Effects of saturated and polyunsaturated fatty acids on interactions with cholesterol versus 7-ketocholesterol in Langmuir monolayers and their potential biological implications.

In this work the Langmuir monolayer technique was used to study interactions between cholesterol (chol) and 7-ketocholesterol (7-KC) with saturated (arachidic acid, AA) and polyunsaturated fatty acids (PUFAs) (ω-3 α-linolenic acid, α-LA,and ω-6 γ- linolenic, γ-LA) in order to get insight into their potential role in atherosclerosis. For this study, surface pressure (π)-area (A) isotherms, compressibility modulus (Cs-1) versus π plots, Brewster angle microscopy (BAM) images and excess functions (Aexc and ΔGexc) were analysed. Different behaviour has been observed. For cholesterol/AA mixed monolayers, components immiscibility occurs, whatever the surface pressure or the mixtures composition is, whereas for the 7-KC/AA mixed system, ideal behaviour was observed at low and high surface pressures for all the investigated compositions. However, the remaining mixed studied systems (sterol/PUFA) exhibit negative deviations from the ideality, although some differences do occurr. The magnitude of these deviations depend on the kind of a PUFA (for ω-3 PUFA greater than for ω-6) - attributed to the different geometry of their acyl chains- and the type of a sterol (for 7-KC greater than for cholesterol).The strength of attractive interactions followed the order: chol/ γ-LA <7-KC/γ-LA < chol/α-LA < 7-KC/ α-LA, postulating the formation of stable complexes of 1:2 stoichiometry for 7-KC/α-LA mixed monolayers and 1:1 stoichiometry for chol/α-LA mixed films. For 7-KC/γ-LA system, the formation of a low stability complex of 2:1 stoichiometry was suggested. The existence of these complexes can play an important role in diminishing the circulating sterols in the blood stream, thus decreasing the probability of atherosclerotic plaques formation.

Global strain field mapping of a particle-laden interface using digital image correlation.

HYPOTHESIS: The ability to identify the stress-strain relations correctly is critical to understanding and modeling any rheological responses of an interface. Langmuir-Pockels (LP) trough is one of the most commonly used tools for studying an interface. Most, if not all, existing studies assume a 1D uniaxial compression during a LP-trough compression experiment. It is hypothesized that the deformation field is far more complex than what is typically assumed. EXPERIMENTS: To examine this hypothesis, we custom-built a glass-bottomed LP trough equipped with a camera to capture a series of optical images asa carbon nanotube (CNT)-laden interface is compressed. A digital image correlation (DIC) technique was then applied to the images to evaluate the global strain field during compression of the CNT laden interface. The DIC-corrected strain data were subsequently analyzed with the surface stress data to quantify the surface shear and dilatational moduli of the CNT-laden interface. FINDINGS: Our experimental findings clearly show, for the first time, the development of a non-uniform and complex 2D strain field during compression. The local strains were further quantified and compared with the usual assumption of 1D uniaxial compression. Although the compressive strain averaged over the whole trough area closely resembles the 1D uniaxial compression strain, the 1D compression assumption underestimates the local strain by about 36% at the center of the trough, where the surface stresses are measured. This is the first study in applying the DIC technique to map out the global strain field asa particle-laden interface is compressed. The method may also be applicable to other systems with similar optical texture, allowing the correct identification of stress-strain relationship of an interface.

Structure of unsaturated fatty acids in 2D system.

The behaviour of Langmuir monolayers corresponding to unsaturated fatty acids belonging to the omega-9 (oleic acid), omega-3 (α-linolenic and stearidonic acids) and omega-6 (linoleic, γ-linolenic and eicosadienoic acids) series was studied in order to get insight into the influence of various factors (such as subphase temperature, length, degree of unsaturation and position of the double bonds in the aliphatic chains) on the molecular conformation of these fatty acids spread at the A/W interface. The obtained results derived from the surface pressure - area isotherms, compressibility modulus and monolayer thickness prove that the lift-off and the limiting areas of investigated films increase both with the number of double bonds and length of the hydrocarbon chain. Also, the monolayers are more compressible (more fluid) upon increasing their tails unsaturation degree. However, eicosadienoic (EA) film is more rigid due to its longer acyl chain. Polyunsaturated fatty acids (PUFAs) monolayers, except for EA, exhibit anomalous temperature dependence (area contraction with increasing temperature), which was attributed to the increased solubility of PUFAs molecules with temperature. The recorded thickness values of the monolayers were compared with those estimated from theoretical molecular conformations in order to establish the orientation and configuration of molecules in different surface states of their monolayer.