Pore Mechanism

For hydrophilic and ionic solutes, diffusion mainly takes place via a pore mechanism in the solvent-filled pores. In a simplistic view, the polymer chains in a highly swollen gel can be viewed as obstacles to solute transport. Applying this obstruction model to the diffusion of small ions in a water-swollen resin, Mackie and Meares [56] considered that the effect of the obstruction is to increase the diffusion path length by a tortuosity factor, 0. The diffusion coefficient in the gel, )3,i2, normalized by the diffusivity in free water, DX1, is related to 0 by... 

Solute diffusion through hydrogels has been described in terms of two basic mechanisms, the partition and pore mechanisms. The partition mechanism... 

If the pore-mechanism applies, the rate of permeation should be related to the probability at which pores of sufficient size and depth appear in the bilayer. The correlation is given by the semi-empirical model of Hamilton and Kaler [150], which predicts a much stronger dependence on the thickness d of the membrane than the solubility-diffusion model (proportional to exp(-d) instead of the 1 Id dependence given in equation (14)). This has been confirmed for potassium by experiments with bilayers composed of lipids with different hydrocarbon chain lengths [148], The sensitivity to the solute size, however, is in the model of Hamilton and Kaler much less pronounced than in the solubility-diffusion model. 

The selectivity ratio K+ Na+ at 25 °C is of the order 104 1 whereas at 0°C it is reduced to only 2 1.543 This dramatic decrease has been interpreted as indirect evidence for the possible role of (137) as a carrier in membranes as, if it provided a pore mechanism, this would be unimpaired by freezing. In a carrier mechanism which necessarily involves a mobile ligand to effect incorporation and transfer of the metal freezing would cause a loss of mobility and so impair the mechanism. The high K+ selectivity of (137) has led to its employment in ion-selective electrodes.543... 

Proposed more than 20 years ago, the stalk intermediate—a highly curved lipid stmcture— provides the most plausible description of the initial fusion stage currently available. The related stalk-pore mechanism (23-25) of fusion is viewed favorably by most researchers. It shows the close relation between fusion and the transition from lamellar into bilayer cubic and hexagonal phases (see Fig. 4 in the section entitled Formation of nonlamellar phases in membrane lipids ). Studies on the rhombohedral phase formed in partially dehydrated lipids provide another insight into the possible structure of fusion stalks (26). 

To understand the function of membrane-active peptides, it is important to know the structure and orientation of the peptide in the membrane. As is evident from Figure 18.1, it is possible to distinguish between, for example, carpet and pore mechanisms of action by determining the peptide s orientation in the membrane. Various techniques, such as electron spin resonance (ESR) [35], infrared (IR) spectroscopy [36-38], circular dichroism (CD) [35, 39,40], and solid-state NMR (SSNMR) [4-7] are used to investigate membrane-active peptides in a quasi-native lipid bilayer environment. In the following sections, methods to determine peptide structure and orientation are presented. 

In a previous publication (7j it was concluded that hydro-phobic solutes, such as progesterone, permeate p-HEMA primarily via the "pore" mechanism. However, for progesterone in p-HEMA with 5.25 mole % EGDMA, it was found that the "partition" mechanism dominates permeation. In this mechanism, it is presumed that the solutes permeate by dissolution and diffusion within the macromolecular segments of the polymer backbone. In the "pore" mechanism Kq values are expected to be less than one and reflect... 

The relatively high fractions of Db/Pt for all steroids suggest that permeation through p-HEMA membrane is dominated by the "pore" mechanism. The high Kd values are consistent with the proposed model. According to the model and data obtained in the p-HEMA membrane, partitioning of hydrophobic solutes is governed predominantly by A type domains. Solute within these domains makes a small contribution to permeability. Solute permeation is dominated by the "pore" mechanism. 

In conclusion, 1) Hydrophilic solutes permeate p-HEMA and p-HEMA crosslinked with lower mole % EGDMA via the "pore" mechanism. The diffusion coefficients of the solutes depend on the molecular size and may utilize the "bulk-like" water in the hydrogels. As the water content of hydrogel increases, the solute permeability increases. 2) Hydrophobic solutes permeate p-HEMA and p-HEMA crosslinked with EGDMA via either the "pore" or "partition" mechanisms. Diffusion coefficients are lower than those of hydrophilic solutes however, steroids can permeate even in p-HEMA with 5.25 mole % EGDMA due to the predominant "partition" mechanism for hydrophobic solute permeation in this membrane. Hydrophilic solutes fail to permeate the high crosslinked hydrogels. 

Figures 32(a and b) show typical microscopic pictures of FFC on polymer-coated iron, and aluminum. FFC develops in the presence of pores, mechanical defects, unprotected cut edges, or residual salt crystals underneath the organic coating. The corrosion filaments start growing perpendicular from a defect into the polymer-coated area. FFC occurs only at moderate humidity (60-95%) and therefore, not under full immersion conditions. FFC has been found to be triggered by anions such as chloride, bromide, and sulfate. The filament growth rate increases with temperature. Like for cathodic delamination on iron and zinc the corrosion kinetics depend strongly on the surface pretreatment and coating composition.

Table 5-2. Moisture movement in solid(s) with capillary size pores. Mechanisms of moisture movement. Representation according to [0.4, 5.1].

Because of their advanced level of development, high sensitivity, and broad applicability, fluorescence spectroscopy with labeled LUVs and planar bilayer conductance experiments are the two techniques of choice to study synthetic transport systems. The broad applicability of the former also includes ion carriers, but it is extremely difficult to differentiate a carrier from a channel or pore mechanism by LUV experiments. However, the breadth and depth accessible with fluorogenic vesicles in a reliable user-friendly manner are unmatched by any other technique. Planar bilayer conductance experiments are restricted to ion channels and pores and are commonly accepted as substantial evidence for their existence. Exflemely informative, these fragile single-molecule experiments can be very difficult to execute and interpret. Another example for alternative techniques to analyze synthetic transport systems in LUVs is ion-selective electrodes. Conductance experiments in supported lipid bilayer membranes may be mentioned as well. Although these methods are less frequently used, they may be added to the repertoire of the supramolecular chemist. 

The essential point of the model which Lindemann has suggested for saturation and overshooting relaxation is the assumption that the Na -pores have two sites or entries for Na one for transport of Na across the membrane by a pore mechanism and a second one which closes the pore if occupied by a Na -ion. The network of this model simply reads... 

Badly broken bones are often repaired through the use of traditional, bioinert materials, such as titanium alloys. The metals are used to either hold broken bones in place to allow union or hold a graft in place. Porous metals can also be used to anchor materials in place, by allowing bone to grow into the pores (mechanical fixation). However, no material is completely bioinert Those that do not cause toxicity, do not degrade, or do not bond with tissue are termed nearly inert. Nearly inert materials do trigger a response fi om the immune system, in which fibrous scar tissue forms around the implant, isolating it from the body. 

Fang, Q. H., Berberian, K., Gong, L. W. et al. 2008. The role of the C terminus of the SNARE protein SNAP-25 in fusion pore opening and a model for fusion pore mechanics. Proc. Natl. Acad. Sci. U.S.A. 105 15388-15392. 

Flux of PEG through IPN was reduced by a factor of 10 with a change of pH from 2.5 to 7.0. Furthermore, rejection of solute through the complexed membrane was higher than seen with the uncomplexed membrane. The change in flux was attributed to a "pore" mechanism similar to that described by Osada. 

---