Membranes order parameters

Thus, the time-resolved measurement of such membrane probes contains information on the dynamics of the hindered probe rotation, often interpreted as the micro-viscosity, and about the hindrance of this rotation, usually interpreted as the static packing arrangement of the lipids or the so-called membrane order [136, 137]. Fluorescence polarisation studies in membranes, however, exhibit some major limitations the experimentally determined steady-state and time-re-solved anisotropies characterize the motional restrictions of the reporter molecule itself and give therefore only indirect information about the dye environment, with the consequence that, if the probe is bound covalently to the lipid (TMA-DPH), this attachment may dominate the recorded depolarisation behaviour. The membrane order parameters obtained from freely mobile probes like (DPH) result from a broad distribution of localisation within the hydrophobic interior, the detailed characterisation of which reveals inherent ambiguities [138]. 

FIGURE 10.3 Profiles of the order parameter (order parameter is plotted in a log scale as a function of nitrox-ide position (n) along the alkyl chain of n-SASL) at 25°C in DMPC membranes with and without 10mol% zeaxanthin. (From Subczynski, W.K. et al., Biochim. Biophys. Acta, 1068, 68, 1991. With permission.)... 

We would like to point out that an order parameter indicates the static property of the lipid bilayer, whereas the rotational motion, the oxygen transport parameter (Section 4.1), and the chain bending (Section 4.4) characterize membrane dynamics (membrane fluidity) that report on rotational diffusion of alkyl chains, translational diffusion of oxygen molecules, and frequency of alkyl chain bending, respectively. The EPR spin-labeling approach also makes it possible to monitor another bulk property of lipid bilayer membranes, namely local membrane hydrophobicity. 

Compared to US and its subsequent variants, the ABF method obviates the a priori knowledge of the free energy surface. As a result, exploration of is only driven by the self-diffusion properties of the system. It should be clearly understood, however, that while the ABF helps progression along the order parameter, the method s efficiency depends on the (possibly slow) relaxation of the collective degrees of freedom orthogonal to . This explains the considerable simulation time required to model the dimerization of the transmembrane domain of glycophorin A in a simplified membrane [54],... 

Fig. 1 Solid-state NMR structure analysis relies on the 19F-labelled peptides being uniformly embedded in a macroscopically oriented membrane sample, (a) The angle (0) of the 19F-labelled group (e.g. a CF3-moiety) on the peptide backbone (shown here as a cylinder) relative to the static magnetic field is directly reflected in the NMR parameter measured (e.g. DD, see Fig. 2c). (b) The value of the experimental NMR parameter varies along the peptide sequence with a periodicity that is characteristic for distinct peptide conformations, (c) From such wave plot the alignment of the peptide with respect to the lipid bilayer normal (n) can then be evaluated in terms of its tilt angle (x) and azimuthal rotation (p). Whole-body wobbling can be described by an order parameter, S rtlo. (d) The combined data from several individual 19F-labelled peptide analogues thus yields a 3D structural model of the peptide and how it is oriented in the lipid bilayer...

From these overall profiles, it is not easy to extract conformational properties, other than that it will be clear that the lipid molecules are strongly anisotropically oriented in the bilayer. For this, other characteristics are much more appropriate. It is possible to define an order parameter which indicates how much the lipid tails are oriented normal to the membrane ... 

Such order parameters can also be obtained from deuterium NMR measurements, and therefore one frequently finds predictions for these order parameters in the literature. The order parameter profile that belongs to the tails of the lipid membrane as given in Figure 5 is shown in Figure 6. 

Figure 9. A comparison of the order parameter profile as found by MC simulations [72] of model 9/10 cis unsaturated chains in a monolayer (the x-line is to guide the eye) with experimental data obtained from NMR experiments (o) on the same chains incorporated into a biological membrane. Redrawn from [72] by permission of the American Institute of Physics...

Many conformations were sampled by the usual MC procedure. The result is of course that there is no preferred orientation of the molecule. Each conformation can, however, be characterised by an instantaneous main axis this is the average direction of the chain. Then this axis is defined as a director . This director is used to subsequently determine the orientational order parameter along the chain. The order is obviously low at the chain ends, and relatively high in the middle of the chain. It was found that the order profile going from the centre of the molecules towards the tails fell off very similarly to corresponding chains (with half the chain length) in the bilayer membrane. As an example, we reproduce here the results for saturated acyl chains, in Figure 10. The conclusion is that the order of the chains found for acyl tails in the bilayer is dominated by intramolecular interactions. The intermolecular interactions due to the presence of other chains that are densely packed around such a chain,... 

From the density profiles one cannot really judge the effect of the double bonds the density profiles for membranes of saturated lipids are very similar to those of unsaturated ones. Therefore it is necessary to consider some of the conformational characteristics of the tails. It is possible to compute the order parameter profile for both the saturated and the unsaturated chains. The order parameter profile for the saturated chain closely follows the results presented in Figure 17 for DMPC membranes for both the SCF and the MD predictions. The order parameter profiles for the unsaturated chain closely follows the MC predictions, as discussed in Figure 9. A pronounced dip is found near the cis double bond. For this reason, we choose here to present complementary data about the conformational properties of the acyl chains. 

Biological membranes fluidity order parameters lipid-protein interactions translational diffusion site accessibility structural changes membrane potentials complexes and binding energy-linked and light-induced changes effects of additives location of proteins lateral organization and dynamics... 

Biological membranes Fluidity and order parameters Determination of the phase transition temperature Effect of additives (e.g. cholesterol)... 

H. Pottel, W. van der Meer, and W. Herreman, Correlation between the order parameter and the steady-state fluorescence anisotropy of l,6-diphenyl-l,3,5-hexatriene and an evaluation of membrane fluidity, Biochim. Biophys. Acta 730, 181-186 (1983). 

TUt angle of the peptide symmetry axis with respect to the membrane normal Azimuthal rotation angle of the peptide Molecular order parameter of the peptide... 

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