Preparation and Kinetic Properties of Cysteine Surfactants

A cysteine surfactant (IV, AS-Cys) was synthesized by coupling cysteine to N-cetyl-N, N,-dimethyl-N-β-aminoethyl-ammonium chloride. Under micellar conditions at pH 8.0, excess AS-Cys cleaved p-nitrophenyl acetate (PNPA) with kψmax=1.04 s-1 (corresponding to &:cat=26.0 L/mol-s), and the formation of S-acetyl-AS-Cys. The latter surfactant underwent intramolecular S→N transfer (A:max=0.44 s-1 for nonmicellar conditions at pH 8.0, k=0.01 s-1 in 5.5X10-3 M micellar cetyltrimethylammonium chloride) affording N-acetyl-AS-Cys. A second mole of PNPA could be cleaved by the free SH group of micellar N-acetyl-AS-Cys (kψmax=1.45 s-1 at pH 8.0, corresponding to kcat=36.3 L/mol s) yielding N, S-diacetyl-AS-Cys. S-Deacetylation of the latter was slow (kψ~5X10-5 s-1) under micellar conditions at pH 8, but could be accelerated by comicellization with imidazole-functionalized surfactants (e.g., kψ=9.2X10-4 s-1). Various mechanistic aspects of these reactions are discussed in detai.