Figure 5.

pH‐dependent chemical shifts of His42. The pK _a values of His42 were determined from ¹⁵N‐decoupled ¹³C‐HSQC spectra of PrgK_19–92 selectively‐labeled with ¹³C/¹⁵N‐histidine. (Left) Assigned spectra recorded at pH 5.2 showing the (A) ¹³C^ɛ1‐¹H^ɛ1 and (B) ¹³C^δ2‐¹H^δ2 signals. Each ¹³C^δ2 signal is a doublet due to scalar coupling with adjacent the ¹³C^γ. (Right) Eleven superimposed spectra recorded as the sample was titrated in small steps from pH 5.2 (red) to 8.7 (light blue). Signals from conformer B diminish in intensity as conformer A is increasingly favored under alkaline conditions. The nomenclature is illustrated for the neutral N^δ1H tautomer of a histidine sidechain