Proton transfer external

The question arises whether an external electric field will have any large influence on the direction of these proton transfers. In the NH3 molecule all three protons are situated in one hemisphere of the electronic cloud, and so give to the molecule a dipole moment. In the (NH4)+ ion, on the other hand, it is generally accepted that the four protons are placed symmetrically at the corners of a tetrahedron. Accordingly, the (NH4)+ ion will have no dipole moment. 

However, not all such proton transfers are diffusion controlled. For example, if an internal hydrogen bond exists in a molecule, reaction with an external acid or base is often much slower. In the following case ... 

The transfer of a proton between an acidic and a basic group within the same molecule is often more complex than the process shown in (1). The proton may be transferred along hydrogen-bonded solvent molecules between the acidic and basic groups if these are too remote to permit formation of an intramolecular hydrogen bond. Alternatively, two inter-molecular proton transfers with an external acid or base may be necessary. Tautomerisation of oxygen and nitrogen acids and bases (3) will be described in Section 6. The reactions are usually quite rapid and fast reaction... 

Over the past 20 years, with the availability of fast reaction techniques (Eigen and de Maeyer, 1963 Hammes, 1974 Bemasconi, 1976), numerous kinetic studies have been made of the reactivity of hydrogen-bonded protons towards an external base (52). The majority of such studies have been made with hydroxide ion as the external base. Some examples of proton transfer to... 

Notice that each step in the overall sequence changes the electronic or steric characteristics of the complex in a way that facilitates the next step.246 This is an important principle that is applicable throughout enzymology For an enzyme to be an efficient catalyst each step must lead to a change that sets the stage for the next. These consecutive steps often require proton transfers, and each such transfer will influence the subsequent step in the sequence. Some steps also require alterations in the conformation of substrate, coenzyme, and enzyme. One of these is the transimination sequence (Eqs. 14-26,14-39). On the basis of the observed loss of circular dichroism in the external aldimine, Ivanov and Karpeisky suggested that a... 

Based on these results, Sakurai and coworkers proposed for the addition of alcohols to the silene 44 the mechanism shown in Scheme 14. In the first stage the silene forms an alcohol-silene complex 48 as suggested by Wiberg61, and this stage is followed by an intramolecular proton migration in 48 (the first-order rate constant, k ) which competes with the intermolecular proton transfer from an additional external alcohol (the second-order rate constant, ko). These two processes give the syn and anti addition products, respectively. 

Although the heterolytic process here is formally a concerted ionic splitting of H2 as often illustrated by a four-center intermediate with partial charges, the mechanism does not have to involve such charge localization. In other words, the two electrons originally present in the H H bond do not necessarily both go into the newly-formed M H bond while a bare proton transfers onto L or, at the opposite extreme, an external base. The term a-bond metathesis is thus actually a better description and may comprise more transition states than the simple four-center intermediate shown above, e.g., initial transient coordination of H2 to the metal cis to L and dissociation of transiently bound H- L as the final step. Examples of this type of activation will be given in this Section. 

Figure 4. Scheme for proton transfer by plastoquinone as a mobile carrier in membrane lipid. Electrons are transferred one by one to a bound plastoquinone A (PQA) which in turn reduces external plastoquinone. When reduced, the anionic plastoquinone takes up protons to become a hydroquinone which is oxidized by the cytochrome bb f complex on the inside of the membrane to release protons. A second quinone, vitamin K, (KQ) is also involved in chloroplast electron transport, but its role in proton movement is not known. 

Figure 6. Scheme to represent known aspects of the plasma membrane NADH oxidase and its association with proton release. The oxidase is activated when hormones or ferric transferrin bind receptors. Oxidase may activate tyrosine kinase which can activate MAP kinases to result in phosphorylation of the exchanger leading to Na+/H+ exchange. Oxidation of quinol in the membrane can also release protons to the outside equal to the number of electrons transferred. External ferricyanide can activate electron flow by accepting electrons at the quinone. G proteins (GTP binding proteins) such as ras-activate electron transport and proton release in some way and may be a link to kinase activation (McCormick, 1993). Semiquinone formation in the membrane could lead to superoxide and peroxide formation by one electron reduction of oxygen. 

In succinyl derivatives, the freely rotating single bond between carbon 2 and 3 allows the terminal carboxyl group to assume many more orientations and as a result this drastically reduces the probability that it will stay in the proper conformation long enough for deacylation to occur. Kirby et al. (46) proposed an intramolecular catalysis of amide bond hydrolysis by a proton transfer from external general acids and also showed that in dilute acid the O-protonated amide is the reactive species that initiates the deacylation process (see Reaction 3). 

CT interaction 132). In the case of substituted benzenes with benzylic hydrogens, proton transfer yields radicals. The main evidence for this CT photoreduction mechanism is the lack of any isotope effect on kt when toluene- -d3 is substituted for toluene-hg. In the absence of any labile protons on the aromatic half of the CT complex, such as with benzene itself, the CT interaction is primarily a quenching one, unless an external proton source is present, in which case the complex is apparently protonated 183>. 

Horsewill et al. (1994) examined the hydrostatic pressure effect on the proton transfer in crystals of a carboxylic acid dimer. Under a hydrostatic pressure, the distance of hydrogen bonds becomes shorter, and this is accompanied by a decrease in the potential barrier to proton transfer. The temperature dependence of the rate of the proton transfer turns out to be of a non-Arrhenius type. The influence of phonon-assisted tunnelling becomes evident as the external pressure increases, especially at lower temperatures. 

Application of high external pressures influences the transition temperature to the antiferroelectric phase (Yasuda et al., 1979 Samara and Semmingsen, 1979). The Tq becomes lower as the applied pressure increases. Under an ultra-high pressure of about 3 GPa, the antiferroelectric transition itself disappears and the high dielectric constant of ca. 200 is maintained even at cryogenic temperatures (Moritomo et al., 1991). Since Raman diffraction measurements under 3-4.5 GPa revealed that squaric acid exists still as an alternating bond form, the tautomerization coupled with intermolecular proton transfer occurs even at low temperatures (Moritomo et al., 1990). 

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