Potentiometric and spectroscopic studies of reactions of histidine with 13-membered amide-based macrocyclic Cu2+ chelates

The Cu2+ complex of a 13-membrred macrocycle, 2,9-dioxo-1,4,7,10-tetraaza-4 ,7-cyclotridecanediacetic acid (abbreviated as LH2), formed histidine adduc ts in which a histidine molecule occupied a secondary coordination site of the macrocyclic Cu2+ chelate. The formation of these adducts was observed a s a change in the d-d band of the macrocyclic Cu2+ chelate in the presence of histidine at different pH values. The compositions of the species were d etermined by potentiometric titration, and the formation constants were obt ained as: log[CuL(hdH)]/[CuL][hdH] = 2.8(0.2) and log[Cu(LH-2)(hdH)]/[Cu(LH -2)][hdH] = 2.6(0.4) (hd = histidinate(-1) ion; LH-2 = L with deprotonated amide nitrogen atoms). These adducts are formed by the coordination of the imidazole ring nitrogen atom to the central metal ion, and are stabilized b y an interaction (such as hydrogen bond formation) between the pendant arms of the macrocyclic chelate and the substrate. (C) 1999 Elsevier Science S. A. All rights reserved.