Surface of biological membranes

The protein-containing colloidal solutions of water-in-organic solvents are optically transparent. Hence, absorption spectroscopy, circular dichroism spectroscopy and fluorescence spectroscopy are found to be convenient for studying biocatalysis [53]. The reversed micelles are interesting models for studying bioconversion, since the majority of the enzymes in vivo act inside or on the surface of biological membranes. 

Thermally or photochemically induced proton transfers represent bistable switching processes and are of interest for information storage. A lateral transfer of information on the surface of biological membranes is thought to occur by fast proton conduction through protonic networks [8.234]. 

The resultant tocopheroxyl radical is relatively stable and, in normal circumstances, insufficiently reactive to initiate lipid peroxidation itself. It has been demonstrated in vitro that a-tocopherol radical can be converted back to a-tocopherol by reduction with ascorbic acid at the surface of biological membranes... 

Interfaces between aqueous phase and the volumes confined by amphiphilic molecules [288]. In vitro, these refer to lipid vesicles and micelles, lipid lamellae, cubic and hexagonal phases, Langmuir-Blodgett (LB) films, which are important in colloid science and in extraction technology. In vivo, these are the surfaces of biological membranes. 

The glycolipids and glycoproteins from the surface of biological membrane form the glycan ligands responsible for the interaction of cellular membranes with sugarbinding proteins known as lectins and galectins, as well as with other receptors. 

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

Our knowledge of biological membrane ultrastructure has increased considerably over the years as a result of rapid advances in instrumentation. Although there is still controversy over the most correct biological membrane model, the concept of membrane structure presented by Davson and Danielli of a lipid bilayer is perhaps the one best accepted [12,13]. The most current version of that basic model, illustrated in Fig. 7, is referred to as the fluid mosaic model of membrane structure. This model is consistent with what we have learned about the existence of specific ion channels and receptors within and along surface membranes. 

The discovery of galvanic electricity (i.e. electrical phenomena connected with the passage of electric current) by L. Galvani in 1786 occurred simultaneously with his study of a bioelectrochemical phenomenon which was the response of excitable tissue to an electric impulse. E. du Bois-Reymond found in 1849 that such electrical phenomena occur at the surface of the tissue, but it was not until almost half a century later that W. Ostwald demonstrated that the site of these processes are electrochemical semipermeable membranes. In the next decade, research on semipermeable membranes progressed in two directions—in the search for models of biological membranes and in the study of actual biological membranes. 

Ionophores are necessary since the lipid components of biological membranes tend to be orientated such that their polar groups face the membrane surfaces while the non-polar hydrocarbon portions occupy the membrane interior. The hydrophobic nature of the centre of the membrane thus acts as a barrier to the passage of ions such as sodium or potassium. 

In 1848 du Bois-Reymond [21] suggested that the surfaces of biological formations have a property similar to the electrode of a galvanic cell and that this is the source of bioelectric phenomena observed in damaged tissues. The properties of biological membranes could not, however, be explained before at least the basic electrochemistry of simple models was formulated. The thermodynamic relationships for membrane equilibria were derived by Gibbs in 1875 [29], but because the theory of electrolyte solutions was formulated first by Arrhenius as late as 1887, Gibbs does not mention either ions or electric potentials. 

Martinek, K., Levashov, A. V, Pantin, V. I., and Berezin, I. V. (1978). Model of biological membranes or surface-layer (active center) of protein globules (enzymes) - reactivity of water solubilized by reversed micelles of aerosol OT in octane during neutral hydrolysis of picrylchloride. Doklady Akademii Nauk SSSR, 238, 626-9. 

A very brief description of biological membrane models, and model membranes, is given. Studies of lateral diffusion in model membranes (phospholipid bilayers) and biological membranes are described, emphasizing magnetic resonance methods. The relationship of the rates of lateral diffusion to lipid phase equilibria is discussed. Experiments are reported in which a membrane-dependent immunochemical reaction, complement fixation, is shown to depend on the rates of diffusion of membrane-bound molecules. It is pointed out that the lateral mobilities and distributions of membrane-bound molecules may be important for cell surface recognition. 

FIGURE 11-1 Biological membranes. Viewed in cross section, all cell membranes share a characteristic trilaminar appearance. When an erythrocyte is stained with osmium tetroxide and viewed with an electron microscope, the plasma membrane appears as a three-layer structure, 5 to 8 nm (50 to 80 A) thick. The trilaminar image consists of two electron-dense layers (the osmium, bound to the inner and outer surfaces of the membrane) separated by a less dense central region. 

These requirements would be fulfilled if SDS were bound to the BSA monolayer in the form of small aggregates or pseudo-micelles. Such aggregates have been demonstrated to be formed as the result of the interaction of SDS and BSA in solution (2). Further, the electrostatic nature of the interaction was demonstrated by the fact that the complex was completely dissociated by adjusting pH to values above 10.0. Therefore, it is suggested that the cause of the marked shift in pK of the ammonium groups of the SDS-BSA surface complex was the presence of aggregates of SDS bound at cationic sites of the protein monolayer. It may be inferred from this hypothesis that the natural result of the interaction of anionic lipids with an interfacial protein film is the formation of a mosaic structure—one of the proposed characteristics of biological membranes. 

Describe the structure of biological membranes and the characteristic functions of lipid-, protein-, and carbohydrate-containing components. Describe the differences between inner and outer membrane surfaces. 

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