Diffusion lipid

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

The result of a typical diffusion measurement is shown in Figure 2. In the 1H-NMR spectrum of a cubic phase of monoolein and 2H20 with 10% Desmopressin, the signals from the aromatic residues (Tyr and Phe) in Desmopressin, appear in a spectral region which does not contain any signals from the lipid. Therefore, the peptide and lipid diffusion coefficients could be determined separately (Table II), and in Figure 2 the spectra from such an experiment are shown. The lipid diffusion coefficient was also determined in a cubic phase in the absence of Desmopressin. 

The Desmopressin diffusion coefficient in the cubic phase at 40 C (D=0.24 x 10-10 m2s-l) is about a factor 9 smaller than in 2H20-solution at 25 C (D=2.25 x 10-10 m s" ), a difference which is larger than what is expected from pure obstruction effects a reduction factor of three is expected from the inclusion of a solute in the water channels of the cubic phase (13). Thus, the results indicate an interaction between the peptide and the lipid matrix and/or membrane surface, especially since the peptide and lipid diffusion coefficients are very similar in the cubic phase (Table... 

Using preformed pores in a DMPC bilayer, Gurtovenko and Vattulainen [82] investigated the translocation of DMPC across a pore. It was shown that multiple lipids diffused across the pore before it dissipated, providing support for pore-mediated flip-flop as mechanism for passive flip-flop. The timescale for pore dissipation was found to be 35-200 ns, at the limits of current computational capability for equilibrium simulations. 

Effect of Ionization on Lipid Diffusion. Passive lipid diffusion of certain drugs is also dependent on whether or not the drug is ionized. Drugs will diffuse more readily through the lipid layer if they are in their... 

Lipids diffuse freely in fluid model membranes. FRAP measurements show full recovery and diffusion coefficients on the order of magnitude of 10 cm /sec. Free diffusion with a similar rate is often observed for lipids in the biomembrane. However, many cell membrane proteins show lower diffusion rates and incomplete recovery after photobleaching. For membrane proteins, dramatically different behavior in model and biological membranes is a common case. In model membranes, membrane proteins also diffuse freely and their diffusion coefficients are often similar to the diffusion coefficients of lipids. On the contrary, in biomembranes, the diffusion of proteins is 2-3 orders of magnitude slower and the fluorescence recovery is often incomplete. This observation points to limitations of the fluid mosaic model as will be discussed below. 

Another aspect of lipid-protein interaction that is conve-luently studied in supported bilayers is the lateral diffusion of proteins and lipids and their influence on each other. The regulatory lipid phosphatidyl-inositol-biphosphate (PIP2) slows the diffusion of syntaxin in supported bilayers (37). Conversely, increasing syntaxin concentrations decrease the diffusion of PIP2 and to a lesser extent that of phosphatidylserine. In another system, in which the transmembrane domain of the fibroblast growth factor receptor was incorporated into supported bilay-ers, lipid and protein diffusion were measured (60). Although protein diffusion was slow (0.006 ttm /s), lipid diffusion was fast (2.6 ttm /s). 

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mammalian plasma membranes. Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (transverse diffusion, or flip-flop) is usually very slow. Proteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a... 

Lipid diffusion. What is the average distance traversed by a membrane lipid in 1 is, 1 ms, and 1 s Assume a diffusion coefficient of 10 cm s i. 

Lipids diffuse laterally (horizontally) rapidly but transversely (vertically) slowly. Proteins diffuse laterally. 

NMR is a valuable technique in the analysis of lipid phases. More specifically, proton, deuterium, carbon-13, fluorine-19, and phosphorus-31 NMR have been utilized for analysis of the dynamic and motional properties of lipids, lipid diffusion, ordering properties, head-group hydration, lipid asymmetry, quantitation of lipid composition, and head-group conformation and dynamics. Cullis et al. and Gruner et al. have shown the importance of P-31 NMR as a tool in the determination of phase properties and lipid asymmetry and the identification of bilayer, hexagonal, and isotropic phases. 

To address these questions, we follow a simplified spherical macroion that is allowed to move concomitantly with lipid diffusion. To do so, we extend our model to include protein diffusion and performed CH-DMC (see Section 2.4) calculations. We studied the same mixed membranes considered in Figure 4, focusing on two typical cases. In the first, the model protein has a diffusion constant much larger than that of lipids in the unperturbed (bare) membrane, with a ratio D = 10 between the two, while in the second, the diffusion constant is comparable to that of the lipids, and D = 2 (see Eq. 10). As we show, these two scenarios lead to different lipid and protein diffusion characteristics. 

Wolf, D.E., Hagopian, S.S., and Isogima, S. (1986). Changes in sperm plasma membrane lipid diffusibility after hyperactivation during in vitro capacitation in the mouse. J. Cell Biol. 

Deverall MA, Garg S, Ltidtke K, Jordan R, Rtihe J, Naumann CA (2008) Transbilayer coupling of obstructed lipid diffusion in polymer-tethered phospholipid bilayers. Soft Matter 4 1899-1908... 

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