Phospholipid-monolayers

Grazing incidence excitation of a fluorescent probe in a phospholipid monolayer can also be used to indicate order. The collective tilt of the molecules in a domain inferred from such measurements is indicative of long-range orientational order [222]. 

Stigter and Dill [98] studied phospholipid monolayers at the n-heptane-water interface and were able to treat the second and third virial coefficients (see Eq. XV-1) in terms of electrostatic, including dipole, interactions. At higher film pressures, Pethica and co-workers [99] observed quasi-first-order phase transitions, that is, a much flatter plateau region than shown in Fig. XV-6. 

The effect is more than just a matter of pH. As shown in Fig. XV-14, phospholipid monolayers can be expanded at low pH values by the presence of phosphotungstate ions [123], which disrupt the stmctival order in the lipid film [124]. Uranyl ions, by contrast, contract the low-pH expanded phase presumably because of a type of counterion condensation [123]. These effects caution against using these ions as stains in electron microscopy. Clearly the nature of the counterion is very important. It is dramatically so with fatty acids that form an insoluble salt with the ion here quite low concentrations (10 M) of divalent ions lead to the formation of the metal salt unless the pH is quite low. Such films are much more condensed than the fatty-acid monolayers themselves [125-127]. 

Folded membranes were generated out of two phospholipid monolayers at the air/subphase interface [158]. A thin Teflon septum separated the two half-cells of the setup which was also made of Teflon. This septum, with an orifice 50-200 jim in diameter, was... 

The popular applications of the adsorption potential measurements are those dealing with the surface potential changes at the water/air and water/hydrocarbon interface when a monolayer film is formed by an adsorbed substance. " " " Phospholipid monolayers, for instance, formed at such interfaces have been extensively used to study the surface properties of the monolayers. These are expected to represent, to some extent, the surface properties of bilayers and biological as well as various artificial membranes. An interest in a number of applications of ordered thin organic films (e.g., Langmuir and Blodgett layers) dominated research on the insoluble monolayer during the past decade. 

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). 

Capacitance measurements of phospholipid monolayers at the ITIES have been proposed as a suitable tool for studying the enzyme activity under the precise control of the electrical state of the monolayer [81]. Kinetics of hydrolysis of phosphatidylcholine... 

Ruckenstein and Li proposed a relatively simple surface pressure-area equation of state for phospholipid monolayers at a water-oil interface [39]. The equation accounted for the clustering of the surfactant molecules, and led to second-order phase transitions. The monolayer was described as a 2D regular solution with three components singly dispersed phospholipid molecules, clusters of these molecules, and sites occupied by water and oil molecules. The effect of clusterng on the theoretical surface pressure-area isotherm was found to be crucial for the prediction of phase transitions. The model calculations fitted surprisingly well to the data of Taylor et al. [19] in the whole range of surface areas and the temperatures (Fig. 3). The number of molecules in a cluster was taken to be 150 due to an excellent agreement with an isotherm of DSPC when this... 

Chesniuk et al. studied the transfer of alkali and alkaline-earth cations across phospholipid monolayers at water-1,2-DCE macrointerfaces by cyclic voltammetry. These authors considered the effect of the cation nature, the concentration of the transferring ion, and the applied potential (at the positive end of the polarization window), and noticed either an enhancement of the current or a blocking of the transfer process [13,14]. The enhancement factors observed were very much larger than in other studies, especially at... 

Phospholipid monolayers in liquid expanded state are likely to modify mostly the interfacial concentrations, and this electrostatic effect can be described by Eq. (6). Taking a 1/2, in agreement with most experimental results, and introducing the approximation A 02 = 0, Eq. (6) simplifies to [59]... 

Phospholipid monolayers at liquid-liquid interfaces influence the charge transfer processes in two ways. On the one hand, the phospholipids constitute a barrier that blocks the process by impeding the transferring species to reach the interface [1,15,48]. On the other hand, the phospholipids modify the electrical potential difference governing the process [60]. While the first influence invariably leads to a decreased rate, the second one might result in either a decreased or an increased rate of charge transfer. The net effect of the phospholipids on the charge transfer process depends on the state of the monolayer, and therefore studies with simultaneous electrochemical and surface pressure control are preferable [10,41,45]. 

The effect of phospholipid monolayers on the rate of charge transfer has been the subject of several experimental studies, but still there is a need for additional experimental evidence. For large molecular areas, the effect on the rate of ion transfer seems to be negligible [5]. An increasing surface concentration of lipids leads to liquid expanded states where the electrostatic effects are noticeable. An enhanced rate of ion transfer across monolayers of pure phospholipids has then been observed both for the cases of tracer [11,12] and supporting electrolyte ion transfer [13,17]. Finally, the blocking effect is dominant in liquid condensed monolayers [15]. 

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. 

A BLM can even be prepared from phospholipid monolayers at the water-air interface (Fig. 6.10B) and often does not then contain unfavourable organic solvent impurities. An asymmetric BLM can even be prepared containing different phospholipids on the two sides of the membrane. A method used for preparation of tiny segments of biological membranes (patch-clamp) is also applied to BLM preparation (Fig. 6.10C). 

Shibata, A., Y. Kiba, N. Akati, K. Fukuzawa, and H. Terada. 2001. Molecular characteristics of astaxanthin and beta-carotene in the phospholipid monolayer and their distributions in the phospholipid bilayer. Chem. Phys. Lipids 113 11-22. 

Since its first important flowering in the hands of purely physical chemists, interest in monolayers at the air-water interface has waxed and waned with a frequency of roughly 25 years. The first resurrection of interest came from biochemistry, primarily during the 1955-65 decade as phospholipid monolayers were studied as models for the cell membrane (see, for example, Chapman, 1968). This is still a very productive field of biophysical research. 

N. L. Thompson, H. M. McConnell, and T. P. Burghardt, Order in supported phospholipid monolayers detected by dichroism of fluorescence excited with polarized evanescent illumination, Biophys. J. 46, 739-747 (1984). 

M. L. Pisarchick and N. L. Thompson, Binding of a monoclonal antibody and its Fab fragment to supported phospholipid monolayers measured by total internal reflection fluorescence microscopy,. Biophys. J. 58, 1235-1239 (1990). 

Pitcher WH III, Keller SL, Huestis WH. Interaction of nominally soluble proteins with phospholipid monolayers at the air-water interface. Biochim Bio-phys Acta 2002 1564 107. 

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