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Mixed 14/16 Helices in the Gas Phase: Conformation-Specific Spectroscopy of Z-(Gly)(n), n = 1, 3, 5.
MedLine Citation:
PMID:  23039317     Owner:  NLM     Status:  Publisher    
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.
Jacob C Dean; Evan G Buchanan; Timothy S Zwier
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-10-5
Journal Detail:
Title:  Journal of the American Chemical Society     Volume:  -     ISSN:  1520-5126     ISO Abbreviation:  J. Am. Chem. Soc.     Publication Date:  2012 Oct 
Date Detail:
Created Date:  2012-10-8     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  7503056     Medline TA:  J Am Chem Soc     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
Department of Chemistry, Purdue University , West Lafayette, Indiana 47907-2084, United States.
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