The Influence of Glycyl Residues on the Flexibility of Peptide Hormones in Solution

13 C nuclear magnetic resonance spectroscopy is used to gain information on the flexibility of the backbone in peptide hormones and peptide hormone analogs. 13 C spin‐lattice relaxation times ( T 1 ) were measured on luliberin, the luteinizing‐hormone‐releasing hormone and des(Gly‐NH 2 ) 10 ‐luliberin‐ N ‐ethylamide in aqueous solution at 25.2 and 67.9 MHz at temperatures of 32°, 40° and 55°C. The 13 C spin‐lattice relaxation times indicate increased flexibility of the peptide backbone in the immediate environment of glycyl residues in luliberin (<Glu‐His‐Trp‐Ser‐Tyr‐Gly‐Leu‐Arg‐Pro‐Gly‐NH 2 ) and the hormone analog des(Gly‐NH 2 ) 10 ‐luliberin‐ N ‐ethylamide (<Glu‐His‐Trp‐Ser‐Tyr‐Gly‐Leu‐Arg‐Pro‐NH‐CH 2 CH 3 ) in aqueous solutions. 13 C NMR spectroscopy is shown to be a sensitive technique for monitoring the time‐averaged conformational flexibility of peptides in solution. Activation energies ( E a ) of about 25 kJ/mol were obtained for rotational reorientation of non‐terminal α‐carbons in the peptide backbone. Rotation of methyl groups was characterized by an E a of 9.6 kJ/mol whereas reorientation of the N‐terminal pyroglutamyl residue showed an E a value of 14.6 kJ/mol. The E a values of individual carbons in the side‐chains of prolyl, arginyl and leucyl residues in the peptides were similar to those obtained for the α‐carbon of the same amino acid residue in the peptide backbone of the hormones.