Characterization of the Interaction Between Caffeine and Soybean Selenoprotein by Spectroscopic and Cyclic Voltammetric Methods

The quantitative interaction of soybean selenoprotein (SSP) with caffeine (CAF) has been studied by fluorescence, cyclic voltammetry and resonance Rayleigh scattering (RRS) spectra. Fluorescence results demonstrate that the quenching of SSP, induced by CAF, can mainly be attributed to static quenching, and the relative contribution of dynamic quenching is about 0.37% by using a Stem–Volmer analysis. The binding constant (Ka) and binding sites (n) of the formed SSP/CAF complex at 293 K calculated from the fluorescence quenching results were found to be 4.29 × 104 L·mol−1 and 1.22, respectively, which is relatively consistent with the corresponding values obtained by cyclic voltammetry. Based on Förster’s theory, the average binding distance (r) between CAF and SSP was found to be 3.03 nm. The interaction mechanism between CAF and SSP was also supported by inspecting the RRS spectra. The effect of CAF on the conformation of SSP was investigated by using the fluorescence phase diagram and circular dichroism spectra. The experimental results show that SSP binding to CAF follows the two-state model and the helical content of SSP decreases after the interaction. In addition, the effect of CAF on the antioxidant activity of SSP was revealed by ultraviolet spectrophotometry. According to the results, SSP/CAF possesses significantly higher antioxidant activity than the control soybean selenoprotein.