A binary catalytic system based on mixed monolayers of a phospholipid and amphiphilic bis(Zn2+-cyclen)

Abstract

Herein, we report on the study of the properties of mixed Langmuir monolayers composed of a synthetic amphiphilic receptor, alkylated bis-cyclic zinc complex of 1,4,7,10-tetraazacyclododecane, and a lipid, distearoyl phosphatidylcholine. The kinetics of a hydrolysis of a model substrate, bis(p-nitrophenyl) phosphate in individual and mixed monolayers of the amphiphilic receptor was studied by using fiber-optical absorption/reflection spectroscopy. The hydrolysis of the organic phosphate in these planar systems proceeded by a two-stage mechanism. This mechanism comprises substrate adsorption on the monolayer via a reaction of the zero order with respect to the adsorbate followed by the pseudo-second-order reaction of the hydrolytic decomposition of the substrate. Unlike the reaction in molecular and colloidal solutions, the process in the monolayer results in the complete decomposition of the model substrate into nitrophenol and phosphate anion. The amphiphilic receptor is directly involved in this reaction to yield a stable complex with the phosphate anion as a resulting product of hydrolysis. An increase in the receptor affinity for the phosphate anion is, most likely, due to the effect of the interface on the strength of the coordination bonds in an intermediate product and the receptor-phosphate complex. Immobilization of the receptor within the lipid matrix increases the rate of substrate decomposition in the monolayer by almost an order of magnitude. We suggested an explanation for the observed effect of the lipid matrix on the catalytic properties of the amphiphilic metallocomplex.