Thermodynamics of interaction of octyl glucoside with phosphatidylcholine vesicles: partitioning and solubilization as studied by high sensitivity titration calorimetry.

The interaction of the surfactant octyl glucoside (OG) with dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC), and soy bean phosphatidylcholine (soy bean PC) was studied using high-sensitivity titration calorimetry. We determined the partition coefficient of OG between water and lipid bilayers and the transfer enthalpy of the surfactant by addition of lipid vesicles to OG monomers or vice versa. Comparison with the micellization enthalpy of the surfactant gives information on differences in the hydrophobic environment of OG in a liquid-crystalline bilayer or a micelle. The average partition coefficient P in mole fraction units for x(e) approximately 0.12-0.2 decreases slightly from 4152 at 27 degrees C to 3479 at 70 degrees C for DMPC and from 4260 to 3879 for soy bean PC, respectively. The transfer enthalpy deltaH(T) of OG into lipid vesicles is positive at 27 degrees C and negative at 70 degrees C. Its temperature dependence is larger for the incorporation of OG into DMPC than into soy bean PC vesicles. It is concluded that OG in DMPC vesicles is better shielded from water than in soy bean PC vesicles or in micelles. Titration calorimetry was also used to determine the phase boundaries of the coexistence region of mixed vesicles and mixed micelles in the systems OG/DMPC, OG/DPPC, OG/DSPC, and OG/soy bean PC vesicles at 70 degrees C in the liquid-crystalline phase. DMPC and soy bean PC solubilization was also studied at 27 degrees C to investigate the effect of temperature. The effective surfactant to lipid ratios at saturation, R(e)(sat), for all PCs studied are in the range between 1.33-1.72 and the ratios at complete solubilization, R(e)(sol), are between 1.79-3.06. At 70 degrees C, the R(e)(sat) values decrease with increasing chain length of the saturated PC. The ratios depend also slightly on temperature and the degree of unsaturation of the fatty acyl chains. For the OG/soy bean PC system, the coexistence range for mixed vesicles and mixed micelles is larger than for the corresponding PCs with saturated chains.