No abstract

The solubilization of POPC lipid bilayers by the nonionic detergent C12EO8 was studied by isothermal titration calorimetry. The characteristic transfer enthalpies for the detergent and the lipid between bilayers and micelles were determined by titration of detergent micelles to lipid membranes and vice versa. For purpose of comparison, the enthalpy and Gibbs free-energy changes for the aggregation of aqueous detergent monomers to micelles as well as for the partitioning into lipid bilayers were analyzed. The phase boundaries between pure bilayers and pure micelles, i.e., the detergent mole fraction, when the bilayers become saturated with detergent and first mixed micelles appear and the mole fraction when the bilayers are completely solubilized and only mixed micelles are present could be easily determined from the titration experiments. The detergent binding to membranes does not follow the mass action law because there are no specific binding sites. An equation using partitioning of detergent between water and bilayers gave good fits to the experimental data. The experiments lead to a consistent set of transfer enthalpies and entropies for the system of monomers, micelles, and bilayers. Suggestions are made about the thermodynamic nature of solubilization and partitioning. Finally, besides the limiting compositions of bilayers and micelles, another composition-driven transition was detected within the mixed micellar range. This can be imagined to correspond to a variation of the micellar shape and size and/or intermicellar interactions.

The lyotropic phase behavior of the dienic lipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) has been investigated by means of IR spectroscopy at 25 °C. Gradual hydration has been realized exposing the lipid to an atmosphere of variable relative humidity (RH). Upon scans of decreasing RH, the liquid crystalline lipid undergoes the chain-freezing transition to a metastable gel state. By storage of the sample at low RH, the gel transforms to a crystalline subgel designated as SGI. Subsequent hydration induces the conversion to a second subgel (SGII). The subgel phases are characterized by the dense packing of the acyl chains as indicated by the correlation field splitting of the CH2 rocking and bending modes. Band shifts of phosphate group vibrations as well as the splitting of the carbonyl stretching mode are correlated with the hydration of the polar region of the bilayer given in terms of the molar ratio of water to lipid. The ν1,3(OH) absorption band of water yields qualitative information about the water−lipid interaction. The drastic sharpening of this band in the SGI phase was attributed to the reduction of water binding sites on the lipid, leading to a more uniform population of water molecules adsorbed onto the lipid headgroup. The external conditions of phase transformation of DODPC were compared with corresponding data of dimyristoylphosphatidylcholine (DMPC) having the same number of subsequent methylene segments in the acyl chains. Apparent differences can be attributed to the influence of the diene groups representing a rigid spacer inserted between the methylene chains and the ester groups of the lipid, i.e., in a position near the polar/apolar interface of the bilayer.

In dilute aqueous mixtures of the detergent C 12 EO 8 and the phospholipid POPC the phase and partition behavior as well as the transfer enthalpies of the respective molecules between the various states (monomers, bilayers, micelles) have been measured by isothermal titration calorimetry [Heerklotz et al., J. Phys. Chem. 1996, 100, 6764]. To derive more information about the molecular interpretation of the thermodynamic data, we performed additional experiments for a series of detergents, C 12 EO n with n ) 3-8 and dependent on temperature (for C 12 EO 8 ). The data can be discussed in terms of a three-stage model (bilayers, coexistence, micelles) considering nonideal mixing within the aggregates. The mixing properties are determined by packing effects controlling the hydration of the headgroups, the water exposure of the hydrocarbon core, and the order of the hydrocarbon chains. Additionally, two types of systematic deviations from the simple three-stage behavior are found for low n and low detergent contents in the bilayer. These effects could be related to special properties of detergents surrounded by lipids only and to solubilization intermediates occurring close to the lytic detergent content.

Hydration properties of mixtures of a zwitterionic lipid, palmitoyloleoylphosphatidylcholine (POPC), and nonionic surfactants (oligo(oxyethylene) dodecyl ether, C12En with n ) 1-8) were studied over a wide range of surfactant/lipid molar ratios RA/L from 0.1 to 2 at T ) 25°C. The adsorption of water by the POPC/C12En mixtures was measured by the isopiestic method at two different relative humidities (RH ) 86.5 and 97%). Deuterium NMR on 2 H2O and 31 P NMR on the phospholipid as well as X-ray diffraction were employed to characterize the phase state of the mixtures. For samples in the LR phase the area requirement of POPC and surfactant molecules and the thickness of the hydrophobic core of the bilayer were estimated from the repeat spacing and the known composition of the sample. The experimental results are compared to data reported previously for pure POPC and C 12En systems under identical conditions. Small C12En concentrations (RA/L ) 0.1 and 0.2) in the membrane tighten the membrane packing. The area per molecule in the bilayer/water interface occupied by the lipid is reduced and that of the surfactant enlarged in the mixture compared to bilayers of the pure components under equal conditions. Further increase of the surfactant concentration causes a significant thinning of the hydrophobic core and a progressive increase of the area requirement of the amphiphilic molecules in the membrane/water interface. Finally, at high surfactant concentrations (R A/L ) 1 and 2) the area requirement of the amphiphilic constituents and the vertical extension of the polar interface region increase with growing ethylene oxide chain length n. The hydration of the lipid is reduced by the presence of C12En to a level comparable to the primary hydration shell. The first two or three oxyethylene groups next to the alkyl chain of the surfactant also show reduced hydration in the mixtures. The remaining EO groups have hydration characteristics very similar to the pure surfactant, with the exception of bilayers with RA/L ) 2 at RH ) 97%.