Berlin 2008 – scientific programme
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BP: Fachverband Biologische Physik
BP 13: Biopolymers
BP 13.5: Talk
Tuesday, February 26, 2008, 18:15–18:30, PC 203
Why ion-terminated, finite polyalanine helices are stable in the gas phase — •Volker Blum1, Joel Ireta1,2, Alexandre Tkatchenko1, and Matthias Scheffler1 — 1Fritz-Haber-Institut, Berlin, Germany — 2Universidad Autonoma Metropolitana-Iztapalapa, Mexico DF, Mexico
The formation of helical secondary structure in peptides is often associated with a “hydrophobic effect”, but a series of landmark experiments indicate an intrinsic helical stability of isolated gas-phase polyalanine peptides, when terminated, e.g., by simple alkali ions [1]. We here quantify the mechanisms that stabilize Li+ ion-terminated helices from first principles, including (i) the direct stabilization by saturating missing H-bonds near the C-terminus, (ii) indirect (de-)stabilization by the absence of H-bond cooperativity in short chains, (iii) the electrostatic presence of the positive ion, which more than offsets the missing H-bond cooperativity, and (iv) the stabilizing effect of non-local correlation (van der Waals) between side chains with increasing helix length. (i), (ii), and (iii) are covered by density-functional theory (DFT) in the PBE generalized gradient approximation; regarding (i), we show how details of the termination affect the stability hierarchy of different helix types (α, 310) For short helices Alan (n=1-10), we capture (iv) by quantum-chemical MP2 perturbation theory; this is used to corroborate a set of semi-empirical C6 corrections to DFT, which are then used to describe the limit of even larger helices. [1] Kohtani, Kinnear, Jarrold, JACS 122, 12377 (2000).