Zwitterionic Osmolytes Resurrect Electrostatic Interactions Screened by Salt

Area of Science:

• Biophysics
• Physical Chemistry

Background:

• Cells use zwitterionic osmolytes to manage osmotic stress from salt.
• These compounds stabilize proteins and enzyme activity in high ionic strength environments.
• The electrostatic influence of osmolytes, however, remains under-explored.

Purpose of the Study:

• Investigate how zwitterionic osmolytes affect electrostatic interactions in saline solutions.
• Determine the role of osmolytes in modulating salt's screening effect on charged surfaces.
• Explore the combined impact of various salts and osmolytes on electrostatic forces.

Main Methods:

• Utilized atomic force microscopy to probe interactions between negatively charged silica surfaces.
• Examined mixtures of five different salts (NaCl, KCl, CsCl, MgCl2, CaCl2) and five zwitterionic osmolytes (betaine, proline, trimethylamine N-oxide, glycine, sarcosine).
• Varied solute concentrations and pH to analyze effects on electrostatic forces.

Main Results:

• All tested osmolytes reduced the screening effect of salts on electrostatic repulsion.
• Osmolytes increased solution screening length and enhanced negative surface charge by desorbing protons and cations.
• These effects are linked to osmolytes' higher molecular polarizability than water.
• An osmolytic Hofmeister effect was observed, dependent on osmolyte hydrophobicity and cation hydration.

Conclusions:

• Zwitterionic osmolytes significantly alter electrostatic interactions in biologically relevant saline solutions.
• Osmolytes enhance electrostatic repulsion by modifying solution properties and surface charge.
• Findings provide a more realistic model of cytosolic Coulomb interactions, highlighting an understudied area.