Rate constant, proton dissociation determination

There are several variants of this method for more complicated reactions. If the reacting species is produced by a preceding chemical reaction, deviations from Eq. (14.6) may be observed for large in, when the reaction is slower than mass transport. From these deviations the rate constant of the chemical reaction can be determined. As an example we consider hydrogen evolution from a weak acid HA, where the reacting protons are formed by a preceding dissociation reaction ... 

The dioxo complexes of W(IV) and Mo(IV), having high pKa values (Table II), are formed via hydrolysis as the rate-determining step (Scheme 4) and the observed rate constants for the inversion along the O-M-O axis for the W(IV) and the Mo(IV) complexes are therefore defined by Eq. (18). These were calculated as a function of pH, using the proton exchange rate constants (Table IV) and the acid dissociation constants (Table II)... 

Table 2. Acid dissociation pK values, 1 = 0.10 M(NaCl), relating to the active site protonation of different plastocyanins, PCu(I), as determined by (a) proton NMR (b) the variation of rate constants (25 °C) with pH for the [FelCN) ] oxidation of PCu(I), 1 = 0.10 M(NaCl), and (c) similar experiments with [Co(phen)3] " as oxidant. The latter is an apparent value only, and is believed to be composite due to reaction occurring at the remote site...

In order to better understand the detailed dynamics of this system, an investigation of the unimolecular dissociation of the proton-bound methoxide dimer was undertaken. The data are readily obtained from high-pressure mass spectrometric determinations of the temperature dependence of the association equilibrium constant, coupled with measurements of the temperature dependence of the bimolecular rate constant for formation of the association adduct. These latter measurements have been shown previously to be an excellent method for elucidating the details of potential energy surfaces that have intermediate barriers near the energy of separated reactants. The interpretation of the bimolecular rate data in terms of reaction scheme (3) is most revealing. Application of the steady-state approximation to the chemically activated intermediate, [(CH30)2lT"], shows that. 

The plot of the pH-dependence (Fig. 18) indicates qualitatively a participation of an intermediate acid-base equilibrium. Evaluation of rate constants kr and kg is made difficult by the inaccessibility of the dissociation constant of reaction (24 b) which corresponds to protonation of a radical anion. ESR would be a suitable method for the determination of the dissociation constants of at least the more stable radical anions. Another possibility for obtaining at least an approximate value of the equilibrium constant is the measurement of the shifts of the half-wave potentials of the more negative wave at potential 3 with pH. Because the half-wave potential of this wave is known to be sensitive to the... 

The rates of dissociation of Gd3+ chelates used or proposed as CAs are generally low at pH = 7.4. The complexes dissociate much faster in acidic solutions, when the proton-assisted dissociation predominates. In order to compare the kinetic stabilities of complexes, some authors suggest use of the first-order rate constants (kobs) obtained in 0.1 M HCl or HC104 solution. Wedeking et al. compared the acid-assisted dissociation rates (kobs) of several acyclic and cyclic Gd3+ complexes with the long term (14 days) deposition of Gd in the whole body of mice [ 15]. They found an inverse proportionality between the dissociation rates (kobs) and the total residual Gd, i.e. the lower the dissociation rate, the less the residual Gd found in the bodies of the mice [15]. The (kobs) values determined by Wedeking et al. and other authors for several Gd3+ and Y3+ complexes are shown in Table 1. 

Fast atom bombardment mass spectrometry has been utilized for the quantitative determination of ionic species, in glycerol/water solutions, which are produced by chemical and enzymic reactions. It is shown that reaction constants can be determined in this manner and that they can be accurately related to those determined by other methods used in the analysis of aqueous solutions. The reactions studied include proton dissociation constants for organic acids, an enzyme equilibrium constant, and enzyme rate constants using natural substrates. 

The rate constants for protonation of the excited singlet states of several compounds were determined by Weller (1961). Although the measurement of excited state equilibrium constants has become more common, there have been relatively few determinations of the rate constants involved. Trieff and Sundheim (1965) investigated the effects of solvent changes on the rates of protonation and deprotonation of 2-naphthol in the S) state. The dissociation rate constant decreased progressively with the addition of methanol or glycerol to the aqueous solution but the protonation rate constant varied in a more complex manner. As mentioned above, Stryer (1966) found both rate constants smaller in D20 than in H20. 

The log(V/K) profile shows the pKs of groups in substrate or enzyme required for binding, as well as ones involved in catalysis (Fig. 5). Thus, if a substrate binds only as a dianion, the V/K profile will decrease a factor of 10 per pH nnits below the pK of the substrate. If a group on the enzyme has a required protonation state for catalysis, this pK will also appear. The pKs seen in V/K profiles may not appear at their true values if the substrate is sticky (that is, reacts to give products as fast or faster than it dissociates). The pK will be displaced ontward on the profile by log (1 + Sr), where Sr is the stickiness ratio (the ratio of the net rate constant to produce the first product and the off-rate constant for the substrate). Comparison of pKs in V/K and pKi profiles allows one to determine stickiness of the substrate. 

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