Detergent solubilization of phosphatidylcholine bilayers in the fluid state: Influence of the acyl chain structure

Seventeen different, chemically defined phosphatidylcholines, dispersed in aqueous medium in the form of large unilamellar vesicles, have been tested for solubilization by the non-ionic detergent Triton X-100. The temperatures (either 20 °C or 45 °C) were such that the bilayers were always in the liquid-disordered state. For each case, the solubilization parameters, D_on (total detergent: lipid mole ratio producing the onset of solubilization) and D₅₀ (total detergent: lipid mole ratio producing 50% solubilization), were determined under equilibrium conditions. Both parameters varied generally in parallel. When double bonds were introduced to the acyl chains, other factors remaining constant, solubilization became more difficult, i.e., more detergent was required. Cis-unsaturated phospholipids required more detergent than the corresponding trans-isomers. Increasing chain length in saturated phospholipids between C12 and C16 decreased moderately the detergent/lipid ratios causing solubilization. Acyl and alkyl phospholipids were equally susceptible to Triton X-100 solubilization. Lipid chain order, as measured by DPH fluorescence polarization, seemed to facilitate solubilization, perhaps because more ordered bilayers have a smaller capacity to accommodate detergent monomers without breaking down into lipid-detergent mixed micelles.