Vesicle lateral diffusion

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

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). 

Analysis of SFV entry has thus shown that the virus binds to receptors on the cell surface and moves by lateral diffusion into coated pits to be internalized by coated vesicles. The endocytosed virus is delivered into endosomes. Here presumably, the viral envelope is activated by the low pH prevailing in this compartment to fuse with the vacuolar membrane. This results in the release of the viral nucleocapsid into the cytoplasm. During normal infection, the virus might not enter into lysosomes although SFV particles have been identified in this compartment using the large loads of virus needed to visualize the entry process by electron microscopy. Even if this were to happen normally, the viral nucleocapsid would escape destruction because of the rapidity of the fusion mechanism. 

Figure 19 shows the pressure effeets on the lateral self diffusion eoeffieient of sonicated DPPC and POPC vesicles. The lateral diffusion coefficient of DPPC in the LC phase decreases 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 1 x 10 cm /s) are approximately constant. There is another sharp decrease in the value of the lateral diffusion coefficient at the... 

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]. 

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. 

A very suitable method for measurement of the lateral diffusion of molecules adsorbed at the foam film surfaces is Fluorescence Recovery after Photobleaching (FRAP) ([491-496], see also Chapter 2). Measurements of the lateral diffusion in phospholipid microscopic foam films, including black foam films, are of particular interest as they provide an alternative model system for the study of molecular mobility in biological membranes in addition to phospholipid monolayers at the air/water interface, BLMs, single unilamellar vesicles, and multilamellar vesicles. 

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... 

Table 2. Lateral diffusion coefficients measured for a variety of probes in vesicles and membranes.

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]. 

Takezoe H, Yu H. Lateral diffusion of photopigments in photoreceptor disc membrane vesicle by the dynamic Kerr effect. Biochemistry 1981 20 5275-5281. 

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. 

The increased temperature stability of the polymerized vesicles is, however, stronger evidence suggesting that complete phase separation has not occurred in the particles synthesized by Nakache et al. In a later paper [23], the authors noted that only a fraction of the vesicles appear to polymerize. If the monomer is evenly distributed among all the vesicles initially, one could expect monomer from non-reacting vesicles to diffuse to vesicles containing a growing radical, much like a typical emulsion polymerization experiment in which growing radicals in micelles are fed by monomer droplets. 

For cell membranes to be effective permeability barriers, they must be flexible and allow relatively free motion of proteins that are embedded in or linked to them. Integral membrane proteins often diffuse laterally, and many receptor-mediated solute-uptake pathways involve endocytosis that entails phospholipid rearrangement in the membrane. Hormone secretion and other protein trafficking processes involve exo-cytosis and it is usual for membrane vesicles to fuse with each other in a process that also involves the lateral diffusion of membrane constituents. The activity of some receptors is strongly linked to the extent of fluidity of the membrane around them. 

The NMR rotating frame spin-lattice relaxation time (Tjp) method, which has been used successfully in our laboratory in studies of pressure effects on diffusion in highly viscous liquids, was used in this study to measure lateral diffusion of the phospholipid molecules in DPPC and POPC vesicles. An advantage of this method is that the diffusion coefficient is found directly from measured quantities without estimations of molecular parameters or the effects of the addition of spin or fluorescence probes to... 

---