# Dresden 2006 – wissenschaftliches Programm

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# AKB: Biologische Physik

## AKB 19: Proteins

### AKB 19.2: Vortrag

### Mittwoch, 29. März 2006, 16:15–16:30, ZEU 260

**On the balance of the enthalpic and the entropic contributions to the stability of the helix: A DFT-GGA study** — •Lars Ismer^{1}, Joel Ireta^{1}, Matthias Scheffler^{1}, and Jörg Neugebauer^{2} — ^{1}Fritz-Haber-Insitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin — ^{2}Max-Planck-Institut für Eisenforschung, Max-Planck-Strasse 1, D-40237 Düsseldorf

Accurate theoretical studies of the thermodynamic stability of isolated
peptide chains may serve as a reliable reference to understand the stability
of the secondary structure of proteins. We have therefore calculated
the free energy difference needed to fold the fully extended structure
(FES) of isolated, infinite polyalanine (Ala) and -glycine (Gly) chains
into various helical conformations such as the 3_{10}-, α-, and π-helix. The calculations were done by employing DFT-GGA, plane waves, pseudopotentials and the quasi-harmonic approximation to estimate the finite temperature effects. We find that entropic contributions to the free energy
strongly reduce the enthalpic stability of the helices at elevated temperatures, leading to a transition to the FES at *T*_{c} ∼ 460 K for Ala and *T*_{c} ∼ 400 K for Gly. Below *T*_{c} the α-helix is the conformation with the lowest free energy. The π-helix shows the strongest temperature dependence resulting in a significant destabilization with respect to the α- and 3_{10}-helix for *T* > 0 K. A detailed analysis showed these thermodynamic trends to be *intrinsic* features of the specific hydrogen bonding network formed by the various helices and to be largely independent of the specific amino acid.