Rate constant, proton dissociation effect

We also have used C fixation to measure equilibria and rates of dissociation as a function of temperature. The conclusions reached from these studies have been reported. The dependence of the dissociation equilibria on pH was consistent with dissociation reactions involving the addition to two protons per subunit, a pH-independent dissociation, and a dissociation upon the loss of one proton per subunit. The rate constants for dissociation were consistent with terms first order in hydrogen and hydroxide ions and a pH-independent path. The equilibrium constants in the range 3-35° at pH 7.2 exhibited no dependence on temperature the association reaction was entropy-driven with A5 = 68 cal moL The rate constants for the pH-independent dissociation followed A// = 6 kcal mol The order of effectiveness of concentrated salts in promoting dena-turation was correlated with their effect on the activity coefficient of ace-tyltetraglycine ethyl ester and suggested that peptide groups became more exposed upon dissociation. 

The most trivial explanation for the effect of electrolytes on rate of proton dissociation is to consider the effect of salts on the dielectric constant of the solution (see also Equation 1). In concentrated salt solutions, a considerable fraction of the water molecules are oriented in an hydration shell around the ions thus, their dielectric constant is smaller than in pure water (Hasted et al., 1948). A decreased dielectric constant will accelerate ion-pair recombination and slow down ion-pair separation. 

The first step was found to be a fast pre-equilibrium (Scheme 12-8). The dependence of the measured azo coupling rate constants on the acidity function and the effect of electron-withdrawing substituents in the benzenediazo methyl ether resulting in reduced rate constants are consistent with a mechanism in which the slow step is a first-order dissociation of the protonated diazo ether to give the diazonium ion (Scheme 12-9). The azo coupling proper (Scheme 12-10) is faster than the dissociation, since the overall rate constant is found to be independent of the naphthol con-... 

Proton dissociation in the excited states commonly occurs much easier than in the ground states, and the great difference in proton dissociation constants by several orders of magnitude is characteristic for photoacids [47]. These dyes exist as neutral molecules and their excited-state deprotonation with the rate faster than the emission results in new red-shifted bands in emission spectra [48]. Such properties can be explored in the same manner as the ground-state deprotonation with the shift of observed spectral effect to more acidic pH values. 

The effect of exchange of lactic, mandelic and sulfosalicylic acids on the relaxation of solvent protons gave rate constants (k) of exchange from 1.73 to 0.701 mol-1 s-1.642 Kinetics of complex formation with mandelic (HMDA) and vanillomandelic acids (HVMDA) gave rate constants (1.09 x 103 and 1.13 x 103 mol-1 s 1 for MDA- and VMDA ) consistent with a dissociative (Eigen) mechanism.438 As in the case of oxalic and malonic acids (Section 33.5.5.5.ii Table 27), species with coordinated hydroxyl are labilized. 

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. 

Actually a very crudal kinetic aspect has apparently always been overlooked so far. The original assumption of a proton mobility comparable to that in free solution which allows for dispersion frequencies in the MHz range must not be made in this special case. It is only applicable to the diffusion-controlled polarization of counterion atmospheres as discussed before. In contrast to those counterions the protons of the Kirkwood-Shumaker model are diemically bound at specific sites and must dissociate before they can jump to another site. Thus the lifetime of protons at a given site has to be taken into account with regard to the relaxation time of the overall fluctuation process. Its effect can be readily estimated on the basis of the rate constant of protolytic dissociation... 

The rate constant of recombination Atb has an upper bound of about 10 1 mol s corresponding to a difiiision-controlled mechanism. Adis denotes the dissociation constant. Since the mechanism requires pA pH in order to yield appreciable dipole mommts we may set Aub ft/ hydrogen-ion concentration). Hence the lifetime of bound effective protons can be estimated to be... 

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