Binding studies of a new copper (II) complex containing mixed aliphatic and aromatic dinitrogen ligands with bovine serum albumin using different instrumental methods

The new copper (II) complex, [Cu(N–N)(L)(EtOH)](NO3)2·2H2O; in which N–N = 2,9-dimethyl-1,10-phenanthroline and L = N,N-dimethyltrimethylenediamine; has been synthesized and characterized by 1H and 13C NMR, absorption spectroscopy and elemental analysis (CHN). The interaction of this complex with bovine serum albumin (BSA) was investigated under physiological condition in 0.01 M phosphate buffer using spectroscopic methods including fluorescence, UV–vis absorption and circular dichroism (CD). The results of fluorescence titration revealed that the complex strongly quench the intrinsic fluorescence of BSA through a static quenching procedure. Binding constants (Kb), association constants (Ka) and the number of binding sites (n ≈ 1) were calculated using modified Stern–Volmer equations. The thermodynamic parameters indicate that the hydrophobic and hydrogen bonding interactions play a major role in BSA–Cu complex association. The process of binding was spontaneous, in which Gibbs free energy change (ΔG) was negative. The distance r0 between donor (BSA) and acceptor (copper complex) was obtained to be 1.9 nm according to Foster’s non-radiative energy transfer theory and showed that the energy transfer from BSA to the Cu(II) complex occurs with high probability. Other results also revealed that binding of the complex could induce the conformational changes in BSA.