Spectroscopic studies on binding of shikonin to human serum albumin

The binding properties on shikonin to human serum albumin (HSA) have been studied for the first time using fluorescence spectroscopy in combination with UV-visible absorbance spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and circular dichroism (CD) spectroscopy. The results of spectroscopic measurements suggested that the hydrophobic interaction is the predominant intermolecular force stabilizing the complex, which is in good agreement with the results of molecule modeling study. And the enthalpy change ΔH° and the entropy change ΔS° were calculated to be -13.86 kJ mol^-1 and 51.16 J mol^-1 K^-1 according to the Vant'Hoff equation. The fluorescence quenching mechanism and the number of binding site (n ≈ 1) were also obtained from fluorescence titration data. The efficiency of Förster energy transfer provided a distance of 2.12 nm between tryophan and shikonin binding site. The alterations of protein secondary structure in the presence of shikonin in aqueous solution were quantitatively calculated from FT-IR and CD spectroscopy with reductions of α helices content about 2.8-5.4% and with increases of β structures about 2.4%. In addition, the effect of common ions on the binding constants of shikonin-HSA complexes was also discussed.