Language
Country/Area
No result found
The mystery of the Hofmeister series: How do these ions affect protein solubility?
The Hofmeister series, also known as the solvent-like series, is a classification based on the solvent-like properties of ions, which describe the effects of ions on proteins during salting-out or salting-in processes. This range of effects was first studied by Franz Hofmeister, who explored the role of cations and anions on protein solubility.
These ions not only affect the solubility of proteins, but also the stability of their secondary and tertiary structures.
In the Hofmeister series, ions can be divided into two categories based on their effect on the structure of water: structure builders (kosmotropes) and structure destroyers (chaotropes). Strongly charged ions interact strongly with water molecules, disrupting hydrogen bonds and inducing electrostatic structuring of nearby water, and are therefore called structure builders. Weakly charged ions can disturb the structure of water and are called structure disruptors.
In this series, the effect of anions appears to be more significant than that of cations. Some of the anions that have the ability to stabilize and enhance protein solubility include:
(Structure Maker) Citrate > Fluoride > Phosphate > Sulfate > Acetate > Methanesulfonate > Chloride > Bromine > Iodine > Tetrafluoroborate > Thiocyanate
For cations, the usual order is:
(Structure maker) Tetramethyl nitrogen > Ammonium acetate > Potassium > Sodium > Calcium > Magnesium > Barium > Protamine
When oppositely charged structure-building cations and anions coexist in solution, they attract each other rather than combining with water; the same is true for structure-destroying cations. Thus, the preferred combinations of ions can be arranged as:
Structure Maker-Structure Maker > Structure Maker-Water > Water-Water > Structure Destroyer-Water > Structure Destroyer-Structure Destroyer
The properties of these combinations have a significant impact on protein stability and solubility. The structure-builder anions paired with the structure-builder cations reduce the structural effect of these ions because they bind to each other so strongly that they do not effectively change the structure of water. The combination of structure-building anions and structure-destroying cations can effectively stabilize proteins.
The mechanism of the Hofmeister series is not fully understood, but it is not due to changes in the general water structure; instead, interactions between ions and proteins and with water molecules in direct contact are likely to be more important. Simulation studies have shown that the change in solvation energy between ions and surrounding water molecules is the underlying mechanism of the Hofmeister series.
The earliest members of the series increase the surface tension of the solvent and reduce the solubility of non-polar molecules (i.e., "salting out"); later salts increase the solubility of non-polar molecules and reduce the structural order of water (i.e., " Salt in").
The salting-out effect is widely used in protein purification, especially in the technique of protein precipitation using ammonium sulfate. However, these salts can also interact directly with proteins and may lead to specific binding, such as the binding of phosphate and sulfate groups to RNase A.
Some ions with strong "salt-in" effects, such as iodide anions and thiocyanate anions, are strong denaturants. They strongly bind to unfolded proteins, thereby shifting the chemical equilibrium toward unfolded proteins and increasing protein Possibility of allergic reactions.
The Hofmeister series has given us a deep understanding of the effects of different ions on protein stability, but... When we consider the effects of different ions on proteins, is it possible to rethink the role of these ions in biochemical processes? Woolen cloth?
