STUDIES OF COPPER COMPLEXES OF PEPTIDES CONTAINING ALPHA-AMINOISOBUTYRIC ACID: PHOTOCHEMISTRY OF COPPER(III), ULTRAVIOLET-VISIBLE SPECTRA OF COPPER(III,II) AND STABILITY CONSTANTS OF THE COPPER(II) COMPLEXES

The photochemical instability of copper(III) complexes of peptides containing the (alpha)-aminoisobutryic acid residue (Aib), NH(,2)C(CH(,3))COOH, contrasts with their excellent thermal stability. The photochemical behavior of copper(III)-Aib peptide complexes is characterized by disappearance quantum yield measurements and by photodecomposition product analysis using reverse phase liquid chromatography. The copper(III) disappearance quantum yield, (phi), depends on the ligand-to-metal-charge-transfer (LMCT) band irradiated and the peptide ligand structure. All copper(III)-peptide complexes studied have two LMCT transitions occurring at 250-265 nm and 365-370 nm for tetrapeptides and at 267-278 nm and 385-395 nm for tripeptides. The two transitions have been tentatively assigned as (sigma)- and (pi)-deprotonated-peptide-nitrogen-to-copper(III) charge transfer. For copper(III)-Aib(,3), (phi) has plateau values of 0.34 and 0.23 moles einstein(\u27-1) for (sigma)- and (pi)-LMCT irradiation, respectively. For copper(III)-G(,2)AibG, the plateau (phi) values are 0.45 and 0.10 mole einstein(\u27-1). Replacement of the (alpha)-carbon methyl groups with hydrogen decreases photoredox reactivity and increases thermal redox reactivity. The photodecomposition products depend on the LMCT band irradiated and on solution pH. For copper(III)-Aib(,3), the difference in the identity of the peptide fragments between (sigma)- and (pi)-LMCT irradiation suggest the copper(II)-amidyl radical states populated by (sigma)-LMCT irradiation are more reactive than are the (pi)-copper(II)-amidyl states. The proton and copper(II) association constants for the Aib(,n) (n = 2-4) and Aib(,n)a (n = 1-3) series of ligands and for G(,2)AibG and Aib(,3)G were measured by potentiometric titration. The copper(II) complexes of the Aib ligands are 2.5 to 40 times more stable thermodynamically than the corresponding glycl complexes. In accord with the stability constant measurements, the visible absorption band maxima for the copper(II)-Aib peptide complexes are blue shifted 30-40 nm relative to the glycyl peptide complexes. This (alpha)-carbon methyl group effect needs to be taken into account in predictions of absorption maxima by rules of average environment