Chirality effects in clusters from chiral samples in helium

Chirality is very common in several branches of science. A chiral object cannot be superimposed onto its mirror image. A human hand is a typical chiral object as are snail houses or screws. Molecules, where a carbon atom binds to four different residual groups are chiral as well and exist in two enantiomers, a left- and a right-handed version. Ribose, the sugar of DNA and all amino acids except glycine are chiral molecules that exist, amazingly, only in one of the two possible enantiomers in living cells on Earth. The present experimental proposal aims at a deeper understanding of the influence of chirality on molecular cluster formation. For some specific amino acids, a strong preference of clusters consisting of only one enantiomer has been reported, even if they were sampled from a 1:1 mixture of both chiral forms. We could confirm this observation for amino acid clusters formed in superfluid helium droplets, although the cluster formation process was very different from formation in water or other solvents. Combining high-resolution mass spectrometry with laser spectroscopy in our helium nano-droplet experiment, we plan to obtain detailed information about the structures of chiral cluster ions and in particular, about weakly bound chiral clusters that are difficult to form in other experiments. Labelling chiral molecules with heavier isotopes will help to unambiguously identify possible homochiral preferences in clusters built from racemic mixtures. In previous experiments, where clusters of amino acids were bombarded with keV ions or low-energy electrons, the loss of neutral fragments with a mass of 18 atomic mass units was observed. This is likely the loss of a water molecule, which may indicate formation of a peptide bond. Clusters of amino acids frozen onto a dust grain in cold interstellar clouds are expected to be exposed to ions or electrons in the above mentioned energy range. Thus, peptide bond formation via such simple and universal processes would be of relevance for astrochemistry and astrobiology and maybe even for the beginning of live on Earth. Photodissociation of these cluster ions in the UV at resonance would be a clear proof for peptide bond formation and will be investigated with a recently commissioned laser system. Two PhD students will perform the experiments utilizing an existing instrument and a new instrument that will be constructed during the first year of this project. The new instrument will be optimized for the laser spectroscopy of complex ions. The two PhD students will be supported and guided by the applicant and experienced postdoc researchers working in the group in Innsbruck. Furthermore, international collaborations with research groups in Germany, the USA and Israel will contribute to the success of this project.

Chirality is very common in nature. A chiral object cannot be superimposed onto its mirror image. A human hand is a typical chiral object as are snail houses or screws. Molecules, where a carbon atom binds to four different residual groups are chiral as well and exist in two enantiomers, a left- and a right-handed version. Ribose, the sugar of DNA and all amino acids except glycine are chiral molecules that exist, amazingly, only in one of the two possible enantiomers in living cells on Earth. The present experimental proposal aimed at a deeper understanding of the influence of chirality on molecular cluster formation. For some specific amino acids, a strong preference of clusters consisting of only one enantiomer has been reported, even if they were sampled from a racemic source, i.e., 1:1 mixture of both chiral forms. We could confirm this observation for amino acid clusters formed in superfluid helium droplets, although the cluster formation process was very different from formation in water or other solvents. We observed this effect also for cluster ions that were formed inside of helium nanodroplets. Subtle differences in the cluster growth as well as the stability of the resulting cluster ions obtained from homochiral and racemic samples of chiral molecules were investigated utilizing high-resolution mass spectrometry. Two novel instruments were developed and built for this project with the primary goal to boost the yield of ions solvated with He atoms. Laser spectroscopy of such helium-tagged ions results in spectra with a minimum matrix shift. We discovered two methods that both are based on the pickup of dopants into multiply-charged helium nanodroplets. In one case, cold ions with up to a few 100 He atoms attached are extracted from large droplets by shrinking the helium nanodroplets in a collision cell filled with helium gas. An alternative method to obtain such helium-tagged ions is splashing of multiply-charged helium droplets via collisions with surfaces. Besides the spectroscopy of cationic atoms and molecules, both methods enable also laser spectroscopy of anions and arbitrary complexes and clusters that can be formed inside helium nanodroplets. The potential of these new techniques was successfully demonstrated by means of several selected molecular and cluster targets.