In our attempt to understand the mechanisms by which ice nucleators catalyze freezing, we biologists have sometimes imagined ice nucleation active proteins as having an attractive force that draws water molecules into the appropriate configuration to form ice. Our simplified model involves water molecules and proteins. But what would happen to the interaction of water molecules and these proteins in the presence of other forces on the water molecules? The forces that can bind water molecules to various degrees are numerous. Gravitation, capillarity and pressure can reduce the potential energy of water. Dissolved materials such as salts or sugars can interact with water through ion-dipole or dipole-dipole attraction and hydrogen bonds. Surface interactions with undissolved materials can also influence water via ion-dipole attraction, van der Waals forces, hydrophobic interactions and hydrogen bonds. A decrease in the boiling point of water with increasing altitude and the freezing-point depression observed for sugar and salt solutions are examples of how these forces act on water.
The recent paper of Thomas Koop and Bernhard Zobrist (reference below) presents an analysis of the influence of solutes on heterogeneous ice nucleation by using approaches from studies on homogeneous nucleation. For biologists, and particularly those of us who have only vague memories of plant or microbial physiology, a quick refresher course on the definition of water activity and water potential will be useful in understanding this paper. One of the objectives of their work is to propose a generic model for estimating the influence of solutes on heterogeneous ice nucleation activity. They compare methods based on the lambda parameter (a parameter whose value changes depending on the nature of the solute) and on water activity. This latter parameter can be estimated directly from the melting point (and they present tables in their paper for this estimation). These authors then use this approach to illustrate how a range of invertebrates balance their investment in decreasing the water activity of their tissues and in forming heterogeneous ice nuclei depending on if they are frost tolerant or if they strive to avoid freezing.
This approach has other applications concerning biology, and in particular in identifying solute conditions that denature heterogeneous ice nuclei. I suppose that this would be revealed by heterogeneous ice nucleation temperatures that were markedly different from those predicted by extrapolation of the water activity vs. ice nucleation curves. In Koop and Zobrist’s paper the behavior of the biological ice nucleator tested (the Snomax formulation of Pseudomonas syringae) suggested that there was no aberrant influence of the solutes used here. I look forward to a future paper for indicators of denaturing effects.
Koop, T., Zobrist, B. 2009. Parameterizations for ice nucleation in biological and atmospheric systems. Phys. Chem. Chem. Phys. 11:10839-10850. http://dx.doi.org/10.1039/b914289d
The recent paper of Thomas Koop and Bernhard Zobrist (reference below) presents an analysis of the influence of solutes on heterogeneous ice nucleation by using approaches from studies on homogeneous nucleation. For biologists, and particularly those of us who have only vague memories of plant or microbial physiology, a quick refresher course on the definition of water activity and water potential will be useful in understanding this paper. One of the objectives of their work is to propose a generic model for estimating the influence of solutes on heterogeneous ice nucleation activity. They compare methods based on the lambda parameter (a parameter whose value changes depending on the nature of the solute) and on water activity. This latter parameter can be estimated directly from the melting point (and they present tables in their paper for this estimation). These authors then use this approach to illustrate how a range of invertebrates balance their investment in decreasing the water activity of their tissues and in forming heterogeneous ice nuclei depending on if they are frost tolerant or if they strive to avoid freezing.
This approach has other applications concerning biology, and in particular in identifying solute conditions that denature heterogeneous ice nuclei. I suppose that this would be revealed by heterogeneous ice nucleation temperatures that were markedly different from those predicted by extrapolation of the water activity vs. ice nucleation curves. In Koop and Zobrist’s paper the behavior of the biological ice nucleator tested (the Snomax formulation of Pseudomonas syringae) suggested that there was no aberrant influence of the solutes used here. I look forward to a future paper for indicators of denaturing effects.
Koop, T., Zobrist, B. 2009. Parameterizations for ice nucleation in biological and atmospheric systems. Phys. Chem. Chem. Phys. 11:10839-10850. http://dx.doi.org/10.1039/b914289d