Research on the Interaction between Pheophorbide and Bovine Serum Albumin

Abstract

The interaction between natural pheophorbide (a superior photosensitizer) and bovine serum albumin (BSA) in physiological condition is investigated by means of UV-Vis, fluorescence and synchronous fluorescence spectra so as to provide the basis for clinical use. Natural pheophorbide was isolated from silkworm excrement. BSA in pH 7.4 Tris buffer mixed with different concentration of pheophorbide was kept at certain temperature for 3 h or under illumination by laser at 630 nm for 20 min. UV-Vis absorption of BSA was enhanced and its fluorescence was quenched by pheophorbide. Illumination of laser at 630 nm intensified the quenching. The mechanism is deemed as mainly static quenching. The binding constants Ka at 300, 310, 320 K are separately 6.93×1012,7.40×1012,6.82×1012 L/mol/s respectively. Number of binding sites n is 1; the binding distance R is 3.70 nm, and that suggests non-radiation energy transfer from BSA to pheophorbide. The thermodynamic parameters of the binding reaction are H=36.7 kJ/mol, S=213 J/mol/K, and G negative value, and indicates that hydrophobic force plays a predominant role in the process, and it is a spontaneous interaction. Synchronous fluorescence spectra show that pheophorbide mainly interacts with tryptophan residue of BSA and leads to the promotion of hydrophobic force. Pheophorbide can bind to serum protein and be transported in vivo, makes no destruction to molecular structure of serum protein, but causes its conformational alteration.