Biomembrane characteristics

Molecular study of lipid bilayer interfaces is necessary for a better understanding of the membrane-drug interaction and DD into biomembranes. The points to be clarified are (1) How can we determine DD sites at the bilayer interface (2) What kind of method is advantageous (3) Is it possible to unambiguously specify the bilayer interfacial portion coupled with drugs (4) What are most important characteristics of DD at the bilayer interface In order to answer these important questions, this chapter has been planned. We will emphasize the significance of the molecular level information obtainable from NMR studies. 

Surfactant has a similar amphoteric structure as lipid, which makes it possible to form a stable membrane the same as a lipid membrane and can be used to embed proteins. A surfactant membrane has many characteristics similar to those of a biomembrane, so that it can retain the bioactivities of proteins well. The process of preparing a sur-factant/protein-modified electrode is simple and viable. There are usually two methods... 

Figure 16 shows the experimental arrangement for the measurement of the surface pressure. The trough (200 mm long, 50 mm wide and 10 mm deep) was coated with Teflon. The subphase temperature was controlled within 0.1 C by means of a jacket connected to a thermostated water circulator, and the environmental air temperature was kept at 18 °C. The surface tension was measured with a Wilhelmy plate of platinum(24.5 x 10.0 x 0.15 mm). The surface pressure monitored by an electronic balance was successively stored in a micro- computer, and then Fourier transformed to a frequency domain. The surface area was changed successively in a sinusoidal manner, between 37.5 A2/molecule and 62.5 A2/molecule. We have chosen an unsaturated phospholipid(l,2-dioleoyl-3-sn-phosphatidyI-choline DOPC) as a curious sample to measure the dynamic surface tension with this novel instrument, as the unsaturated lipids play an important role in biomembranes and, moreover, such a "fluid" lipid was expected to exhibit marked dynamic, nonlinear characteristics. The spreading solution was 0.133 mM chloroform solution of DOPC. The chloroform was purified with three consecutive distillations. 

Many ESR studies (see Section 3.3.6) using different systems have shown that phase separation in lipid layers may lead to a domain-like lateral structure. The area of domain formation can be extended over several hundred angstroms. In this context, charge-induced domain formation in biomembranes is of special interest for the medicinal chemist. In particular, the addition of Ca2+ to negatively charged lipids leads to domain formation [106]. Each lipid component is expected to have a characteristic spontaneous curvature. The Ca2+-induced domains lead to protrusions in the... 

In addition, the reversibility of phase transition in lipid-water systems has been studied [30]. It was observed that the relaxation times in the transition region and the lifetimes of the metastable phases are similar, and sometimes significantly longer than the times characteristic of the biomembrane processes. The question arises as to the physiological significance of the equilibration that occurs a long time after lipid phase transition. 

The traditional fluorescence and electron-spin resonance methods for recording molecular collisions do not allow the study of translational diffusion and rare encounters of molecules in a viscous media because of the short characteristic times of these methods. To measure the rate constants of rare encounters between macromolecules and to investigate the translation diffusion of labelled proteins and probes in a medium of high viscosity (like biomembranes), a new triplet-photochrome labeling technique has been developed (Mekler and Likhtenshtein, 1986 Mekler and Umarova, 1988 Likhtenshtein, 1993 Papper and Likhtenshtein, 2001). 

In the following, a unique oscillation of membrane current observed with a liquid membrane system by the present authors [32,37], which has characteristics similar to those of the oscillation at a biomembrane with so-called sodium channel , will be introduced as an example, and the mechanisms for the oscillation will be clarified by using VCTIES, taking into consideration ion transfer reactions and adsorptions at two W/M interfaces in the membrane system. In this connection, various oscillations other than this example and the elucidation of their mechanisms were described elsewhere [32,37,38]. 

Dermal absorption, the process by which a substance is transported across the skin and taken up into the living tissue of the body (USEPA, 1992), is a complex process. The skin is a multilayered biomembrane with particular absorption characteristics. It is a dynamic, living tissue and as such its absorption parameters are susceptible to constant changes. Upon contact with the skin, a portion of the substance can penetrate into the non-viable stratum comeum and may subsequently reach the viable epidermis, the dermis and, ultimately, the vascular network. During the absorption process, the compound may be subject to biotransformafion (Noonan and Wester, 1989). The stratum comeum provides the skin its greatest barrier function against hydrophilic compounds, whereas the viable epidermis is most resistant to highly lipophilic compounds (Flynn, 1985). 

These membranes thus have some of the characteristics of biological membranes. Biomembranes are highly fluid and, because of their low viscosity, they exhibit high diffusivities. They are also highly permselective, although for different reasons. However, both types of membranes can behave as chemical pumps . 

On the other hand, the sugar moiety of the glycosphingolipids represents the hydrophilic part of the molecule. The solubility in water is also dependent on the number of monosaccharide residues bound in the molecule. Furthermore, such strongly anionic components as neuraminic acids, and sulfate or phosphate groups, influence the ratio of the solubility in water and fat. These two kinds of solubility characteristics are probably essential in the biological roles of the glycosphingolipids when they function as biomembrane components. 

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. 

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