.

The results support a substantial contribution of the superoxide anion radical (O₂-), consistent with a mainly Type I photodynamic therapy (PDT) pathway. This radical is proposed to arise via electron transfer from intracellular donors to triplet-state BCD, forming the intermediate BCD+ radical. Molecular docking and molecular dynamics simulations of BCD2+ bound to human serum albumin (HSA) suggest that tyrosine residues in albumin are plausible electron donors (binding constant Kb,HSA = 1.5 × 105 M-1). The calculated BCD-tyrosine distance (< 1 nm) is consistent with feasible electron transfer.

These findings support a substantial Type I contribution to BCD photocytotoxicity, while indicating that a minor contribution from singlet oxygen (1O₂), characteristic of Type II PDT, cannot be excluded and merits further study.