Circular Dichroism of Model Peptides Emulating the Amphipathic α-Helical Regions of Intermediate Filaments

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Abstract
The a and d positions of the heptad repeats (abcdefg) found in the α-helical sections of intermediate-filament proteins are hydrophobic, and the remaining locations are almost exclusively hydrophilic and often charged. Two synthetic peptides that maximize these features were designed, synthesized, and investigated by circular dichroism for α-helix formation in water and in 50% trifluoroethanol (TFE). A 14-residue peptide, AcNLEELKKKLEELKGNH2 (NLEKG14), had mean residue ellipticities at 222 nm ([θ]222) of −18 400 ± 1000 and −37 200 ± 1900 deg cm2 dmol-1, in water at 2 °C and in 50% TFE at 2 °C, respectively. A longer version of NLEKG14, AcNLEELKKKLEELKQQLEELKKKLEELKQQNH2 (NLEKQ29), had [θ]222 of −43 000 ± 2200 deg cm2 dmol-1 in water and in 50% TFE at 2 °C. Using −43 000 deg cm2 dmol-1 as [θ]222 for a 100% helix, NLEKG14 in 50% TFE at 25 °C was estimated to be 77% helix. This estimate was confirmed by two-dimensional 1H NMR studies of NLEKG14 in 50% TFE. Comparison with the sequences and conformations found in IF proteins indicates that the α-helical regions in the proteins may be exceptionally stable, but the high values for the ellipticity of α-helices now revealed allow for significant portions of the protein rod regions to be occupied by conformations other than α-helix.