Calcium ions affect the exchange network but not the structure of a small peptide (melanostatin) in solution: A H-1 and C-13 NMR spectroscopic 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 N OESY maps and the temperature coefficients of the amide 1H NMR chemical shi fts were measured at increasing concentrations of calcium. A 1:1 complex wi th 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 evide nce was also provided for the occurrence of a minor species with the metal bound to the Leu carbonyl and with different stoichiometry. Upon metal comp lexation, substantial changes in the intrinsic chain flexibility of the pep tide and in the exchange rates between water and amide protons were detecte d.