A basic property of any compound is solubility in water and in aqueous solutions, which determines a compound’s availability and ability to be used in biochemical or industrial applications. Dr. Jan Heyda, together with colleagues from Pennsylvania State University, recently published a contribution on this topic in Nature Chemistry.
From our daily kitchen routines, we expect that solubility increases with temperature: sugar dissolves well in hot tea, and salt in hot water. Caffeine solubility grows from 20 g/l at 25°C to 600 g/l in boiling water. In contrast, the solubility of nonpolar compounds, such as oils, decreases as temperature increases. Similarly, in a beaker, we can control the solubility of more complex molecules such as enzymes.
A cell, however, cannot use temperature variation to control the solubility, stability, and function of its enzymes. Instead, nature has developed softer controlling mechanisms, e.g., a variation of the concentration of osmolytes or ions in the enzyme environment. ‘‘There is a tight, over century-long link between the dissolution of enzymes in salt solutions and Prague. Franz Hofmeister, Professor of Pharmacology at the First Faculty of Medicine, conducted the first systematic study on this topic in 1888, in which he ordered ions according to their ability to precipitate egg white proteins. In our study, we focus on the thiocyanate anion (SCN-), which increases solubility,’’ Dr. Heyda explains. Prague citizens and visitors can see a plaque commemorating Hofmeister’s work on a wall at the First Faculty of Medicine in U Nemocnice street.
Even after a century, the investigation of ion effects on solubility is leading to novel, unexpected findings. Joint experimental and theoretical collaboration of teams from Pennsylvania State University and UCT Prague has revealed the paradoxical behavior of polyethylene oxide. While the solubility of the polymer increases in the presence of sodium thiocyanate (NaSCN), the solubility of the monomer in the presence of the same salt decreases.
“In order for this work to succeed, the connection of computer simulations with two types of experiments was essential,’’ Dr. Heyda notes. “In the first experiment, we monitored the strength of the binding of the thiocyanate anion to every single monomer unit [-CH2-CH2-O-]. In the other experiment, the averaged water structure in the vicinity of polyethylene oxide molecule was measured. Employing computer simulations, we were able to refine this information again to the level of individual monomer units.”
With this information in hand, Dr. Heyda and colleagues have proven that the thiocyanate anion is depleted from the regions with well-ordered water structures (terminal groups), while the anion is attracted to regions where a water structure is perturbed. Such behavior is depicted in the figure below. This explains why sodium thiocyanate lowers the solubility of dimethoxyethane (monomer), slightly increases the solubility of the dimer, and significantly increases the solubility of longer polyethylene oxides. Importantly, an analogous behavior was also observed for chemically more complex acrylamide-based polymers, which are chemically close to peptides.
Reference
Rogers, B.A., Okur, H.I., Yan, C. et al. Weakly hydrated anions bind to polymers but not monomers in aqueous solutions. Nat. Chem. (2021). https://doi.org/10.1038/s41557-021-00805-z