Figure 2.

¹³C CP-MAS spectra of His-27 in VM+ membrane-bound BM2(1–33) as a function of pH. The spectra were measured at 243 or 263 K (A) and 303 or 313 K (B) under 7–14.5-kHz MAS. At low temperature, the Cδ2 chemical shift differed significantly between low and high pH, with cationic histidine (∼117 ppm, +) present at low pH and neutral tautomers (∼114 ppm, τ) present at high pH. At high temperature, a Cδ2 exchange peak was observed at ∼118 ppm, indicating interconversion among all three histidine species.