Thermodynamics of sodium dodecyl sulfate partitioning into lipid membranes

The partition equilibria of sodium dodecyl sulfate (SDS) and lithium dodecyl sulfate between water and bilayer membranes were investigated with isothermal titration calorimetry and spectroscopic methods (light scattering, P-31-nuclear magnetic resonance) in the temperature range of 28degreesC to 56degreesC. The partitioning of the dodecyl sulfate anion (DS-) into the bilayer membrane is energetically favored by an exothermic partition enthalpy of DeltaH(D)(0) = -6.0 kcal/mol at 28degreesC. This is in contrast to nonionic detergents where DeltaH(D)(0) is usually positive. The partition enthalpy decreases linearly with increasing temperature and the molar heat capacity is DeltaC(P)(0) = -50 +/- 3 cal mol(-1) K-1. The partition isotherm is nonlinear if the bound detergent is plotted versus the free detergent concentration in bulk solution. This is caused by the electrostatic repulsion between the DS- ions inserted into the membrane and those free in solution near the membrane surface. The surface concentration of DS- immediately above the plane of binding was hence calculated with the Gouy-Chapman theory, and a strictly linear relationship was obtained between the surface concentration and the extent of DS- partitioning. The surface partition constant K describes the chemical equilibrium in the absence of electrostatic effects. For the SDS-membrane equilibrium K was found to be 1.2 x 10(4) M-1 to 6 x 10(4) M-1 for the various systems and conditions investigated, very similar to data available for nonionic detergents of the same chain length. The membrane-micelle phase diagram was also studied. Complete membrane solubilization requires a ratio of 2.2 mol SDS bound per mole of total lipid at 56degreesC. The corresponding equilibrium concentration of SDS free in solution is C-D,f(sol) similar to 1.7 mM and is slightly below the critical micelles concentration (CMC) = 2.1 mM (at 56degreesC and 0.11 M buffer). Membrane saturation occurs at - 0.3 mol SDS per mol lipid and the equilibrium SDS concentration is C-D,f(sat) similar to 2.2 mM +/- 0.6 mM. SDS translocation across the bilayer is slow at ambient temperature but increases at high temperatures.