Lipids lateral diffusion coefficient

Diffusion is the random movement of a particle because of an exchange of thermal energy with its environment. Membrane lipids and proteins participate in highly anisotropic translational and rotational diffusion motion. Translational diffusion in the plane of the membrane is described by the mean square lateral displacement after a time At (r ) = TD At. Lipid lateral diffusion coefficients in fluid phase bilayers are typically in the range Dj 10 to 10 cm /s (3). 

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. ... 

In the fluid state, the lateral diffusion coefficient of lipids in the bilayer structure is 0( 10 1 ) m2 s-1 (the symbol O is used to indicate order of magnitude). Interestingly, it has been shown that the diffusion coefficients of phospholipids may differ greatly from the inner to the outer leaflet of the biomembrane layer [4,5]. Again, this is related to the differences in chemical... 

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). 

Substituting hx = 3.6 cm and K ip/w = K - into Eq. 28 Johnson et al. calculated solute lateral diffusion coefficients in stratum corneum bilayers from macroscopic permeability coefficients. Measurements with highly ionized or very hydrophilic compounds were not performed because of the possible transport along a nonlipoidal pathway. Comparison of the computed Aat values with experimentally determined data for fluorescent probes in extracted stratum corneum lipids [47] showed a highly similar curve shape. The diffusion coefficient for the lateral transport showed a bifunctional size dependence with a weaker size dependence for larger, lipophilic compounds (> 350 Da), than... 

There is an abrupt decrease in the lateral diffusion coefficient of DPPC upon the phase transition from the GI phase to the Gi phase. This is because the acyl-chain region is being packed even more efficiently in the Gi phase than in the GI phase, and the hydrocarbon volume in the Gi phase is smaller than in the GI phase. Also, in the Gi phase, the lipid acyl-chains from the opposing bilayer leaflets interdigitate. In order for a phospholipid molecule to diffuse it has to circumvent the nearby interdigitated molecules which hinder diffusion. 

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. 

Several studies show that the values of the lateral diffusion coefficient (D, cm2 s 1) in foam films stabilised by phospholipid(s) depend on two main groups of factors. The first is related to the type of the film, its thickness and radius, and the lipid composition of film monolayers. The second is related to the dependence of the surface diffusion within the limits of given film type and composition on the molecular characteristics of the lipid(s) building the film (molecular charge, length, lipid phase, etc.). 

The above estimates are for fluid-like systems. In gel-like systems with features of frozen order, the time scales are much longer. For example, the lateral diffusion coefficient in a gel-like one-component membrane is about 10 -10 cm /s (43), whereas in a fluid membrane it is usually 10 cm /s. In a similar maimer, the diffusion of matter inside lipid droplets is a much slower process compared with lipid interfaces caused by entanglement effects, as the situation is largely similar to a polymer melt. This effect is the case inside LDL. It has been estimated that the diffusion coefficient for cholesterol esters inside LDL particles is roughly 10 cm /s (44) and is intermediate to diffusion in fluid- and gel-like membranes. 

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... 

A third technique for studying foam films is the fluorescence recovery after photobleaching (FRAP). This techniques was applied by Clarke et al. [36] for lateral diffusion in foam films, and involves irreversible photobleaching by intense laser light of fluorophore molecules in the sample. The time of redistribution of probe molecules (which are assumed to be randomly distributed within the constitutive membrane lipids in the film) is monitored. The lateral diffusion coefficient, D, is calculated from the rate of recovery of fluorescence in the bleaching region due to the entry of unbleaching fluoroprobes of adjacent parts of the membranes. 

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 term exdmer is used when the excited dye forms a transient fluorescent dimeric complex with another fluorophore of the same kind. The exdmer fluorescence is usually red shifted with respect to that of the monomer (see Fig. 6.28) The most widely used types of eocdmer-forming probes are pyrene (see Fig. 6.28) and perylene and their derivatives. The ratio of the maxima of the excimer to the monomer spectra can be used to judge the efficiency of exdmer formation. This (Ex/Mo)-ratio depends on the concentration of the dye and is controlled by the diffusion properties. It allows, when using pyrene or perylene labeled fatty acids or phospholipids (see Fig. 6.28), the estimation of the probe s lateral diffusion coefficients in lipid bilayer membranes. Thus, membrane fluidity can be measured by monitoring the fluorescence spectra of such an exdmer probe. 

It has to be remembered that a lipid exhibits very slow motion. Typically the lateral diffusion coefficient of a standard phosphocholine is around 10 cm /s (for water this coefficient is around 2.5 x 10 cm /s). In other words, a lipid undergoes a displacement roughly equivalent to 1 A/ns. In a standard bio-membrane the average distance between lipids at the surface is around 7-8 A, i.e., the time necessary to simply exchange the location of two neighbour lipids will be in the 10-100 ns range (Figure 9.13). 

Table 9.1 A comparison of dynamical properties of lipids. Relaxation times of the wobbling motion of the hydrophobic chain and the rotational motion of the headgroup (PN vector) and lateral diffusion coefficient, D.

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. 

Fluorescence recovery after photobleaching has been used to determine lateral diffusion in block copol5uner bilayers (Fig. 21). The experiments 5deld a time constant x from which the lateral diffusion coefficient D = P/2x is calculated. In the fluid La phase, lipid molecules have lateral diffusion coefficients of... 

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.

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]. 

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