Trapping biomolecular ions in superfluid helium droplets

Helium droplet spectroscopy is a well-established and important technique for achieving fundamental insight into the properties of embedded dopant molecules. Superfluid He droplets provide a gentle, weakly interacting, and cryogenic environment for the investigated molecules which are cooled internally and translationally to the droplet (equilibrium) temperature of 0.37 K. Measured spectra often reveal a high resolution that is comparable to the resolution observed in spectra of isolated molecules in the gas phase, even for larger species like C60 molecules. Therefore, it is promising to use this technique in the investigation of large biomolecules, i. e. to enable the acquisition of detailed information on the intrinsic properties of proteins and peptides. Furthermore, obtained spectra contain information on the interaction between the dopant and the droplet and in this way, elementary properties of the He droplets themselves can be studied. This is interesting in terms of looking at, for example, superfluidity on a microscopic scale. The experiment presented in this thesis is a new approach to investigate large biomolecules under well-defined conditions and at temperatures in the sub-Kelvin regime. The He droplet isolation technique is combined with established gas-phase techniques such as electrospray ionization and quadrupole mass filtering and provides for the possibility of studying mass–to–charge selected cold biomolecular ions in He droplets. The experiment can quite flexibly be combined with various experimental techniques and also be enhanced by additional methods to allow for more precise selection of the investigated ion species which are trapped. Ions can be incorporated in the He droplets via pickup, which is conceptually similar to pickup experiments performed on neutral species. Here, gas-phase ions are produced by elecrospray ionization. They are mass–to–charge selected and accumulated in a linear ion trap where they are stored by a trapping potential of a few electronvolts. Traversing He droplets can pick up the ions, remove them from the trap due to the high kinetic energy of the massive He droplets and thereby, make them accessible for further investigation by the experimental technique of choice. Having mass– to–charge or even conformation selected gas-phase ions stored in an ion trap provides the great advantage of a “clean” doping of the droplets with well-defined species. This doping technique yields higher intensities of ion-doped droplets than alternative experimental approaches for doping He droplets with large biomolecules. It has the further advantage of being applicable to a large variety of molecular species which can be incorporated in superfluid He droplets via pickup, such as cluster ions. Thus, the ion-pickup opens the helium droplet isolation technique to a wide range of new, interesting species.