Our group has been studying chiral recognition in gas phase using mass spectrometry for more than 10 years. We are interested in gas phase studies of fundamental interactions because the gas phase avoids complications and masking effects that may arise upon solvation. Therefore, the results of gas phase experiments can be directly compared with those of high-level computational studies. In chapter 2, I studied the roles of hydrogen bonding and pi stacking in gas phase chiral recognition between aromatic crown molecules and aromatic amines. High affinity between host and guest doesn't necessarily result in better recognition. If the affinity is too high, both host enantiomers will bind to the chiral guest very tightly so little discrimination is observed. In order to build an efficient chiral recognition system, we need to select a host and guest that have intermediate binding affinity. Hydrogen bonding is another significant factor that controls the host-guest affinity. In the case of host 1, more hydrogen bonds results in better recognition. We also find that the degree of chiral recognition is greater in the gas phase than in solution. Modeling at the B3LYP/6-31G* level is qualitatively correct, but quantitative agreement with experiment is poor. Inspired by Rekharsky's work which shows successful induced chiral recognition with an achiral host (cucurbituril) in solution, we tested the possibility of applying cucurbiturils as gas phase chiral recognition containers in chapter 5. Conferring chirality on cucurbiturils makes the chiral recognition happen in a restricted space, which might strengthen or hinder the discrimination. By comparing our results with Rekharsky's, we showed the role of solvent in this chiral recognition process. In the gas phase, the enantiodiscrimination does not happen between the "leaving MP" and the "approaching" stronger chiral binder. Because hydrophobic effects are absent in the gas phase, it is possible that the hydrophobic methyl substituent of 2-methylpiperazine and the stronger chiral binder might not be simultaneously included inside the cavity. Therefore, we do not observe enantiodiscrimination in gas phase. The dissociation experiment for the CB ternary complex shows that sec-butylamine binds externally to the CB host. Further, the heterochiral diastereomer is more stable than the homochiral diastereomer. This conclusion is consistent with Rekharsky's result in solution. For more than 15 years, the most common ionization method in our lab has been electrospray ionization. However, ESI is subject to problems with ion suppression, especially when the sample is a mixture or it has a high concentration of salt. The easily ionized molecules tend to scavenge the available charges in the spray solution and dominate the resulting ion population even though other compounds may be present in high abundance. Nanoelectrospray usually yields cold ionization, and analyte suppression can be greatly reduced at nanospray flow rates. Therefore, we constructed a porous polymer monolith (PPM) nanospray emitter similar to that described by Oleschuk et al. and characterized the properties of the PPM emitter. This work is described in chapter 3. Our tests show that this PPM nanospray emitter possesses some special analytical properties: decreased ion suppression, quite stable spray, strong signal intensity and good reproducibility in emitter performance. Chapter 4 deals with the application of the new CRAFTI method to negative ions. CRAFTI stands for cross-sectional areas by Fourier transform ICR. The CRAFTI technique measures collision cross sections, providing a probe of the gas phase conformations of supramolecular complexes. Our preliminary work has shown that CRAFTI is applicable to positive ions, so we further demonstrate the application of the newly-developed method to negative ions in this work. Based on the fact that the experimental cross sections correlate linearly with the theoretical values, we have obtained evidence that CRAFTI is a valid method for negative ions. However, some problems remain. First, we are still working to understand the physical meaning of the CRAFTI cross sections. The absolute values we obtain are generally greater than those obtained from momentum transfer cross section calculations modeled in helium. Second, the precision of the measurements (currently about 2-3%) is still larger than we desire. We need to carefully tune the excitation and isolation amplitudes to make the signal strong and monoisotopic for weak ions. CRAFTI is a very promising and attractive method because FT-ICR provides accurate mass-to-charge measurement along with the cross section measurement. In other words, one technique is sufficient to obtain the shape, size and mass of a molecule simultaneously.
College and Department
Physical and Mathematical Sciences; Chemistry and Biochemistry
BYU ScholarsArchive Citation
Fang, Nannan, "Gas Phase Chiral Recognition, Characterization of Porous Polymer Monolith Nanospray Ionization, and the Negative Mode CRAFTI Method Using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry" (2009). All Theses and Dissertations. 1921.
gas phase, chiral recognition, nanospray ionization, collision cross section, FTICR, mass spectrometry