Lipid translational diffusion coefficient

The results indicate that diffusion and accumulation of receptors in the contact zone is the rate-determining step and a diffusion coefficient was calculated for the receptor accumulation of between 1 and 2 x 10 m s , in close agreement with that for the lipid translational diffusion coefficient measured in vesicle membranes [10], thereby supporting earlier theoretical predictions [11]. 

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). 

Table 1 Translational diffusion coefficients of lipids and order parameters in some membrane phases...

The fluidity of LB monolayers on solid supports was characterized by measuring the long-range (> 10 xm) translational diffusion of dissolved fluorescent lipids using fluorescence pattern photobleaching recovery (FPPR). In this technique, an area of the membrane is illuminated with a spatially striped intensity. After irreversibly photobleaching, fluorescence recovery occurs, as unbleached molecules from nonilluminated stripes move into the illuminated stripes . In fluid-like LB films, translational diffusion coefficients... 

A theoretical prediction for the receptor translational diffusion coefficient Dr based on viscous interactions of proteins with membrane lipids was derived by Saffman and Delbruck (1975). In their hydrodynamic model, a solitary cylindrical protein (i.e., a protein at infinite dilution) of radius s embedded in a membrane of thickness h and viscosity 17 m,... 

Fluorescein-labeled proteins are also used to measure the translational mobility of proteins and lipids by the Fluorescence Recovery After Photo-bleaching technique [54-59]. The uniformly labeled fluorescent sample is flashed with an intense light source to bleach a spot, thus producing a concentration gradient. The rate of recovery of fluorescence in that bleached area is measured and used to calculate the diffusion coefficient of the probe dye into the bleached zone. Such diffusion coefficient measurements have been used to determine the association constants of proteins in cells [60], to measure the exchange of tubulin between the cytoplasm and the microtubules [61,62], to study the polymerization-depolymerization process of actin [63-65] and to monitor the changes that occur upon cell maturation [66,67]. 

The theoretical description of translational diffusion in a lipid bilayer depends on the size of the diffusing particle. Theoretical descriptions based on fluid hydrodynamic theory (51, 52) have been shown to be applicable to particles whose radius in the plane of the bilayer is significantly larger than the radius of the lipid molecules that constitute the bilayer, in which case the diffusion coefficient may be given by ... 

The diffusion coefficients and translational movements of proteins are important in considering the release of proteins from hydrogel matrix devices and other delivery vehicles, and in membrane transport, as far as this can be considered to be a passive diffusion process. Changes in shape during membrane transport in a lipid environment may also have to be considered. Table 11.6 gives some values of diffusion coefficient of a number of therapeutic peptides and proteins. 

Recently, we have shown precisely by using fluorescence polarization and 270 MHz NMR methods, that methylmercury chloride is not particularly lipid soluble. In fact it has a partition coefficient lipid/water of about 2, but a translational diffusion rate of 20 x 10 9 seconds (1,2). In other words it diffuses rapidly, but it does not partition very well in membranes. 

Under normal conditions the membrane bilayer is in a fluid state. Membrane proteins can migrate within the plane of the membrane with diffusion coefficients of about 10 cm sec while lipids diffuse with coefficients of about 10 cm sec . Overall behaviour might be considered, therefore, in thermodynamic terms. But generalized deductions relating fluidity of the membrane to enzyme activity are difficult to make for several reasons. For example, motion in a given lipid molecule may include rapid rotations but slow lateral movement. Also, increased disorder in a bilayer may not correlate with increased translational motion. Moreover, all membranes so far examined have shown transbilayer asymmetry while there is evidence in several cases for at least small areas of concentration of certain lipids, i.e. micro-lateral heterogeneity. These sorts of consideration complicate the interpretation of experiments designed to show how the bilayer lipids affect membrane enzyme activities at a molecular level. 

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