“…However, lysine, understood to be the second most basic, has not showed any stable zwitterionic forms in the gas phase [21]. Histidine, though with basicity lower than lysine, has potentially multiple zwitterion moieties due to its imidazole side chain [22,23]. Thus, it would be interesting to know whether or not histidine possesses zwitterions in the gas phase and subsequently a systematic search was performed to identify them.…”