Protein Adsorption on Clay Minerals: Implications for Ice Nucleation

Ice-nucleating particles (INPs) play a critical role in cloud microphysics by initiating ice formation in mixed-phase clouds. Ice nucleation (IN) on mineral dust, which is the most abundant atmospheric INP type, is controlled by rare surface sites that may not represent the average mineral surface. Soil dusts containing biogenic material have shown to contribute to IN at even higher temperatures than pure mineral dusts. The question therefore arises how interactions between minerals and organic macromolecules, such as proteins, modify the IN ability of either. To date, the potential of proteins to directly adsorb onto mineral surfaces and contribute to IN remain largely unknown.

Using an experimental bottom-up approach, we investigate protein adsorption on a clay mineral and its implications for immersion freezing with the Super DRoplet Ice Nuclei Counter Zurich (S-DRINCZ) offering the option of parallel cooling several well plates. The clay mineral kaolinite (0.1–0.01 wt%), which exhibits a median freezing temperature T(50) of −7.5 °C at a concentration of 0.1 wt% was mixed with the protein ferritin (0.1–0.0025 wt%), which shows a slightly higher T(50) of −6.9 °C at the same concentration in its pure form, and the mixtures were analyzed with respect to their IN ability. Complementary UV/VIS spectroscopy is employed to determine the adsorption capacity onto kaolinite, while transmission electron microscopy (TEM) combined with EDX spectroscopy is used to localize ferritin on mineral surfaces to identify preferential adsorption sites via the iron- rich core of the protein. These results provide new insights into how mineral–protein interactions modify IN in atmospheric dust particles.