Fig. 5

The sc(i) → mc(i - 4) hydrogen bonds stabilize polyQ helices. a Peptides used to determine the contribution of sc(i) → mc(i − 4) hydrogen bonds to the stability of helical secondary structures and the effect of shielding. b Projection of the fully helical structure of peptide L₄Q₁₆ in the color code used in a, where Lys residue areas are shown in gray, double circles represent the PGAS N-capping motif, Leu residues are displayed in white, and Gln residues in black, with an indication of the sc(i) → mc(i − 4) hydrogen bonds in purple and of the shielding of the hydrogen bond between residues Q1 and L1 by the side chain of L1, as a blue shade. One unshielded, weaker hydrogen bond between Q1 and Q5 is represented by a dashed purple arrow. c Helicity of the peptides listed in a as determined by circular dichroism (CD) at pH 7.4 and 277 K, with an indication of the number of shielded hydrogen bonds that can occur in each peptide according to the model shown in b. The vertical bars represent the values of helicity obtained after scaling the experimental spectra by factors 0.9 and 1.1 to account for experimental error in the determination of peptide concentration. d Regions of the ¹H,¹⁵N heteronuclear single quantum correlation (HSQC) spectrum, measured at pH 7.4 and 278 K, of peptides L₄Q₁₆ (see also Fig. 2b) and L1-4A containing the Hε₂₁ side chain resonances. e Region of the total correlation spectroscopy (TOCSY) spectrum of peptides L₄Q₁₆ and L1-4A measured at pH 7.4 and 278 K illustrating the β and γ ¹H resonances of the Gln side chains