Nuclear‐Magnetic‐Resonance Study of Aggregations and Conformations of Melanostatin and Related Peptides

Abstract

The 1H and 13C NMR spectra of melanostatin (Pro-Leu-Gly-NH2) and related peptides (Pro-Leu-Gly, Z-Pro-Leu-Gly, Z-Pro-Leu-Gly-NH2 and Z-Pro-Leu-Gly-OCH3, where Z = benzyloxycarbonyl) were analyzed in a variety of solvents. At physiological pH, the melanostatin molecule is N-protonated in aqueous solution. The concentration dependences of the chemical shifts of amide-proton and carbonyl-carbon resonances and of proton spin-lattice relaxation times were observed in relation to molecular aggregations. In dimethylsulfoxide solution, aggregations were observed for N-protonated melanostatin and Pro-Leu-Gly prepared with HCl and for the Na salt of Z-Pro-Leu-Gly but not for N-protonated melanostatin prepared with HClO4 or HNO3, unprotonated melanostatin, Z-Pro-Leu-Gly-NH2, or Z-Pro-Leu-Gly-OCH3. The leucine NH and glycine CO groups of N-pronated melanostatin are involved in the intermoledular H-bonds of aggregates. The leucine NH group of N-protonated Pro-Leu-Gly also forms the intermolecular H-bond. The solvent and temperature dependences of the chemical shifts of amide-proton and carbonyl-carbon resonances were measured to determine intramolecular H-bonding. In dimethylsulfoxide solution, N-protonated melanostatin molecules in part take the .beta.-turn structure and the trans carboxamide NH proton and carbonyl oxygen of the proline residue form an intramolecular hydrogen bond.