Protonic acids association

Throughout these sections it has been assumed that protonation and association equilibria are established on time scales much shorter than those for the kinetic steps. For the usual protonations and ion-pairings that assumption will always be true, except when very rapid reactions are being studied by certain techniques presented in Chapter 11. On the other hand, if carbon acids are involved, or any sluggish association reactions, the assumption of rapid prior equilibria may not hold true. 

Especially in dicotyledonous plant species such as tomato, chickpea, and white lupin (82,111), with a high cation/anion uptake ratio, PEPC-mediated biosynthesis of carboxylates may also be linked to excessive net uptake of cations due to inhibition of uptake and assimilation of nitrate under P-deficient conditions (Fig. 5) (17,111,115). Excess uptake of cations is balanced by enhanced net re-lea,se of protons (82,111,116), provided by increased bio.synthesis of organic acids via PEPC as a constituent of the intracellular pH-stat mechanism (117). In these plants, P deficiency-mediated proton extrusion leads to rhizosphere acidification, which can contribute to the. solubilization of acid soluble Ca phosphates in calcareous soils (Fig. 5) (34,118,119). In some species (e.g., chickpea, white lupin, oil-seed rape, buckwheat), the enhanced net release of protons is associated with increased exudation of carboxylates, whereas in tomato, carboxylate exudation was negligible despite intense proton extrusion (82,120). 

The addition of protons to the carbonyl group is an important process because of the role it plays in the acid catalyzed reactions of many types of carbonyl compounds. Because of the difference in electronegativity, it is likely that the proton is associated specifically with the oxygen rather than with the carbonyl group as a whole. 

Fairman, W. A., Sonders, M. S., Murdoch, G. H and Amara, S. G. (1998) Arachidonic acid elicits a substrate-gated proton current associated with the glutamate transporter EAAT4. Nat. Neurosci. 1,105-113. 

The crucial requirement of excited-state proton transfer (ESPT) is suggested by the failure of 1-naphthyl methyl ether to undergo self-nitrosation under similar photolysis conditions. The ESPT is further established by quenching of the photonitrosation as well as 1-naphthol fluorescence by general bases, such as water and triethylamine, with comparable quenching rate constants and quantum yield. ESPT shows the significance in relation to the requirement of acid in photolysis of nitrosamines and acid association is a photolabile species. 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

In general, semiconductor electrodes adsorb in aqueous solutions water molecules, hydronium ions, and hydroxide ions in addition to various solute ions. As a result, the dissociation-association equilibria of the adsorbed hydronium ions and water molecules produce, in the proton dissociation-association reactions of Eqns. 9-69 and 9-70, the acidic and basic proton levels, respectively, on the electrode interface as shown in Fig. 9-21 ... 

The way in which the proton is associated with the alumina-silica catalyst is a matter of some doubt. Thomas (78) assumes the aluminium to be tetrahedral when linked with tetrahedral silicon, the extra valence electron being supplied by hydrogen from water contained in the catalyst (Fig. 21a). Both aluminium hydroxide and silicic acid are very weak acids because of the affinity of oxygen for the hydrogen (83), and a coordination of aluminium with the hydroxyl oxygen contained in the catalysts... 

There are thus two classes of acids on surfaces of metal oxides Lewis acids and Brdnsted acids (which are also termed proton acids). The weight of evidence (1-8) shows that strong Brpnsted acids are the primary seat of catalytic activity for skeletal transformations of hydrocarbons. In the solids under review, they consist of protons associated with surface anions. 

Fig. 4.4 Hypothetical model showing the modulation of glutamate transporter by arachidonic acid. Interactions of glutamate with its receptor result in depolarization and Ca2+ entry into the cell. Ca2+-mediated stimulation of PLA2 results in breakdown of neural membrane phospholipids and the release of arachidonic acid. Arachidonic acid not only modulates proton conductance associated with neuronal excitability, but also provides eicosanoids, which may control the glutamate transporter (modified from Fairman and Amara, 1999)...

The site of nucleophilic substitution under acid conditions is also decided by the protonated specie formed. It has been proposed that with fairly strong acids the proton is associated with an imidazole nitrogen giving rise to the cationic forms 18 and 1965 in which the... 

In this analysis, two outliers were detected, 2,2-dimethylpropanoic acid and 3-sulfopropanoic acid they were excluded from the final model. The physically reasonable nature of the relation is illustrated by noting that a decrease in pK (increase in acidity, less tendency to hold the proton) is associated with a smaller positive charge on the COO carbon and less negative charge on the RO oxygen. 

Ubiquinone functions as a carrier in the mitochondrial electron transport chain it is responsible for the proton pumping associated with complex I (Brandt, 1999) and is directly reduced by the citric acid cycle enzyme succinate dehydrogenase (Lancaster, 2002). As shown in Figure 14.8, it undergoes two single-electron reduction reactions to form the relatively stable semiquinone radical, then the fully reduced quinol. In addition to its role in the electron transport chain, it has been implicated as a coantioxidant in membranes and plasma lipoproteins, acting together with vitamin E (Section 4.3.1 Thomas etal., 1995, 1999). 

The initial proton release associated with H2 cleavage is promoted by a base. For hydrogen evolution, net proton uptake from the medium is necessary. Conversely, for Hj oxidation protons are transferred from the active site to solution. The transfer of protons within a protein is considered to involve small (<1 ) movements of the amino acids that participate in the pathway (Williams, 1995). The proton transfer would involve a rotation of each individual donor and acceptor. Analyses of the crystal structures have suggested proton transfer pathways for the NiFe and Fe-only hydrogenases. 

Bis Complexes. The dicyano complexes [Fe(CN)2 (diimine)2] are protonated in strong acid to give stable mono and diprotonated species. Protonation is associated with major color changes, typically dark violet to yellow, and these complexes have been used as acid-base indicators in nonaqueous titrations. They also show strong solvatochromatic charge-transfer bands. For example, the absorbance maximum of [Fe(CN)2(bipy)2] in water is 515 nm and in pyridine, 629 nm. The solvatochromism of a variety of [Fe(diimine)2(CN)2] and [Fe(diimine)(CN)4] complexes has been established, ... 

Fig. 5. Model for proton transfer in the bacterial reaction center of Rb. sphaeroides based on its known crystal structure. Some amino-acid residues are shown in the protein interior. See text for detaiis. Figure source Paddock, Rongey, McPherson, Juth, Feher and Okamura (1994) Pathway of proton transfer in bacterial reaction centers Role of aspartate-L213 in proton transfers associated with reduction of quinone to dihydroquinone. Biochemistry 33 743.

Part V is devoted to the study of H transfers in organic and organometallic reactions and systems. In Ch. 18 Koch describes kinetic studies of proton abstraction from CH groups by methoxide anion, of the reverse proton transfer from methanol to hydrogen bonded carbanion intermediates, and of proton transfer associated with methoxide promoted dehydrohalogenation reactions. Substitutent effects, kinetic isotope effects and ah initio calculations are treated. Of great importance is the extent of charge delocalization in the carbanions formed which determine the kinetic and thermodynamic acidities. 

This chapter is divided into three sections Proton transfer from carbon acids to methoxide ion proton ttransfer from methanol to carbanion intermediates proton transfer associated with methoxide promoted dehydrohalogenation reactions. 

Zeolites derive their acidity or catalytic activity from the proton associated with the framework aluminum. Replacement of aluminum by the other three-valent elements could also lead to the generation of Bronsted acid sites. However, one needs to exercise caution in proclaiming unusual catalytic properties before one thoroughly understands the nature of these materials. For example, many studies have been reported on the nature of the acidity of boron ZSM-5 (5—6—7-8). Although physical measurements (8-9) indicate that the protonic site associated with the... 

Enzyme catalyzed mechanisms represent fundamentally familiar reactions from organic chemistry (Figure 2.17). Acid-base catalysis is associated with the donation or subtraction of protons. Acid catalysis is a process in which partial proton transfer from an acid lowers the free energy of the reaction transition state, while base catalysis is a process in which partial proton subtraction by a base lowers the free energy of the reaction transition state. Concerted acid-base catalysis, where both processes occur simultaneously, is a common enzymatic mechanism. 

Although protons may be present in calcined cracking catalysts (8) and some authors have considered these protons as responsible for catalyst activity (21,22), the present considerations indicate that the Lewis acid which has also been suggested (23, 24) is entirely responsible for catalyst activity. Thus, the proton acidity of different acids bears little or no necessary relationship to their activity as acid catalysts (26), and silica-magnesia cracking catalysts which are actually basic in their aqueous solutions and do not exchange protons with alkali metal ions are active cracking catalysts. In the catalyst chemistry to be discussed, a proton may or may not be present, but it does not contribute to the activity of the cata,lyst. The presence or absence of a proton is associated with the manner in which water functions as... 

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