THEME 1: Protonation isomers in ion mobility

This project looks at the use of ion mobility (IM) in conjunction with mass spectrometry and liquid chromatography, in order to evaluate its use for the separation and characterization of small molecules (organic compounds, lipids or glycans). Ion mobility can serve as a filtering tool, resulting in the separation of ions of interest from interfering ions, based on their charge state. Since an ion’s mobility also depends on its mass and shape/size, different classes of chemical compounds often have different mobilities. This can be used when complex mixtures of molecules are being investigated, e.g. in chemical synthesis. Similar to the optimization and selection of some parameters in LC separation, a precise tuning of the ion mobility parameters is needed: e.g. the analogous parameter to the stationary phase in IM being the drift gas, and the mobile phase which is “pulling” the molecule through resembling the electrical field. Like retention times and mass-to-charge ratios, drift times are analyte-dependent and can be used as an additional determinant of analyte identity.

The main focus of this project will be on an important phenomenon where a single chemical entity shows two different drift times due to the formation of gas-phase charge isomers (protomers). The occurence of protomers has important implications for ion mobility characterization of such analytes, and also for the interpretation of their fragmentation behaviour (CID, MS/MS) in the gas phase. Benzocaine, the ethyl ester of para-aminobenzoic acid, which finds an application as a local anesthetic, is found to adopt in its protonated form at least two populations of distinct structures in the gas phase and their relative intensities strongly depend on the properties of the solvent used in the electrospray ionization (ESI) process. Ion mobility results allow for an unambiguous identification of two protomeric species - the N- and O-protonated form. Density functional theory (DFT) calculations link these structures to the most stable solution and gas-phase structures, respectively, with the electric properties of the surrounding medium being the main determinant for the preferred protonation site. The fact that the N-protonated form of benzocaine can be found in the gas phase is owed to kinetic trapping of the solution phase structure during transfer into the mass spectrometer.

A combination of such experimental approaches with DFT calculations will target similar molecules, including negatively and multiply charged ones, which show this fascinating phenomenon only visible in some ionization methods.