Amphiphilic urocanic acid derivatives as catalysts of ester hydrolysis

Abstract

The hydrolysis of p-nitrophenyl acetate at pH 8 and 25°C is demonstrated to be a good test reaction to study the influence of the structure of several long-chain derivatives of urocanic acid on the imidazole ring reactivity in organized media. Cetyltrimethylammonium bromide micelles including (E)-dodecyl urocanate gave an approximately 7-fold rate enhancement over (Z)-dodecyl urocanate, 30-fold over (E)-urocanic acid and 4790-fold over CTABr. The behavior of (E)-dodecyl urocanate in the presence of excess substrate was also been investigated. The value of k_obs decreased by 37% as the substrate:catalyst ratio was increased from 1:10 to 10:1. Under the same conditions, the total activity was restored when one equivalent of chloral was added to one equivalent of (E)-dodecyl urocanate. Chloral significantly enhanced the deacylation rate of N-acetyl dodecyl urocanate, giving a good reaction turnover.