Binding of Cu and Cu with peptides: Peptides = oxytocin, Arg -vasopressin, bradykinin, angiotensin-I, substance-P, somatostatin, and neurotensin.

The intrinsic binding ability of 7 natural peptides (oxytocin, arg -vasopressin, bradykinin, angiotensin-I, substance-P, somatostatin, and neurotensin) with copper in 2 different oxidation states (Cu ) derived from different Cu precursor sources have been investigated for their charge-dependent binding characteristics. The peptide-Cu complexes, [M - (n-1)H + nCu ] and [M - (2n-1)H + nCu ], are prepared/generated by the reaction of peptides with CuI solution/Cu-target and CuSO solution and are analyzed by using matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. The MALDI mass spectra of both [M - (n-1)H + nCu ] and [M - (2n-1)H + nCu ] complexes show no mass shift due to the loss of ─H atoms in the main chain ─NH of these peptides by Cu and Cu deprotonation. The measured m/z value indicates the reduction of Cu oxidation state into Cu during MALDI processes. The number and relative abundance of Cu bound to the peptides are greater compared with the Cu bound peptides. Oxytocin, arg -vasopressin, bradykinin, substance-P, and somatostatin show the binding of 5Cu , and angiotensin-I and neurotensin show the binding of 7Cu from both CuI and Cu targets, while bradykinin shows the binding of 2Cu , oxytocin, arg -vasopressin, angiotensin-I, and substance-P; somatostatin shows the binding of 3Cu ; and neurotensin shows 4Cu binding. The binding of more Cu with these small peptides signifies that the bonding characteristics of both Cu and Cu are different. The amino acid residues responsible for the binding of both Cu and Cu in these peptides have been identified based on the density functional theory computed binding energy values of Cu and the fragment transformation method predicted binding preference of Cu for individual amino acids.