Structural insights for designed alanine-rich helices: Comparing NMR helicity measures and conformational ensembles from molecular dynamics simulation

The temperature dependence of helical propensities for the peptides Ac-ZGG- (KAAAA)(3)X-NH2 (Z = Y or G, X A, K, and D-Arg) were studied both experimentally and by MD simulations. Good agreement is observed in both the absolute helical propensities as well as relative helical content along the sequence; the global minimum on the calculated free energy landscape corresponds to a single alpha-helical conformation running from K4 to A18 with some terminal fraying, particularly at the C-terminus. Energy component analysis shows that the single helix state has favorable intramolecular electrostatic energy due to hydrogen bonds, and that less-favorable two-helix globular states have favorable salvation energy. The central lysine residues do not appear to increase helicity; however, both experimental and simulation studies show increasing helicity in the series X = Ala -> Lys -> D-Arg. This C-capping preference was also experimentally confirmed in Ac-(KAAAA)(3)X-GY-NH2 and (KAAAA)(3)X-GY-NH2 sequences. The roles of the C-capping groups, and of lysines throughout the sequence, in the MD-derived ensembles are analyzed in detail. (C) 2008 Wiley Periodicals, Inc.