Hydrated membrane/channel system

Perhaps the most appealing example of a hydration-dehydration molecular system is that of an ionic channel across a low dielectric lipid bilayer or cell membrane. With reconstitution methods, it is possible to observe one molecular channel or conducting unit at a time that is, to see the protein switching between open and closed states. 

Four metal cations, Na+, K+, Mg2+ and Ca2+, are of primary importance. Na+ and K+ are ubiquitous in biological systems where they are employed to control membrane potential as well as intracellular levels of hydration. Several types of Na+ and K+ transporting channels are known to exist and some, notably the inwardly... 

The air management system is shown in Fig. 7.1b. A side channel compressor is used for low pressure experiments (below 130 kPa), while a centralized air compression plant is used to study the effect of ah pressure on stack performance (between 130 and 250 kPa). An important issue to be considered is the cell humidification to guarantee that the stack works properly, since the electrolyte membrane needs to be continuously hydrated (see Sects. 3.2 and 4.5). That humidification is... 

Efforts to stabilize BLMs by the use of polymerizable lipids have been successful, but the electrochemical properties of these membranes were greatly compromised and ion channel phenomena could not be observed [21]. Microfiltration and polycarbonate filters, polyimide mesh, and hydrated gels have been used successfully as stabilizing supports for the formation of black lipid films [22-25] and these systems were observed to retain their electrical and permeability characteristics [24]. Poly(octadec-l-ene-maleic anhydride) (PA-18) was found to be an excellent intermediate layer for interfacing phospholipids onto solid substrates, and is sufficiently hydrophilic to retain water for unimpeded ion transfer at the electrode-PA-18 interface [26]. Hydrostatic stabilization of solventless BLMs has been achieved by the transfer of two lipid monolayers onto the aperture of a closed cell compartment however, the use of a system for automatic digital control of the transmembrane pressure difference was necessary [27]. 

For a vapor-equilibrated membrane (i.e., one that is in contact with water vapor only), the physical model proposes that there is water in the ionic domains but none in the collapsed channels except for the bound water hydrating the few sulfonic acid sites present. Furthermore, the sulfonic acid sites that make up the collapsed channels are always fluctuating, but the elusters are elose enough together to form a transport pathway after the pereolation threshold has been reached. Due to the nature of the collapsed ehaimels, the membrane is treated as a homogenous single-phase system. In this sense, the water vapor does not penetrate into the cluster-network, but instead dissolves into the membrane. Thus, the vapor-equilibrated membrane transport mechanism is similar to the single-phase transport models mentioned previously. 

For the transport properties, the conductivity is explained based on the pereolation eoneept where the percolation threshold occurs around X = 2 and is correlated with enough clusters being hydrated and connected by hydrated sulfonie acid sites to form a complete conductive pathway across the membrane [34,41]. With the addition of more water into the system, more elusters and channels form and the pathways become less tortuous the eonductivity increases. The electro-osmotic coefficient depends on the type... 

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