Dataset Information

Structure-reactivity effects on primary deuterium isotope effects on protonation of ring-substituted alpha-methoxystyrenes.

ABSTRACT: Primary product isotope effects (PIEs) on L(+) and carboxylic acid catalyzed protonation of ring-substituted alpha-methoxystyrenes (X-1) to form oxocarbenium ions X-2(+) in 50/50 (v/v) HOH/DOD were calculated from the yields of the alpha-CH(3) and alpha-CH(2)D labeled ketone products, determined by (1)H NMR. A plot of PIE against reaction driving force shows a maximum PIE of 8.7 for protonation of 4-MeO-1 by Cl(2)CHCOOH (DeltaG(o) = 1.0 kcal/mol). The PIE decreases to 8.1 for protonation of 4-MeO-1 by L(3)O(+) (DeltaG(o) = -2.8 kcal/mol) and to 5.1 for protonation of 3,5-di-NO(2)-1 by MeOCH(2)COOH (DeltaG(o) = 13.1 kcal/mol). The PIE maximum is around DeltaG(o) = 0. Arrhenius-type plots of PIEs on protonation of 4-MeO-1 and 3,5-di-NO(2)-1 by L(3)O(+) and on protonation of X-1 by MeOCH(2)COOH in 50/50 (v/v) HOH/DOD give similar slopes and intercepts. These were used to calculate values of [(E(a))(H) - (E(a))(D)] = -1.2 kcal/mol and (A(H)/A(D)) = 1.0 for the difference in activation energy for reactions of A-H and A-D and for the limiting PIE at infinite temperature, respectively. These parameters are consistent with reaction of the hydron over an energy barrier. There is no evidence for quantum mechanical tunneling of the hydron through the barrier. These PIEs suggest that the transferred hydron at the transition state lies roughly equidistant between the acid donor and base acceptor and contrast with the recently published Brønsted parameters [Richard, J. P.; Williams, K. B. J. Am. Chem. Soc. 2007, 129, 6952-6961], which are consistent with a product-like transition state. An explanation for these seemingly contradictory results is discussed.

SUBMITTER: Tsang WY

PROVIDER: S-EPMC2825562 | biostudies-literature | 2009 Oct

REPOSITORIES: biostudies-literature

ACCESS DATA

Publications

Structure-reactivity effects on primary deuterium isotope effects on protonation of ring-substituted alpha-methoxystyrenes.

Tsang Wing-Yin WY Richard John P JP

Journal of the American Chemical Society 20091001 39

Primary product isotope effects (PIEs) on L(+) and carboxylic acid catalyzed protonation of ring-substituted alpha-methoxystyrenes (X-1) to form oxocarbenium ions X-2(+) in 50/50 (v/v) HOH/DOD were calculated from the yields of the alpha-CH(3) and alpha-CH(2)D labeled ketone products, determined by (1)H NMR. A plot of PIE against reaction driving force shows a maximum PIE of 8.7 for protonation of 4-MeO-1 by Cl(2)CHCOOH (DeltaG(o) = 1.0 kcal/mol). The PIE decreases to 8.1 for protonation of 4-Me ...[more]

PMID: 19788330

Similar Datasets

Project description:The L-lactate-flavocytochrome b2-ferricyanide electron-transfer system from the yeast Hansenula anomala was investigated by rapid-reaction techniques. The kinetics of reduction of oxidized flavocytochrome b2 by L-lactate and L-[2H]lactate were biphasic both for flavin and haem prosthetic groups and at all concentrations tested. The first-order rate constants of the rapid and slow phases depended upon substrate concentrations, a saturation behaviour being exhibited. Substitution of the C alpha-H atom by 2H was found to cause appreciable changes in the rate constants for the initial reduction of flavin and haem (phase I), which were respectively about 3-fold and 2-fold less than with L-lactate. In contrast, no significant isotope effect was noted on the apparent reduction rate constants of the slow phase, phase II. Under steady-state conditions an isotope effect of 2.0 was found on the overall electron transfer from L-lactate to ferricyanide. These transient reduction results were discussed in terms of a kinetic model implying intra- and inter-protomer electron exchanges between flavin and haem b2, all of which have been experimentally described. Computer simulations indicate that the reaction scheme provides a reasonable explanation of the fast-reduction phase, phase I (in absence of acceptor). The pseudo-first-order rate constant for oxidation of reduced haem b2 in flavocytochrome b2 increased with increasing ferricyanide concentration in a hyperbolic fashion. The limiting value at infinite ferricyanide concentration, which was attributed to the intramolecular electron-transfer rate from ferroflavocytochrome b2 to the iron of ferricyanide within a complex, was 920 +/- 50 s-1 at pH 7.0 and 5 degrees C. Stopped-flow and rapid-freezing measurements showed haem b2 and flavin to be 90 and 44% oxidized respectively under steady-state conditions in presence of ferricyanide. Simulation studies were carried out to check the participation of the proposed reduction sequence in the overall catalytic reaction together with the role of reduced haem b2 (Hr) and flavin semiquinone (Fsq) as electron donors to ferricyanide. When the rate of the intramolecular electron-transfer exchange between Fsq and ferricyanide was adjusted to 200 s-1, simulated data accounted for molar activities defined under various conditions of L-lactate, [2H]lactate and ferricyanide concentrations. Simulation studies were extended to data obtained using cytochrome c as acceptor and reaction catalysed by Saccharomyces cerevisiae flavocytochrome b2. The differences in reactivity observed for Hr and Fsq with ferricyanide and cytochrome c were discussed in terms of redox potentials, electrostatic interactions, distances and accessibility of the participating groups.

Dataset Information

Structure-reactivity effects on primary deuterium isotope effects on protonation of ring-substituted alpha-methoxystyrenes.

Publications

Structure-reactivity effects on primary deuterium isotope effects on protonation of ring-substituted alpha-methoxystyrenes.

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure