Fig. 4: Introduction of a pH-sensitive conformational switch.

a The side chains of Gln and Glu at pH 2.8, but not that of Glu at pH 7.4 can donate a hydrogen to the main chain CO. b QxE variants of L₄Q₁₆ with pH-sensitive Glu_i+4 → X_i interactions shown in red. c Helical propensities at pH 7.4 (black) and 2.8 (dark red) compared to those of L₄Q₁₆ at pH 7.4 (orange). d Residue-specific differences in helical propensity due to substitution of Gln by Glu at pH 7.4 (black) and pH 2.8 (dark red). e–g Gln side chain N_ε2-H_ε21 regions of the ¹H-¹⁵N HSQC spectra of QxE variants at pH 7.4 (left) and 2.8 (right) with the spectrum of L₄Q₁₆ overlaid as an orange shade. The spectra of variants Q1E and Q4E with ¹⁵N labeling of only the Gln in position i+4 to the mutated residue are superimposed in green. h QM/MM-derived hydrogen bond electron densities for the Glu_i+4 → Leu_i interaction. Mean values for the Gln_i+4 → Leu_i interaction¹⁴ are shown as an orange line. First and second panels: normalized histograms showing the distribution of the electron density ρ(r) of the main chain to main chain interaction in the absence (white background) and in the presence (gray) of the side chain to main chain hydrogen bond. Third panel: electron densities for the side chain to main chain hydrogen bond. Fourth panel: electron densities for the bifurcated hydrogen bond. i Natural population analysis (NPA) charges on the donor hydrogen atom in Glu and Gln, showing that its charge depends on whether it participates in the hydrogen bond (lower values) or not. j, k pH-dependent helicity and thermal stability for peptide E(P3-7)_n. Top: Helical projections. Center: CD spectra (278 K) of peptides E(P3-7)_n and (P3-7)_n at the indicated conditions. Bottom: thermal denaturation of peptides E(P3-7)_n and (P3-7)_n monitored by measuring the mean residue ellipticity at 222 nm. l 2D CACO NMR spectra of E(P3-7)₃. m Residue-specific helical propensities for peptide E(P3-7)₃ at pH 2.8.