Solute effects on the colloidal and phase behavior of lipid bilayer membranes: ethanol-dipalmitoylphosphatidylcholine mixtures.

By means of the scanning differential calorimetry, x-ray diffractometry, and the dynamic light scattering, we have systematically studied the phase and packing properties of dipalmitoylphosphatidylcholine vesicles or multibilayers in the presence of ethanol. We have also determined the partial ternary phase diagram of such dipalmitoylphosphatidylcholine/water/ethanol mixtures. The directly measured variability of the structural bilayer parameters implies that ethanol binding to the phospholipid bilayers increases the lateral as well as the transverse repulsion between the lipid molecules. This enlarges the hydrocarbon tilt (by up to 23 degrees) and molecular area (by < or = 40%). Ethanol-phospholid association also broadens the interface and, thus, promotes lipid headgroup solvation. This results in excessive swelling (by 130%) of the phosphatidylcholine bilayers in aqueous ethanol solutions. Lateral bilayer expansion, moreover, provokes a successive interdigitation of the hydrocarbon chains in the systems with bulk ethanol concentrations of 0.4-1.2 M. The hydrocarbon packing density as well as the propensity for the formation of lamellar gel phases simultaneously increase. The pretransition temperature of phosphatidylcholine bilayers is more sensitive to the addition of alcohol (initial shift: delta Tp = 22 degrees C/mol) than the subtransition temperature (delta Ts reversible 5 degrees C/mol), whereas the chain-melting phase transition temperature is even less affected (delta Tm = 1.8 degrees C/mol). After an initial decrease of 3 degrees for the bulk ethanol concentrations below 1.2 M, the Tm value increases by 2.5 degrees above this limiting concentration. The gel-phase phosphatidylcholine membranes below Tm are fully interdigitated above this limiting concentration. The chain tilt on the fringe of full chain interdigitation is zero and increases with higher ethanol concentrations. Above Tm, some of the lipid molecules are solubilized by the bound ethanol molecules. More highly concentrated ethanol solutions (> 7 M) solubilize the phosphatidylcholine bilayers with fluid chains fully and result in the formation of mixed lipid-alcohol micelles

Ion distribution profiles between polar and charged membranes as observed by anomalous X-ray diffraction

The distribution profiles of rubidium ions near polar, zwitterionic 1,2-di stearoyl-sn-glycero-3-phospho choline (DSPC) and charged, anionic 1,2-di stearoyl-sn-glycero-3-phospho glycerol (DSPG-) membranes were determined by means of anomalous X-ray diffraction close to the K-absorption edge of $\rm Rb ^+$, which was measured to be 15.225 keV. The samples were kept in the L$_\beta'$-phase at 30$\;\rm{}^\circ C$. The DSPC suspensions in 2 M RbCl were examined at 15.125 keV, 15.196 keV, 15.210 keV, the $\rm DSPG ^-$ suspensions in 0 M RbCl and 2 M RbCl at 14.667 keV and 15.196 keV. This gave up to seven peaks in the powder-like diffraction pattern and revealed, for the first time directly, different rubidium ion distributions in either of the investigated systems: rubodium ions are enriched by 100% in the proximity of the charged $\rm DSPG ^-$ headgroups and depleted by 25% from the vicinity of the zwitterionic DSPC headgroups. The latter phenomenon results in rubidium ion concentration maximum in the aqueous compartment between polar, but uncharged bilayers