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Published on: July 27, 2022
Observation of zwitterion formation in the gas-phase H/D-exchange with CH(3)OD: solution-phase structures in the gas
Nick C Polfer1, Robert C Dunbar, Jos Oomens
1FOM-Institute for Plasma Physics Rijnhuizen, Nieuwegein, The Netherlands. polfer@rijnh.nl
Abstract:
Infrared spectroscopy of gas-phase singly deuterated [Trp + K](+) (formed by H/D exchange with CH(3)OD) shows that some (approximately 20%) kinetically stable zwitterionic (ZW) conformer is formed, based on the diagnostic antisymmetric CO stretch of the deprotonated carboxylate moiety, upsilon(as)(CO(2)(-)), 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 ND(3) 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 CH(3)OD 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.
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