title: Vibrational spectroscopy and conformational structure of protonated polyalanine peptides isolated in the gas phase.
abstract: The conformational structures of protonated polyalanine peptides, Ala(n)H(+), have been investigated in the gas phase for n = 3, 4, 5, and 7 using a combination of resonant-infrared multiphoton dissociation (R-IRMPD) spectroscopy in the NH and OH stretch regions and quantum chemical calculations. Agreement between theoretical IR and experimental R-IRMPD spectral features has enabled the assignment of specific hydrogen-bonded conformational motifs in the short protonated peptides and revealed their conformational evolution under elevated-temperature conditions, as a function of increasing chain length. The shortest peptide, Ala(3)H(+), adopts a mixture of extended and cyclic chain conformations, protonated, respectively, at a backbone carbonyl or the N-terminus. The longer peptides adopt folded, cyclic, and globular charge-solvated conformations protonated at the N-terminus, consistent with previous ion-mobility studies. The longest peptide, Ala(7)H(+), adopts a globular conformation with the N-terminus completely charge-solvated, demonstrating the emergence of "physiologically relevant" intramolecular interactions in the peptide backbone. The computed conformational relative free energies highlight the importance of entropic contributions in these peptides.
authors.affiliation: Physical and Theoretical Chemistry Laboratory, Chemistry Department, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom. timothy.vaden@chem.ox.ac.uk
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Vaden
authors.forename: Tjalling S J A
authors.initials: TS
authors.lastname: de Boer
authors.forename: John P
authors.initials: JP
authors.lastname: Simons
authors.forename: Lavina C
authors.initials: LC
authors.lastname: Snoek
authors.forename: Sándor
authors.initials: S
authors.lastname: Suhai
authors.forename: Béla
authors.initials: B
authors.lastname: Paizs
mesh_terms: D005740:Gases / Q000737:chemistry*; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D013057:Spectrum Analysis; D014732:Vibration
chemical_list: D005740:Gases; D010455:Peptides; C019529:polyalanine