A comprehensive research on Lactone Sophorolipid (LSL) and Soy Protein Isolate (SPI) interacting mixture

Abstract Soy protein isolate (SPI) is a promising biological material, and lactone sophorolipid (LSL) is a non-toxic, completely biodegradable amphiphilic functional biosurfactant. A comprehensive research of interaction mode and mechanism on LSL and SPI composite system was studied using surface tension, turbidity, fluorescence spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, circular dichroism (CD) spectroscopy, dynamic light scattering (DLS), isothermal microcalorimetric titration (ITC) and molecular docking methods. The properties of surface activity, foam stability, antibacterial ability and so on were evaluated in detail. Compared with a single pure substance, the performance of the composite system shows different performance optimizations in different aspects. For the SPI/LSL systems with different mass ratios studied, the surface activity, foaming performance and foam stability of the composite system with a mass ratio of 1:50 are more prominent than those of other ratios. The fluorescence distribution showed the increase of LSL leading to a significant change in the fluorescence distribution of SPI, the quenching mechanism is mainly static quenching mechanism, and the combination of LSL and SPI is an spontaneous endothermic process. UV–Vis spectroscopy showed that LSL changed the conformation of SPI. The CD spectrum indicated that the combination of LSL and SPI increases the α-helical structure of protein, and is beneficial to the secondary structure of protein. The molecular model presents the conformation of the SPI/LSL complex, indicating that the hydrophobic force and hydrogen bonds are considered to be the main interaction forces between SPI and LSL, which may lead to the flexible conformation of the protein. This research enriches our understanding of the interaction mechanism between SPI and LSL, and provides some important and interesting information for the development and application of biosurfactant-protein mixed systems with environmentally friendly, non-toxic and non-polluting properties.