Fig. 3: Gln side chain to main chain hydrogen bonds can be accepted by different residues.

a Top: Sequence, numbering, and representation, as helical projection, of the L₃XQ₁₆ variants studied in this work. Bottom: residue-specific helical propensity of the L₃XQ₁₆ variants. The type of residue X (position 10) is indicated by the colored circles. Left: helical profile of the 7 most helical single variants. Center: helical profile of the least helical single variants and L₄Q₁₆. Right: Helical profile of the outlier variants (X = T, S) and L₄Q₁₆ (spectra in Supplementary Fig. 4b). b Effect of the rotameric state of the acceptor on the interaction of atom H_ε21 with H₂O. Left: two frames of the L₄Q₈ Charmm36m trajectory showing Gln4 involved in a bifurcated hydrogen bond with Leu 4 in either the mt (top) or tp (bottom) rotamer. Right: radial distribution function for Gln4 H_ε21 in the frames where Leu 4 populates the mt (red) or tp (gold) rotamer (shades show the 95% CI obtained from 10 block bootstrapping). c The frequency of the side chain to main chain hydrogen bond is strongly correlated with the solvent accessibility surface area (SASA) of H_ε21, which depends on the type of residue X. d Multiple regression correlating intrinsic helicity (x1) (Pace and Scholtz scale²⁹) and SASA (x2) with the average helicity (y): the data were standardized to estimate the relative weight of each variable in defining the model. e Measured versus predicted average L₃XQ₁₆ helicity. f Squared r correlation scores (r²) for the multiple regression shown in e, using different reported scales for intrinsic amino acid helicity and H_ε21 SASA values derived from two sets of 1 μs MD trajectories independently generated with different force fields. The results are shown for all and apolar (L, I, V, F, Y, M, W, A) residues in position X. g Number of ScanProsite-identified protein sequences in UniprotKB (including the Swiss-Prot and TrEMBL databases) containing the (P3-7)_n motif with an increasing number of Gln_i+4 → Ω_i pairs (Ω = W, L, Y, F, I, M; X = any amino acid). h Representative (P3-7)_n-like natural sequences with UniprotKB annotation score = 5.