Protons, in membranes

As a matter of fact, the difference in understanding the role of protons in membrane phosphorylation reflects the existence of two different principal approaches to the problem of energy coupling in biomembranes that involve numerous concrete models. The first of them is most clearly expressed in the widely accepted Mitchell chemiosmotic concept [39, 40, 45, 46]. According to Mitchell s postulate, X represents a transmembrane difference in the electrochemical potential of hydrogen ions, A/Ih+- This value can be presented in the form of the sum of two separately measured quantities A/Ih+ = Aq> -h (RT/F) ln([H ]i /[H ], , t), where Aq> = — [Pg.117]

Note that H20(l) is formed in the low-temperature fuel cells when temperature is below 100°C and pressure is 1 bar. If temperature is above I00°C and pressure is ambient, H20(g) is formed instead of H20(I). A mixture of H20(g) and H20(l) can also be formed when the temperature is around 100°C. In these half-reactions, Pt/C represents carbon support slurry with particles of about a micron in size with Pt nanoparticles deposited on the carbon. Nanoparticles are used to increase the surface area of the electrode. In the H+(m) symbol, m represents a proton conductive membrane. While the properties of H+(m) and H+(aq) might be different, this fact is usually ignored in most of the studies because the chemical potential (Gibbs energy of formation) of proton in membrane is not known. 

Tocanne JF, Teissie J. (1990) Ionization of phosphoHpids and phospholipid-supported interfacial lateral diffusion of protons in membrane model systems. Biochim Biophys Acta 1031 111-142. 

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

Why has nature chosen this rather convoluted path for electrons in Complex 111 First of all. Complex 111 takes up two protons on the matrix side of the inner membrane and releases four protons on the cytoplasmic side for each pair of electrons that passes through the Q cycle. The apparent imbalance of two protons in ior four protons out is offset by proton translocations in Complex rV, the cytochrome oxidase complex. The other significant feature of this mechanism is that it offers a convenient way for a two-electron carrier, UQHg, to interact with the bj and bfj hemes, the Rieske protein Fe-S cluster, and cytochrome C, all of which are one-electron carriers. 

The most promising fuel cell for transportation purposes was initially developed in the 1960s and is called the proton-exchange membrane fuel cell (PEMFC). Compared with the PAFC, it has much greater power density state-of-the-art PEMFC stacks can produce in excess of 1 kWA. It is also potentially less expensive and, because it uses a thin solid polymer electrolyte sheet, it has relatively few sealing and corrosion issues and no problems associated tvith electrolyte dilution by the product water. 

As with batteries, differences in electrolytes create several types of fuel cells. The automobile s demanding requirements for compactness and fast start-up have led to the Proton Exchange Membrane (PEM) fuel cell being the preferred type. This fuel cell has an electrolyte made of a solid polymer. 

This proton exchange membrane is used in both hydrogen and methanol fuel cells, in which a catalyst at the anode produces hydrogen from the methanol. Because the membrane allows the protons, but not the electrons, to travel through it, the protons flow through the porous membrane to the cathode, where they combine with oxygen to form water, while the electrons flow through an external circuit. 

Complex 1 850 kDa (probably a dimer in membrane) About 40 1 FMN covalently bound, bound 16-24 Fe-S atoms in 5 to 7 centers Spans membrane, NADH site on matrix face, UQ site in membrane 0.06 UQ Pumps protons out of matrix during electron transporl/2e"... 

Complex III 280 kDa 11 28 type hemes (b and bg) bound to same mitochondrially coded peptide 1 C heme (cytochrome c,) 1 Fe-S center Rieske factor Spans membrane, cytochrome b, and b in membrane, cytochrome c, and Fe-S center on outer face 0.25-0.53 Pumps protons out of matrix during electron transport/2e"... 

In rat liver mitochondria, in state 4, the AP was estimated to be about 220 mV, with the membrane potential representing about 90% of this (Nicholls, 1974 Appendix 3). Similar values have been reported for human and rat skeletal muscle mitochondria in state 4 (Stumpf et al., 1982). The control of the rate of electron transport is not only determined by the availability of ADP, but also of Pj oxidizable substrates, and oxygen. There is evidence for futile cycling of protons in intact normal rat hepatocytes (Brand et al., 1993). Recently, Porter and Brand (1993) found a correlation between the proton permeability of the inner membrane of liver mitochondria and body size in animals from the mouse (20 g) to horses (150 kg) with a decrease in permeability with increasing weight of several-fold at a constant... 

It is interesting to compare the thermal-treatment effect on the secondary structure of two proteins, namely, bacteriorhodopsin (BR) and photosynthetic reaction centers from Rhodopseudomonas viridis (RC). The investigation was done for three types of samples for each object-solution, LB film, and self-assembled film. Both proteins are membrane ones and are objects of numerous studies, for they play a key role in photosynthesis, providing a light-induced charge transfer through membranes—electrons in the case of RC and protons in the case of BR. 

The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

These main objectives can be reached only by modifying the structures and compositions of primarily the anode (methanol electrode) and secondarily the cathode (oxygen electrode) as discussed in Sections 111 and IV, respectively. In addition. Section IV discusses the conception of new proton exchange membranes with lower methanol permeability in order to improve the cathode characteristics. Section V deals with the progress in the development of DMFCs, while in Section VI the authors attempt to make a prognosis on the status of DMFC R D and its potential applications. 

Finally, a simple method for a rapid evaluation of the activity of high surface area electrocatalysts is to observe the electrocatalytic response of a dispersion of carbon-supported catalyst in a thin layer of a recast proton exchange membrane.This type of electrode can be easily obtained from a solution of Nafion. As an example. Fig. 11 gives the comparative... 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

S. R. Narayanan, A. Kindler, B. Jeffries-Nakamura, W. Chun, H. Frank, M. Smart, S. Surampudi, and G. Halpert, in Proc. of the First International Symposium on Proton Conducting Membrane Fuel Cells, Ed. by S. Gottesfield, G. Halpert, and A. R. Landgrebe, The Electrochemical Society, Pennington, NJ, PV 95-23, 1995, pp. 261-266. 

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