title: Crystallographic characterization of 12-helical secondary structure in β-peptides containing side chain groups.
abstract: Helices are the most extensively studied secondary structures formed by β-peptide foldamers. Among the five known β-peptide helices, the 12-helix is particularly interesting because the internal hydrogen bond orientation and macrodipole are analogous to those of α-peptide helices (α-helix and 3(10)-helix). The β-peptide 12-helix is defined by i, i+3 C═O···H-N backbone hydrogen bonds and promoted by β-residues with a five-membered ring constraint. The 12-helical scaffold has been used to generate β-peptides with specific biological functions, for which diverse side chains must be properly placed along the backbone and, upon folding, properly arranged in space. Only two crystal structures of 12-helical β-peptides have previously been reported, both for homooligomers of trans-2-aminocyclopentanecarboxylic acid (ACPC). Here we report five additional crystal structures of 12-helical β-peptides, all containing residues that bear side chains. Four of the crystallized β-peptides include trans-4,4-dimethyl-2-aminocyclopentanecarboxylic acid (dm-ACPC) residues, and the fifth contains a β(3)-hPhe residue. These five β-peptides adopt fully folded 12-helical conformations in the solid state. The new crystal structures, along with previously reported data, allow a detailed characterization of the 12-helical conformation; average backbone torsion angles of β-residues and helical parameters are derived. These structural parameters are found to be similar to those for i, i+3 C═O···H-N hydrogen-bonded helices formed by other peptide backbones generated from α- and/or β-amino acids. The similarity between the conformational behavior of dm-ACPC and ACPC is consistent with previous NMR-based conclusions that 4,4-disubstituted ACPC derivatives are compatible with 12-helical folding. In addition, our data show how a β(3)-residue is accommodated in the 12-helix, thus enhancing understanding of the diverse conformational behavior of this flexible class of β-amino acids.
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: D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry*; D017433:Protein Structure, Secondary
chemical_list: D010455:Peptides