Imidazole–Imidazole Hydrogen Bonding in the pH-Sensing Histidine Side Chains of Influenza A M2

[Image: see text] The arrangement of histidine side chains in influenza A M2 tetramer determines their pK(a) values, which define pH-controlled proton conduction critical to the virus lifecycle. Both water-associated and hydrogen-bonded imidazole–imidazolium histidine quaternary structures have been proposed, based on crystal structures and NMR chemical shifts, respectively. Here we show, using the conduction domain construct of M2 in lipid bilayers, that the imidazole rings are hydrogen bonded even at a pH of 7.8 in the neutral charge state. An intermolecular 8.9 ± 0.3 Hz (2h)J(NN) hydrogen bond is observed between H37 N(ε) and N(δ) recorded in a fully protonated sample with 100 kHz magic-angle spinning. This interaction could not be detected in the drug-bound sample.