Mid-IR Spectroscopy of Gas-Phase Biomolecules: Experiment, Theory, and Instrument Development
Examensarbete för masterexamen
Experiment With the purpose of understanding weak interactions between amino acids, two infrared (IR) spectroscopy experiments are performed in the range of 550 − 1850 cm−1 (5.4 − 18 μm) on gas-phase proton-bound amino acid dimers. In both, electrospray ionization (ESI) is used to deliver the dimers to the gas phase as ions, where they are trapped in an ion cyclotron resonance (ICR) mass spectrometer. In the first, the spectra of Met2H+, MetTrpH+, and Trp2H+are obtained with IR multiple photon disassociation (IRMPD) using the CLIO free electron laser as an IR light source. In the second, homo- and heterochiral Asn2H+ are similarly studied with IRMPD, this time using FELIX as the IR light source. The resulting spectra contain enough information to infer the location of the surplus proton. Theory For each of the molecules, a conformational search is performed using molecular dynamics (MD) simulations with varied initial conditions. The conformers are then optimized and their IR spectra are determined with the density functional theory (DFT) B3LYP functional. Energy calculations are done with a Gaussian-4 (G4MP2) method when computationally feasible, and with a complete basis set (CBS-4M) method otherwise. The room temperature predicted IR spectra are obtained by summing up the IR spectra of individual conformers multiplied by their Boltzmann factors. Agreement between experimental and predicted spectra is good with the exception of the region near 1500 cm−1. These frequencies correspond to vibrational bending modes of the protonated amino group, suggesting that the intermolecular hydrogen bond cannot be approximated as harmonic. Finally, the intermolecular interactions are classified with the noncovalent interaction (NCI) method. Instrument Development Development of a high-resolution mass spectrometer meant for use in infrared-ultraviolet (IR-UV) ion-dip spectroscopy of gas-phase biomolecules began in 2018 at the Department of Physics, University of Gothenburg. At the outset of this work, design of the instrument and its enclosing vacuum chamber were partially finished, and all major parts were acquired. Designs of interior components such as sample holder, airlock, nozzle mount, and skimmer flange have been made. The main chamber and its supporting structure have been assembled. Today, the main chamber reaches an ultimate pressure of 2.5 × 10−7 mbar, which is well sufficient for operation.
IRMPD , dimer , gas-phase , methionine , tryptophan , asparagine