Investigating the structures and properties of DMS-selected amino acid ions

Abstract

This thesis outlines gas-phase studies of ionized amino acids in which their structures and interactions within ion cluster complexes are probed using mass spectrometry, differential ion mobility, and laser spectroscopy. Chapter 1 discusses the motivation and background of this thesis, while chapter 2 introduces the methods applied in the research project. In chapter 3, ultraviolet photodissociation (UVPD) spectroscopy of molecular ions following separation within a differential mobility spectrometry mass spectrometer (DMS-MS) is used to investigate the various gas-phase conformations of two protonated aromatic amino acids, tryptophan (Trp) and tyrosine (Tyr). Isomeric forms of target ions are isolated using DMS-MS, and UVPD is performed to map out their respective UVPD spectra. Vibronic spectra of the protonated amino acid conformers are simulated at the density functional theory (DFT) level to assist the identification of prototropic isomers (protomers) of ions observed in the experiments. Assigning the experimental spectra to the calculated vibronic spectra, we conclude that the most likely protonation site of Trp and Tyr is on the nitrogen of the amine in the gas phase. In chapter 4, DMS-MS is employed to study the structural stability of protonated arginine (Arg)n clusters (n=1−4) resulting from salt-bridge interactions. The ion-solvent interactions between ions and various gaseous molecules monitored in the DMS cell are analyzed to define cluster stability under different gaseous environments. Further understanding is gained by simulating the electrostatic potential maps of protonated Arg monomer and clusters − this provides insight into the charge distribution around a molecule induced by relevant molecular interactions. Both experimental and computational findings suggest that the protonated Arg monomer is stabilized with the single protonation site on the nitrogen of the guanidino group, and that salt-bridges exist in the gas phase structures of protonated (Arg)n (n=2−4).