Relation of the transition-state structure for the water-catalyzed hydrolysis of 1-acetylimidazolium ion to solvent hydrophobicity: proton inventories in water-acetonitrile mixtures

Abstract

The transition-state structure for the water-catalyzed hydrolysis of 1-acetylimidazolium ion has been probed in solvent systems which may mimic the hydrophobic nature of an enzyme's active site. The kinetic solvent deuterium isotope effects, k/sub H/sub 2/O//k/sub D/sub 2/O/, are 2.58, 2.49, and 2.10 in water, in 0.5 vol fraction of acetonitrile in water and 0.9 vol fraction of acetonitrile in water, respectively. The proton inventory investigations suggest all three solvent systems entertain a transition-state structure composed of a catalytic proton bridge between the reorganizing substrate and a water molecule acting as a general-base catalyst. A compression of the transition-state structure in the solvent system containing the largest amount of acetonitrile is suggested to be responsible for the diminished kinetic solvent deuterium isotope effect. The reaction has been shown to be second order with respect to water.