Phospholipids at the interface

A theoretical approach based on the electrical double layer correction has been proposed to explain the observed enhancement of the rate of ion transfer across zwitter-ionic phospholipid monolayers at ITIES [17]. If the orientation of the headgroups is such that the phosphonic group remains closer to the ITIES than the ammonium groups, the local concentration of cations is increased at the ITIES and hence the current observed due to cation transfer is larger than in the absence of phospholipids at the interface. This enhancement is evaluated from the solution of the PB equation, and calculations have been carried out for the conditions of the experiments presented in the literature. The theoretical results turn out to be in good agreement with those experimental studies, thus showing the importance of the electrostatic correction on the rate of ion transfer across an ITIES with adsorbed phospholipids. 

The intestinal absorption of dietary cholesterol esters occurs only after hydrolysis by sterol esterase steryl-ester acylhydrolase (cholesterol esterase, EC 3.1.1.13) in the presence of taurocholate [113][114], This enzyme is synthesized and secreted by the pancreas. The free cholesterol so produced then diffuses through the lumen to the plasma membrane of the intestinal epithelial cells, where it is re-esterified. The resulting cholesterol esters are then transported into the intestinal lymph [115]. The mechanism of cholesterol reesterification remained unclear until it was shown that cholesterol esterase EC 3.1.1.13 has both bile-salt-independent and bile-salt-dependent cholesterol ester synthetic activities, and that it may catalyze the net synthesis of cholesterol esters under physiological conditions [116-118], It seems that cholesterol esterase can switch between hydrolytic and synthetic activities, controlled by the bile salt and/or proton concentration in the enzyme s microenvironment. Cholesterol esterase is also found in other tissues, e.g., in the liver and testis [119][120], The enzyme is able to catalyze the hydrolysis of acylglycerols and phospholipids at the micellar interface, but also to act as a cholesterol transfer protein in phospholipid vesicles independently of esterase activity [121],... 

The transmembrane domain may be made up of one or many transmembrane elements. Generally, the transmembrane elements include 20-25 mostly hydrophobic amino acids. At the interface with aqueous medium, we often find hydrophilic amino acids in contact with the polar head groups of the phospholipids. In addition, they mediate distinct fixing of the transmembrane section in the phospholipid double layer. A sequence of 20-25 hydrophobic amino acids is seen as characteristic for membrane-spaiming elements. This property is used in analysis of protein sequences, to predict possible transmembrane elements in so-called hydropathy plots". 

A decrease in occupied area of the head group results in an increase in packing density of the molecules (45) exhibits only an expanded phase, (46) both a liquid and a solid-like phase, and (47) forms only a condensed film. Monolayer properties of many natural phospholipids and synthetic amphiphiles are described in the literature37 38. Especially the spreading behaviour of diacetylenic phospholipids at the gas-water interface was recently described by Hupfer 120). 

Penetration of electrolytes into both the air-water interface and films of dipalmitoyl lecithin is accompanied by a relatively small surface potential increase, whereas hydrolysis of CaCl% produces accumulation of Ca(OH)t and related species at the interface (l). Although in the absence of ionic lipids a correlation between interfacial ionic populations of the electrolyte and the surface potential changes is not yet possible, the marked surface potential effects of CaCU accompanying the presence of small quantities of acidic phospholipids in dipalmitoyl lecithin films suggest that the acidic lipid contaminants are still the only certifiable species whose interaction with CaCl2 produces an appreciable surface potential increase. Surface radioactivity and IR absorption spectra of dipalmitoyl lecithin in the presence of CaCU produced no evidence of Ca -dipalmitoyl lecithin interaction. 

Air-Water Interface. Organized films of surfactants and phospholipids at the air-water interface are of interest in biophysics, general interfacial chemistry, and have relevance to the self-assembling aggregates, which are viewed as having potential applications in non-linear optics and as microelectronic devices (122). FT-IR spectroscopy has recently been applied to the problem of obtaining information about amphiphiles at the air-water interface. 

The structure of the interfacial layers in food colloids can be quite complex as these are usually comprised of mixtures of a variety of surfactants and all are probably at least partly adsorbed at interfaces which even individually, can form complex adsorption layers. The layers can be viscoelastic. Phospholipids form multi-lamellar structures at the interface and proteins, such as casein, can adsorb in a variety of conformations [78]. Lecithins not only adsorb also at interfaces, but can affect the conformations of adsorbed casein. The situation in food emulsions can be complicated further by the additional presence of solid particles. For example, the fat droplets in homogenized milk are surrounded by a membrane that contains phospholipid, protein and semi-solid casein micelles [78,816], Similarly, the oil droplets in mayonnaise are partly coated with granular particles formed from the phospho and lipo-protein components of egg yolk [78]. Finally, the phospholipids can also interact with proteins and lecithins to form independent vesicles [78], thus creating an additional dispersed phase. 

Various dynamic processes have been investigated using computer simulations of phospholipids. These include the dynamics of the alkyl chain movement of the phospholipid, the structure of water at the interface, diffusion of small molecules, interactions of phospholipids with water, dmgs, peptides, and proteins, and the effect of unsaturation or the presence of cholesterol on the phospholipid conformation. 

The FRAP method has been applied to the measurements of molecular lateral diffusion of molecules adsorbed at the interface of equilibrium common thin foam films and of black foam films [39-43], Initially Clark et al. reported FRAP measurement of surface diffusion of the fluorescence probe 5-N(octadecanoyl)aminofluorescein incorporated into foam films stabilised with NaDoS [39]. Then followed the measurements of protein-stabilised foam films where the protein was covalently labelled with fluorescein [40,41], Studies of FRAP measurements of surface lateral diffusion in equilibrium phospholipid common thin foam films and black foam films were also reported [42,43]. 

A question of interest here is the origin of the DMPC molecules building up the bilayer, considering the low monomer concentration in the DMPC suspension and the small volume of the drop in the cell. However, as indicated in Section 3.4.3, NBF can be formed only at close packing at the interface (r ). A possible mechanism is the vesicle degradation at the surfaces, i.e. at the solution/air interface. An evidence of this mechanism are the kinetic studies of insoluble phospholipid monolayer of Ivanova et al. [291]. Nevertheless, NBF formation from vesicle suspensions needs further research. 

The alveolar surface represents a thin liquid film formed at the interface between the alveolar gas phase and a liquid hypophase covering the epithelium. This film is stabilised by the alveolar surfactant (AS), consisting mainly of phospholipids and proteins. AS plays an important role in alveolar stabilisation in the process of breathing. It is known that AS components exist as individual molecules and as various lipid and protein/lipid micellar structures present in the so-called hypophase and, according to some researchers, form a continuous lipid monolayer at the water/air interface [e.g. 1-4]. 

Numerous techniques have been employed to examine the monolayer structure of phospholipids at the air/water interface including surface tension, fluorescence, neutron and X-ray reflection, and IR and Raman spectroscopy. In contrast, very few techniques are suitable to examine monolayers at the oil/water interface. Surface tension and fluorescence microscopy [46-48] have shed some light on these buried monolayers, but most other surface techniques are hampered because of effects from the bulk liquids. Since VSFS is insensitive to the bulk, it is an excellent technique for probing these monolayers. 

Positioned at the interface between the air and tear film, the lipid layer is produced by the meibomian glands with contributions from the glands of Zeis and Moll. Most of this layer consists of low-polarity lipids, such as wax and cholesterol esters, with traces of triglycerides. A thin polar portion, adjacent to the tear-aqueous layer, may contain surfactant phospholipids needed to spread lipid film over aqueous layers. The main purpose of the lipid layer appears to be to reduce evaporation of the tear film. 

Emulsifying properties. One of the major functions of commercial lecithins is to emulsify fats. In an oihwater system, the phosphohpid components concentrate at the oUrwater interface. The polar, hydrophilic parts of the molecules are directed toward the aqueous phase, and the nonpolar, hydrophobic (or lipophilic) parts are directed toward the oil phase. The concentration of phospholipids at the oihwater interface lowers the surface tension and makes it possible for emulsions to form. Once the emulsion is formed, the phosphohpid molecules at the surface of the oil or water droplets act as barriers that prevent the droplets from coalescing, thus stabilizing the emulsion (159). 

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