The binding reaction between Vitamin B12 (B12, cyanocobalamin) and human serum albumin (HSA) was investigated by fluorescence quenching, UV-vis absorption and circular dichroism (CD) spectroscopy. Under simulative physiological conditions, fluorescence quenching data revealed that the quenching constants (Ksv) are 3.99 x 10(4), 4.33 x 10(4), 4.76 x 10(4) and 5.16 x 10(4) M(-1) at 292, 298, 304 and 310 K, respectively. The number of binding sites, n is almost constant around 1.0. On the basis of the results of fluorescence quenching the mechanism of the interaction of B12 with HSA has been found to be a dynamic quenching procedure. Thermodynamic parameters DeltaHTheta= -13.38 kJ mol(-1), DeltaSTheta=66.73 J mol(-1) K(-1) were calculated based on the binding constant. These suggested that the binding reaction was enthalpy and entropy driven, and the electrostatic interaction played major role in stabilizing the reversible complex. The binding distance r=5.5 nm between HSA and B12 was obtained according to Forster theory of energy transfer. The effect of B12 on the conformation of HSA was analyzed by synchronous fluorescence and CD spectroscopy. Synchronous spectra indicated that the polarity around the tryptophan (Trp214) residues of HSA was decreased and its hydrophobicity was increased; however, the alpha-helix content of the protein was predominant in the secondary structure but the CD spectra indicated that B12 induced minor conformational changes of HSA.
