Aromatic protonation. 12. Solvent isotope effects in water-water-d2 mixtures on the detritiation of trimethoxybenzene

Ketonization equilibria of phenol in aqueous solution

The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions, 2 –> 1 and 3 –> 1, were measured. The rates of the reverse reactions, 1 –> 2 and 1 –> 3, were determined from the rates of acid-catalyzed hydron exchange at the ortho- and para-positions of 1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2t and -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of 2 and 3 through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, τ(2) = 260 μs and τ(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profiles.

Energetics of enzyme catalysis. I. Isotopic experiments, enzyme interconversion, and oversaturation

An enzyme-catalyzed interconversion of one substrate, S, and one product P, by an enzyme that exists in two forms E1 and E2 where E1 binds S and E2 binds P, is considered S + E1 in equilibrium E1S in equilibrium E2P in equilibrium E2 + P. Under reversible conditions (where the concentrations of S and P are not far removed from their equilibrium values) it is shown that, in addition to the usual unsaturated and saturated behaviour there exists a third regime at high substrate concentration: the oversaturated region. In this region, the rate-limiting transition state is the interconversion of the unliganded forms of the enzyme: E1 and E2. Expressions for six different experiments involving deuterium, tritium and 14C labels are presented. By considering the results from these experiments, the nature and importance of the enzyme interconversion steps can be elucidated.

Energetics of proline racemase: transition-state fractionation factors for the two protons involved in the catalytic steps

The isotope effects for the interconversion of L-proline and D-proline, catalyzed by proline racemase, have been determined in the saturated region with both [2-2H]proline and [2-3H]proline. The deuterium fractionation factors for each of the protons in flight have been obtained from two kinds of experiment: by measuring the rate of racemization of one [2-2H]proline enantiomer as it racemizes into an equilibrated pool of unlabeled proline and by measuring the deuterium content of a proline sample at the optical rotation maximum that occurs when an equimolar mixture of one deuterium-labeled enantiomer and the other unlabeled enantiomer runs to equilibrium. The tritium fractionation factors for each of the protons in flight have been determined from measurements of the rate of loss of tritium to the solvent as one [2-3H]proline enantiomer runs to equilibrium. Good agreement is found among the fractionation factors determined by each method. The deuterium fractionation factors for the two protons are not identical: that for the proton derived from L-proline is 0.375 and that for the proton derived from D-proline is 0.44. This difference has been confirmed by a double-competition experiment in which the optical rotation of a mixture of DL-[2-2H]proline and unlabeled DL-proline is followed with time. The rotation (initially zero) passes through a maximum, from which the ratio of the two fractionation factors (0.86) is obtained. These data, coupled with the equilibrium fractionation factor for the 2-position of proline (which has been determined to be 1.17), provide the transition-state factors for each of the in-flight protons, and delineate the nature of the transition state(s) for the enzyme-catalyzed racemization.