Observation of zwitterion formation in the gas-phase H/D-exchange with CH(3)OD: solution-phase structures in the gas phase.
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.
Polfer
Nick C
NC
FOM-Institute for Plasma Physics Rijnhuizen, Nieuwegein, The Netherlands. polfer@rijnh.nl
Dunbar
Robert C
RC
Oomens
Jos
J
D003903:Deuterium / Q000737:chemistry*; D041961:Deuterium Exchange Measurement / Q000379:methods*; D005740:Gases / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D007700:Kinetics; D013058:Mass Spectrometry; D008958:Models, Molecular; D008968:Molecular Conformation; D013055:Spectrophotometry, Infrared
D005740:Gases; D006859:Hydrogen; D003903:Deuterium