Interaction of an acidic sophorolipid biosurfactant with phosphatidylcholine model membranes

Sophorolipids (SLs) constitute a group of unique biosurfactants (BS) in the light of their outstanding properties, among which their antimicrobial activities stand out. SLs can exist mainly in an acidic and a lactonic form, both of which display inhibitory activity. Given the amphipathic nature of SLs it is feasible that these antimicrobial actions are the result of the perturbation of the physicochemical properties of targeted membranes. Thus, in this work we have carried out a biophysical study to unveil the molecular details of the interaction of an acidic SL with a model phospholipid membrane made of 1,2-dipalmitoy-sn-glycero-3-phosphocholine (DPPC). Using differential scanning calorimetry it was found that SL altered the phase behaviour of DPPC at low molar fractions, producing fluid phase immiscibility with the result of formation of biosurfactant-enriched domains within the phospholipid bilayer. Fourier-transform infrared spectroscopy showed that SL interacted with DPPC increasing ordering of the phospholipid acyl chain palisade and hydration of the lipid/water interface. Small angle X-ray scattering showed that SL did not modify bilayer thickness in the biologically relevant L_α fluid phase. SL was found to induce contents leakage in 1-palmitoy-2-oleoy-sn-glycero-3-phosphocholine (POPC) unilamellar liposomes, at sublytic concentrations below the cmc. This SL-induced membrane permeabilization at concentrations below the onset for membrane solubilization can be the result of the formation of laterally segregated domains, which might contribute to provide a molecular basis for the reported antimicrobial actions of SLs.