Fig. 3.

His³⁷ sidechains reorient at low pH but remain static at high pH at physiological temperature. (A) 303 K ¹³C-¹⁵N dipolar couplings. At pH 8.5, a 1:1 combination of Cγ-Nδ1 and Cε1-Nδ1 couplings reaches the rigid limit. At pH 4.5, the dominant Cγ-Nδ1 coupling is motionally averaged. (B) Cδ2-Hδ2 coupling at 308 K is motionally averaged at pH 4.5 but in the rigid limit at pH 8.5. (C) Measured order parameters at pH 4.5. (D) Two-site jump of imidazolium at low pH. A 45° reorientation around the Cβ-Cγ bond fits the observed order parameters.