Heterogeneous ice nucleation by biomolecules

Abstract

In recent research, the role of biomolecules in heterogeneous ice nucleation has gained prominence, as they serve as models for atmospheric ice nucleating agents. While established methods for biomolecule synthesis exist, distinguishing ice nucleation efficiencies from their oligomers poses challenges. This study demonstrates that the constant cooling rate approach is inadequate for accurately assessing Apoferritin subforms' ice nucleation rates due to data uncertainties. Isothermal measurements are employed to differentiate ice nucleating efficiencies of Apoferritin subforms using serial-diluted samples. Furthermore, this approach extends to investigating the ice nucleating efficiency of DNA origami tiles, a novel biomolecular model. By fitting data to classical nucleation theory (CNT), plausible model parameters are derived for both Apoferritin and DNA origami tiles. Finite element simulations are used to explore the influence of temperature gradients on isothermal freezing rates inside droplets of varying characteristics. Frozen fraction analysis of microlitre and nanolitre droplets highlights Apoferritin oligomers as the most effective ice nucleation particles (INPs), followed by dimers and monomers. Determining CNT parameters for each Apoferritin subform was challenging due to data ambiguities. However, through isothermal measurements, CNT parameters for each Apoferritin subform are successfully determined such as the free energy barrier constant, kinetic factor, and shape factor. These parameters show Apoferritin clusters as the most efficient subform, attributed to their complex shape, which contains active sites like pores, cavities, and a larger surface area. Isothermal measurements further confirm that DNA origami tiles have a higher ice nucleation efficiency over Apoferritin dimers and monomers due to their larger surface area and smaller contact angle with water. Notably, factors such as droplet volume, contact angle with the substrate, and cooling rate should be considered for an accurate freezing rate depiction; neglecting them leads to skewed results. With an increase in these factors, the measured isothermal freezing rate also increases over time, requiring more time for droplets to achieve a uniform inner temperature. In conclusion, isothermal measurements are a valuable tool for distinguishing ice nucleation efficiencies, even for mixed molecules like Apoferritin subforms. DNA origami tiles emerge as efficient INPs, holding promise as intriguing candidates for future ice nucleation research, particularly in designing tailored INPs for focused studies.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Alison C Rust (Supervisor) & Walther Schwarzacher (Supervisor)