Protons, chemical

Diuretics This indicates the unique property of capsaicin-sensitive primary afferent neurons to release mediators (neuropeptides and others) from both peripheral and central nervous system terminals upon adequate stimulation. Capsaicin and other chemical (protons) or physical (heat) stimuli release mediators from both peripheral and... 

The reversible one-electron transfer to form an anion radical (R ) is followed by an irreversible chemical protonation to form /f H, which is subsequently reduced itself (the reduction potential of the species / H, has been shown to be more positive3 than that of the parent, R) and then undergoes another irreversible protonation reaction. In a protic solvent, the reactions proceed rapidly to the final product, / H2. In a rigorously purified aprotic solvent, the intermediate anion radical R , has an appreciable lifetime and reacts only slowly, principally with adventitious impurities in the solvent. Thus, the stability of aromatic anion radicals can be taken as a measure of the protic character of a solvent. 

Aza analogues of sesquifulvalene 12 and their vinylogues 17 and 18 can be described to a flrst approximation by a covalent resonance form (A) and a dipolar one (B), whereas structures of type 19 may only exist as betaines. Comparison of the chemical proton shifts observed for compounds 12 and 17-19 with those of their corresponding precursors 31, 36-38 (ASH, Scheme 6) deserves a brief comment. 

A second advantage of water is that in addition to being able to dissolve electrolytes by the physical forces involved in solvation, it is also able to undergo chemical proton-transfer reactions with potential electrolytes and produce ionic solutions. Water is able to donate protons to, and to receive protons from, molecules of potential electrolytes. Thus, water can function as both a source and a sink for protons and consequently can enter into ion-forming reactions with a particularly large range of substances. This is why potential electrolytes often react best with water as a partner in the proton-transfer reactions. Finally, water is stable both chemically and physically at ambient temperature, unlike many organic solvents which tend to evaporate (Table 4.24) or decompose slowly with time. 

Figure 18.21. Proton Transport by Cytochrome C Oxidase. Four "chemical" protons are taken up from the matrix side to reduce one molecule of O2 to two molecules of H2O. Four additional "pumped" protons are transported out of the matrix and released on the cytosolic side in the course of the reaction. The pumped protons double the efficiency of free-energy storage in the form of a proton gradient for this final step in the electron-transport chain.

Hexadeuterated ABA. Experiments that involved the feeding of [3, 5,7- Hgj-ABA to tomato shoots revealed that the C-3 protonation in the isomerization to PA took place from the a-face (si-face) where the chemical protonation-deprotonation is difficult [92]. The isomerization to PA may be catalyzed enzymatically, although the enzyme involved has not yet been identified. 

The basic character of benzidine affords an alternate mechanism to influence the average position of the bead. Protonation of the amine functional groups by simple addition of trifluoroacetic acid (TFA) to the solution also generates positive charges on the benzidine unit and forces the bead to encircle the biphenol station. Neutralization with base (pyridine) returns the system to its initial state by removing the positive charges on the benzidine station (Scheme 6). These phenomena were verified in careful JH-NMR spectroscopic experiments [7], Thus, chemical (proton transfer) reactions are also useful to control the sliding motion of the macrocyclic bead in this rotaxane. 

If C-l is a C13 nucleus, the C13-H side band of the proton at C-l introduces magnetic non-equivalence and the coupling between equivalent chemical protons in the ring appears as sub-structure on these C13... 

In the electron transport chain, CcO receives electrons from cytochrome c, a water-soluble heme protein, on the cytoplasmic side of the membrane, and transfers them through a series of electron transfer steps to the active site, which contains a heme iron and a copper, where the electrons are used to reduce the molecular oxygen. The protons needed for this reaction are taken from the mitochondrion matrix side throngh two proton-conducting channels. In addition to these chemical protons, four more protons, per every oxygen molecule reduced, are translocated across the membrane. The overall enzymatic reaction of CcO is... 

This finding inunediately suggested that one chain can be used to deliver chemical protons for oxygen chanistry in BNC, and another to translocate pumped protons. The majority of currently discussed models of CcO are based on this idea. However, it is not clear yet how the enzyme controls the gating of protons at Glu242 site to be delivered for chemistry and for pumping. 

FIGU RE 4.5 Computer simulations have revealed two bifurcated chains of water molecules in the catalytic site of CcO [21]. One branch connects Glu242 to the binuclear center, and another connects Glu242 to F op D of heme a3, indicating that pumped protons can be channeled using one branch, while chemical protons can use the other. Most of the current models of CcO pump... 

Some other theoretical possibilities have been discussed by Siegbahn et al. [30], Olsson and Warshel [31], and Xu and Voth [32] recently. Siegbahn et al. proposed that the PLS is not His291, but rather a nearby Prop A of heme a3. The energetics of their model raises questions however they report that the chemical proton has no driving force left after the first proton transfer to Prop A has happened. Our most recent calculations indicate that there is a remaining driving force for the second proton. Still, the identity of the PLS remains unknown. 

This finding immediately suggested that one branch of the structure can be used to supply protons to the binuclear center for chemistry, and another branch for pumping protons. Following this proposal, Wikstrom and coworkers suggested [33] that water orientation in these chains maybe used for gating pumped and chemical protons by the water-gate mechanism. 

This finding suggests a possible mechanism of proton transfer in this region one branch of the water cluster can be used by pumped protons the other by chemical protons (Figure 4.14). There is driving force for both of these reactions [39]. The mechanism of gating of chemical and pumped protons, however, is not clear. 

One possibility is that the barrier for proton transfer of the pumped proton is lower than that for the chemical proton hence, the rate of pumped proton transfer is faster than for chemical proton. In order to evaluate this proposal, one needs to study the energy profile along the proton transfer path. 

Fig. 8 A power stroke model for a bacterial flagellar motor that has been described in terms of two asymmetric sawtooth potentials whose spatial periods are out of phase with one another. In the original depiction [24], only the long green arrows pointing downward in the shaded reaction windows were drawn for the chemical (proton binding and release) transitions, but this approximation represents a logically impossible limit...

These facts can be explained if we consider the coexistence of a chemical pyrrole polymerization parallel to the formation of the polymer on the electrode. It is known that acidification of a pyrrole solution in an organic solvent gives a polymeric powder with alternating saturated and unsaturated pjnrole rings [21,136-9] (Figure 10.12). The kinetics of this homogeneous chemical proton-catalyzed polymerization of... 

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