Water channel models

The simple water charmel models can explain the ionomer peak and the small-angle upturn in the scattering data of fhe unoriented samples as well as of the oriented films. Interestingly, the helical structure of backbone segments is responsible for fhe sfabilify of fhe long cylindrical charmels. The self-diffusion behavior of wafer and protons in Nation is well described by the water channel model. The existence of parallel wide channels af high wafer uptake favors large hydrodynamic confributions to electro-osmotic water transport and hydraulic permeation. 

Neuman RD, Ibrahim TH (1999) Novel structural model of reversed micelles The open water-channel model. Langmuir 15 10-12... 

Fig. 6.4 Inverted micelles model by Gierke (top left), polymer bundles model (top right), and parallel water-channel model (bottom) (Reproduced from Refs. [16, 20] with pmnissiffli of the American Chemical Society (copyright 2004) and Nature Publishing Group)...

Secretory epithelia control transport of water and solutes from the subluminal compartment (blood) into the lumen or body exterior. At present, there is no single unifying model for transepithelial fluid or water transport. In some epithelia, transcellular routes of fluid transport via water channels may predominate [88a], However, in other types of epithelia, such as the cervical-vaginal epithelia, transport of fluids usually occurs via the paracellular route [1, 14], In the latter, movement of fluid can be driven by three main mechanisms (Figure 15.1C) ... 

Weber and Newman do the averaging by using a capillary framework. They assume that the two transport modes (diffusive for a vapor-equilibrated membrane and hydraulic for a liquid-equilibrated one) are assumed to occur in parallel and are switched between in a continuous fashion using the fraction of channels that are expanded by the liquid water. Their model is macroscopic but takes into account microscopic effects such as the channel-size distribution and the surface energy of the pores. Furthermore, they showed excellent agreement with experimental data from various sources and different operating conditions for values of the net water flux per proton flux through the membrane. 

As shown in Figure 16b, the 2-D rib models deal with how the existence of a solid rib affects fuel-cell performance. They do not examine the along-the-channel effects discussed above. Instead, the relevant dimensions deal with the physical reality that the gas channeFdiffusion media interfaces are not continuous. Instead, the ribs of the flow-channel plates break them. These 2-D models focus on the cathode side of the fuel-cell sandwich because oxygen and water transport there have a much more significant impact on performance. This is in contrast to the along-the-channel models that show that the underhumidification of and water transport to the anode are more important than those for the cathode. 

FIGURE 11-46 Structure of an aquaporin, AQP-1. The protein is a tetramer of identical monomeric units, each of which forms a transmembrane pore (derived from PBD ID 1J4N). (a) Surface model viewed perpendicular to the plane of the membrane. The protein contains four pores, one in each subunit. (The opening at the junction of the subunits is not a pore.) (b) An AQP-1 tetramer, viewed in the plane of the membrane. The helices of each subunit cluster around a central transmembrane pore. In each monomer, two short helical loops, one between helices 2 and 3 and the other between 5 and 6, contain the Asn-Pro-Ala (NPA) sequences found in all aquaporins, and form part of the water channel, (c) Surface representation of a single subunit, viewed in the plane of the membrane. The near side of the AQP-1... 

Jung J, Preston G M, Smith B, Guggino W, Agre P. 1994. Molecular structure of the water channel through aquaporin CHIP. The hourglass model. J Biol Chem 269 14648-14654. 

T. H. Ibrahim and R. D. Neuman. Nanostructure of open water-channel reversed micelles. i. H -NMR spectroscopy and molecular modeling. Langmuir, 20(8) 3114—3122, 2004. 

Tsang, Y.W., and C.F. Tsang. 1987. Channel model of flow through fractured media. Water Resour. Res. 

Examples showing that metal speciation is important to metal toxicity include arsenic, copper, selenium, and chromium. While ionic copper (Cu2+) and CuClj are highly toxic, Q1CO3 and Cu-EDTA have low toxicity (Morrison et al, 1989). Toxicity tests show that As(III) is about 50 times more toxic than As(VI). Trivalent chromium is much less toxic than hexavalent chromium, probably because Cr(VI) is much smaller and the chemical structure of chromate is similar to sulfate. A special channel already exists in biomembranes for sulfate transport. While modeling metal speciation is not always possible, and redox equilibrium is not achieved in all natural waters, geochemical modeling of equilibrium species distribution remains one of the methods of discerning metal speciation. 

P-08 - Channel model for the theoretical study of aspirin adsorption on clinoptilolite. Water influence... 

Note that many of the molecules produced have few internal polar fimctional groups to which ions may bind. Instead, it is more likely that ion-water-channel interactions escort the ion through the pore. To that end. many of the models can then be viewed as methods to pull water into the lipidic core of a bilayer membrane and thereby stabilize ions in transport. Recent studies of molecular dynamics simulations of ion transportation in human aquaporin-1 and in the bacterial glycerol facilitator GlpF revealed the key role of water in the stabilization of ions in transit and in the molecular selectivity of channels. Synthetic compoxmds form less-defined stmctures than these complex proteins but apparently act as efficiently as more complex natural materials. It is likely that continued study of synthetic systems will continue to reveal the general details underlying all transport processes. 

The size and size distribution of the water channels can be measured precisely by the modem instrument, which makes the water and solute transport theories based on the black box approach look more obsolete. A new transport model that matches the advancement of the modem characterization method is called for. As well, the precise control of the water channel size and its distribution, as well as the surface roughness, is required for the future membrane design. 

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