Proton dissociation-replacement reactions

In order to account for the above behavior we have replaced the conventional rate equations by a Smoluchowski equation, describing diffusion in the potential of mutual interaction. In this picture one introduces explicitly the R 0 /H distance distribution, thus obtaining a partial (instead of an ordinary) differential equation for the time evolution of proton dissociation. The fact that the observed reaction is reversible in the excited-state, has promoted the development of the theory of reversible diffusion influenced reactions [14], as an (almost obvious) extension of the theory of (irreversible) diffusion influenced reactions [15, 16]. 

FIGURE 19.29 A mechanism for the pyruvate kinase reaction, based on NMR and EPR studies by Albert Mildvan and colleagues. Phosphoryl transfer from phosphoenolpyrnvate (PEP) to ADP occurs in four steps (a) a water on the Mg ion coordinated to ADP is replaced by the phosphoryl group of PEP (b) Mg dissociates from the -P of ADP (c) the phosphoryl group is transferred and (d) the enolate of pyruvate is protonated. (Adapted from Mildvan, A., 1979. Advances in Eiizymology 49 103-126.)... 

Removal of the proton produces a negative charge in the position trans to where the Cl- leaves, which enhances the process by a trans effect (see Section 20.9). After the dissociation of Cl- from the transition state, the reaction with H20 is rapid. A proton from H20 replaces the one that was lost from the central nitrogen atom, and the remaining OH- completes the coordination sphere of the Pd2+. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

Thymidylate synthase (TS) is the enzyme that converts 2-deoxyuridine monophosphate into thymidine monophosphate. This is a key step in the biosynthesis of DNA. This enzymatic reaction of methylation involves the formation of a ternary complex between the substrate, the enzyme, and tetrahydrofolic acid (CH2FAH4). The catalytic cycle involves the dissociation of this complex and the elimination of FAH4. It is initiated by pulling out the proton H-5, thus generating an exocyclic methylene compound. As the release of a F" " ion is energetically forbidden, the fluorine atom that replaces the proton H-5 cannot be pulled out by the base. This leads to inhibition of the enzyme (Figure 7.2). 

The decreases in the pK of phenylacetic acid occasioned by replacement of the meta-hydrogen with Cl or N02 are -0.18 and -0.34, respectively, substantially less than for the benzoic acids. On the other hand, for the phenols the differences amount to -0.90 and -1.53, much greater than for the benzoic acids. The Hammett equation asserts that for reactions such as the dissociation of protons from phenylacetic acids or from phenols, the changes in AG occasioned by meta substitutions are proportional to the o values, i.e., to the changes in AG for the standard reaction - dissociation of a proton from benzoic acid.1, J... 

In substitution reactions, solutions of a salt and an acid with the same anion are fed through alternate compartments of an array of cation-exchange membranes. The dissociated metal ions from the salt are removed and replaced by protons to generate the free acid. For example, amino acids are produced from their sodium salts in this way. Compared with conventional neutralization and recovery techniques, the membrane-mediated process is considerably simpler and gives a higher yield of the purified product. 

An electron moves from to Qg in about 200 p,s [28-31,51]. Excitation of the reaction center by a second photon sends another electron from P to Q, and then on to Qg with similar kinetics. The fully reduced Qg now probably picks up two protons from the solvent, dissociates from the reaction center as the quinol (QgH2), and is replaced by a fresh molecule of ubiquinone. Electrons from OgH2 return to P" via a Cyt bc complex and a high-potential, c-type cytochrome. This cyclic electron flow drives proton translocation across the chromatophore membrane, and is coupled to the formation of ATP. 

The different reaction paths described by the model are shown in scheme 8, For clarity, ionic charges and the proton transfer step that follows displacement by a hydroxylic molecule have been omitted. The intermediates that maintain association with the leaving group may be regarded as ion pairs. An unsolvated ion pair acquires solvation before formation of the substitution product and in its solvated form may undergo dissociation to yield a symmetrically solvated ion. The possibility of reaction at the stage of the solvated ion pair makes race-mization with partial inversion the expected stereochemical outcome. However, where a mechanism exists for preferential replacement of the counter-anion on the front side of the carbonium ion, in the manner 13... 

In the triple layer model the surface reactions for protonation and dissociation of the surface functional group are Eqs. (6.6) and (6.7) as written for the constant capacitance model, where h is replaced by I, . The reactions for adsorption of the background electrolyte in the P-plane are... 

In the cathodic reduction of substrates with appropriate leaving groups X, these groups can be replaced by hydrogen in a reaction which is termed cathodic cleavage . A mechanism is assumed involving the reduction of the substrate to a radical anion that dissociates into a radical and X, sometimes both reactions occur in a single step (dissociative electron transfer) [153]. The radical is further reduced to an anion that is subsequently protonated (Eq. 19). 

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