Vibrational Spectroscopy and Conformational Structure of Protonated Polyalanine Peptides Isolated in the Gas Phase

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 112; no. 20; pp. 4608 - 4616
• Main Authors: Vaden, Timothy D, de Boer, Tjalling S. J. A, Simons, John P, Snoek, Lavina C, Suhai, Sándor, Paizs, Béla
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 22.05.2008
• Subjects: A: Kinetics, Spectroscopy; Gases - chemistry; Models, Molecular; Peptides - chemistry; Protein Conformation; Spectrum Analysis; Vibration
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp800069n

The conformational structures of protonated polyalanine peptides, AlanH+, 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, Ala3H+, 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, Ala7H+, 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.