Water structure and chaotropicity: their uses, abuses and biological implications

Area of Science:

• Physical Chemistry
• Biophysical Chemistry
• Solution Chemistry

Background:

• The concept of "water structure" is frequently used to explain various aqueous phenomena.
• Solute hydration is often understood through proposed structural changes in bulk water.

Purpose of the Study:

• To examine the origins of attributing solute hydration to water structure changes.
• To evaluate the classification of solutes as chaotropic or kosmotropic in relation to water structure.
• To reassess the applicability of "water structure" concepts to complex biological phenomena.

Main Methods:

• Literature review and conceptual analysis of "water structure" theories.
• Examination of the chaotropic/kosmotropic classification and its link to solvent structure.
• Analysis of solute activity and solvation water behavior.

Main Results:

• The study expresses doubt that complex effects on macromolecules (e.g., Hofmeister, osmolyte) are solely due to bulk solvent structure changes.
• Chaotropicity, in its original sense, is argued to stem from solute activities impacting macromolecules.
• Deviations of solvation water from bulk-like behavior are identified as a key factor.

Conclusions:

• Complex phenomena like Hofmeister and osmolyte effects may not be simply explained by bulk solvent structure modification.
• Chaotropicity, when correctly interpreted, is derived from solute-macromolecule interactions and altered solvation shells.
• Chaotropicity remains a significant and biologically relevant parameter for understanding solution behavior in biological systems.