Ternary Cu(II) complexes, Cu(H−1L)(ACys−) and Cu(H−2L)(ACys−); L = peptides, ACys− = N-acetyl-cysteinate. Analogous complexes to the intermediates in the transport of Cu(II) from Cu(H−2L) to cysteine

The Cu(II) in Cu(H−2L) has been postulated to be successively transported to cysteine (Cys) as follows; Cu(H−2L) ⇄ Cu(H−2L)(Cys∗−) ⇄ Cu(H−1L)(Cys∗−) → Cu(H−1L)(Cys−), where Cys∗− denotes the monodentate Cys−. N-acetyl-cysteinate (ACys−) complexes Cu(H−2L)(ACys−) and Cu(H−1L)(ACys−), having similar coordination modes to Cu(H−2L)(Cys∗−) and Cu(H−1L)(Cys∗−), respectively, exhibited the S → Cu(II) charge transfer absorption at 325–355 nm and the d–d absorption at 530–610 nm. A linear interrelation existed between the energies of the CD and d–d absorptions. Cu(H−2L)(ACys−) were in rapid equilibrium with Cu(H−1L)(ACys−). Upon forming the ternary complex, pKc2 of the parent Cu(H−1L) was raised to more than 1.0. The formation constants (K) of the Cu(H−1L)(ACys−) species from Cu(H−1L) were bigger than those of Cu(H−2L)(ACys−) from Cu(H−2L). The linear free-energy relationship existed between the free-energy change (ΔG) and the entropy change (ΔS) for the ternary complex formation. The rate constants (k1+) for the Cu(H−1L)(Cys−) formation closely correlated with the K values for Cu(H−2L)(ACys−). The ternary complexes containing ACys are considered to be analogous complexes to the intermediates in the transport of Cu(II) from peptides to cysteine.