Membranes dynamic phase behavior model

The dynamic phase behavior model of Hazel emphasizes that the membrane must remain suitably poised between propensities for forming both bilayer (lamellar) and hexagonal II structures. Although excessive formation of hexagonal phases at high temperatures is disruptive of cellular function—and potentially of lethal consequence to the cell—under normal conditions cellular membranes must possess domains in which hexagonal II structures can be assembled. These structures are essential components of such normal membrane functions as membrane fusion during exo- and endocytosis and membrane traffick-... 

The lipidic cubic phase has recently been demonstrated as a new system in which to crystallize membrane proteins [143, 144], and several examples [143, 145, 146] have been reported. The molecular mechanism for such crystallization is not yet clear, but the interfacial water and transport are believed to play an important role in nucleation and crystal growth [146, 147], Using a related model system of reverse micelles, drastic differences in water behavior were observed both experimentally [112, 127, 128, 133-135] and theoretically [117, 148, 149]. In contrast to the ultrafast motions of bulk water that occurs in less than several picoseconds, significantly slower water dynamics were observed in hundreds of picoseconds, which indicates a well-ordered water structure in these confinements. 

As an example of a membrane model, phospholipid monolayers with negative charge of different density were used. It had already been found ( ) and discussed O) that the physical and biological behavior of phospholipid monolayers at air-water interfaces and of suspensions of liposomes are comparable if the monolayer is in a condensed state. Two complementary methods of surface measurements (using radioactivity and electrochemical measurements), were used to investigate the adsorption and the dynamic properties of the adsorbed prothrombin on the phospholipid monolayers. Two different interfaces, air-water and mercury-water, were examined. In this review, the behavior of prothrombin at these interfaces, in the presence of phospholipid monolayers, is presented as compared with its behavior in the absence of phospholipids. An excess of lipid of different compositions of phos-phatidylserine (PS) and phosphatidylcholine (PC) was spread over an aqueous phase so as to form a condensed monolayer, then the proteins were inject underneath the monolayer in the presence or in the absence of Ca. The adsorption occurs in situ and under static conditions. The excess of lipid ensured a fully compressed monolayer in equilibrium with the collapsed excess lipid layers. The contribution of this excess of lipid to protein adsorption was negligible and there was no effect at all on the electrode measurements. 

The accumulation and distribution of licpiid water in the polymer electrolyte membrane fuel cell (PEMFC) is highly dependent on the porous gas diffusion layer (GDL). The accmnulation of liquid water is often simply reduced to a relationship between liquid water saturation and capillary pressure however, recent experimental studies have provided valuable insights in how the microstmcture of the GDL as well as the dynamic behavior of the liquid play important roles in how water will be distributed in a PEMFC. Due to the importance of the GDL microstmcture, there have been recent efforts to provide predictive modeling of two-phase transport in PEMFCs including pore network modehng and lattice Boltzmann modeling, which are both discussed in detail in this chapter. Furthermore, a discussion is provided on how pore-scale infonnation is used to coimect microstmcture, transport and performance for macroscale upscaling. 

Computer simulations of both equilibrium and dynamic properties of small solutes indicate that the solubility-diffusion model is not an accurate approximation to the behavior of small, neutral solutes in membranes. This conclusion is supported experimentally [57]. Clearly, packing and ordering effects, as well as electrostatic solute-solvent interactions need to be included. One extreme example are changes in membrane permeability near the gel-liquid crystalline phase transition temperature [56]. Another example is unassisted ion transport across membranes, discussed in the following section. 

Numerous models are based on statistical mechanics, molecular dynamics, and other types of macroscopic phenomena. These models are valuable because they provide a fundamental understanding of behaviors of related species and of conduction through different proton-water complexes. Almost all microscopic models treat the membrane as a two-phase system. Although these models provide valuable information, they are usually too complex to be integrated into an overall fuel cell model. How the membrane structure changes as a function of water content is still under investigation and unclear. 

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