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Secondary Structure of Ac-Alan-LysH+ Polyalanine Peptides (n = 5,10,15) in Vacuo: Helical or Not?

MPS-Authors
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Rossi,  Mariana
Theory, Fritz Haber Institute, Max Planck Society;

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Blum,  Volker
Theory, Fritz Haber Institute, Max Planck Society;

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Kupser,  Peter
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Helden,  Gert von
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Bierau,  Frauke
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Meijer,  Gerard
Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Scheffler,  Matthias
Theory, Fritz Haber Institute, Max Planck Society;

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Citation

Rossi, M., Blum, V., Kupser, P., Helden, G. v., Bierau, F., Pagel, K., et al. (2010). Secondary Structure of Ac-Alan-LysH+ Polyalanine Peptides (n = 5,10,15) in Vacuo: Helical or Not? The Journal of Physical Chemistry Letters, 1(24), 3465-3470. doi:10.1021/jz101394u.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0011-2576-4
Abstract
The polyalanine-based peptide series Ac-Alan-LysH+ (n = 5−20) is a prime example that a secondary structure motif that is well-known from the solution phase (here: helices) can be formed in vacuo. Here we revisit the series members n = 5,10,15, using density functional theory (van der Waals corrected generalized gradient approximation) for structure predictions, which are then corroborated by room temperature gas-phase infrared vibrational spectroscopy. We employ a quantitative comparison based on Pendry’s reliability factor (popular in surface crystallography). In particular, including anharmonic effects into calculated spectra by way of ab initio molecular dynamics produces remarkably good experiment−theory agreement. We find the longer molecules (n = 10,15) to be firmly α-helical in character. For n = 5, calculated free-energy differences show different H-bond networks to still compete closely. Vibrational spectroscopy indicates a predominance of α-helical motifs at 300 K, but the lowest-energy conformer is not a simple helix.