title: Mixed 14/16 helices in the gas phase: conformation-specific spectroscopy of Z-(Gly)n, n = 1, 3, 5.
abstract: Single-conformation ultraviolet and infrared spectroscopy has been carried out on the neutral peptide series, Z-(Gly)(n)-OH, n = 1,3,5 (ZGn) and Z-(Gly)(5)-NHMe (ZG5-NHMe) in the isolated environment of a supersonic expansion. The N-terminal Z-cap (carboxybenzyl) provides an ultraviolet chromophore for resonant two-photon ionization (R2PI) spectroscopy. Conformation-specific infrared spectra were recorded in double resonance using resonant ion-dip infrared spectroscopy (RIDIRS). By comparing the experimental spectra with the predictions of DFT M05-2X/6-31+G(d) calculations, the structures could be characterized in terms of the sequence of intramolecular H-bonded rings of varying size. Despite the enhanced flexibility of the glycine residues, a total of only six conformers were observed among the four molecules. Two conformers for ZG1 were found with the major conformation taking on an extended, planar β-strand conformation. Two conformers were observed for ZG3, with the majority of the population in a C11/C7/C7/π(g-) structure that forms a full loop of the glycine chain. Both ZG5 molecules had their population primarily in a single conformation, with structures characteristic of the first stages of a "mixed" β-helix. C14/C16 H-bonded rings in opposing directions (N → C and C → N) tie the helix together, with nearest-neighbor C7 rings turning the backbone so that it forms the helix. φ/ψ angles alternate in sign along the backbone, as is characteristic of the mixed, C14/C16 β-helix. The calculated conformational energies of these structures are unusually stable relative to all others, with energies significantly lower than the PGI/PGII conformations characteristic of polyglycine structures in solution and in the crystalline form, where intermolecular H-bonds play a role.
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, USA.
authors.forename: Jacob C
authors.initials: JC
authors.lastname: Dean
authors.forename: Evan G
authors.initials: EG
authors.lastname: Buchanan
authors.forename: Timothy S
authors.initials: TS
authors.lastname: Zwier
mesh_terms: D005740:Gases / Q000737:chemistry; D005998:Glycine / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D015394:Molecular Structure; D010455:Peptides / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
chemical_list: D005740:Gases; D010455:Peptides; D005998:Glycine