Abstract: The interaction between vitamin D3 (VD3) and soy protein isolate (SPI) was studied by multiple spectroscopies (?uorescence, synchronous fluorescence, ultraviolet-visible (UV-Vis), and Fourier infrared spectroscopy). The fluorescence spectroscopy results showed that the intrinsic fluorescence of SPI was statically quenched by VD3. The binding constants (KA) at different temperatures were 1.245 × 104 (293 K), 1.250 × 104 (293 K) and 3.531 × 104 (306 K) L/mol, and the numbers of binding sites (n) were 0.973 3, 0.992 4 and 1.094 2, respectively. The binding distance (r) was 2.92, and the binding promoted fluorescence quenching of the protein through non-radiative energy transfer. According to analysis of the thermodynamic data, the reaction of VD3 with SPI was a spontaneous endothermic process, and the interaction was dominated by electrostatic interaction and hydrophobic interaction. The synchronous fluorescence and UV-Vis spectra illustrated that the addition of VD3 changed the conformation of SPI, and the microenvironment of aromatic amino acid residues changed from hydrophobic to hydrophilic status. Fourier transform infrared spectroscopy revealed that VD3 could indude secondary structure changes in SPI.

Key words: soy protein isolate, vitamin D3, ?uorescence spectroscopy, synchronous fluorescence, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy