Short-chain oligopeptides with copper(II) binding properties: the impact of specific structural modifications on the copper(II) coordination abilities

A series of linear tetrapeptides containing two histidyl residues in position 2 and 4, namely DHGH, DHGDH, KHGH, KHGdH, Ac-DHGH-NH2, Ac-DHGdH-NH2, Ac-KHGH-NH2, and Ac-KHGdH-NH2, were syn- thesized and characterised. Their copper(II) binding properties were investigated in depth through a vari- ety of physicochemical methods. Potentiometric titrations were first carried out to establish the stoichiometry and the stability of the resulting copper(II)–peptide complexes. The copper(II) chromoph- ores that are formed in the various cases in dependence of pH were subsequently characterised by exten- sive spectroscopic analysis (UV–Vis, EPR, CD) in strict correlation with potentiometric data. The effects of the nature of the first amino acid (Lys versus Asp) and of N-terminal amino group protection on copper(II) binding were specifically addressed. On turn, the careful comparison of the copper(II) coordination abil- ities of the linear peptides with those of their cyclic analogs provided insight into the effects of cyclization on the overall metal binding properties.