Stability of the Cu(II) Complexes of Tripeptides, Cu(H−2L), in Dynamic Aspects; L = Tripeptides Composed of Various Combinations of α-, β-, and γ-Amino-Acid Residues. Stopped-Flow Kinetic Studies on the Reaction of Cu(H−2L) with Cysteine

Abstract

Abstract The stability of the fused chelate-ring structures of Cu(H−2L) concerning dynamic aspects was evaluated based on the rate constant (k1+) for the formation of ternary complexes, Cu(H−1L)(Cys−), from Cu(H−2L) and cysteine (Cys), where L denotes tripeptides composed of various combinations of α-, β-, and γ-amino-acid residues. The ligand-exchange reaction was examined by the stopped-flow spectrophotometric method. The Cu(H−2L) species, having 5–6–5- and 6–5–5-membered fused-chelate rings, were kinetically fairly stable; k1+ = 2–3 × 104 M−1 s−1. Other Cu(H−2L) species, having 5–5–5-, 5–5–6-, 5–5–7-, 5–6–6-, and 6–5–6-membered rings, reacted rapidly with Cys to form the Cu(H−1L)(Cys−) species; k1+ ≥ 1 × 105 M−1 s−1. The Cu(H−1L)(Cys−) species, upon forming, successively reacted with Cys to afford a binary complex, Cu(Cys−)2. The Cu(H−1L)(Cys−) species, with the β-Ala residue at the N-terminus, were kinetically labile to be rapidly transformed to Cu(Cys−)2.