Membranes comparison with experiment

Bader, S. (2003) Modelling of chemical osmosis across clay membranes comparison with experiments, submitted to Advances in Water Resources... 

A consideration relevant for later comparison of the results of simulations to experimental data concerns the experimental accuracy. The transport coefSc-ients are notoriously difficult to measure and the steady state flux method outlined above is not very precise large values for the transport coefficients are easier to determine and reported values are often of respectable precision, but even for high values (e,g., D 10 cmVs) the accuracy is typically not better than 10-20%, while it decreases for barrier membranes (e.g., D 10 cm /s and smaller) to that of an order-of-magnitude estimate. Hence, comparison with experiments will be made taking account of these variabilities. 

For emulsion liquid membrane systems, a model which contains six differential and algebraic equations is developed by Huang et al in Chapter 8. The model takes into account five steps in the transport process. For comparison with experiment, the arsenic concentration in the external phase versus time is measured for the removal of arsenic from water in an ELM system. Excellent correlation of the experimental data with the theoretical predictions is obtained. 

To date, a number of simulation studies have been performed on nucleic acids and proteins using both AMBER and CHARMM. A direct comparison of crystal simulations of bovine pancreatic trypsin inliibitor show that the two force fields behave similarly, although differences in solvent-protein interactions are evident [24]. Side-by-side tests have also been performed on a DNA duplex, showing both force fields to be in reasonable agreement with experiment although significant, and different, problems were evident in both cases [25]. It should be noted that as of the writing of this chapter revised versions of both the AMBER and CHARMM nucleic acid force fields had become available. Several simulations of membranes have been performed with the CHARMM force field for both saturated [26] and unsaturated [27] lipids. The availability of both protein and nucleic acid parameters in AMBER and CHARMM allows for protein-nucleic acid complexes to be studied with both force fields (see Chapter 20), whereas protein-lipid (see Chapter 21) and DNA-lipid simulations can also be performed with CHARMM. 

Experimental Results and Comparisons with the Classical Lipid Barrier Model. Some typical experimental data are presented in Figure 1 for the transport of g-estradiol. In each of the experiments a lag-time of 1.5 to 2.5 hours were followed by linear steady state fluxes. The effective permeability coefficient, Peff> was calculated from such data using Equation 1 under sink conditions (i.e., Cj/K Cr/Kr where, Kj is the partition coefficient between membrane and donor phase and Kr the partition coefficient between membrane and receiver phase.)... 

The dependence of the interaction force between two undulating phospholipid bilayers and of the root-mean-square fluctuation of their separation distances on the average separation can be determined once the distribution of the intermembrane separation is known as a function of the applied pressure. However, most of the present theories for interacting membranes start by assuming that the distance distribution is symmetric, a hypothesis invalidated by Monte Carlo simulations. Here we present an approach to calculate the distribution of the intermembrane separation for any arbitrary interaction potential and applied pressure. The procedure is applied to a realistic interaction potential between neutral lipid bilayers in water, involving the hydration repulsion and van der Waals attraction. A comparison with existing experiments is provided. 

To conclude, we presented a new method to account for the effect of the thermal fluctuations on the interactions between elastic membranes, based on a predicted intermembrane separation distribution. It was shown that for a typical potential, the distribution function is asymmetric, with an asymmetry dependent on the applied pressure and on the interaction potential between membranes. Equations for the pressure, root-mean-square fluctuation, and asymmetry as functions of the average distance (and the parameters of the interacting membranes) were derived. While no experimental data are available for two interacting lipid bilayers, a comparison with experimental data for multilayers of lipid bilayer/water was provided. The values of the parameters, determined from the fit of experimental data, were found within the ranges determined from other experiments. 

One of the conventional methods for establishing the existence of active transport is to analyze the effects of metabolic inhibitors. The second is to correlate the level or rate of metabolism with the extent of ion flow or the concentration ratio between the interior and exterior of cells. The third is to measure the current needed in a short-circuited system having similar solutions on each side of the membrane the measured flows contribute to the short-circuited current. Any net flows detected should be due to active transport, since the electrochemical gradients of all ions are zero (Ai// = 0, cD = c,). Experiments indicate that the level of sodium ions within the cells is low in comparison with potassium ions. The generalized force of chemical affinity shows the distance from equilibrium of the /th reaction... 

Clegg RM, Vaz WLC. Translational diffusion of proteins and lipids in artificial lipid bilayer membranes. A comparison of experiment with theory. In Progress in Protein-Lipid Interactions, Vol. I. Watts A, De Pont JJHHM, eds. 1985. Elsevier, Amsterdam, The Netherlands, pp. 173-229... 

Indeed, these situations can be observed in living cell structures. During electron microscopic observation of freeze-thawed cell nuclei, rapidly frozen specimens had the original tissue structure. When these specimens were thawed rapidly, no difference could be seen in comparison with the control. Only a minor increase in the affinity to dyes and a slight condensation of chromosome have been observed. On the contrary, when specimens are thawed slowly many large cavities are observed, which indicates that the cellular materials are forced out by ice crystals. In these specimens, a very serious rupture of nuclear membrane was also observed. Similar results have been obtained with freeze-fractured electron microscopic observation of rapidly frozen red blood cells. In these experiments, membrane structure had been damaged in the regions where ice crystals and red blood cell membranes were in close contact. 

Note that diffusion models and hydraulic permeation models have their own caveats the membrane is neither a homogeneous acid solution, nor is it the well-structured porous rock. Critical comparison of the results of the two approaches with each other and with experiments, is of crucial importance for understanding the membrane functioning within the cell and developing the strategies on water management and optimized membrane properties. 

Many substances cross biological membranes according to their lipid solubility. Other polar molecules, such as amino acids and glucose, cross the membranes more rapidly than expected according to their solubUity in lipids. Cations, such as Na" and K, also cross membranes rapidly in spite of their hydrophilic nature. This passive transport of substances at higher rates than predicted from their lipid solubility is termed facilitated diffusion. That proteins are directly involved in facilitated diffusion was shown by comparison of experiments with natural membranes and synthetic membranes produced with phospholipid films. With phospholipid films all molecules, except water, diffuse according to lipid solubility and molecular size. Ions are essentially impermeable. The addition of membrane proteins, however, frequently allowed many polar and charged species to penetrate the membrane at rates comparable to natural membranes. 

In experiments with amphotericin pores in lipid bilayer membranes (63) and with single macroscopic capillaries (64 66) filled with carefully purified aqueous solutions of potassium chloride, 1/f noise was found to be negligible in comparison with other noise components. The conclusion was that 1/f noise in electrolytes is absent with an upper limit on a of about 10 3. De Vos et al. (67) suggested that occasionally reported results on measurable 1/f noise in electrolytes (see, for instance, reference 68) stem from uncontrolled contamination of the samples by inclusions of solid, liquid, or gaseous phases. 

For comparison, similar experiments were conducted using a conventional fixed-bed reactor module. The membrane reactor module was replaced with a quartz tube and the reactor effluent was analyzed for products and reactants. In some cases, a conventional reactor was simulated by conducting experiments in the membrane reactor without sweeping by blocking off the shell side of the reactor. 

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