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Coordination of cooper(II) to cyclic peptides with cysteinic disulfide bridge: Complex stability and visible absorption spectra

Solutions containing copper(II) and an -S-S- bridged cyclic oligopeptide (general formula Cys-(Gly)n-Cys, CG(n)C, 0 less-than-or-equal-to n less-than-or-equal-to 4) in aqueous KCl (0.1 mol dm-3) were studied at 25-degrees-C by potentiometric titration methods and absorption spectrometry. The ligands studied were found to give rise to a number of complexes ; besides several (3 to 5) monoligand complexes, the formation of at least one (up to 5) bis-ligand species was detected for each ligand. Stability constants of all detected complex species (4 to 11 per ligand) were determined. From the acidity constants of individual complexes (computed from stability constants) the hapticities of the ligands were inferred. Visible absorption spectra of all examined solutions contain one peak whose position shifts hypsochromically (almost- equal-to 740 nm ... 540 nm) with increasing deprotonation and hapticity, indicating changes in the coordination geometry of the copper(II) ion, probably from tetragonal towards distorted tetrahedral. With CG2C, and additional absorption peak (588 nm) was osbserved.

cysteinic peptides ; cyclic peptides ; complex ; stability ; absorption spectra

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