Greatly enhanced binding of a cationic porphyrin towards bovine serum albumin by cucurbit[8]uril.

Binding affinity towards serum albumin and intracellular proteins is of importance for a photodynamic therapy (PDT) sensitizer to selectively localize in tumours and efficiently induce cell death. In this paper, it was found that cucurbit[8]uril (CB8) can greatly improve the binding affinity of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP), a promising PDT photosensitizer, towards bovine serum albumin (BSA). Absorption, fluorescence emission, (1)H NMR, dynamic light scattering, atomic force microscope, as well as protein photocleavage measurements suggest that the binding enhancement originates from the formation of a ternary complex of CB8·TMPyP·tryptophan residues. This finding opens up a new approach for the development of more efficient PDT agents.