Kinetics and mechanism of the Cu2+ induced hydrolysis of nitrile groups inthe side chain of tetraazamacrocycles. Models for nitrilases

A series of mono-N-functionalized tetraaza macrocycles having a nitrile gro up in their side chain have been synthesized and the kinetics and mechanism of the Cu2+ induced hydrolysis has been studied. Two factors were systemat ically varied: the length of the side chain and thus the distance between C u2+ and the nitrile group, as well as the rigidity of the macrocycle by int roducing an additional ethylene bridge. The mechanism of the hydrolysis pro ceeds by an intramolecular attack of a coordinated OH- onto the nitrile gro up in a five or six center transition state. The intramolecular nature of t he reaction has been proven (a) by the pH dependence of the hydrolysis, whi ch in some cases has a plateau at high pH values, (b) by the competitive in hibition with SCN-, and (c) by the spectral changes observed at high PH. Th e sequence of Cu2+ induced hydrolysis rates is the following: flexible macr ocycle with a short chain > rigid macrocycle with a short chain > flexible macrocycle with a longer chain similar to rigid macrocycle with a longer ch ain. The length of the side chain, which determines whether a five or six c enter transition state is formed, is the most important factor. The fastest hydrolysis has a half-life time of about 50 ms at pH 12.5 and 25 degreesC and indicates the efficiency of the metal ion. The rigidity of the macrocyc le also influences the reactivity since in the rigid complexes on one side the Cu2+ ion is less accessible for OH- to give the reactive intermediate a nd on the other side the transition state is less reactive because of topol ogical aspects.