Abstract: In this work, the interaction between curcumin (Cur) and Cynanchum auriculatum Royle ex Wigh protein (CAP) or soy protein isolate (SPI) was characterized by a combination of steady-state fluorescence spectroscopy, far-ultraviolet circular dichroism (CD) spectroscopy, differential scanning calorimetry (DSC), and dynamic light scattering (DLS). The protective effects of the two proteins on the chemical stability of Cur were further investigated. Results showed that Cur quenched the intrinsic fluorescence of the proteins in a static manner. The complexation of Cur with either protein was a spontaneous reaction characterized by a decrease in Gibbs free energy and mainly driven by hydrophobic interaction. Cur bound more readily to SPI than to CAP. Fourier-transform infrared spectroscopy (FTIR) analysis suggested that in addition to hydrophobic interactions, hydrogen bonding might also be involved in the formation of Cur-protein complexes. Cur binding resulted in a reduction in the α-helix structure in CAP with a paralleled increase in the contents of three other secondary structures, as well as reduced the thermal stability of CAP. Compared with SPI, Cur-SPI complex contained less α-helix and β-turn structures and presented a higher thermal stability. The complexation of Cur with CAP and SPI afforded nanoparticles with a diameter of 159.98 and 244.34 nm, respectively. The thermal stability of Cur at 40 and 90 ℃ was improved remarkably after being bound with CAP or SPI, and the protective effect of SPI appeared to be more pronounced.

Key words: Cynanchum auriculatum Royle ex Wight protein, soy protein isolate, curcumin, interaction, thermal stability