Comparative Studies on the Stability of a Chelate-Ring Unit in Dynamic Aspects. Chelate-Rings of the Cu(II) Complexes Composed of [N-(Glycyl)], [N-(β-Alanyl)], and [N-(2-Aminoethyl)] Moieties

The kinetic stability of a chelate-ring unit in Cu(H−1GlyGly), Cu(H−1β-AlaGly), and Cu(EDMA) complexes has been inspected by stopped-flow spectroscopy. Those three Cu(II) complexes reacted with cysteine (CysH) or penicillamine (PesH) to form primarily ternary complexes, formulated as Cu(H−1L)(Rs−); L = GlyGly and β-AlaGly, and Cu(EDMA)(Rs−); where Rs− represents the aminothiolates. The chelate-ring units composed of [N-(Gly)]-, [N-(β-Ala)]-, and [N-(2-aminoethyl)]-moieties in Cu(H−1GlyGly), Cu(H−1β-AlaGly), and Cu(EDMA) were conserved, respectively, in the corresponding ternary complexes. The rate of the ternary complex formation was very rapid, so much so that the rate constants, k1+, could not be determined by the conventional stopped-flow techniques. The ternary complexes upon forming reacted with the RsH to produce the binary Cu(Rs−)2 species. Thus, the observables were limited to the rate constants, k2+, for the transformation from Cu(H−1L)(Rs−) to Cu(Rs−)2. The kinetic stabilities of those chelate-ring units were compared based on the rate constant, k2+, and equilibrium constant, K2. The stability was arranged as follows: [N-(Gly)]- > [N-(2-aminoethyl)]- > [N-(β-Ala)]-unit. Though Cu(EDMA) was kinetically stable, the Cu(II) in Cu(EDMA)(Rs−) was rapidly delivered by a nucleophilic attack of RsH to form Cu(Rs−)2. On the contrary, Cu(H−1GlyGly) was kinetically less stable than Cu(EDMA), but delivery of the Cu(II) from Cu(H−1L)(Rs−), by the proton-assisted mechanism, was comparatively slow.