Coordination of Cu2+ Ions to C2 Symmetric Pseudopeptides Derived from Valine

Abstract

The acid−base and coordination properties of a family of pseudopeptidic ligands with C2 symmetry derived from valine (4a−e) have been studied using a variety of techniques as a model for metal coordination in peptides and proteins. The Cu2+ cation has been selected for coordination studies, although, for comparison, some results for Zn2+ are also presented. Good agreement has been obtained between the results obtained by potentiometric titrations, spectroscopic analysis, and mass spectrometry (ESI) studies. These results highlight the potential for the use of ESI MS for characterizing the nature of the complex species formed. Clearly, the Cu2+ complexes are much more stable than the Zn2+ complexes. While the role of the aliphatic spacer seems to be very minor in the case of the Zn2+ complexes, revealing the ability of this cation to accommodate different coordination environments, this role is critical in the case of Cu2+. Different complexes with 1:1 or 2:2 Cu2+:L stoichiometries can be formed according to the length of the spacer and the basicity of the media. This is fully illustrated by the resolution of the X-ray structures of two different Cu2+ complexes corresponding to the ligands containing a spacer with two methylene groups (ligand 4a, complex 6a [Cu2(H−1L)2](ClO4)2 with a 2:2 stoichiometry) and a propylene spacer (4b, complex 5b [CuH−2L]·CH3CH2OH with a 1:1 stoichiometry).