Rate constant, proton dissociation

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. 

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

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-... 

The occurrence of proton transfer reactions between Z)3+ ions and CHa, C2H, and NDZ, between methanium ions and NH, C2HG, CzD , and partially deuterated methanes, and between ammonium ions and ND has been demonstrated in irradiated mixtures of D2 and various reactants near 1 atm. pressure. The methanium ion-methane sequence proceeds without thermal activation between —78° and 25°C. The rate constants for the methanium ion-methane and ammonium ion-ammonia proton transfer reactions are 3.3 X 10 11 cc./molecule-sec. and 1.8 X 70 10 cc./molecule-sec., respectively, assuming equal neutralization rate constants for methanium and ammonium ions (7.6 X 10 4 cc./molecule-sec.). The methanium ion-methane and ammonium ion-ammonia sequences exhibit chain character. Ethanium ions do not undergo proton transfer with ethane. Propanium ions appear to dissociate even at total pressures near 1 atm. 

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 first indication that A-acyloxy-A-alkoxyamidcs reacted by an acid-catalysed process came from preliminary H NMR investigations in a homogeneous D20/ CD3CN mixture, which indicated that A-acetoxy-A-butoxybenzamide 25c reacted slowly in aqueous acetonitrile by an autocatalytic process according to Scheme 4 (.k is the unimolecular or pseudo unimolecular rate constant, K the dissociation constant of acetic acid and K the pre-equilibrium constant for protonation of 25c).38... 

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)... 

Attempts to optimize the stability and SOD activity of C-substi-tuted (R = methyl and fused cycloalkyl) [Mn(II)[15]aneN5)Cl2] complexes 95 have shown that increasing the number of hydrocarbon substituents greatly increases the kinetic stability of the complex toward dissociation via protonation (Fig. 19). (443). There is also some enhancement of thermodynamic stability. The trans-fused endohexano Mn(II) complex 96 has a faster dismutation rate constant (9.09 X 107M-1 s1, pH 7.4) and a 10 times higher thermodynamic stability than the unsubstituted complex. 

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-... 

Cu (H G ) in Figure 1. The two protonated nickel(III) complexes then undergo substitution reactions for the terminal peptide nitrogen with rate constants of 0.94 s 1 and 17 s 1, respectively (21). It is interesting that the corresponding nickel(II) complexes have similar but somewhat larger rate constants. Thus, Ni (H gG )H dissociates with a rate constant of... 

Table V Rate Constants for the Solvent and Protonation Pathways in Complex Dissociation (30) ...

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...

Table 9. Acid dissociation pK, values relating to the protonation at the remote binding site of different plasto-cyanins [49, 101] as obtained from the variation of rate constants with pH 25 °C, 1=0.10 M(NaCl)...

Being valid these assumptions, we can consider the dissociation of the ionic cluster [A Lref], where 1 is a metal ion, a proton, or another cationic species binding the unknown A with a reference molecule ref, whose affinity for 1 is known. The dissociation channels are shown in Scheme 3, where and k2 are the rate constants for the two dissociation processes. 

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