Liquid crystalline bilayers

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

As discussed in the preceding sections, fluid, globular micelles are formed from monoalkyl surfactants, whereas the liquid-crystalline bilayer structure is formed from a variety of dialkyl amphiphiles and from single-chain amphiphiles with rigid hydrophobic segments. It may then be asked what structure is expected from amphiphiles with three alkyl chains. 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

There is a large cholesterol concentration gradient in cells from 0-5 mol% in the ER membrane to 25-40 mol% in the plasma membrane [12], Cholesterol has a condensing effect on liquid-crystalline bilayers, causing increased rigidity and thickness [13]. At high concentrations, cholesterol induces an intermediate liquid-ordered phase between the gel and liquid-disordered phases [13]. A number of... 

The interactions obviously differed between the lipid bilayers and the natural membranes. Furthermore, cholesterol slightly hinders the drug partitioning into the liquid-crystalline bilayers, in agreement with several previous reports, and the drug molecules interact electrostatically with membrane proteins at the hydrophilic interface adjacent to the polar headgroups of the phospholipid molecules (7). 

Tracey, A.S. and K. Radley. 1985. A vanadium-51 nuclear magnetic resonance investigation of vanadate oxyanions in a lyotropic liquid crystalline bilayer system. Can. J. Chem. 63 2181-2184. 

For a phase-separated region to exist, lipids have to move into and out of various phases. The lateral diffusion constant in liquid crystalline bilayers is about l(T8cm2/s, which corresponds to an exchange frequency between lipid-lipid nearest neighbors of about 106/s. A necessary precondition for the detection of phases by NMR technique is that the proportion of observable species in the phase is sufficiently large. 

Figure 18.3 The orientation of a peptide in the membrane can be described by the tilt angle x and the azimuthal angle p. x is the angle between the bilayer normal (n) and the peptide long axis, p describes a rotation around the peptide long axis and must be defined with respect to a reference group as indicated by the white circle. In liquid-crystalline bilayers, peptides can usually also rotate around the membrane normal (shown by the dashed arrow). Three characteristic peptide orientations are shown in the S-state the peptide lies flat on the membrane surface with charged amino acids facing the water in the T-state the peptide is inserted with an oblique tilt into the membrane, possibly in a dimeric state (shown as a second peptide in white) and in the inserted l-state the peptide has a transmembrane orientation. In this state, the peptide may self-assemble into pores (shown here as a barrel-stave pore together with additional white peptides).

The classic X-ray diffraction work of Small et al. [5,207,208] pointed out the existence of inverted (reverse) bile salt micelles within mixed bile salt-phospholipid liquid crystalline bilayers. The aggregates were considered to consist of 2-4 molecules (of cholate) with their hydrophilic sides facing inwards bound by hydrogen bonds between the hydroxyl groups, leaving their hydrophobic sides facing outwards to interact with the acyl chains of the phospholipid. At saturation, about 1 molecule of cholate was present for every 2 molecules of lecithin. Appreciably more bile salts... 

The cytoplasm of bacteria is surrounded by a cell envelope which is composed of several layers. The inner layer, the cytoplasmic membrane, is in direct contact with the cytoplasm. It consists of a liquid-crystalline bilayer of phospholipids in which proteins are embedded. 

Fig. 1 Logarithm of applied pressure (log P) plotted versus the lamellar repeat period (d) for subgel bilayers of DPPC and liquid-crystalline bilayers of EPC, DAPC, and 1 1 EPC MOPC, The repeat periods in excess water (with no applied pressure) are shown on the x-axis. Data are taken from [19,21, 24, 25]...

We have also analyzed the interactions between gel and liquid crystalline bilayers composed of the second most common membrane phospholipid PE. Figure 5 shows electron density profiles of bacterial PE (BPE), which is in the liquid crystalline phase at room temperature. The figure shows profiles of BPE at zero applied pressure and in 60% PVP, the same conditions shown for the profiles of EPC (Fig. 2). Each profile shows two bilayers, with the intervening fluid space. The bilayer on the left is centered at the origin with the high density head group peaks located at 20 A. Note that the bilayers for BPE in water and in 60% PVP nearly superimpose, indicating that, as was the case for EPC (Fig. 2), the amount of osmotic stress... 

Upon cooling organisms from the growth temperature, the first membrane lipid phase transition that is likely to occur is that from hexagonal-II or other non-la mellar phase to liquid-crystalline bilayer. There is no evidence that this transition results in irreversible damage... 

Before proceeding further, a brief sketch of the structure of biomembranes is given to aid the following discussion. These biological constructs are composed primarily of lipid molecules, proteins, and water. The liquid-crystalline bilayer phase of lipids is the primary construct of membranes and... 

In principle, the partition coefficient can be calculated from this decrease in 7ni [108], However, ITC experiments and also experiments using fluorescent probes have shown that the partition coefficient is a function of OG concentration in the bilayer, decreasing with increasing surfactant saturation of the bilayer. The approach using DSC curves therefore leads to incorrect results. A further increase of the OG concentration to values where partial or complete micellization for liquid-crystalline bilayers is observed, still leads to DSC curves with a clear endo-thennic peak, which is now almost independent of total OG concentration. Obviously, the mixed micelles fonned at higher temperatures convert to bilayer phases upon cooling and the phase behavior at lower temperature is more complicated. 

Akutsu et al. (1980) have reported P-NMR studies of lipid-containing viruses (e.g., bacteriophage PM2) that are spherical in shape with a hydrated diameter of 600 A and possessing a lipid bilayer. The virus contains only four proteins, namely, proteins I, II, III, and IV. The representative P-NMR spectra are shown in Fig. 14, where a 60% sucrose solvent was used to eliminate the influence of the overall rotational motion of the virus on the spectrum. It is clear firom the spectra A and C that there are two major components an axial symmetric powder pattern superimposed on the broad component. Akutsu and co-workers assigned these two components to a liquid-crystalline bilayer and the DNA inside the virus. Simulated spectrum B b d on the spectra of extracted lipids and T4 phage, which is known to have no lipid membrane, is in good agreement with the observed spectrum A or C. In the presence of 4-6 Af urea, the nucleocapsid was... 

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