Protons Transport Differently

The starting point to elucidate the way the proton moves in solution is to consider its movement through the solvent at a steady state—constant velocity—and at a concentration so low that there is no interionic interaction (zeroth approximation). This occurs when the electric driving force ze X balances the Stokes viscous force, 6nrr v. Thus, the Stokes mobility is 

What radius should one use Suppose one takes a rough-and-ready measure to consider the hydronium ion. Since it has approximately the same radius as that of a K ion, one would expect its mobility to be approximately the same, i.e., about 5 x 10 cm v s . It is here that one encounters the great anomaly—the nonconforming 

One can pick up a clue as to the reason for this anomaly in mobility if one asks What is the proton s mobility in other, related solvents This rather vital question was addressed and solved in a Ph.D. thesis by an Austrian student, Hanna Rosenberg, more than 40 years ago. She foimd that if, for example, methanol was added to water, the anomalous mobility of the proton was decreased (Fig. 4.120). When methanol was replaced by other, larger alcohols (no water present), she was astounded to find that the anomalous mobility was greatly reduced until by the time n-propanol was reached, the difference between HCI and LiCl was greatly reduced (Table 4.29). 

The proton is indeed anomalous in its conductance and mobility. These properties do not vary with temperature in the expected, regular way. There is not the expected near-sameness for hydronium and deuterium ion mobilities. The conductance of protons in aqueous-non-aqueous media is wholly dependent on the mole-fraction of water present. 

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A more recent view of proton transport is that of Kreuer, who, compared with the Zundel-based view, describes the process on different structural scales within phase separated morphologies. The smallest scale is molecular, which involves intermolecular proton transfer and the breaking and re-forming of hydrogen bonds. When the water content becomes low, the relative population of hydrogen bonds decreases so that proton conductance diminishes in a way that the elementary mechanism becomes that of the diffusion of hydrated protons, the so-called vehicle mechanism . 

One view to explain different P/O ratios for different classes of organisms is to consider variability in both the molecular mechanism as well as the stoichiometry of proton transport and ATP synthesis with the source of the enzyme [67]. However, considering our molecular mechanism and the energetics of the oxidative phosphorylation process, we believe that a universality in the mechanistic, kinetic and thermodynamic characteristics of the system is operative. 

The Fe-S Reaction Center (Type I Reaction Center) Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria, but the process differs in several respects and involves additional enzymatic reactions (Fig. 19-47b). Excitation causes an electron to move from the reaction center to the cytochrome bei complex via a quinone carrier. Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis, just as in purple bacteria and in mitochondria. 

Another possible scheme (3.3b) is based on a suggestion that protons transported by the F0 factor and included in H20 molecule in the ATP synthesis differ from one another, and A/ZH is consumed for transforming the / j factor to a conformation making ATP release from the catalytic site easier [26], The second scheme (3.3b) is characterized by its simplicity and conciseness if compared with scheme (3.3a). However, it possesses a serious drawback in that the conjugation idea is completely neglected, no matter if it is energetic or chemical. This contradicts the facts observed and, therefore, may not be accepted. 

The functional and morphological heterogeneity of a lamellar system of chloroplasts indicates that pH values in different compartments (in granal and intergranal thylakoids) differ. This type of structure makes it difficult to measure local pH values at different sites. Therefore, mathematical models taking into account the spatial structure of chloroplasts provide a tool for studying the effect of diffusion restrictions on pH distributions over the thy lakoid on the rates of electron transport, proton transport, and ATP synthesis. The rate of ATP synthesis depends on the osmotic properties of a chloroplast-incubation medium and, therefore, on topological factors. 

The fact that the pKa of the various oxidation states of quinones differ dramatically has been exploited in a recent design of artificial systems that mimic the light-driven transmembrane proton transport characteristic of natural photosynthesis [218]. Triad artificial reaction centers structurally related to 41 were vectorially inserted into the phospholipid bilayer of a liposome (vesicle) such that the majority of the quinone moieties are near the external surface of the membrane, and the majority of the carotenoids extend inward, toward the interior surface. The membrane... 

The difference between omeprazole and SCH 28080 in their ability to inhibit gastric H /K -ATPase is dependent on their inhibition kinetics. In contrast to omeprazole, SCH 28080 competes with the high affinity K -site on the gastric H /K -ATPase. Its effect on Na /K -ATPase activity is much less pronounced in comparison with its effect on gastric H /K -ATPase activity [159, 160]. SCH 28080 is a protonatable weak base (pK = 5.6) which accumulates in acidic compartments in the same way as omeprazole on the lumenal, acidic side of the parietal cell membrane in a protonated form [161]. However, SCH 28080 is chemically stable and active by itself after protonation [162] and does not need an acid-induced transformation such as required by omeprazole-like irreversible inhibitors. Therefore, in proton transport studies, SCH 28080 inhibits the initial rate of HVK" -ATPase mediated H accumulation and the steady state proton concentration. This is in contrast to omeprazole, which first needs accumulation of acid within gastric vesicles to generate an interior of low pH to facilitate the acid-induced transformation prior to being able to inhibit the HVK -ATPase [163]. SCH 28080 binds to the lumenal side of H /K" -ATPase [161,... 

The initial emphasis on evaluation and modeling of losses in the membrane electrolyte was required because this unique component of the PEFC is quite different from the electrolytes employed in other, low-temperature, fuel cell systems. One very important element which determines the performance of the PEFC is the water-content dependence of the protonic conductivity in the ionomeric membrane. The water profile established across and along [106]) the membrane at steady state is thus an important performance-determining element. The water profile in the membrane is determined, in turn, by the eombined effects of several flux elements presented schematically in Fig. 27. Under some conditions (typically, Pcath > Pan), an additional flux component due to hydraulic permeability has to be considered (see Eq. (16)). A mathematical description of water transport in the membrane requires knowledge of the detailed dependencies on water content of (1) the electroosmotic drag coefficient (water transport coupled to proton transport) and (2) the water diffusion coefficient. Experimental evaluation of these parameters is described in detail in Section 5.3.2. 

Proton transport in carbonic anhydrase as an example of the difference between microscopic and phenomenological LFERs 212... 

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