Flexibility in peptide molecules and restraints imposed by hydrogen bonds, the Aib residue, and core inserts

The first segments of this article review some of the early crystal structure determinations of beta-bends, gamma-bends, large conformational changes in cyclic peptides upon complexation with metal ions, and both the stability and drastic change in conformation as a function of the solvent used for growing crystals. More recent structure analyses have concerned helical peptides containing the Aib and/or Dpg residues and associated conformational problems such as 310-/alpha-helix transitions, helix aberrations, reversals, severe curving of the helix, unfolding, and hydration of backbone. Ion channels occurring in three crystal forms of zervamicin and possible ion transport and gating mechanisms are described. Finally, hydrogen bonding patterns are presented in supramolecular assemblies of retropeptides with core inserts consisting of oxalyl, adipoyl, suberoyl and polymethylene moieties.