Cationic metallovesicles: catalysis of the cleavage of p-nitrophenyl picolinate and control of copper(II) permeation

Ligand surfactants 1-3 have been synthesized. They feature a 2-(hydroxymethyl)pyridine as the ligand subunit and different lipophilic backbones which allow the formation, in aqueous solution, of different aggregates: vesicles in the case of 1 and 2, micelles in the case of 3. In the presence of Cu2+ ions, these aggregates are catalysts of the hydrolytic cleavage of activated esters of amino acids. Those made of surfactants 1 and 2 constitute one of the first examples of hydrolytic metallovesicles. The kinetic investigation was focused on the hydrolysis of the p-nitrophenyl ester of picolinic acid (PNPP): in line with previous results, the kinetic effects were attributed to the pseudo-intramolecular transacylation of a metal-ion-coordinated hydroxy group of the ligand subunit via the formation of a ternary complex comprising surfactant, Cu2+, and PNPP. The observed accelerations were shown to be strongly dependent on the nature of the aggregate and, for vesicles, on the structure of the lipophilic backbone. This modulation of the reactivity has been attributed to (i) the different tendencies of the ligand amphiphiles in the aggregates to form nonproductive ternary complexes involving two ligand molecules and one Cu2+ ion and (ii) the rate of flip-flop of the surfactants which is suggested to be responsible for the transport of the Cu2+ ions across the vesicular membrane.