Lipid bilayers head-group orientation

The authors reported that the hydrophobic peptide had little influence on the lipid structure, as lipid lateral diffusion rates, lipid conformations, and head group orientations were identical to a neat bilayer. The distance distribution of the phospholipid atoms surrounding the peptide was rather broad, pointing to the absence of special interactions between the peptide and the surrounding phospholipids. 

Figure 22.3 The basic construction of phosphodiglyceride molecules within lipid bilayers. The fatty acid chains are embedded in the hydrophobic inner region of the membrane, oriented at an angle to the plane of the membrane surface. The hydrophilic head group, including the phosphate portion, points out toward the hydrophilic aqueous environment.

Figure 6. V ariation of the orientational order parameter 5 along the hydrocarbon chains of the lipids of DMPC lipid bilayers, according to MD simulations of Berger et al. [58]. The line is drawn to guide the eye. The spheres are experimental values obtained by Seelig and Seelig [59] using 2H-NMR spectroscopy. (Numbering of C-atoms from the head group to the CH3 terminal group). Redrawn from [58] by permission of the Biophysical Society...

The properties of membranes commonly studied by fluorescence techniques include motional, structural, and organizational aspects. Motional aspects include the rate of motion of fatty acyl chains, the head-group region of the phospholipids, and other lipid components and membrane proteins. The structural aspects of membranes would cover the orientational aspects of the lipid components. Organizational aspects include the distribution of lipids both laterally, in the plane of the membrane (e.g., phase separations), and across the membrane bilayer (phospholipid asymmetry) and distances from the surface or depth in the bilayer. Finally, there are properties of membranes pertaining to the surface such as the surface charge and dielectric properties. Fluorescence techniques have been widely used in the studies of membranes mainly since the time scale of the fluorescence lifetime coincides with the time scale of interest for lipid motion and since there are a wide number of fluorescence probes available which can be used to yield very specific information on membrane properties. 

The exact dimensions of a phospholipid bilayer membrane in terms of the in-plane area and the height of the lipid molecules as well as the thickness of the water layer that is associated with them is dependent on the chemical identity of the phospholipid head group, the length and the degree of saturation of the acyl chains, and the degree of hydration. This information may be obtained from a combination of small-angle X-ray diffraction by MLV or oriented multi-bilayer samples of phospholipids in excess water, electron and/or neutron density profiles across lipid bilayers, and atomic level molecular dynamics simulations of hydrated lipid bilayers. H-NMR studies on selectively deuter-ated phospholipids have also been important in elucidating acyl... 

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