Supramolecular Assembly of Proline Mediated by Alkali Metal Cations
Rayoon Chang1,2, Waldemar Hoffmann1,2, Mateusz Marianski2, Jongcheol Seo2, Gert von Helden2 and Kevin Pagel1
1Institut für Chemie und Biochemie, Freie Universität Berlin, Takustr. 3, 14195, Berlin, GER
2Molekülphysik, FHI der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, GER
The subtle interplay of non-covalent interactions often drives individual monomers to form exceptionally stable supramolecular structures, as observed in inorganic, as well as small biomolecules such as nucleobases and amino acids. For the amino acids the formation of stable structures, supported by chiral enrichment, is, however, very rare and often restricted to one specific cluster size. The only amino acid that behaves differently is proline – amongst very stable supramolecular structures in the gas phase, a unique oscillation trend in homochirality is observed for a wide range of cluster sizes. To date, these clusters have mainly been studied via ion mobility-mass spectrometry (IM-MS), a method which separates ions based on differences in their mass, charge, size and shape. However, IM-MS only provides information regarding the geometrical size, and is therefore insensitive towards detailed intra- and inter-molecular interactions. Infrared (IR) spectroscopy, on the other hand, can reveal diagnostic molecular vibrations, and is thus a versatile tool to study such interactions. We therefore coupled IM-MS with gas-phase IR spectroscopy in the mid-IR region (1000-1800 cm-1) to deduce structural details of the isolated, alkali metal cationized proline clusters in the gas phase. Our experimental results are in good agreement with ab initio calculations, which also show that the formation of highly symmetric proline clusters strongly depends on the nature of the alkali metal cations.