Permeation paths

Rgure 6.6. The potential energy (Ep) along the permeation path of two molecules of different sizes representing hydrogen and methane reproduced from Koresh and Soffer (1987). 

Crystallographic studies led to the high-resolution structure of KcsA by Doyle et al. (1998b), an achievement that lent a firm structural foundation to more than three decades of functional work on K+ channels. The crystal structure revealed that KcsA is formed by the association of four subunits, contributing equally to form a water-filled pore. Each subunit has two transmembrane segments, TM1 in the periphery of the complex and TM2 lining the permeation path. Toward the extracellular face of the channel is the selectivity filter, where intimate contact with the permeant ions takes place. 

Johnson JP, Jr., Zagotta WN. The carboxyl-terminal region of cyclic nucleotide-modulated channels is a gating ring, not a permeation path. Proc Natl Acad Sci USA 2005 102 2742-7. 

The permeability of the cracked structure is highly dependent on the existence of open cracks, and open cracks can be major permeation paths. Here, shape of open cracks is approximated by elliptic, and one dimensional water permeation in the elliptic tube is assumed (Figure 2). In the case of single open crack problem, one open crack exists in the unit cell. The flow rate of water of a unit cell Au is given as a function of maximum crack... 

Figure 7.5 Permeation paths through unfilled and filled rubber...

Filler materials render barrier properties to composite systems by forcing the gas molecules to travel around the filler particles, thus making the permeation path tortuous compared to a monolithic polymer. For thin, plate-Uke fillers for example, inorganic clay platelets, GO sheets, and so on, the path can be depicted as shown in Figure 8.8 (Nielsen, 1967). 

By substituting the position-dependent terms in Equation 10.3 and Equation 10.4 into Equation 10.2, the permeability coefficient can be obtained by integrating along the permeation path ... 

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