The Cu2+ complex of a 13-membered macrocycle, 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclotridecanediacetic acid (abbreviated as LH2), formed histidine adducts in which a histidine molecule occupied a secondary coordination site of the macrocyclic Cu2+ chelate. The formation of these adducts was observed as 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 determined by potentiometric titration, and the formation constants were obtained 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 by an interaction (such as hydrogen bond formation) between the pendant arms of the macrocyclic chelate and the substrate. © 1999 Elsevier Science S.A.