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-glycyl-L-histidyl-L-alanine ([H3L]2+) has been studied in aqueous solution at t = 25-degrees-C and I = 0.1 mol dm-3 by potentiometric, visible spectrophotometric and circular dichroism measurements. All the experimental techniques show that the complex [CuLH-2]- is predominant over a wide pH range, while the monodentate [CuLH-3]2- is formed in the acidic region and a further deprotonated [CuLH-3]2- species exists at pH higher than 10. The formation constants of the three above complexes are reported and a structure is proposed on the basis of spectroscopic results. The structure of [CuLH-2]- species very probably involves four nitrogen donors in the plane, excluding coordination by the L-alanine residue, while it seems likely that [CuLH-3]2- is formed by simple dissociation of the N-1 pyrrole hydrogen.