COORDINATION OF COPPER(II) TO CYCLIC-PEPTIDES WITH A CYSTEINIC DISULFIDE BRIDGE - COMPLEX STABILITY AND VISIBLE ABSORPTION-SPECTRA

Solutions containing copper(II) and an -S-S- bridged cyclic oligopepti de (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 spect rometry. The ligands studied were found to give rise to a number of co mplexes; besides several (3 to 5) monoligand complexes, the formation of at least one (up to 5) bis-ligand species was detected for each lig and. Stability constants of all detected complex species (4 to 11 per ligand) were determined. From the acidity constants of individual comp lexes (computed from stability constants) the hapticities of the ligan ds 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, in dicating changes in the coordination geometry of the copper(II) ion, p robably from tetragonal towards distorted tetrahedral. With CG2C, and additional absorption peak (588 nm) was osbserved.