Electrostatics Trigger Interfacial Self-Assembly of Bacterial Ice Nucleators.

Ice active bacteria can catalyze water freezing at high subzero temperatures using ice nucleating proteins (INPs) located at their outer cell walls. INPs are the most effective ice nucleators known and are of significant interest for agriculture, climate research, and freeze/antifreeze technologies. The aggregation of INPs into large ice nucleation sites is a key step for effective ice nucleation. It has been proposed that ice active bacteria can drive the aggregation of INPs and thereby trigger ice nucleation. However, the mechanism of INP aggregate assembly and the molecular processes behind the activation are still unclear. Both biochemical pathways and activation through electrostatics have been proposed based on experiments with lysed ice active bacteria. For a more direct view on the assembly of INPs, we follow the structure and water interactions of a synthetic model INP of the well-studied ice bacterium Pseudomonas syringae at the air-water interface as a function of the subphase pH. By combining sum frequency generation spectroscopy with two-dimensional infrared spectra, we conclude that self-assembly and electrostatic interactions drive the formation of ordered INP structures capable of aligning interfacial water.

Structural Insight into the Self-Assembly of a Pharmaceutically Optimized Insulin Analogue Obtained by Small-Angle X-ray Scattering.

B29Nε-lithocholyl-γ-l-ßGlu-desB30 human insulin [NN344] belongs to a group of insulins with fatty acid or sterol modifications. These insulin analogues have been found to form subcutaneous depots upon injection and hereby have a protracted release profile in vivo. In the present study, B29Nε-lithocholyl-γ-l-Glu-desB30 human insulin was investigated using in-solution small-angle X-ray scattering (SAXS) at chemical conditions designed to mimic three stages during treatment in vivo: in-vial/pen, postinjection, and longer times after injection. We found that the specific insulin analogue formed a mixture of mono- and dihexamers under in-vial/pen conditions of low salt and stabilizing phenol. At postinjection, conditions mimicking a subcutaneous depot, B29Nε-lithocholyl-γ-l-Glu-desB30 human insulin, formed very long straight soluble hexamer-based rods stacked along the Zn(II)-axis. The self-assembly was triggered by an increase in salt concentration when going from vial to physiological conditions. Mimicking longer times after injection and the additional removal of phenol caused the length of the rods to decrease significantly. Finally, we found that the self-assembly could be controlled by varying the amount of modification at the interaction interface by making mixed hexamers of B29Nε-lithocholyl-γ-l-Glu-desB30 and desB30 human insulin. This opens extra possibilities for controlling the release profile of very-long-acting insulins.