Observation of Zwitterion Formation in the Gas-Phase H/D-Exchange with CH3OD: Solution-Phase Structures in the Gas Phase

Infrared spectroscopy of gas-phase singly deuterated [Trp + K]+ (formed by H/D exchange with CH3OD) shows that some (∼20%) kinetically stable zwitterionic (ZW) conformer is formed, based on the diagnostic antisymmetric CO stretch of the deprotonated carboxylate moiety, υas(CO2−), at 1680 cm−1. A majority of the deuterated [Trp + K]+ is found to be in the charge solvation (CS) conformation, with deuterium exchange occurring on both the acid and amino groups, which is consistent with H/D scrambling. Interestingly, H/D exchange with the more basic ND3 reagent did not result in the stabilization of a kinetically stable zwitterion, although it is not clear yet what causes this observation. The result for CH3OD shows that H/D exchange can in fact alter the structure of the analyte and, hence, care needs to be taken when interpreting gas-phase H/D exchange studies. Moreover, this result shows the possibility of forming solution-phase structures that are thermodynamically disfavored in the gas phase, thus opening a new area of study.