PH dependence of rates

Figure 5. The pH dependence of rate constant of dopachrome and melanin formation (Q) imidazole-Cu (O) PVIm-Cu (A) PIPo—Cu (9) tyrosinase 30°C, air, phosphate buffer... 

Lasaga, 1984), the rates of dissolution can be expected to increase with ajj+ acidic solutions. The experiments, in fact, denote the pH dependency of rate constants as simple proportionalities of type ... 

In the surface complexation model, Stumm and co-workers (Furrer and Stumm, 1983, 1986 Stumm and Furrer, 1987 Stumm and Wieland, 1990) suggested that adsorption or desorption of protons on an oxide surface polarizes the metal-oxygen bonds, weakening the bonding between the cation and the underlying lattice and explaining the pH-dependence of rates. Surface complexation reactions for an oxide mineral can be written as follows (Schindler, 1981) ... 

PZC of V2O5 at pH 3 was derived [86] from pH dependence of rate of dissolution in chloric VII acid. 

The main emphasis of studies of proton transfer by means of proton resonance has been on their mechanism. Concentration- and pH-dependence of rates to determine the individual order of reactions do not come within the scope of this review. In some cases proton resonance methods have been able to solve the dual problem of mechanism and temperature dependence of individual rate. 

The lower efficiency of the bipyridine complex compared to its ter-pjTidine analogue was proposed to be due to its dimerization at neutral pH. A related complex that cannot dimerize, [Cu"(neocuproine) (H20)2] (8, Fig. 10), was proved to be much more efficient than both bpy and tpy complexes (334). The half-life of ApA with respect to the trans-esterification step was 3 min (ApA, 0.5 mM complex, 10 mM) at neutral pH and 37°C. The reaction is first order with respect to the copper complex and, as in the two previous examples, a pH dependence of rate constants is observed (optimal reactivity at pH 7). The... 

A study of photochemical aspects of the nitroprusside anion in relation to its use in pharmacy gives some kinetic information, including the pH dependence of rates. 

L213DN mutant RCs. Comparison of the theoretical and experimental pH dependencies of rate constant kAB in L213DN RCs shows major discrepancies between them (i) Alteration of AspL to Asn leads to pH dependence of kAB in all regions studied, from pH 5 to > 9, while calculations predict pH dependence only at pH > 8. (ii) The model predicts that protonation of Glu n low pH leads to the disappearance of electrostatic retardation and therefore to an increase in the rate constant of electron transfer in mutant RCs in comparison with Wt RCs. However, the experimental data [12,13] do not support this prediction (Fig. 2C). The model calculations of the electron transfer rate and the experimental values differ by more than two orders of magnitude at high pH. 

Many reactions are catalyzed by Br0nsted acids and/or bases. Since the reaction mechanism and the nature of the rate-determining step may depend on pH, dependence of rate on pH may be quite complex. 

Fig. 10.6. pH dependence of rate constant for two redox species at ( ) GC, ( ) oxygen plasma treated and (A) electrochemically polarized diamond electrodes... 

It is obvious that the reaction is accelerated markedly by water. However, for the first time, the Diels-Alder reaction is not fastest in water, but in 2,2,2-trifiuoroethanol (TFE). This might well be a result of the high Bronsted acidity of this solvent. Indirect evidence comes from the pH-dependence of the rate of reaction in water (Figure 2.1). Protonation of the pyridyl nitrogen obviously accelerates the reaction. 

Much of the study of kinetics constitutes a study of catalysis. The first goal is the determination of the rate equation, and examples have been given in Chapters 2 and 3, particularly Section 3.3, Model Building. The subsection following this one describes the dependence of rates on pH, and most of this dependence can be ascribed to acid—base catalysis. Here we treat a very simple but widely applicable method for the detection and measurement of general acid-base or nucleophilic catalysis. We consider aqueous solutions where the pH and p/f concepts are well understood, but similar methods can be applied in nonaqueous media. 

The kinetic dependence of the reaction was explained in terms of a reaction between PhB(OH)3 and PhHg+. From analysis of the concentration of the species likely to be present in solution it was shown that reaction between these ions would yield an inverse dependence of rate upon molecular acid composition in buffer solutions, as observed for a tenfold change in molecular acid concentration, and that at high pH this dependence should disappear as found in carbonate buffers of pH 10. The form of the transition state could not be determined from the available data, and it would be useful to have kinetic parameters which might help to decide upon the likelihood of the 4-centre transition state, which was one suggested possibility. 

The pH dependence of the rate suggests that nucleophilic attack by the sulphur atom of the sulphoxide on the undissociated hydroperoxide is unimportant while nucleophilic attack by the hydroperoxide anion on the sulphoxide is the preferred reaction under these conditions. Changing the alkyl substituent of the sulphoxide had little effect on the rate and thus the reaction probably proceeds in a similar manner as for oxidation by peracids... 

FIGURE 2 pH dependance of the erosion rate of PCPP versus time. Discs of PCPP were formulated into 1.4-cm-diameter discs 1 mm thick by compression molding, and placed into 0.1 M phosphate buffers at various pH values at 37°C. The cumulative percentage of the polymer which degraded was measured by absorbance at 250 nm. 

The pH-dependence of the inactivation rate indicated the participation of both a basic and an acidic group in the reaction with 40. The latter could be explained by the formation at the active site of the highly reactive epoxide 1,2-anhydroconduritol F (42) which is subsequently activated by the acidic... 

The pH-dependence of the rate is complicated. The rate increases very rapidly between pH values of 8 and 10 and the authors correlate this with the literature value of p bh+ of benzylamine of 9.34. The rate levels off at pH 11-12, but at pH 13-14 a reaction the rate of which is linearly dependent on hydroxyl ion concentration occurs. 

This mechanism does not account for the pH dependence of the reaction rate. According to our experiments the rate of reaction between hydrogen peroxide and peroxydisulphate strongly depends on the acidity and at about pH = 5 a maximum can be observed. 

The last part of Eq. (1) is derived from the pH dependence of permeability, given a pH gradient between the two sides of the intestinal barrier, based on the well known Henderson-Hasselbalch equation. Direct measurement of in situ intestinal perfusion absorption rates confirmed this pH dependence [14]. 

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

The pH dependence of the rate of modification shows that the pKa of Asp-32 is less than 3.165 It is seen in the high-resolution crystal structures that the carboxyl groups of the two aspartate residues are hydrogen-bonded to each other. This is similar to the ionization of maleic acid, which has pKa values of 1.9 and 6.2 (equation 16.30). 

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