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Free fatty acids in biomembranes have been proposed to be a central component in several cellular control and regulatory mechanisms. To elucidate some fundamental elements underlying this, we have applied molecular dynamics simulations and experimental density measurements to study the molecular packing and structure of oleic acid (HOA) and stearic acid (HSA) in fluid bilayers of dimyristoylphosphatidylcholine (DMPC). The experimental data show a small but consistent positive excess volume for fatty acid concentrations below 10 mol %. At higher concentrations the fatty acids mix ideally with fluid DMPC. The simulations, which were benchmarked against the densitometric data, revealed interesting differences in the structure and location of the fatty acids depending on their protonation status. Thus, the protonated (uncharged) acid is located rather deeply in the membrane with an average position of the carboxy group near the second carbon segment of the lipid chains with a typical end-to-end distance of 16−18 Å. This structure of the fatty acid brings about a rather tight lateral packing in the mixed membrane and a moderate ordering and hence stretching of the lipid chains. Deprotonation of the fatty acids is associated with a pronounced movement of their carboxy group to a more hydrated position at the membrane interface and a lateral expansion driven by the mutual repulsion of the anions. These changes increase both the disorder and the degree of interdigitation of the lipid chains, and they make the membrane thinner by 2−3 Å. [ABSTRACT FROM AUTHOR]

We have used synchrotron X-ray reflectivity measurements to investigate the structure of n-dotriacontane (n-C32H66or C32) films deposited from the vapor phase onto a SiO2-coated Si(100) surface. Our primary motivation was to determine whether the structure and growth mode of these films differ from those deposited from solution on the same substrate. The vapor-deposited films had a thickness of ∼50 Å thick as monitored in situby high-resolution ellipsometry and were stable in air. Similar to the case of solution-deposited C32 films, we find that film growth in vacuum begins with a nearly complete bilayer adjacent to the SiO2surface formed by C32 molecules aligned with their long axis parallel to the interface followed by one or more partial layers of perpendicular molecules. These molecular layers coexist with bulk particles at higher coverages. Furthermore, after thermally cycling our vapor-deposited samples at atmospheric pressure above the bulk C32 melting point, we find the structure of our films as a function of temperature to be consistent with a phase diagram inferred previously for similarly treated solution-deposited films. Our results resolve some of the discrepancies that Basu and Satija (Basu, S.; Satija, S. K. Langmuir2007, 23, 8331) found between the structure of vapor-deposited and solution-deposited films of intermediate-length alkanes at room temperature. [ABSTRACT FROM AUTHOR]