Vesicles lipid lateral diffusion

Ellena, J.F., L.S. Lepore, and D.S. Cafiso (1993) Estimating lipid lateral diffusion in phospholipid vesicles from 13C spin-spin relaxation. J. Rhys. Chem. 97, 2952-2957. 

The interaction between bacterial lipopolysaccharides (EPS) and phospholipid cell membranes was studied by various physical methods of deep rough mutant EPS (ReEPS) of Escherichia coH incorporated in phospholipid bilayers as simple models of cell membranes. SS P-NMR spectroscopic analysis suggested that a substantial part of ReEPS is incorporated into l,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers when mixed multilamellar vesicles were prepared. Furthermore the lipid lateral diffusion coefficients measurements at various molar ratios of ReEPS/egg-PC/POPG indicated that the incorporated ReEPS reduces the diffusion coefficients of the phospholipids in the membrane. EUV formed by the ReEPS from Salmonella enterica, eventually in mixture with dilauroyl phosphatidylcholine (DEPC), have been prepared and characterized by DES, SANS and EPR. PFGSE NMR measurements have shown that water permeability through the lipid bilayer is low at room temperature. However, above a transition temperature centered at 30-35 °C, the water permeability increases. ... 

R. Fato, M. Battino, G. P. Castelli, and G. Lenaz, Measurement of the lateral diffusion coefficients of ubiquinones in lipid vesicles by fluorescence quenching of 12-(9-anthroyl) stearate, FEES Lett. 179, 238-242 (1985). 

The above data suggest that a crosslinked bilayer vesicle is essentially a single polymer molecule (really two, one in each half of the bilayer). In other words the polymerization of the lipid monomers exceeded a gel-point. This concept raises the question of what mole fraction of bis-substituted lipid is necessary to achieve a gel-point for a bilayer composed of a crosslinker lipid, i.e. bis-lipid, and a mono-substituted lipid. Approximately 30% of the lipids in a bilayer vesicle of SorbPCs must be bis-SorbPC (4) in order to produce a polymerized vesicle that could not be dissolved by detergent or organic solvent [29], A complementary study of Kolchens et al. found that the lateral diffusion coefficient, D, of a small nonreactive lipid probe in a polymerized bilayer of mono- and bis-AcrylPC was dramatically reduced when the mole fraction of the bis-AcrylPC, was increased from 0.3 to 0.4 [24]. The decreased freedom of motion of the probe molecule indicates the onset of a crosslinked bilayer in a manner consistent with a 2-dimensional gel-point. 

Is it possible to use simple bilayer vesicles (liposomes) to test the involvement of other modes of motion than lateral diffusion of lipids (e.g., motions across the bilayer that would be important in the transmission of signals from the cell interior to the external cell surface) ... 

Note, however that the concepts about the lipid membrane as the isotropic, structureless medium are oversimplified. It is well known [19, 190] that the rates and character of the molecular motion in the lateral direction and across the membrane are quite different. This is true for both the molecules inserted in the lipid bilayer and the lipid molecules themselves. Thus, for example, while it still seems possible to characterize the lateral movement of the egg lecithin molecule by the diffusion coefficient D its movement across the membrane seems to be better described by the so-called flip-flop mechanism when two lipid molecules from the inner and outer membrane monolayers of the vesicle synchronously change locations with each other [19]. The value of D, = 1.8 x 10 8 cm2 s 1 [191] corresponds to the time of the lateral diffusion jump of lecithin molecule, Le. about 10 7s. The characteristic time of flip-flop under the same conditions is much longer (about 6.5 hours) [19]. The molecules without long hydrocarbon chains migrate much more rapidly. For example for pyrene D, = 1.4x 10 7 cm2 s1 [192]. 

Fig. 13.—Stages of vesicle and tubule formation from a one-component bilayer. Stages A and B are energetically favourable if allowed by packing, but require free lateral diffusion of lipids in the bilayer and water flow across the bilayer. Stage C involves fusion. Stage D is energetically unfavourable for a bilayer in the fluid-state. A mechanism of vesicle and alveoli formation from a membrane similar to that shown here has been found to occur in a variety of cell types and in micropino-...

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

There is independent physical evidence for non-uniform distribution and restriction from transmembrane diffusion of a-Toc in lipid membranes. Differential scanning calorimetry results indicated that it partitioned into the most fluid domains in lipid vesicles. Fluorescence studies showed that a-Toc has a very high lateral diffusion rate in egg lecithin but it does not take part in transbilayer (flip-flop) migration even over many hours . It is not known if this behavior of a-Toc extends to natural biomembranes where actual structures and conditions may dramatically change migration phenomena. 

Reactivity in aggregates may be used to get useful infonnation on mobility in these systems. Vesicles are particularly amenable to these studies because, as mentioned earlier, mobihty in these aggregates is lower than in micelles. For instance, it is estimated that above T, lateral diffusion of the lipids within the plane of the vesicle bilayer is very fast (diffusion coefficient of 10 cm s , in the fluid phase), though three orders of magnitude slower than in an aqueous medium. Accordingly, randomization of a hpid in a leaflet of the bilayer of a 500 A vesicle will occur in milliseconds, whereas the slow transverse (flip-flop) movement from one leaflet to another may take up to several days [1, 7, 60]. 

Very little is known about the motions of lipid bilayers at elevated pressures. Of particular interest would be the effect of pressure on lateral diffusion, which is related to biological functions such as electron transport and some hormone-receptor interactions. Pressure effects on lateral diffusion of pme lipid molecules and of other membrane components have yet to be carefully studied, however. Figure 9 shows the pressure effects on the lateral self diffusion coefficient of sonicated DPPC and POPC vesicles [86]. The lateral diffusion coefficient of DPPC in the liquid-crystalline (LC) phase decreases, almost exponentially, with increasing pressure from 1 to 300 bar at 50 °C. A sharp decrease in the D-value occurs at the LC to GI phase transition pressure. From 500 bar to 800 bar in the GI phase, the values of the lateral diffusion coefficient ( IT0 cm s ) are approximately constant. There is another sharp decrease in the value of the lateral diffusion coefficient at the GI-Gi phase transition pressure. In the Gi phase, the values of the lateral diffusion coefficient ( 1-10"" cm s ) are again approximately constant. 

Little is known about the diffusional properties of ferredoxin and plastocyanin along the thylakoid surface. Using lipid vesicles and applying the technique gf FPy P Fragate et al (53b) estimated a lateral diffusion coefficient of 5 X 10 cm s for plastocyanin. For various reasons it is problematical to apply this value to the in vivo situation as discussed in detail in ref. 13. 

In contrast to surfactants, lipids adsorbed on hydrophilic surfaces can be expected to form planar bilayers, due to their large spontaneous radius of curvature. A double chain amphiphile forming a bilayer on silica was already discussed in chapter 3.1.2 in the context of 2H NMR investigations of water soluble amphiphiles. Bilayers from water insoluble lipid amphiphiles have been adsorbed to large spherical silica particles by condensation of unilamellar vesicles from aqueous solution, and a series of studies explored different NMR methods suitable for the measurement of lateral diffusion coefficients in such supported bilayers . 

Figure 6.5 Temperature dependence of the lateral diffusion coefficient of the fluorescence probe di018 (see Reference 49) in large vesicles of pure DMPC (curve 1), of a 1/1 mixture of DMPC and of a butadiene lipid before polymerization (curve 2), of the same mixture after polymerization of the butadiene lipid (curve 3), of the butadiene lipid before polymerization (curve 4) and of this lipid after polymerization (curve 5). T, Tg, Tg and Tg correspond to the transition temperatures of the lipids or lipid mixture. Polymerization is seen to reduce the lateral diffusion coefficient. Reproduced from Reference 49 with permission of American Physical Society.

Lipids also undergo rapid lateral motion in membranes. A typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second. At that rate, a phospholipid could travel from one end of a bacterial ceil to the other in less than a second or traverse a typical animal ceil in a few minutes. On the other hand, transverse movement of lipids (or proteins) from one face of the bilayer to the other is much slower (and much less likely). For example, it can take as long as several days for half the phospholipids in a bilayer vesicle to flip from one side of the bilayer to the other. 

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