Bilayer flexibilities

A criticism often aimed at the use of extrinsic fluorescent probes is the possible local perturbation induced by the probe itself on the microenvironment to be probed. There are indeed several cases of systems perturbed by fluorescent probes. However, it should be emphasized that many examples of results consistent with those obtained by other techniques can be found in the literature (transition temperature in lipid bilayer, flexibility of polymer chains, etc.). To minimize the perturbation, attention must be paid to the size and shape of the probe with respect to the probed region. 

Hence we conclude that a mesoscopic description for the size of an onion is quantitatively possible and that the mesoscopic determinants are bilayer flexibility and bilayer interaction. 

Having established that bilayer flexibility and bilayer interaction are the mesoscopic determinants, the next question is whether these determinants can be coupled to molecular parameters. In fact, this has been done to quite some extent. In general, bilayer flexibility can be shown (both experimentally as well as theoretically by simulation methods) to be directly related to bilayer thickness, lateral interaction between heads and tails of the surfactants, type of head group (ethoxylate, sugar, etc.), type of tail (saturated, unsaturated) and specific molecular mixes (e.g. SDS with or without pen-tanol). The bilayer interaction is known to be related to characteristics such as classical electrostatics. Van der Waals, Helfrich undulation forces (stemming from shape fluctuations), steric hindrance, number, density of bilayers, ionic strength, and type of salt. Two examples will be dicussed. 

This range yields more highly tmncated cones. The main mesophase stmcture obtained from these units is a flexible bilayer such as that fonned in vesicles and liposomes. These arrangements are often obtained from doublechain surfactants such as lecithin, double tailed cationic surfactants and AOT. 

In some of these models (see Sec. Ill) the surfactants are still treated as flexible chains [24]. This allows one to study the role of the chain length and chain conformations. For example, the chain degrees of freedom are responsible for the internal phase transitions in monolayers and bilayers, in particular the hquid/gel transition. The chain length and chain architecture determine the efficiency of an amphiphile and thus influence the phase behavior. Moreover, they affect the shapes and size distributions of micelles. Chain models are usually fairly universal, in the sense that they can be used to study many different phenomena. 

In previous studies (56,57) a bilayer was modeled with atomic detail of the hydrocarbon chain, including the proper dihedral angle functions and using flexible bond angles, but the aqueous layer was not represented. 

Modeling Pardaxin Channel. The remarkable switching of conformation in the presence of detergents or phospholipid vesicles (5) suggests that pardaxin is a very flexible molecule. This property helps to explain the apparent ability of pardaxin to insert into phospholipid bilayers. In addition, it is consistent with the suggestion that the deoxycholate-like aminoglycosteroids (5,7) present in the natural secretion from which pardaxin is purified (5) serve to stabilize its dissociated conformation. The question of the mechanism by which pardaxin assembles within membranes is important for understanding pore formation and its cytolytic activity (5). 

Unsaturations of lipids play a key role in lipid homeostasis, where organisms adapt to temperature variations of the environment. Plants and animals maintain physiological functions by reversibly altering the composition and conformation of lipid molecules of the cell membrane. To achieve this, they extensively and elegantly use the unsaturations (double bonds) present in their side chains. This is the process by which cell membranes adjust their flexibility (fluidity) of the bilayer and adapt themselves to perturbations in temperature, pressure, and other variations in the natural environment [11-14]. They remain indispensable for the poikilothermism exhibited by fishes, invertebrates, and amphibians [15, 16]. Commercially,... 

Structural properties of both AFA-PLN and WT-PLN bound to SER-CAla after reconstitution in a functional lipid bilayer environment were examined by 13C solid-state NMR.241 Chemical-shift assignments in all domains of AFA-PLN provide direct evidence for the presence of two terminal ot-helices connected by a linker region of reduced structural order that differs from previous findings on free PLN. A combination of the spectroscopic data with biophysical and biochemical data using flexible protein-protein docking simulations provides a structural basis for understanding the interaction between PLN and SERCala.244 Using a... 

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