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BP: Fachverband Biologische Physik
BP 4: Protein Structure and Folding
BP 4.5: Talk
Monday, March 26, 2007, 18:30–18:45, H43
Why are pi-helices so seldomly observed in proteins ? — Lars Ismer1, Joel Ireta2, and •Joerg Neugebauer1 — 1Max-Planck-Institut fuer Eisenforschung, Max-Planck-Strasse 1, D-40237 Duesseldorf — 2Fritz-Haber-Institut, Faradayweg 4-6, D-14195 Berlin
Three different helical secondary structure motifs are observed in proteins: the alpha-, the 3-10-, and the pi-helix. While the alpha- and the 3-10-helix show occurences of about 80 % and 20 % respectively, the pi-helix is, however, only found in exceptional cases. The existing explanations herefore given in literature are rather qualitative and based on empirical assumptions. We here present a free energy analysis of infinitely long poly-L-alanine and -glycine helices which is based on DFT-GGA and the harmonic approximation and which is free of any empirical input parameters. We show, that the rarity of the pi-helix can be explained as an entropic effect, which is intrinsic and exists even in the absence of any environmental aspects, like solvents. By means of elasticity theory we show that the origin of the instability is due to geometric peculiarities of the pi-helix and independent of the amino acid sequence.