Glycylsarcosyl-L-histidylglycine, a peptide with a ''breakpoint'' in complex formation: Hydrolysis of the complex [CuH(-1)L]

Coordination of Cu(II) with glycylsarcosyl-L-histidylglycine to form the series of [CuL] and [CuH(-1)L] was studied potentiometrically and spectroscopically. [CuL] includes the Cu(N2O)O chromophore, in which Cu(II) coordinates with nitrogens from the terminal amino and imidazole groups and a carbonyl oxygen of the Gly-Sar bond, and shows a d-d transition band at 696 nm in the absorption spectrum and at 648 nm in the CD spectrum. The predominant species around pH 8 was [CuH(-1)L] with Cu(N3O) chromophore, in which Cu(II) coordinates with three nitrogens from a terminal amino, a deprotonated Sar-His peptide bond, and an imidazole of a histidyl residue, and shows the d-d transition band at 652 and 580 nm in the absorption and CD spectra, respectively. This species was hydrolyzed at the Gly-Sar peptide bond, yielding the Cu(II) complex of sarcosyl-L-histidylglycine.