POTENTIOMETRIC AND SPECTROSCOPIC STUDY OF COPPER(II) COMPLEXES WITH GLYCYL-GLYCYL-L-HISTIDYL-L-ALANINE IN AQUEOUS-SOLUTION

The complex formation between copper(II) and the tetrapeptide glycyl-g lycyl-L-histidyl-L-alanine (H3L!2+) has been studied in aqueous solut ion at t = 25-degrees-C and I = 0.1 mol dm-3 by potentiometric, visibl e spectrophotometric and circular dichroism measurements. All the expe rimental techniques show that the complex CuLH-2!- is predominant ove r a wide pH range, while the monodentate CuLH-3!2- is formed in the a cidic region and a further deprotonated CuLH-3!2- species exists at p H higher than 10. The formation constants of the three above complexes are reported and a structure is proposed on the basis of spectroscopi c results. The structure of CuLH-2!- species very probably involves f our nitrogen donors in the plane, excluding coordination by the L-alan ine residue, while it seems likely that CuLH-3!2- is formed by simple dissociation of the N-1 pyrrole hydrogen.