title: Crystallographic characterization of helical secondary structures in alpha/beta-peptides with 1:1 residue alternation.
abstract: Oligomers that contain both alpha- and beta-amino acid residues in a 1:1 alternating pattern have recently been shown by several groups to adopt helical secondary structures in solution. The beta-residue substitution pattern has a profound effect on the type of helix formed and the stability of the helical conformation. On the basis of two-dimensional NMR data, we have previously proposed that beta-residues with a five-membered ring constraint promote two different types of alpha/beta-peptide helix. The "11-helix" contains i, i+3 CO...H-N hydrogen bonds between backbone amide groups; these hydrogen bonds occur in 11-atom rings. The alpha/beta-peptide "14/15-helix" contains i, i+4 CO...H-N hydrogen bonds, which occur in alternating 14- and 15-atom rings. Here we provide crystallographic data for 14 alpha/beta-peptides that form the 11-helix and/or the 14/15-helix. These results were obtained for a series of oligomers containing beta-residues derived from ( S,S)- trans-2-aminocyclopentanecarboxylic acid (ACPC) and alpha-residues derived from alpha-aminoisobutyric acid (Aib) or l-alanine (Ala). The crystallized alpha/beta-peptides range in length from 4 to 10 residues. Nine of the alpha/beta-peptides display the 11-helix in the solid state, three display the 14/15-helix, and two display conformations that contain both i, i+3 and i, i+4 CO...H-N hydrogen bonds, but not bifurcated hydrogen bonds. Only 3 of the 14 crystal structures presented here have been previously described. These results suggest that longer alpha/beta-peptides prefer the 14/15-helix over the 11-helix, a conclusion that is consistent with previously reported NMR data obtained in solution.
authors.affiliation: Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
authors.forename: Soo Hyuk
authors.initials: SH
authors.lastname: Choi
authors.forename: Ilia A
authors.initials: IA
authors.lastname: Guzei
authors.forename: Lara C
authors.initials: LC
authors.lastname: Spencer
authors.forename: Samuel H
authors.initials: SH
authors.lastname: Gellman
mesh_terms: D002264:Carboxylic Acids / Q000737:chemistry; D018360:Crystallography, X-Ray; D003521:Cyclopropanes / Q000737:chemistry; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry*; D017510:Protein Folding; D017433:Protein Structure, Secondary; D012996:Solutions
chemical_list: D002264:Carboxylic Acids; D003521:Cyclopropanes; D010455:Peptides; D012996:Solutions; C521993:cis-beta-aminocyclopropanecarboxylic acid