Interaction of detergent with complex mimics of bacterial membranes

Detergents are valuable tools to extract membrane proteins for biophysical, biochemical, and structural scrutiny. The detergent-driven solubilization of bilayers made from a single lipid species is commonly described in terms of pseudo-phase diagrams and a three-stage model accounting for three ranges comprising (i) intact vesicles, (ii) vesicle/micelle co-existence, or (iii) mixed micelles. Moreover, the pseudo-phase boundaries thus determined can often be quantitatively rationalized in terms of the molecular shapes of the lipid and the detergent used. Yet, it has remained unclear to what extent this approach can be applied to multi-component lipid membranes that more closely mimic the compositional complexity of cellular membranes. Here, we studied how lipid mixtures composed of palmitoyl oleoyl phosphatidylethanolamine (POPE), palmitoyl oleoyl phosphatidylglycerol (POPG), and tetraoleoyl cardiolipin (TOCL) are solubilized by the commonly used zwitterionic detergent lauryldimethylamine N-oxide using isothermal titration calorimetry. While phase diagrams of the diverse lipid mixtures showed the typical ranges of the three-stage model, we found that POPG-rich POPE/POPG bilayers are more difficult to solubilize than POPG-poor POPE/POPG bilayers. In turn, POPE/POPG/TOCL bilayers became increasingly resistant to detergent with increasing TOCL content. Since POPG is nearly cylindrically shaped and TOCL adopts inverted cone-like shapes under current buffer conditions, our solubilization data do not align with shape-based arguments. Instead, additional electrostatic interactions between lipids and detergents lead to non-additive mixing behavior affecting the resilience of complex lipid bilayers against solubilization.