Spectroscopic Studies on the Binding of Cobalt(II) 1,10-Phenanthroline Complex to Bovine Serum Albumin

The binding interaction of the cobalt(II) 1,10-phenanthroline complex (Co(phen) (3) (2+) , phen = 1,10-phenanthroline) with bovine serum albumin (BSA) was investigated by fluorescence spectroscopy combined with UV-Vis absorption and circular dichroism measurements under simulative physiological conditions. The experiment results showed that the fluorescence intensity of BSA was dramatically decreased owing to the formation of Co(phen) (3) (2+) -BSA complex. The corresponding association constants (K (a)) between Co(phen) (3) (2+) and BSA at four different temperatures were calculated according to the modified Stern-Volmer equation. The enthalpy change (Delta HA degrees) and entropy change (Delta SA degrees) were calculated to be -2.73 kJ mol(-1) and 82.27 J mol(-1) K-1, respectively, which suggested that electrostatic interaction and hydrophobic force played major roles in stabilizing the Co(phen) (3) (2+) -BSA complex. Site marker competitive experiments indicated that the binding of Co(phen) (3) (2+) to BSA primarily took place in site I of BSA. A value of 4.11 nm for the average distance r between Co(phen) (3) (2+) (acceptor) and tryptophan residues of BSA (donor) was derived from Forster's energy transfer theory. The conformational investigation showed that the presence of Co(phen) (3) (2+) resulted in the change of BSA secondary structure and induced the slight unfolding of the polypeptides of protein, which confirmed the microenvironment and conformational changes of BSA molecules.