Hydrophilic pore

Due to the issues mentioned here and the fact that, in fuel cells the wetting property may change within the DL, it is necessary to have another technique that can differentiate between hydrophobic and hydrophilic pores within a sample. This information would allow researchers to improve the design of diffusion materials. 

As sfafed previously, fhe capillary pressure dafa, fhe overall pore disfribu-tion (when ocfane is fhe working fluid), and fhe hydrophilic pore distribution (when water is the working fluid) can be obfained through this technique. In addition, these measurements can be used with different compression pressures of fhe sample DL and wifh a wude range of temperatures inside the system [200,201]. For more information regarding this technique, please refer to fhe paper by Volfkovich ef al. [198]. 

Aquaporins help water to pass through biological membranes. They form hydrophilic pores that allow H2O molecules, but not hydrated ions or larger molecules, to pass through. Aquaporins are particularly important in the kidney, where they promote the reuptake of water (see p. 328). Aquaporin-2 in the renal collecting ducts is regulated by antidiuretic hormone (ADH, vasopressin), which via cAMP leads to shifting of the channels from the ER into the plasma membrane. 

Using a similar approach, Notman et al. [81], determined the free energy for pore formation in bilayers composed of ceramide, as a model for the stratum corneum of the skin, both in the presence and in the absence of DMSO. Without DMSO, the bilayer was in the gel phase, and interestingly, a hydrophobic pore was observed with a high free-energy barrier ( 60 kj/mol). In the presence of DMSO, the bilayer was more fluid, and the more typical hydrophilic pore was observed, with a much smaller activation energy of 20kJ/mol. This work provided a thermodynamic and structural explanation for the enhanced permeability of skin by DMSO. 

Kandasamy, S.K., Larson, R.G. Cation and anion transport through hydrophilic pores in lipid bilayers. J. Chem. Phys. 2006, 125, 074901. 

Leontiadou, H., Mark, A.E., Marrink, S.J. Molecular dynamics simulations of hydrophilic pores in lipid bilayers. Biophys. J. 2004, 86, 2156-64. 

Figure 3. A supramolecular arrangement of a helices of voltage-gated Na" channel proposed by Numa [Noda et al. (1984)). Four helix bundles of segment 2 from each repeat unit constitute the wall of hydrophilic pore through which ions pass. All the other helices 1, 3-6, cover the pore and stabilize it in bilayer membranes. The ion pair 1 is a mimic from oligoether carboxylate and dioctadecyidimethylammonium representing hydrophilic helix 2 and all the other hydrophobic helices 1 and 3-6, respectively.

Cyclodextrin-substituted molecular channel approaches have now been extended to include acyl substituents through a covalent bond formation. Stearoyl and methyl cholate-substituted cyclodextrins 10 and 11, respectively, have been synthesized. It may be worthwhile commenting on the molecular design of methyl cholate-substituted a-cyclodextrin. All of the ether groupings are convergent at the inner side of the steroidal backbone of a bent structure to make the molecule amphiphilic. Once the cyclodextrin derivative is incorporated into the membrane phase, it may easily be expected that the ether parts are assembled inside the channel in the sea of hydrophobic lipid molecules and the hydrophobic steroidal skeletons cover its outside to stabilize the inner hydrophilic pore (Figure 13). 

Ion channels provide hydrophilic pores through which select ions can diffuse, moving down their electrical or chemical concentration gradients they are characteristically unsaturable and have very high flux rates. 

As long as fuel cells are using liquid electrolytes like phosphoric acid or concentrated caustic potash, the catalyst utilization is usually not limited by incomplete wetting of the catalyst. Provided the amount of electrolyte is sufficiently high, the hydrophilic porous particles are not only completely flooded but due to their expressed hydrophilicity are wetted externally by an electrolyte film that together with the whole electrolyte-filled hydrophilic pore system establishes the ionic contact of an electrode to the respective counterelectrode. 

Metal cations dissolve in water and are said to be hydrophilic ( water loving ). Cell membranes exclude water and are described as hydrophobic ( water hating ). Gramicidin A lodges in the cell membrane because the outside of the molecule is hydrophobic. Na+ and K+ pass through each hydrophilic pore at a rate of 107 ions/s. The pore is selective for monovalent cations it excludes anions and more highly charged cations. 

Figure 6. Schematic representation of glucose in the pore of the facilitative glucose transporter. The amphipathic a-helices of the membrane spanning domains are thought to form a hydrophilic pore through which glucose moves via hydrogen bonding to amino acids. The importance of the hydrogen bonds at positions 1,3, and 6 of the glucose molecule for efficient transport (see text) is shown.

Rzayev J, Hillmyer MA (2005) Nanoporous polystyrene containing hydrophilic pores from ABC triblock copolymer precursor. Macromolecules 38 3-5... 

The pharmacokinetics of PVP is well understood as a result of this agent s experimental use to determine the properties of pores in biological membranes. PVP molecules can readily penetrate hydrophilic pores in membranes if they are small enough, and they are also taken up by pinocytotic vesicles. Apparently, PVP is not detectably bound to membrane surfaces and hence does not provide long-lasting viscosity enhancement beyond the normal residence time in the tears. 

FIGURE 26.3 Schematic model for hydrophobic and hydrophilic pore formation hy electroporation. 

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