Solution structures of cyclic and dicyclic analogues of growth hormone releasing factor as determined by two-dimensional NMR and CD spectroscopies and constrained molecular dynamics

Solution structures were determined for a linear analogue of growth hormone releasing factor (GRF), and cyclic and dicyclic analogues in which the side chains of aspartyl and lysyl residues spaced at positions i‐(i + 4) were joined to form a lactam. The four analogues were [Ala15]‐GRF‐(1–29)‐NH2 and its cyclo8–12, cyclo21–25, and dicyclo8–12;21–25 derivatives. The peptides were studied in two solvent systems: 75% methanol/25% water at pH 6.0; and 100% water at pH 3.0. CD spectroscopy was used to assess the overall α‐helical content. Nuclear magnetic resonance spectroscopy was used to determine the structures in more detail. Nearly complete proton resonance assignments were made for each of the peptides, in both solvents. Nuclear Overhauser effects were converted into distance constraints and applied in the molecular dynamics program CHARMM to evaluate the range of low‐energy structures that satisfied the nmr data. In 75% methanol, all of the peptides are comprised of a single α‐helical segment with fraying of one to three residues at each end. The linear analogue has a tendency to kink. In water, the analogues have two helical segments with flexible regions between them and at the termini of the peptides. The linear analogue is helical at residues 7–14 and 21–28. In the cyclo8–12 analogue, the N‐terminal helical region extends to include residues 7–19, while the other helical region is slightly shortened. In the cyclo21–25 analogue, the C‐terminal helical region is extended to include residues 19–28, while the N‐terminal helical region is destabilized. The dicyclic analogue has the largest N‐terminal helix, spanning residues 7–20, but its helical segment at residues 21–28 is not well ordered. All of the analogues exhibit substantial biological activity. The cyclic and dicyclic analogues show dramatically increased resistance to degradation during incubation with human plasma. The i−(i + 4) lactam, therefore, appears to be a synthetic means of stabilizing a local α‐helical conformation, which may be of general use in the design of active, stable peptides.