Copper(II)-cis,cis-1,3,5-triaminocyclohexane complex-promoted hydrolysis of dipeptides: kinetic, speciation and structural studies

Abstract.

The hydrolysis of glycylglycine (GylGly), glycyl-L-leucine (GlyLeu), L-leucylglycine (LeuGly) and glycyl-DL-serine (GlySer) promoted by a copper(II)-cis,cis-1,3,5-triaminocyclohexane complex [Cu(II)TACH] was investigated at 70 °C and pH 7–10, using HPLC. The observed pseudo-first-order rate constants (k _obs) and rate enhancing factors (REF) were as follows: 4.1×10^–3 h^–1 (REF=23) for GylGly, 1.6×10^–3 h^–1 (REF=21) for GlyLeu, 5.1×10^–3 h^–1 (REF=64) for LeuGly and 9.2×10^–2 h^–1 (REF=47) for GlySer [pH 8.1, dipeptide 2 mM, copper(II) 2 mM and TACH 2 mM]. Based on the pH dependence and dipeptide concentration dependence of the initial rates and speciation of the Cu(II)-TACH-dipeptide system at 25 °C and I=0.1, the reactions proceed via the formation of a ternary complex [Cu(TACH)(dipeptide)]⁺ as an intermediate followed by OH^–-dependent and OH^–-independent paths to give amino acid(s). GylGly, GlyLeu and LeuGly preferred the OH^–-dependent path, while GlySer preferred the OH^–-independent path. The latter can be explained by the intramolecular attack of the amide carbonyl group coordinated with its oxygen atom by the OH group in the serine residue. The X-ray crystal structure of [Cu(TACH)(GlyGly)]BPh₄·MeOH confirmed that GlyGly coordinates to copper(II) ion with its terminal amino N and amide O atoms. The crystal structures of [Cu(TACH)(Gly)]BPh₄ and [Cu₂(TACH)₂(OH)₂](ClO₄)₂·NaClO₄·H₂O are also reported. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-002-0368-9.