Berlin 2012 – scientific programme
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BP: Fachverband Biologische Physik
BP 1: Proteins I
BP 1.11: Talk
Monday, March 26, 2012, 12:45–13:00, H 1058
Stabilization of peptide helices with respect to length and vibrational free energy — •Mariana Rossi, Volker Blum, and Matthias Scheffler — Fritz-Haber-Institut, Faradayweg 4-6, 14195 Berlin
We here address the helix-forming alanine-based Ac-Alan-LysH+ polypeptide series in the gas phase, for which experiments [1] have indicated helical onset at n=8. We quantify helix stabilization wrt. peptide length and temperature [harmonic approximation for vibrational free energies (FE)], which are effects that can be dissected and accurately benchmarked in the gas phase. After an initial force-field screening, we fully relax thousands of conformers using density-functional theory with the van der Waals (vdW) corrected [2] PBE exchange-correlation potential. α-Helices are the lowest energy structures at n ≈ 7−8 on the potential energy surface, but only barely. Interestingly, helices are systematically stabilized over globular conformers by inclusion of vibrational FE at 300K. The vibrational entropy is the leading stabilizing term at 300K, but also zero-point-energies favor helical structures by a significant amount. For n≥8, the α-helix should be the only accessible conformer in the FE surface at 300K, in agreement with experiment [1] and with our own quantitative comparison [3] of calculated ab initio anharmonic IR spectra to experimental IR multiphoton dissociation (IRMPD) data for Ac-Alan-LysH+, n=5, 10, 15. [1] Tkatchenko and Scheffler, PRL 102, 073055 (2009); [2] Kohtani and Jarrold, JACS 108, 8454 (2004); [3] Rossi et al., JPCL 1, 3465 (2010).