Hamburg 2009 – wissenschaftliches Programm

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MO: Fachverband Molekülphysik

MO 2: Biomoleküle 1

MO 2.4: Vortrag

Montag, 2. März 2009, 14:45–15:00, VMP 6 HS-F

Ac-Ala_nLysH⁺ in vacuo: secondary structure of polyalanine peptides part II (Density Functional Theory) — •Mariana Rossi, Volker Blum, Peter Kupser, Gert von Helden, Frauke Bierau, Gerard Meijer, and Matthias Scheffler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Predicting the structure and stability of peptides is a challenge, and in vacuo studies provide unique, well-defined conditions to match state-of-the-art experiments and theory. We here present a combined force field (OPLS) / density-functional theory (DFT) study of the polyalanine peptides Ac-Ala_nLysH⁺ (n=5,10,15), for which earlier experiments indicate helical secondary structure [1]; the results are verified by comparing to room-temperature multiphoton IR vibrational spectra measured using the FELIX free-electron laser (part I). For Ac-Ala₅LysH⁺, we pre-screen O(10⁵) candidate structures in the force field, and then use DFT in the generalized gradient approximation with van der Waals corrections (GGA+vdW) in the FHI-aims code [2] to refine a wide range of low-energy conformers. We show (i) how the LysH⁺ termination acts to stabilize helical structures, and (ii) a close energetic competition between different helical motifs (e.g. α, 3₁₀, mixed) in the Ala₅ part. The vibrational spectra are consistent with an α-helical motif. We address systematic frequency shifts between theoretical (harmonic) and the measured spectra, and the role of anharmonicities through extensive ab initio molecular dynamics simulations. [1] R. Hudgins, et al., JACS 120, 12974 (1998). [2] V. Blum, et al., Comput. Phys. Comm., accepted (2008).