• pH effects on the molecular structure and charging state of b-Escin biosurfactants at the air-water interface

      Glikman, Dana; Rey, Natalia García; Richert, Manuela; Meister, Konrad; Braunschweig, Björn (Elsevier, 2021-09-20)
      Saponins like b-escin exhibit an unusually high surface activity paired with a remarkable surface rheology which makes them as biosurfactants highly interesting for applications in soft matter colloids and at interfaces. We have applied vibrational sum-frequency generation (SFG) to study b-escin adsorption layers at the air-water interface as a function of electrolyte pH and compare the results from SFG spectroscopy to complementary experiments that have addressed the surface tension and the surface dilational rheology. SFG spectra of b-escin modified air-water interfaces demonstrate that the SFG intensity of OAH stretching vibrations from interfacial water molecules is a function of pH and dramatically increases when the pH is increased from acidic to basic conditions and reaches a plateau at a solution pH of > 6. These changes are attributable to the interfacial charging state and to the deprotonation of the carboxylic acid group of b-escin. Thus, the change in OAH intensity provides qualitative information on the degree of protonation of this group at the air-water interface. At pH < 4 the air-water interface is dominated by the charge neutral form of b-escin, while at pH > 6 its carboxylic acid group is fully deprotonated and, consequently, the interface is highly charged. These observations are corroborated by the change in equilibrium surface tension which is qualitatively similar to the change in OAH intensity as seen in the SFG spectra. Further, once the surface layer is charge neutral, the surface elasticity drastically increases. This can be attributed to a change in prevailing intermolecular interactions that change from dominating repulsive electrostatic interactions at high pH, to dominating attractive interactions, such as hydrophobic and dispersive interactions, as well as, hydrogen bonding at low pH values. In addition to the clear changes in OAH intensity from interfacial H2O, the SFG spectra exhibit drastic changes in the CAH bands from interfacial b-escin which we relate to differences in the net molecular orientation. This orientation change is driven by tighter packing of b-escin adsorption layers when the b-escin moiety is in its charge neutral form (pH < 4).
    • 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.
    • Toward Understanding Bacterial Ice Nucleation

      Lukas, Max; Schwidetzky, Ralph; Eufemio, Rosemary J.; Bonn, Mischa; Meister, Konrad (American Chemical Society, 2022-01-27)
      Bacterial ice nucleators (INs) are among the most effective ice nucleators known and are relevant for freezing processes in agriculture, the atmosphere, and the biosphere. Their ability to facilitate ice formation is due to specialized ice-nucleating proteins (INPs) anchored to the outer bacterial cell membrane, enabling the crystallization of water at temperatures up to −2 °C. In this Perspective, we highlight the importance of functional aggregation of INPs for the exceptionally high ice nucleation activity of bacterial ice nucleators. We emphasize that the bacterial cell membrane, as well as environmental conditions, is crucial for a precise functional INP aggregation. Interdisciplinary approaches combining high-throughput droplet freezing assays with advanced physicochemical tools and protein biochemistry are needed to link changes in protein structure or protein–water interactions with changes on the functional level.