Reprotonation rate concentration

The binding of pyranine to phosphatidylcholine (lecithin) vesicles as a function of the probe and electrolyte concentration has been investigated [103], The binding of the probe to the internal leaflet of lecithin small unilamellar vesicles (SUVs) was found to be larger than that to the external leaflet. The addition of salt up to 2 M did not prevent binding, even at low probe concentrations. The ground-state reprotonation rate constant was found to depend on the probe content per vesicle. 

It should be noted that the rate of racemization (or the rate of hydrogen exchange in Section 10.1.1) is exactly the same as the rate of enolization, since the reprotonation reaction is fast. Hence, the rate is typical of a bimolecular process and depends upon two variables, the concentration of carbonyl compound and the concentration of acid (or base). 

The following data would appear to substantiate this premise. At high nitroaromatic concentrations Reaction 12 should be able to compete with the reprotonation of the carbanion and the rate of ionization should become equal to the rate of oxygen absorption. Since the stoichiometry of the oxidation did not change on adding the nitroaromatic catalysts, the assumption that the absorption of only one molecule of oxygen occurred for each electron transfer step is legitimate. 

At higher concentrations of acetaldehyde, bimolecular trapping of the enolate in Scheme 4.7 will become faster, so, at some stage, this will compete effectively with the reprotonation of the enolate. When the bimolecular capture of enolate by another acetaldehyde molecule becomes much faster than the reprotonation of the enolate, i.e. when /c4[CH3CHO] k, 1 + k-2[H+] + /c 3[BH+], another limiting approximation to the complex rate equation predicted from the mechanism (Equation 4.17) is obtained, Equation 4.19 ... 

The outward and inward H pathways have been shown to operate in p,s and ms time scales, respectively, so that their contributions to the total Aip formation can be easily measured. On the other hand, the rate of transfer from the Asp-96 carboxylic group to the Schiff base is of the same order of magnitude as the rate of reprotonation of this carboxylate by cytoplasmic ions. To measure the Atp contributions of these two steps separately, one may specifically decelerate the Asp-96 carboxylate reprotonation by decreasing the concentration in the medium. Under such conditions, both steps seem to make an almost equal contribution to energy conservation [20]. 

This is the simplest explanation for the observation that when L and M have come to an equilibrium which contains these species in comparable amounts, the concentration of L decreases to near zero even while M remains at its maximal accumulation. Recent measurements of the quasi-equilibrium which develops in asp96asn bacteriorhodopsin before the delayed reprotonation of the Schiff base confirm this kinetic paradox [115]. Two M states have been suggested also on the basis that the rise of N did not correlate with the decay of M [117]. In monomeric bacteriorhodopsin the two proposed M states in series have been distinguished spectroscopically as well [115]. It is well known, however, that kinetic data of the complexity exhibited by this system do not necessarily have a single mathematical solution. Thus, assurance that a numerically correct model represents the true behavior of the reaction must come from testing it for consistencies with physical principles. It is encouraging therefore that the model in Fig. 5 predicts spectra for the intermediates much as expected from other, independent measurements, and the rate constants produce linear Arrhenius plots and a self-consistent thermodynamic description [116]. 

Figure 2. Dependence of the observed rate of reprotonation on the two-dimensional concentration of pyranine anion in the hydration layer. The rate constant (y) was calculated from the experiments carried out as in Figure 1. The concentration of the anion was calculated from the amplitude of the transient and the dye-lipid ratio of the preparation. The magnitude of the amplitude was varied by modulation of the excitation pulse energy by glass filters. Measurements were carried out in the absence of sucrose, A, and in the presence of 0.57-M sucrose (1.5 X 107 dyn/cm2), B.

When the carbanion decomposes more readily than it reprotonates, kinetic behaviour intermediate between that of the carbanion and bimolecular mechanism is predicted. For only a small extent of substrate ionisation in low conjugate acid concentration (k-i s>, [6h]), general base catalysis is observed. At constant buffer ratio, an increase in base concentration causes a linear increase in observed rate coefficient until / [6h] approaches/ .2. Under this condition the rate coefficient attains a maximum with further increase in base concentration, the kinetics parallel the carbanion mechanism and specific base catalysis is observed . ... 

The kinetics of intercalation and deintercalation of alkali metal ions were investigated in pressure-jump experiments while monitoring the electrical conductivity of the samples (32). These studies indicate biphasic kinetics whose magnitudes are in milliseconds the rates of the fast and slow components increased with increased concentrations of the metal ions. The forward and reverse rates depend on the interlayer distances, and the fast and slow components have been attributed to the ingress of ions into the galleries and interlayer diffusion, respectively. Similar biphasic kinetics on millisecond-second time scales were also observed in pressure-jump experiments for the deprotonation-reprotonation of a-ZrP (33). In the latter case, the slow and fast components have been attributed to deprotonation from the surface and from the interlayer regions of the solid, respectively. 

Intramolecular aldol reaction of (14) to give (16), promoted by lyate ion, has been found to proceed by rate-determining deprotonative formation of enolate intermediate (15) the intramolecular addition (/cc) occurs more rapidly than reprotonation of (15) by H2O or D2O ( hoh or A dod), c/ hoh = 35. However, when the reaction is catalysed by high concentrations of 3-substituted quinuclidine buffers, the enolate addition is rate determining and competitive with reprotonation of (15) k-Q /k (lmol ) increases from 7 to 450 as the acidity of the buffer acid increases from p/(bh = 11-5 to 7.5. The unexpectedly small Marcus intrinsic value for addition of (15) to the carbonyl group has been attributed to favourable interactions between the soft-soft acid-base pair. 

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