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In disperse solution, "osmotic stress" is a restricted case of preferential interactions
1Department of Biochemistry, Brandeis University, 415 South Street, MS 009, Waltham, MA 02254-9110, USA.
Summary
The concept of osmotic stress in biochemistry is flawed. Cosolvent exclusion, not water activity, drives biochemical equilibria in solutions, challenging prior interpretations.
Area of Science:
- Biochemistry
- Physical Chemistry
- Solution Chemistry
Background:
- The "osmotic stress" model interprets cosolvent effects on biochemical equilibria by quantifying "water molecules participating in the reaction."
- This interpretation attributes the effect to lowered solvent water activity caused by the cosolvent.
Purpose of the Study:
- To critically evaluate the "osmotic stress" concept in disperse solutions.
- To elucidate the accurate thermodynamic driving forces behind biochemical equilibria in the presence of cosolvents.
Main Methods:
- Thermodynamic analysis of cosolvent-biomolecule interactions.
- Examination of the free energy changes associated with cosolvent exclusion.
- Analysis of chemical potential perturbations.
Main Results:
- The "osmotic stress" model is fundamentally erroneous for disperse solutions.
- Cosolvent exclusion, requiring a positive free energy change, implies interaction, contradicting the inert assumption.
- Reduced water activity alone does not influence equilibria when the reacting surface is solvent-exposed.
- Observed effects are a specific instance of preferential interactions, driven by cosolvent exclusion free energy.
Conclusions:
- The "number of water molecules" is not a direct measure of participation but reflects mutual perturbations of cosolvent and protein chemical potentials.
- Biochemical equilibria in disperse solutions with cosolvents are governed by preferential interactions and the thermodynamics of exclusion, not simply water activity changes.