Internal Proton Transfer Leading to Stable Zwitterionic Structures in a Neutral Isolated Peptide

No solution for the zwitterion: An acid‐ and base‐containing pentapeptide was designed to explore the possibility of zwitterion formation in the gas phase, in the absence of a net charge. Internal proton transfer between peptide side chains in vacuo gave a zwitterion (highlighted in yellow; the canonical form is highlighted in blue), which was identified by gas‐phase IR spectroscopy.