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

The 1H and 13C nuclear magnetic resonance 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 analysed 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‐protonated melanostatin are involved in the intermolecular hydrogen bonds of aggregates. The leucine NH group of N‐protonated Pro‐Leu‐Gly also forms the intermolecular hydrogen bond. The solvent and temperature dependences of the chemical shifts of amide‐proton and carbonyl‐carbon resonances were measured to determine intramolecular hydrogen bonding. In dimethylsulfoxide solution, N‐protonated melanostatin molecules in part take the β‐turn structure and the trans carboxamide NH proton and carbonyl oxygen of the proline residue form an intramolecular hydrogen bond.