Spectroscopic Probes of Conformational Isomerization of Biological Molecules in a Cold Ion Trap

The function of biologically active molecules depends both on their structure and on their conformational dynamics. In solution, intramolecular interaction with the solvent will influence different biological processes, i.e. protein folding. However secondary structure (helices, sheets) is heavily influenced by intramolecular forces and hence it is important to isolate biological molecules in order to study their intrinsic behavior. Moreover, gas phase studies on biomolecules provide critical information that can serve as benchmarks to test the accuracy of theoretical predictions. The main focus of this thesis is the investigation of the potential energy surfaces of biomolecules in the gas phase. Biomolecular ions are produced in the gas phase via nano-electrospray, mass-selected and guided into a cold, 22-pole ion trap where they are cooled via collisions with cold helium. A variety of double-resonance techniques based on photofragmentation detection are then applied to obtain information on the molecule stable conformations, the potential barriers separating stable conformations and their connectivity. We applied these techniques on molecules of increasing size, starting with protonated phenylalanine and proceeding to the 7amino acid peptides, Ac-Phe-(Ala)5-LysH+ and the 12-residue peptide Ac-Phe-(Ala)3-(Gly)4-(Ala)3-LysH +. As a first step, electronic spectra and conformation-specific IR-UV double resonance spectra are measured. These measurements, in combination with DFT calculations, allow the assignment of two stable conformers in the case of the protonated phenylalanine and four in the case of the seven residue peptide. In the second part of this work we focused on the conformational isomerization of these molecules by performing infrared and ultraviolet hole-filling spectroscopy in the ion trap. We demonstrated that we can induce isomerization between stable conformers of each molecule via vibrational excitation. After energy dissipation, the excited molecules are redistributed among the initially identified conformers, but no new minima were detected. In the last part of this thesis, the fractional populations and the isomerization quantum yields are determined through infrared induced population transfer spectroscopy. Protonated phenylalanine reveals conformational selectivity in its isomerization while the relaxation of the infrared excitation leads to the equilibrium distribution in the case of the 12-residue glycine-containing peptide. The steps that occur during the energy dissipation are discussed in this thesis.