Surface Chemistry Study of the Interactions of Sesame Oil with Meibomian Films

A possible approach for the treatment of meibomian gland disease (MGD) can be the supplementation of meibomian gland secretion (MGS) with nonpolar lipids (NPL) rich plant oils. Sesame oil (SO), approximately equal in monounsaturated fat (oleic acid, 40% of total) and polyunsaturated fat (linoleic acid, 42% of total), has shown multiple health benefits due to its anti-inflammatory and antioxidant effects. Thus, the interactions between SO and MGS in surface layers deserve further study. Therefore, pseudobinary films were formed with controlled MGS/SO molar ratios (0%, 10%, 30%, 50%, and 100% SO) at the air/water surface of the Langmuir trough over phosphate buffered saline (pH 7.4) subphase. Surface pressure (π)-area (A) isotherms and Brewster angle microscopy observations showed nonideal interactions where SO aggregates with MGS and complements the NPL stratum of the meibomian layers. The analysis of stress relaxation transients with Kohlrausch–Williams–Watts equation revealed that the supplementation of fixed amount of MGS with excess lipids via SO altered the dilatational elasticity of the films as reflected by the increase of the exponent β. Thus, SO with its unique combination of high oxidative stability and abundance of long polyunsaturated acyl chains might be a useful supplement to MGS layers.

Nucleation of Surfactant–Alkane Mixed Solid Monolayer and Bilayer Domains at the Air–Water Interface

We investigated the wetting transitions of tetradecane and hexadecane droplets in dodecyltrimethylammonium bromide (C12TAB), tetradecyltrimethylammonium bromide (C14TAB), and hexadecyltrimethylammonium bromide (C16TAB) aqueous solutions. By varying the surfactant concentration, the formation of mixed monolayers of a surfactant and an alkane was observed at the air–water interface. Depending on the combination of surfactant and alkane, these wetting monolayers underwent another thermal phase transition upon cooling either to a frozen mixed monolayer (S1) or a bilayer structure composed of a solid monolayer of a pure alkane rested on a liquid-like mixed monolayer (S2). Based on the phase diagrams determined by phase modulation ellipsometry, the difference in the morphology of the nucleated S1 and S2 phase domains was also investigated using Brewster angle microscopy. Domains of the S1 phase were relatively small and highly branched, whereas those of the S2 phase were large and circular. The difference in domain morphology was explained by the competition of the domain line tension and electrostatic dipole interactions between surfactant molecules in the domains.

Comparing C2=O and C2=S Barbiturates: Different Hydrogen-Bonding Patterns of Thiobarbiturates in Solution and the Solid State

Carbonyl-centered hydrogen bonds with various strength and geometries are often exploited in materials to embed dynamic and adaptive properties, with the use of thiocarbonyl groups as hydrogen-bonding acceptors remaining only scarcely investigated. We herein report a comparative study of C2=O and C2=S barbiturates in view of their differing hydrogen bonds, using the 5,5-disubstituted barbiturate B and the thiobarbiturate TB as model compounds. Owing to the different hydrogen-bonding strength and geometries of C2=O vs. C2=S, we postulate the formation of different hydrogen-bonding patterns in C2=S in comparison to the C2=O in conventional barbiturates. To study differences in their association in solution, we conducted concentration- and temperature-dependent NMR experiments to compare their association constants, Gibbs free energy of association ∆Gassn., and the coalescence behavior of the N-H‧‧‧S=C bonded assemblies. In Langmuir films, the introduction of C2=S suppressed 2D crystallization when comparing B and TB using Brewster angle microscopy, also revealing a significant deviation in morphology. When embedded into a hydrophobic polymer such as polyisobutylene, a largely different rheological behavior was observed for the barbiturate-bearing PB compared to the thiobarbiturate-bearing PTB polymers, indicative of a stronger hydrogen bonding in the thioanalogue PTB. We therefore prove that H-bonds, when affixed to a polymer, here the thiobarbiturate moieties in PTB, can reinforce the nonpolar PIB matrix even better, thus indicating the formation of stronger H-bonds among the thiobarbiturates in polymers in contrast to the effects observed in solution.

Designing a Useful Lipid Raft Model Membrane for Electrochemical and Surface Analytical Studies

A model biomimetic system for the study of protein reconstitution or drug interactions should include lipid rafts in the mixed lipid monolayer, since they are usually the domains embedding membrane proteins and peptides. Four model lipid films composed of three components: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), cholesterol (Chol) and sphingomyelin (SM) mixed in different molar ratios were proposed and investigated using surface pressure measurements and thermodynamic analysis of the monolayers at the air–water interface and imaged by Brewster angle microscopy. The ternary monolayers were transferred from the air–water onto the gold electrodes to form bilayer films and were studied for the first time by electrochemical methods: alternative current voltammetry and electrochemical impedance spectroscopy and imaged by atomic force microscopy. In excess of DOPC, the ternary systems remained too liquid for the raft region to be stable, while in the excess of cholesterol the layers were too solid. The layers with SM in excess lead to the formation of Chol:SM complexes but the amount of the fluid matrix was very low. The equimolar content of the three components lead to the formation of a stable and well-organized assembly with well-developed raft microdomains of larger thickness, surrounded by the more fluid part of the bilayer. The latter is proposed as a convenient raft model membrane for further physicochemical studies of interactions with drugs or pollutants or incorporation of membrane proteins.

Interactions between Phosphatidylcholine and Kaempferol or Myristicin: Langmuir Monolayers and Microelectrophoretic Studies

Flavonoid compounds are known for their antibacterial, anti-inflammatory, and anticancer properties. Therefore, they can influence membrane properties that interest us, modifying both their structure and functions. We used kaempferol (K) and myricetin (M) as representatives of this group. We investigated the influence of the abovementioned compounds on model cell membranes’ properties (i.e., Langmuir monolayers and liposomes). The basic research methods used in these studies were the Langmuir method with Brewster angle microscopy and microelectrophoresis. The π–A isotherms were registered for the pure components and mixtures of these compounds with phosphatidylcholine (PC) in appropriate volume ratios. Using mathematical equations, we established that kaempferol, myricetin, and the lipids formed complexes at 1:1 ratios. We derived the parameters characterizing the formed complexes, i.e., the surfaces occupied by the complexes and the stability constants of the formed complexes. Using the microelectrophoretic method, we determined the dependence of the lipid membranes’ surface charge density as a function of the pH (in the range of 2 to 10) of the electrolyte solution. The presented results indicate that the PC membrane’s modification with kaempferol or myricetin affected changes in the surface charge density and isoelectric point values.

Thermodynamic and Mechanical Properties of DMPC/Cholesterol Mixed Monolayers at Physiological Conditions

One of the main known effects of cholesterol is to rigidify the cell membrane throughout the so-called condensing effect. Although many studies have been done in mixtures of cholesterol with different membrane lipids, there are not many studies in a wide concentration range of cholesterol or at physiological conditions. In this work, we studied mixtures of DMPC/Cholesterol monolayers to determine the effect of cholesterol, from very low to physiological concentrations and two pHs. We use a Langmuir balance and Brewster angle microscopy to study their thermodynamic behavior at 37.0 ± 0.1°C at the air/solution interface. From the analysis of the (π−A) isotherms, we determined the excess area and the compressibility elastic modulus to determine the monolayers mechanical properties. Surprisingly, we found three main effects of cholesterol: The first one is a fluidization effect of the monolayer at all cholesterol concentrations. The second effect is the so-called condensing effect that appears due to the non-ideality of the mixture. The third effect is a stiffness of the monolayer as the cholesterol concentration increases. These effects are stronger in pure water, pH ≈ 6.6, than on buffer at physiological pH = 7.4. We also found that all mixtures are thermodynamically stable at all concentrations at a surface pressure of 30.1 ± 1.6 and 27.4 ± 3.2 mN/m in pure water and buffer, respectively. Furthermore, we compared this stability with a fatty acid monolayer that shows a much lower surface pressure equilibrium value that DMPC or its mixtures with cholesterol, indicating a possibly reason why double chain lipids are better than single chain lipids to made up the cell membrane.

Interactions of Meibum and Tears with Mucomimetic Polymers: A Hint towards the Interplay between the Layers of the Tear Film

Recent clinical findings suggest that mucomimetic polymers (MMP) can alter not only the texture of the aqueous tear but also the spreading and structure of the tear film (TF) lipid layer, thereby allowing for their synchronized performance in vivo. Thus, we aimed to evaluate in vitro (i) the capability of pharmaceutically applicable MMP to ensure the formation of post-evaporative ferning patterns (a characteristic feature of the “healthy” tear colloid) and (ii) the MMP interactions with human meibum films accessed in the course of blink-like deformations via Langmuir surface balance and Brewster angle microscopy (BAM). Four MMP were used- hyaluronic acid (HA), cross-linked hyaluronic acid (CHA), carboxymethyl cellulose (CMC) and gellan gum (GG)- at the concentrations of 0.0001%, 0.001%, 0.01%, 0.05% and 0.1%. Significant differences were observed in the MMP fern formation capability: CHA (≥0.001%) > HA (≥0.01%) = CMC (≥0.01%) > GG (≥0.05%). All MMP affected the spreading of meibum, with BAM micrographs revealing thickening of the films. CHA was particularly efficient, showing concentration-dependent enhancement of tear ferning and of meibomian layer structure, surfactant properties and viscoelasticity. Thus, endogenous and exogenous MMP may play key roles for the concerted action of the TF layers at the ocular surface, revealing novel routes for TF-oriented therapeutic applications.

How the replacement of cholesterol by 25-hydroxycholesterol affects the interactions with sphingolipids: The Langmuir Monolayer Study complemented with theoretical calculations

In this paper, a representative of chain-oxidized sterols, 25-hydroxycholesterol (25-OH), has been studied in Langmuir monolayers mixed with the sphingolipids sphingomyelin (SM) and ganglioside (GM 1 ) to build lipid rafts. A classical Langmuir monolayer approach based on thermodynamic analysis of interactions was complemented with microscopic visualization of films (Brewster angle microscopy), surface-sensitive spectroscopy (polarization modulation–infrared reflection–absorption spectroscopy) and theoretical calculations (density functional theory modelling and molecular dynamics simulations). Strong interactions between 25-OH and both investigated sphingolipids enabled the formation of surface complexes. As known from previous studies, 25-OH in pure monolayers can be anchored to the water surface with a hydroxyl group at either C(3) or C(25). In this study, we investigated how the presence of additional strong interactions with sphingolipids modifies the surface arrangement of 25-OH. Results have shown that, in the 25-OH/GM 1 system, there are no preferences regarding the orientation of the 25-OH molecule in surface complexes and two types of complexes are formed. On the other hand, SM enforces one specific orientation of 25-OH: being anchored with the C(3)–OH group to the water. The strength of interactions between the studied sphingolipids and 25-OH versus cholesterol is similar, which indicates that cholesterol may well be replaced by oxysterol in the lipid raft system. In this way, the composition of lipid rafts can be modified, changing their rheological properties and, as a consequence, influencing their proper functioning.

Condensing Effect of Cholesterol on hBest1/POPC and hBest1/SM Langmuir Monolayers

Human bestrophin-1 protein (hBest1) is a transmembrane channel associated with the calcium-dependent transport of chloride ions in the retinal pigment epithelium as well as with the transport of glutamate and GABA in nerve cells. Interactions between hBest1, sphingomyelins, phosphatidylcholines and cholesterol are crucial for hBest1 association with cell membrane domains and its biological functions. As cholesterol plays a key role in the formation of lipid rafts, motional ordering of lipids and modeling/remodeling of the lateral membrane structure, we examined the effect of different cholesterol concentrations on the surface tension of hBest1/POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and hBest1/SM Langmuir monolayers in the presence/absence of Ca2+ ions using surface pressure measurements and Brewster angle microscopy studies. Here, we report that cholesterol: (1) has negligible condensing effect on pure hBest1 monolayers detected mainly in the presence of Ca2+ ions, and; (2) induces a condensing effect on composite hBest1/POPC and hBest1/SM monolayers. These results offer evidence for the significance of intermolecular protein–lipid interactions for the conformational dynamics of hBest1 and its biological functions as multimeric ion channel.

Surface Dilatational Properties and Foam Performance of Surfactant-Nanoparticle Foaming System under Ultra-High Salinity

We have studied the surface dilatational properties of aqueous foaming dispersions containing mixtures of silica nanoparticles (Ludox CL) and sulfobetaine (LHSB) in Tahe formation water. The effects of temperature and pH on the surface dilatational modulus and time shift were studied by oscillating drop module (ODM). The ODM results show that the surface dilatational modulus of mixtures of CL and LHSB is large and increases with the decrease of surface area deformation, which results from hydrophobic interaction between adsorbed mixtures. Under test conditions, the Gibbs stability criterion E > σ/2 against foam coarsening is fulfilled. Results of Brewster angle microscopy (BAM) show that an uniform adsorption layer is established at the air-water interface. Temperature and pH-value influence the dilatation modulus of the surface by hydrophobic interaction or adsorption. Time shift has a similar variation trend. This is a surprising feature. It suggests that LHSB adsorbed on CL can respond to surface tension gradient. The time shift difference results from the response of LHSB at different adsorption sites. In addition, sand pack tests show that compared to LHSB, a finer foam was produced by the mixtures CL/LHSB due to the capillary-induced \snap-off". Thus, higher pressure difference and higher oil recovery could be achieved.