Interface biomembrane-water

The quantum theory of chemical reactions in polar media can be used as the basis for the theory of charge transfer at the interface between two dielectric media—oil/water and biomembrane/water by this theory one can express the electron transfer rate in terms of the dielectric properties of the medium and the characteristics describing the electronic properties of reactants. 

Models for biological membranes have either been realized as planar lipid monolayers at the gas-water interface (3) or as bi-molecular lipid membranes (BLM) (4) and spherical liposomes (vesicles), respectively (5 6) (Figure 2.). All these models that are only composed of lipid molecules exhibit a diminished stability compared to natural cell membranes. Obviously the protein part besides being functionally important plays a role in terms of stability of biomembranes. This is the case not only for the integral but especially for the boundary proteins ( 7). 

In addition to the long-chain fatty acid molecules described above, a large number of studies have appeared that use IRRAS to study phospholipid monolayers as models of biomembrane interfaces. Mitchell and Dluhy [26] reported the first IRRAS spectra of 1,2-distearoyl-5 -glycero-3-phosphocholine (DSPC), l,2-dimyristoyl-i -glycero-3-phosphocholine (DMPC), and 1,2-di-palmitoyl-,v/ -glycero-3-phosphocholine (DPPC) monolayers at the air-water... 

The study of electron transfer (ET) at the polarized oil (0)/water (W) (or liquid/ liquid) interface is useful for understanding not only certain catal)rtic reactions in two-phase systems (e.g., liquid membranes, microemulsions, micelles, etc.) but also energy conversion processes occurring at biomembranes. In 1979, Samec etal. [1,2] reported, as the first example, an ET between ferrocene (Fc) in nitrobenzene (NB) and Fe(CN)6 in W ... 

The construction of biomembrane systems from polymers containing phosphatidylethanolamine and -choline analogues in the side chains by spreading at the air/water interface and by the Langmuir-Blodgett technique is of great interest [5,101,249,250]. Eor instance, multilayers of dipalmitoyl-DL-a-phosphatidylethanol methacrylamide polymerized imder y radiation resemble a natural phosphoHpid structure and show a fine lamellar structure with a periodicity the average monolayer was estimated to be 32 A thick [251]. 

Early studies on monolayers of chiral molecules like 2-hydroxyalkanes, amphiphilic amino acids, 2-methylhexacosanoic acid esters, and hydroxy-hexadecanoic acid and its esters have been reviewed. The interesting question about monolayers of chiral molecules is whether the parameters which can be determined and the phase transitions are different for pure enantiomers and racemates. For components of biomembranes like phosphatidylcholines 10 this appears not to be the case," but for synthetic compounds like iV-(a-methylbenzyl-stearamide) 11 specific interactions between the molecules of the enantiomers are observed (Chart 2). ° In recent years, advanced techniques have been developed to probe the order in monolayers at the air-water interface, including surface X-ray diffraction, and microscopic techniques, viz. fluorescence microscopy, and Brewster angle microscopy (BAM). The X-ray diffraction technique has been used to identify homochiral and heterochiral two-dimensional domains in mono-layers of racemic amphiphilic amino acids on subphases containing glycine. Fluorescence microscopy requires the introduction in the monolayer of a small... 

However, a variety of nonpolar molecules can be transported across biomembranes by simple diffusion. The molecules encounter little or no barrier at the bilayer-water interface and readily partition into the bilayer interior. The permeabilities of a variety of molecules have been found to correlate with their... 

Experimental Results. The DLVO theory, which is based on a continuum description of matter, explains the nature of the forces acting between membrane surfaces that are separated by distances beyond 10 molecular solvent diameters. When the interface distance is below 10 solvent diameters the continuum picture breaks down and the molecular nature of the matter should be taken into account. Indeed the experiment shows that for these distances the forces acting between the molecularly smooth surfaces (e.g., mica) have an oscillatory character (8). The oscillations of the force are correlated to the size of the solvent, and obviously reflect the molecular nature of the solvent. In the case of the rough surfaces, or more specifically biomembrane surfaces, the solvation force displays a mono tonic behavior. It is the nature of this solvation force (if the solvent is water, then the force is called hydration force) that still remains a puzzle. The hydration (solvation) forces have been measured by using the surface force apparatus (9) and by the osmotic stress method (10, II). Forces between phosphatidylcholine (PC) bilayers have been measured using both methods and good agreement was found. 

The manner in which protons diffuse is a reflection of the physical properties of the environment, the geometry of the diffusion space, and the chemical composition of the surface that defines the reaction space. The biomembrane, with heterogeneous surface composition and dielectric discontinuity normal to the surface, markedly alters the dynamics of proton transfer reactions that proceed close to its surface. Time-resolved measurements of fast, diffusion-controlled reactions of protons with chromophores and fluorophores allow us to gauge the physical, chemical, and geometric characteristics of thin water layers enclosed between phospholipid membranes. Combination of the experimental methodology and the mathematical formalism for analysis renders this procedure an accurate tool for evaluating the properties of the special environment of the water-membrane interface, where the proton-coupled energy transformation takes place. 

In this chapter, two subjects of our study were described. One was concerned with the catalysis by enzymes entrapped in water pools and photomerization at the level of a biomembrane model in vivo. Based on the study of the activity of yeast HK in the water pools, the activity of HK can be seen in noncharged polyoxyethylene mantles with relatively low micropolarity in which almost all the water molecules are bound up with EO chains. This suggests that yeast HK can work more actively in the vicinity of mitochondrial membranes in vivo. The photomerization of cysteine in the water pool with UV irradiation shows that cysteine is easily converted into cystine with lower Wg. This suggests that active oxygen is generated at the interface of the biomembrane rather than in bulk aqueous solution in vivo and SH groups of proteins in the cell membrane are oxidized similarly with UV irradiation. 

Another similarity between the interfaces of electrodes and biomembranes in contact with aqueous solutions is the existence of an ordered water layer at their respective surfaces. Considerable attention has been given to understanding the structure of water at biomembrane surfaces. Studies of... 

Not only do facial amphiphiles act at the oil/water interface, natural facial amphiphiles also interact strongly with lipid bilayers such as cell membranes. Depending on the nature of the facial amphiphile, its interaction with biomembranes can lead to membrane bending, to pore formation, or even complete dissolution of the membrane. The dissolution of membranes by facial amphiphiles leads to cell death, and therefore the secretion of bile acids to the intestine of vertebrates is tightly regulated. The carpet model describes the mechanism of membrane dissolution by facial amphiphiles. Pore formation and membrane bending by facial amphiphiles are described in the next sections. 

The stractural and functional complexity of biomembranes has ehal-lenged researehers to develop simpler artificial models to mimie their properties. Amphiphilic block copolymers are of particular interest, beeause of the dual environmental affinity that is associated with covalently bound hydrophobie and hydrophilic blocks. These strive to minimize their eontaet, and therefore drive self-assembly into assemblies with different arehi-teetures. Based on their chemical specificity, as for example the hydrophilie-to hydrophobie ratio, amphiphilic block copolymers can self-assemble in dilute aqueous solutions into micelles, vesicles, tubes, wire-like structures, nanopartieles, or planar membranes at water-air interfaces. Synthetic membranes have greater mechanical stability than phospholipids because of the higher moleeular weight (Mw) of amphiphilic block copolymers, and thus are thicker and stiffer than lipid bilayers. 

In contrast to polymersomes, there are various models of planar membranes monolayers at the water-air interface, free-standing bilayers, and solid-supported membranes. The functionality of proteins in natural membranes strongly depends on their mobility in the matrix, and this is thus an essential prerequisite for artificial membranes to mimic the dynamic environment of biomembranes in order to serve as templates for biomolecules.Therefore, the building blocks forming a bio-inspired membrane need to possess high flexibility to compensate the hydrophobic mismatch between the size of the biomolecules, and the membrane thickness. Furthermore, a variety of membrane properties (thickness, polarity, and surface charge) have to be considered for the successful insertion/attach-ment of biomolecules. Decoration of polymer membranes with biomolecules, either on their surfaces or inside the bilayers, can be achieved by various approaches, such as physical adsorption, insertion, and covalent binding. Compared to physical immobilization of biomolecules on... 

It is widely known that liquid water departs considerably from its average bulk behavior due to the presence of adjacent interfaces, be they organic, such as biomembranes and proteins, or inorganic, such as clays and ion exchangers [2,3]. 

One of the principal components of surfactant-based systems is water, which is often neglected, and sometimes only the melting point of this component in the system is reported. The behavior of water is sensitive to the presence of adjacent interfaces of different types, such as biomembranes, proteins, and inorganic compounds [170]. Properties of water molecules depart considerably from their average bulk values when there are solutes or interfaces in the neighborhood. Water in very small volumes plays a dominant role as the medium that controls structure, function, dynamics, and thermodynamics near biological membranes or in other confined regions of space [171]. 

The results discussed in this section indicate that the hydrophilic properties of polyphenols facilitate their localization and accumulation at the interface of biomembranes and lipoproteins, thereby suggesting two advantages as antioxidants (a) inhibition of attack by flee radicals in the aqueous phase and (b) efiective repair of lipophilic radicals (such as a-tocopheroxyl radicals). Thus, in addition to thermodynamic (redox potentials) and kinetic (rate constants, stability of the antioxidant-derived radical) properties, solubility properties may determine antioxidant effectiveness in recycling mechanisms at lipid-water interfaces. Also, they may explain that low amounts of polyphenols, concentrated at the lipid-water interfaces, may achieve a concentration high enough to act as antioxidants in biological systems. 

More macroscopic phenoipena of biomembranes are trackable by simplification with abandonment of atomic details in the calculations. In a simulation at oil-water interfaces, water and oil molecules are represented by particles labeled w and o , respectively. An amphiphilic molecule such as a lipid is represented by a chain of two w particles followed by five o particles. A completely repulsive interaction is assumed between o and w, and a Lennard-Jones potential is assumed between particles of the same kind. Thus, this simulation is even simpler than the pioneering simulations of atomic chain models described in Section 2.1. Thus, more macroscopic phenomena can be studied easily. Starting from a i patially random distribution of water, oil, and the amphiphilic molecules, the monolayers and micelles composed of the amphiphilic molecules were spontaneously made at oil-water interfaces as shown in Figure 3. Depletion layers between monolayers and micelles were observed, suggesting the repulsion between biomembranes by the solvation force. 

Water plays a unique role as a solvent and the behavior of water near an interface is important given the technological importance of the chemistry of aqueous solutions that takes place at surfaces (such as biomembranes, electrodes... 

Most dyes have different charge distributions in their ground and excited states. Hence absorption and emission spectra are often sensitive to solvent polarity. Dyes which have particularly large differences are very sensitive probes of local polarity. There are many such dyes used in the biomembrane field to probe the local environment of the membrane-water interface. Recent papers by Morawetz (13a), Osada (13b), and by Turro (13c) have shown that such dyes are also useful for examining aqueous solutions of polymers. 

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