Kinetics of base hydrolysis of α-amino-acid esters in mixed-ligand complexes with [glycylglycinato(2–)] copper(II)

Abstract

Amino-acid esters(E) interact with [Cu(gly-glyO)] to give mixed-ligand complexes according to equilibria (i) and [Cu(gly-glyO)]+ E ⇌[Cu(gly-glyO)E](i), [Cu(gly-glyO)]+ HE⁺⇌[Cu(gly-glyO)E]+ H⁺(ii)(ii), where HE⁺ represents the protonated ester [graphic omitted]H₃CH(R)CO₂R′ and gly-glyO^2– is the dianion of glycylglycine, NH₂CH₂CO[graphic omitted]CH₂CO₂-. The ternary complex is only formed over a narrow pH range (8.5–10) since at higher pH there is evidence for the following competing equilibrium. Rate constants have been determined by pH-stat at [Cu(gly-glyO)E]+ OH^–⇌[Cu(gly-glyO)(OH)]^–+ E 25 °C and I= 0.1 mol dm^–3(K[NO₃]) for the processes (iii) and (iv) where A^– is the anion NH₂CH(R)CO₂^– and [Cu(gly-glyO)E]+ H₂O [graphic omitted] [Cu(gly-glyO)A]^–+ R′OH + H⁺(iii)[Cu(gly-glyO)E]+ OH^– [graphic omitted] [Cu(gly-glyO)A]^–+ R′OH (iv) E = gly-OMe, gly-OEt, L–α–ala-OEt, or L–phe–OEt (ala = alanine, phe = phenylalanine). The ester ligands in the ternary complexes undergo base hydrolysis ca. 50 times faster than the free esters E, consistent with the formation of a mixed-ligand complex involving a unidentate ester species.