Classifying Surfactants with Respect to Their Effect on Lipid Membrane Order

We propose classifying surfactants with respect to their effect on membrane order, which is derived from the time-resolved fluorescence anisotropy of DPH. This may help in understanding why certain surfactants, including biosurfactants such as antimicrobial lipopeptides and saponins, often show a superior performance to permeabilize and lyse membranes and/or a better suitability for membrane protein solubilization. Micelle-forming surfactants induce curvature stress in membranes that causes disordering and, finally, lysis. Typical detergents such as C12EO8, octyl glucoside, SDS, and lauryl maltoside initiate membrane lysis after reaching a substantial, apparently critical extent of disordering. In contrast, the fungicidal lipopeptides surfactin, fengycin, and iturin from Bacillus subtilis QST713 as well as digitonin, CHAPS, and lysophosphatidylcholine solubilize membranes without substantial, overall disordering. We hypothesize they disrupt the membrane locally due to a spontaneous segregation from the lipid and/or packing defects and refer to them as heterogeneously perturbing. This may account for enhanced activity, selectivity, and mutual synergism of antimicrobial biosurfactants and reduced destabilization of membrane proteins by CHAPS or digitonin. Triton shows the pattern of a segregating surfactant in the presence of cholesterol.