Acyl transfer reactions in functional micelles studied by proton magnetic resonance at 270 MHz

Abstract

The course of micelle-catalysed reactions at millimolar concentrations in aqueous solution may be followed by Fourier transform n.m.r. Micelles of the histidine-bearing surfactant (4a) react rapidly with p-nitrophenyl acetate in phosphate-buffered media between pD 7 and 8, entirely by acyl transfer to the imidazole with no concurrent general-base catalysis. The resulting acylimidazole breaks down by a pH-independent route between pD 7.0 and 7.9 forming both acetate ion and monoacetyl phosphate, the latter by reaction with the counter-ion of the buffer. In similar reactions of p-nitrophenyl N-acetylphenylalanine with (4a) no acylimidazole intermediate is observed. The surfactant hydroxamic acid (5a) is also readily acylated by p-nitrophenyl acetate in the same pH region, and the resulting acylhydroxamate is unstable to hydrolysis. This latter reaction is pH-dependent and exhibits a solvent isotope effect of 2.7 at pH(pD) 7.9. Addition of (5a) to the acylimidazole intermediate derived from (4a) causes rapid acyl transfer, although the latter does not affect the rate of acylhydroxamate hydrolysis.