• Ice Nucleation Activity of Perfluorinated Organic Acids.

      Schwidetzky, Ralph; Sun, Yuling; Fröhlich-Nowoisky, Janine; Kunert, Anna T; Bonn, Mischa; Meister, Konrad (ACS Publications, 2021-03-31)
      Perfluorinated acids (PFAs) are widely used synthetic chemical compounds, highly resistant to environmental degradation. The widespread PFA contamination in remote regions such as the High Arctic implies currently not understood long-range atmospheric transport pathways. Here, we report that perfluorooctanoic acid (PFOA) initiates heterogeneous ice nucleation at temperatures as high as −16 °C. In contrast, the eight-carbon octanoic acid, perfluorooctanesulfonic acid, and deprotonated PFOA showed poor ice nucleating capabilities. The ice nucleation ability of PFOA correlates with the formation of a PFOA monolayer at the air−water interface, suggesting a mechanism in which the aligned hydroxyl groups of the carboxylic acid moieties provide a lattice matching to ice. The ice nucleation capabilities of fluorinated compounds like PFOA might be relevant for cloud glaciation in the atmosphere and the removal of these persistent pollutants by wet deposition.
    • Specific Ion–Protein Interactions Influence Bacterial Ice Nucleation

      Schwidetzky, Ralph; Lukas, Max; YazdanYar, Dr. Azade; Kunert, Dr. Anna T.; Pöschl, Dr. Ulrich; Domke, Katrin F.; Fröhlich-Nowoisky, Dr. Janine; Bonn, Prof Dr. Mischa; Koop, Prof. Dr. Thomas; Nagata, Dr. Yuki; et al. (European Chemical Societies Publishing, 2021-01-19)
      Ice nucleation-active bacteria are the most efficient ice nucleators known, enabling the crystallization of water at temperatures close to 0 °C, thereby overcoming the kinetically hindered phase transition process at these conditions. Using highly specialized ice-nucleating proteins (INPs), they can cause frost damage to plants and influence the formation of clouds and precipitation in the atmosphere. In nature, the bacteria are usually found in aqueous environments containing ions. The impact of ions on bacterial ice nucleation efficiency, however, has remained elusive. Here, we demonstrate that ions can profoundly influence the efficiency of bacterial ice nucleators in a manner that follows the Hofmeister series. Weakly hydrated ions inhibit bacterial ice nucleation whereas strongly hydrated ions apparently facilitate ice nucleation. Surface-specific sum-frequency generation spectroscopy and molecular dynamics simulations reveal that the different effects are due to specific interactions of the ions with the INPs on the surface of the bacteria. Our results demonstrate that heterogeneous ice nucleation facilitated by bacteria strongly depends upon the nature of the ions, and specific ion–protein interactions are essential for the complete description of heterogeneous ice nucleation by bacteria.