Membrane gel-state

So-called bouquet molecules , based on a central crown ether or cyclodextrin unit equipped with pendant arms that are also capable of complexing cations and are long enough to traverse a lipid bilayer, have been studied in vesicles with a Na" / Li" gradient across the membrane using both Li and Na NMR. Such systems show a one-for-one exchange of Na" for Li" (antiport). These molecules were found to transport Na" at similar rates in fluid-and gel-state membranes this suggests that ion... 

For PC, PE, and PG, the glycerol backbone is oriented perpendicular to the bilayer surface, while the polar head-groups are almost parallel to the membrane surface. Neutron scattering experiments of selectively deuterated lipid headgroups in liquid crystalline and gel state membranes determine the mean label position with an accuracy of up to 1 A and provide independent support for the almost parallel headgroup orientation of PC, PE, and PG. 

Major determinants of membrane fluidity may be grouped within two categories [53] (1) intrinsic determinants, i.e., those quantifying the membrane composition and phase behavior, and (2) extrinsic determinants, i.e., environmental factors (Table 1). In general, any manipulation that induces an increase in the molal volume of the lipids, e.g., increase in temperature or increase in the fraction of unsaturated acyl chains, will lead to an increase in membrane fluidity. In addition, several intrinsic and extrinsic factors presented in Table 1 determine the temperature at which the lipid molecules undergo a transition from the gel state to liquid crystalline state, a transition associated with a large increase in bilayer fluidity. 

Gel of collodion, PVC, etc. Solution (with low ) of ion exchangers or complex in suitable solvent (apparently dry) "solid-state membrane electrode K, nh , Ca bf4, no3... 

Cholesterol s presence in liposome membranes has the effect of decreasing or even abolishing (at high cholesterol concentrations) the phase transition from the gel state to the fluid or liquid crystal state that occurs with increasing temperature. It also can modulate the permeability and fluidity of the associated membrane—increasing both parameters at temperatures below the phase transition point and decreasing both above the phase transition temperature. Most liposomal recipes include cholesterol as an integral component in membrane construction. 

At sufficiently low temperature, the liquid state of the bilayer is not stable and the membrane abruptly freezes into the so-called gel state. The structure of a... 

A carrier mechanism was excluded for these molecules when ion-transport activity was undiminished in membranes in the gel state. The transport rates... 

Comparison of rates in the gel and liquid state of the membrane. In the case of channel mechanism, ions can pass through a structured pathway which is more or less similar to the bulk aqueous phase, while carriers encapsulate ions and transfer in the membrane. Therefore, the former is insensitive to gel-liquid phase transition of the membrane, while the gel state inhibits the carrier transport. 

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

Electrophoretic mobility and 31P-NMR measurements were made to investigate the binding of the alkaline earth cations to membranes formed from phosphatidyl choline molecules with either saturated or unsaturated hydrocarbon chains. Calcium and magnesium bind to the same degree to membranes formed from lipids with unsaturated chains. These phosphatidylcholine molecules are present in the liquid crystalline state at all temperatures under consideration. Calcium binds more strongly than magnesium to membranes formed from lipids with saturated chains, even when the lipids are in the liquid crystalline state. The selectivity is enhanced when the temperature is lowered and the saturated chain lipids are in the gel state. 

We report here the results of a study of the adsorption of the alkaline earth cations to bilayer membranes formed from phosphatidylcholines with saturated chains dipalmitoyl phosphatidyl choline (DPPC) and dimyristoyl phosphatidyl choline (DMPC). Our salient result is that the adsorption of calcium is distinct from the other alkaline earth cations in two respects. First, only calcium adsorbs significantly more strongly to PCs with saturated chains than to phosphatidyl cholines with unsaturated chains, even when all lipids are present in the liquid crystalline state. Second, when the membranes are present in the frozen or gel state, the binding of calcium is significantly enhanced. We used two independent techniques to demonstrate this unique behavior of calcium. 

The rate and character of the molecular motions of both the molecules embedded in the lipid bilayer and lipid molecules themselves are strongly dependent on the temperature [19, 203], At a certain temperature tm, the gel-liquid crystal phase transition is known to occur for the membrane made of a synthetic lipid. For example, tm = 41.5 °C for the membranes from DPL. In the vesicles formed by a mixture of lipids, e.g. egg lecithin, the phase transition occurs smoothly rather than jumpwise and starts below 0 °C. Note that the permeability of lipid membranes increases notably upon transition from the liquid crystal state to the gel state [204]. 

This is in sharp contrast to the conditions in stratum corneum where the lipid membranes are almost impermeable to water. As a consequence of these facts, we expect the bulk of lipids that form the skin barrier to be in a crystalline (gel) state, that is, to have long carbon chains (C > 20 0) to comply with the physical requirement that the transition temperature should be higher than normal skin temperature (>35°C). A physiological mixture of ceramides, free fatty acids (FFA), and cholesterol is indeed needed for a normal barrier function. 

In the 1970s, the fluid mosaic concept emerged as the most plausible model to account for the known structure and properties of biological membranes [41]. The fact that membranes exist as two-dimensional fluids (liquid disordered) rather than in a gel state (solid ordered) was clearly demonstrated by Frye and Ededin [42], who showed that the lipid and protein components of two separate membranes diffuse into each other when two different cells were fused. Since that time, numerous studies have measured the diffusion coefficient of lipids and proteins in membranes, and the diffusion rates were found to correspond to those expected of a fluid with the viscosity of olive oil rather than a gel phase resembling wax. 

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