Esterolytic Reactivities of Novel Copper(Ii) Complexes with Reduced N-Salicylate Threonine Schiff Bases as Carboxylase Models

Abstract

Three new bi- or tetranuclear copper (II) complexes (complexes 1, 2 and 3) with reduced N-salicylate threonine were synthesized and characterized by single crystal X-ray diffraction, X-ray powder diffraction, UV-vis, and thermogravimetric analysis. Single-crystal diffraction analysis reveals that these complexes have similar coordination modes and conformations which were linked into multidimensional networks through some weak interactions. These three complexes were employed as a model of carboxylesterases to promote the hydrolytic cleavage of p-nitrophenyl picolinate (PNPP) and p-nitrophenyl acetate (PNPA). Esterolytic reactivities of the as-prepared complexes show that in the hydrolysis of p-nitrophenyl acetate (PNPA) these three complexes provide 51, 45, 39-fold rate acceleration as compared to spontaneous hydrolysis of PNPA at pH 7.0, respectively. Under selected conditions, excess three orders of magnitude rate enhancement were observed for the catalytic hydrolysis of another carboxylic ester, p-nitrophenyl picolinate (PNPP). Furthermore, these complexes efficiently promote the PNPA hydrolysis in micellar solution of cetyltrimethylammonium bromide (CTAB) or bis(hexadecylhexmethyl ammonium)ethane bromide (16-6-16) giving rise to excess two orders of magnitude rate enhancement which is approximate 2.0 -4.0 times higher than that in the buffer. However, the introduction of micelles of n-lauroylsarcosine sodium (LSS) or double-chained C 22/8 surfactant did not increase the rate or even inhibited the hydrolysis reaction in contrast with buffered aqueous medium.

Keywords: Reduce amino acid Schiff bases, Copper(II) complexes, Esterolysis, Micellar effects