title: Solution, solid phase and computational structures of apicidin and its backbone-reduced analogs.
abstract: The recently isolated broad-spectrum antiparasitic apicidin (1) is one of the few naturally occurring cyclic tetrapeptides (CTP). Depending on the solvent, the backbone of 1 exhibits two gamma-turns (in CH(2)Cl(2)) or a beta-turn (in DMSO), differing solely in the rotation of the plane of one of the amide bonds. In the X-ray crystal structure, the peptidic C==Os and NHs are on opposite sides of the backbone plane, giving rise to infinite stacks of cyclotetrapeptides connected by three intermolecular hydrogen bonds between the backbones. Conformational searches (Amber force field) on a truncated model system of 1 confirm all three backbone conformations to be low-energy states. The previously synthesized analogs of 1 containing a reduced amide bond exhibit the same backbone conformation as 1 in DMSO, which is confirmed further by the X-ray crystal structure of a model system of the desoxy analogs of 1. This similarity helps in explaining why the desoxy analogs retain some of the antiprotozoal activities of apicidin. The backbone-reduction approach designed to facilitate the cyclization step of the acyclic precursors of the CTPs seems to retain the conformational preferences of the parent peptide backbone.
authors.affiliation: GlaxoSmithKline Cambridge Chemistry Laboratory, Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK. michael.j.kranz@gsk.com
authors.forename: Michael
authors.initials: M
authors.lastname: Kranz
authors.forename: Peter John
authors.initials: PJ
authors.lastname: Murray
authors.forename: Stephen
authors.initials: S
authors.lastname: Taylor
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Upton
authors.forename: William
authors.initials: W
authors.lastname: Clegg
authors.forename: Mark R J
authors.initials: MR
authors.lastname: Elsegood
mesh_terms: D003198:Computer Simulation; D018360:Crystallography, X-Ray / Q000379:methods; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008958:Models, Molecular; D015394:Molecular Structure; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry*; D010456:Peptides, Cyclic / Q000138:chemical synthesis / Q000737:chemistry*; D011487:Protein Conformation
chemical_list: D009842:Oligopeptides; D010456:Peptides, Cyclic; C102351:apicidin