Binding of Cu + and Cu 2+ with peptides: Peptides = oxytocin, Arg 8 ‐vasopressin, bradykinin, angiotensin‐I, substance‐P, somatostatin, and neurotensin

The intrinsic binding ability of 7 natural peptides (oxytocin, arg 8 ‐vasopressin, bradykinin, angiotensin‐I, substance‐P, somatostatin, and neurotensin) with copper in 2 different oxidation states (Cu I/II ) derived from different Cu +/2+ precursor sources have been investigated for their charge‐dependent binding characteristics. The peptide‐Cu I/II complexes, [M − ( n ‐1)H +  nC u I ] and [M − (2 n ‐1)H +  nC u II ], are prepared/generated by the reaction of peptides with CuI solution/Cu‐target and CuSO 4 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 +  nC u I ] and [M − (2 n ‐1)H +  nC u II ] complexes show no mass shift due to the loss of ─H atoms in the main chain ─NH of these peptides by Cu + and Cu 2+ deprotonation. The measured m / z value indicates the reduction of Cu I/II oxidation state into Cu 0 during MALDI processes. The number and relative abundance of Cu + bound to the peptides are greater compared with the Cu 2+ bound peptides. Oxytocin, arg 8 ‐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 2+ , oxytocin, arg 8 ‐vasopressin, angiotensin‐I, and substance‐P; somatostatin shows the binding of 3Cu 2+ ; and neurotensin shows 4Cu 2+ binding. The binding of more Cu + with these small peptides signifies that the bonding characteristics of both Cu + and Cu 2+ are different. The amino acid residues responsible for the binding of both Cu + and Cu 2+ 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 2+ for individual amino acids.