In this work we report the stereo-dependent collision-induced dissociation (CID) of proton-bound complexes of tryptophan and 2-butanol. The dissociation efficiency was measured as a function of collision energy in single collision mode. The homochiral complex was found to be less stable against CID than a heterochiral one. Additional gas dependence measurements were performed with diastereomeric complexes that confirm the findings.
KEYWORDSamino acids, chiral recognition, collision induced dissociation, gas phase, mass spectrometry
| INTRODUCTIONMass spectrometry (MS) is essentially an achiral method and cannot, in its baseline configuration, discriminate between enantiomers because they have the same mass and fragmentation pattern. This limitation can be overcome by placing the sample in a chiral environment, as first demonstrated by Fales and Wright, 1 and later refined by Tao and Cooks and other groups. 2 The developmental history of MS as a modern, cutting-edge tool in chiral analysis is comprehensively described in a number of recent reviews. [3][4][5] MS-based gas-phase experiments allow studies of the interaction of isolated molecules and ion-molecule pairs in order to observe subtle chiral effects without the influence of a solvent. Several of these techniques rely on the differential interaction of enantiomers in diastereomeric complexes. 6-9 Tao and Cooks developed a kinetic method for enantiomeric analysis. 2 In this method, a chiral analyte and a chiral reference compound are complexed with a transition-metal ion. The transition-metal induces sterically dependent multipoint interactions between the analyte and the reference molecules, which results in different free energies for the trimeric cluster ions formed by the enantiomers of the analyte. This small difference can be converted into detectable differences in fragment-ion branching ratios. 10,11 In a recent study, Zehnacker-Rentien and co-workers investigated systems without a transition metal and detected differences in the collision-induced dissociation rates of hetero-and homochiral complexes of camphor and alanine. 12 The observation of a higher fragmentation efficiency for the heterochiral complex, despite it being calculated to be slightly more stable than the homochiral system, was explained by the presence of a second, lower-energy, conformer of the homochiral complex. The chiral effects of molecular and ionic complexes have been widely studied with different theoretical and experimental approaches in gas-phase spectroscopy. 13 The previous chiral recognition experiments were performed in ion trap configurations, with numerous collisions with a target gas over a distribution of kinetic energies. We recently embarked on a program using ion beams and thin targets in order to study chiral interactions in single,