Bilayer normal

Figure 4 Comparison of average distances from the bilayer center along the bilayer normal for deuterated methyl and methylene groups distributed throughout the DPPC molecule computed from constant-pressure MD calculations and neutron diffraction measurements on gel and liquid crystalline phase DPPC bilayers.

Figure 5 Electron density distributions along the bilayer normal from an MD simulation of a fully hydrated liquid crystalline phase DPPC bilayer. (a) Total, lipid, and water contributions (b) contributions of lipid components in the interfacial region.

Figure 10 Elastic incoherent structure factors for lipid H atoms obtained from an MD simulation of a fully hydrated DPPC bilayer, and quasielastic neutron scattering experiments on DPPC bilayers at two hydration levels for (a) motion in the plane of the bilayer and (b) motion m the direction of the bilayer normal.

Several works have been reported for macroscopically orientated biological membranes.106-109 The biomembrane alignment can be carried out mechanically or magnetically. The first one relies on the deposition of lipid bilayers on the surface of a rigid support (glass plates) such that the bilayer normal is perpendicular to the surface of the support itself. Small peptides and the lipid bilayers can be dissolved in organic solvents which are successively removed under vacuum.105 The re-hydration of the system in a chamber of an optimized temperature, humidity and time gives rise to the desired orientation. 

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...

Here 6 is the instantaneous angle between a given C-D bond vector and the axis of rotational symmetry of the molecules, i.e., the bilayer normal. The brackets denote an average over the time scale of the experiment 10 s) so that Sen is the time-averaged orientation of the particular C—D bond with respect to the bilayer normal. 

The second order parameter is termed average alkyl chain order , Smoi. It is calculated in the same way as SCD, with the difference that the angle 0 is taken between the long molecular axis and the bilayer normal. It is related to SCD by Eq. 6.7 ... 

For a chain parallel to the bilayer, normally Smoi = 1 and decreases to zero for a fully unordered, isotropic chain. For a chain oriented parallel to the bilayer the value of mol is —b.5. 

Robinson et al. [69] studied the influence of cholesterol on molecular ordering of phospholipids by MD simulation. They used a more detailed description of the phospholipids including the head groups and charges. The simulated system contained two cholesterol molecules and 18 DMPC molecules in each leaflet of the bilayer (10 mol% cholesterol) and was simulated after equilibrium for 400 ps employing NVT conditions. They observed an increase in the fraction of trans conformations of the lipid alkyl chains with a decrease in kinks. Also, the dynamic and conformation of the flexible cholesterol side chains was characterized and it was found that they had a smaller tilt angle than the lipid chains with respect to the bilayer normal. 

Fig. 6.12 Change in distribution of phosphorus (a), nitrogen (b), and carbonyl oxygen (c) positions along the bilayer normal in pure DPPC and in membranes with 11 and 50 mol% cholesterol.

Fig. 6.13 Distribution of the angle between P-N vector and bilayer normal in pure DPPC membrane (solid line) and in membranes containing cholesterol 11 mol% (dotted line), 50 mol% structure A (dash dot line), and structure B (dashed line). When cosine is positive, the P—N vector points into the water layer.

The low-temperature gel phase corresponds closely to that of the crystalline dihydrate and is thus denoted the Lc/ phase. The detailed stmcture of this phase and the other phases discussed below is treated in detail elsewhere (see article Lipids, Phase Transitions of). The Lg/ phase is characterized by extended hydrocarbon chains that are tilted slightly with respect to the bilayer normal. These chains are packed very tightly, and rotation about their long axes is very severely restricted. The polar headgroup contains only a few bound water molecules, and its motion is also severely restricted. 

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