Attack or Retreat: Two Modes of Membrane Solubilisation by Surfactants

Abstract

Solubilization of a lipid membrane by a micelle-forming surfactant starts with the appearance of mixed micelles coexisting with mixed membranes and aqueous phase. This occurs at a characteristic membrane composition (often given as the mole ratio Resat) and a characteristic aqueous concentration of surfactant that is typically somewhat below the CMC of the pure surfactant. Thermodynamically, both criteria are fully equivalent because they linked by the partition coefficient. However, there are two scenarios when it comes to the mechanism of solubilization. On one hand, the surfactant may destroy the membrane by inducing a critical curvature strain superseding the mechanical stability of the bilayer. On the other hand, the surfactant may de-mix from the membrane and associate in the aqueous phase to form micelles that extract lipid from the membrane. Time-resolved fluorescence anisotropy data of DPH derivatives agree with other parameters reflecting membrane order in that there is a characteristic, minimum order of a given membrane (at a certain temperature) and most detergents have to disorder the membrane to this critical state before micelles appear and solubilization proceeds. However few, mainly bio- and bioanalogous surfactants were found to solubilize by demixing without critical disordering of the membrane. This may account for the superior performance of the latter in solubilizing membrane proteins in their active state.