Secondary structure of Ac-Ala$_n$-LysH$^+$ polyalanine peptides ($n$=5,10,15) in vacuo: Helical or not?

TL;DR

This study investigates whether polyalanine peptides in vacuo adopt helical structures, using density-functional theory and infrared spectroscopy, revealing that longer peptides favor helices while shorter ones are more flexible.

Contribution

It demonstrates that in vacuo, polyalanine peptides of length 10 and 15 predominantly form helices, while shorter peptides like n=5 show competing conformations, supported by combined theoretical and experimental analysis.

Findings

Longer peptides (n=10,15) favor helical structures.

Shorter peptide (n=5) has competing conformations.

Infrared spectra confirm theoretical predictions.

Abstract

The polyalanine-based peptide series Ac-Ala_n-LysH+ (n=5-20) is a prime example that a secondary structure motif which is well-known from the solution phase (here: helices) can be formed in vacuo. We here revisit this conclusion for n=5,10,15, using density-functional theory (van der Waals corrected generalized gradient approximation), and gas-phase infrared vibrational spectroscopy. For the longer molecules (n=10,15) \alpha-helical models provide good qualitative agreement (theory vs. experiment) already in the harmonic approximation. For n=5, the lowest energy conformer is not a simple helix, but competes closely with \alpha-helical motifs at 300K. Close agreement between infrared spectra from experiment and ab initio molecular dynamics (including anharmonic effects) supports our findings.