Vibrational Spectroscopy and Conformational Structure of Protonated Polyalanine Peptides Isolated in the Gas Phase |
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Authors: | Vaden Timothy D de Boer Tjalling S J A Simons John P Snoek Lavina C Suhai Sándor Paizs Béla |
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Affiliation: | Physical and Theoretical Chemistry Laboratory, Chemistry Department, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom, and German Cancer Research Center, Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany. |
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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. |
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