Calcium ions affect the exchange network but not the structure of a small peptide (melanostatin) in solution: a 1H- and 13C-NMR study

The interaction of calcium ions with the peptide hormone melanostatin (Pro-Leu-Gly-NH2) was investigated by H-1 and C-13 NMR spectroscopy in [D-6]DMSO containing H2O (1%). Chemical shifts, spin-lattice relaxation rates, H-1 NOESY maps and the temperature coefficients of the amide 1H NMR chemical shifts were measured at increasing concentrations of calcium. A 1:1 complex with the metal coordinated to the carbonyl moieties of Pro and Gly (K-d = 17 +/- 2 mM(-1)) was shown to be the major species in solution, although evidence was also provided for the occurrence of a minor species with the metal bound to the Leu carbonyl and with different stoichiometry. Upon metal complexation, substantial changes in the intrinsic chain flexibility of the peptide and in the exchange rates between water and amide protons were detected