Proteolytic enzymes: models for electrophilic assistance by NH in acylation

Abstract

The effect of increasing dioxan content of the medium has been employed to examine the possibility of intramolecular electrophilic assistance by NH in alkaline hydrolysis of the methyl esters of 2-acetamidobenzoic (I), pyrrole-2-carboxylic (VII), and benzimidazole-4(7)-carboxylic (V) acids. Significant enhancement of rate constant is observed over isomers where no participation is possible. Enhancements increased steeply with increasing dioxan concentration in agreement with a ‘built-in’ solvation by the NH of the incipient alkoxide ion (derived from the carbonyl oxygen) of the transition-state. Heterocyclic types [(VII) and (V)] exhibited maxima in their rate enhancements as the dioxan content increased. Since the NH to alkoxide bond distance in the heterocyclic types is larger than normally found for direct hydrogen-bonding the maximum is argued to arise from a water bridge acting as a relay between electrophile NH and incipient alkoxide ion of the transition-state.