Membrane, 5-region model

The above iso-pH measurements are based on the 2% DOPC/dodecane system (model 1.0 over pH 3-10 range). Another membrane model was also explored by us. Table 7.16 lists iso-pH effective permeability measurements using the soy lecithin (20% wt/vol in dodecane) membrane PAMPA (models 17.1, 24.1, and 25.1) The negative membrane charge, the multicomponent phospholipid mixture, and the acceptor sink condition (Table 7.1) result in different intrinsic permeabilities for the probe molecules. Figure 7.40 shows the relationship between the 2% DOPC and the 20% soy iso-pH PAMPA systems for ketoprofen. Since the intrinsic permeability of ketoprofen in the soy lecithin membrane is about 20 times greater than in DOPC membrane, the flat diffusion-limited transport region of the log Pe... 

In the classic model of synaptic vesicle recycling in nerve terminals, synaptic vesicles fuse completely with the plasma membrane and the integrated vesicle proteins move away from the active zone to adjacent membrane regions (Fig. 9-9A). In these regions, clathrin-mediated synaptic vesicle endocytosis takes place rapidly after neurotransmitter release (within seconds) [64]. The process starts with the formation of a clathrin-coated pit that invaginates toward the interior of the cell and pinches off to form a clathrin-coated vesicle [83]. Coated vesicles are transient organelles that rapidly shed their coats in an ATP/chaperone dependent process. Once uncoated, the recycled vesicle fuses with a local EE for reconstitution as a synaptic vesicle. Subsequently, the recycled synaptic vesicle is filled with neurotransmitter and it returns to the release site ready for use. This may be the normal pathway when neurotransmitter release rates are modest. Clathrin/ EE-based pathways become essential when synaptic proteins have been incorporated into the presynaptic plasma membrane. 

The effect of ellipticine derivatives on membranes and model membranes continues to be of interest to Sautereau and co-workers (184-186), who included 3ip-NMR techniques in their study (185). The ellipticine derivatives, such as 5, are deeply embedded in the acyl chain region of cardiolipin-containing model membranes. Sautereau et al. (186) studied the effects of elliptinium (5) on Streptococcus pneumoniae and concluded that the toxicity of 5 is related to its intracellular concentration. 

By "inert it means that the membrane is a separator but not a catalyst. Many membrane reactor modeling studies consider only those cases where the membrane is catalydcally inert and the catalyst is packed most often in the tubular (feed) region but sometimes in the annular (permeate) region. When it is assumed that no reaction takes place in the membrane or membrane/support matrix, the governing equations for the membrane/support matrix are usually eliminated. The overall eff ect of membrane permeation can be accounted for by a permeation term which appears in the macroscopic balance equations for both the feed and permeate sides. Thus, the diffusional gradient term... 

The above discussions pertain to models assuming three regions the dense phase, bubble phase and separation side of the membrane. The membrane is assumed to be inert to the reactions. There are, however, cases where the membrane is also catalytic. In these situations, a fourth region, the membrane matrix, needs to be considered. The mass and heat balance equations for the catalytic membrane region will both contain reaction-related terms. 

The membrane region is then split into two sub-regions one region where the concentration of B is zero and the other region where B is present as the case in the model presented above. The more general model of Harold et al. [1989] can be extended to account for the depletion of B by considering the aforementioned two sub-regions. 

In a somewhat similar paper, diffusion through a 2D porous solid modeled by a regular array of hard disks was evaluated [65] using non-equilibrium molecular dynamics. It was found that Pick s law is not obeyed in this system unless one takes different diffusion constants for different regions in the flow system. Other non-equilibrium molecular dynamics simulations of diffusion for gases within a membrane have been presented [66]. The membrane was modeled as a randomly... 

Figure 3. Two models describing the microphases of swollen Nation membranes. Top Gierke s [48] suggestion of aqueous inverse spherical micelles connected by water-filled cylindrical channels. Bottom Yeager and Steck s [49] three-region model of a water/ionomer mixture without regular structure. Regions A, B and C are the hydrophobic polymer, the solvent bridges and the hydrophilic regions, respectively.

Figure 29.1a shows the density profile of each segment along the normal to the membrane. The distribution of the lipid headgroups has two peaks at z = 2a. We show free energy profile AG(z) for the permeation of a water particle in Fig. 29.1b. AG(z) has a positive value in the membrane region of —2a < z < 2a and maximum value of 6 kcal/mol (>s) at the center of the membrane z = 0. Similar results have been found in atomistic simulations [7-13] the free energy barrier for the water permeation was found to be 6-20 kcal/mol around the center of the membrane. However, taking into account the fact that the CG water particle represents to 42 water molecules, the present CG model underestimates the free energy barrier significantly.

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