Acidic Sophorolipid Biosurfactant Protects Serum Albumin Against Thermal Denaturation

Sophorolipids (SLs) constitute a group of unique biosurfactants in the light of their out-standing properties, among which their physicochemical characteristics, and antimicro-bial activities stand out. SLs can exist mainly in acidic and lactonic forms, both of which display inhibitory activity. This study explores the interaction of non-acetylated acidic SL with bovine serum albumin (BSA). SL significantly enhances BSA's thermal stability, in-creasing its midpoint unfolding temperature from 61.9 °C to approximately 76 °C and ΔH from 727 to 1054 kJ mol−1 at high concentrations, indicating cooperative binding. FTIR analysis confirms SL's protective effect against thermal unfolding, enabling BSA to main-tain its helical structure at 70 °C, distinguishing it from other surfactants that cause dena-turation. Furthermore, SL fundamentally alters the sequence of thermal unfolding events; β-aggregation precedes helical domain unfolding, suggesting protective binding to BSA's helical regions. Computational docking reveals high-affinity binding (Kd = 14.5 μM). Uniquely, SL binds between BSA domains IB and IIIA, establishing hydrophobic interac-tions, salt bridges, and hydrogen bonds, thus stabilizing the protein's 3D structure. This distinct binding site is attributed to SL's amphipathic character. This work deepens the understanding of the molecular characteristics of SL-protein interactions and contributes to improving the general knowledge of this outstanding biosurfactant.