Spectroscopic studies on the interaction of Congo Red with bovine serum albumin

The binding interaction of Congo Red (CGR) with bovine serum albumin (BSA) was investigated by spectroscopic techniques including fluorescence spectroscopy, UV–vis absorption, and circular dichroism (CD) spectroscopy under simulative physiological conditions. Fluorescence data revealed that the fluorescence quenching of BSA by CGR was the result of the formation of a BSA–CGR complex, and the corresponding binding constants (Ka) at the four different temperatures (292, 298, 304, and 310 K) were obtained according to the modified Stern–Volmer equation. The thermodynamic parameters ΔH and ΔS were calculated to be −12.67 kJ mol−1 and 58.60 J mol−1 K−1, respectively, which suggested that both hydrophobic force and hydrogen bond played major roles in stabilizing the BSA–CGR complex. Site marker competitive experiments showed that the binding of CGR to BSA primarily took place in site I of BSA. The distance r between CGR (acceptor) and tryptophan residues of BSA (donor) was calculated to be 3.89 nm based on Förster's non-radioactive energy transfer theory. The conformational investigation showed that the presence of CGR resulted in the change of BSA secondary structure and induced the slight unfolding of the polypeptides of protein, which confirmed some micro-environmental and conformational changes of BSA molecules.