Salt screening and specific ion adsorption determine neutral-lipid membrane interactions

Area of Science:

• Biophysics
• Physical Chemistry
• Materials Science

Background:

• Biological membranes are fundamental to cellular function.
• Macromolecular interactions at interfaces are crucial in biological systems.
• The Hofmeister series describes ion-specific effects on protein and lipid behavior.

Purpose of the Study:

• Investigate the swelling of lipid lamellar phases in monovalent salt solutions.
• Clarify the roles of electrostatic and van der Waals forces in salt-induced membrane interactions.
• Explain ion specificity (Hofmeister effects) at macromolecular interfaces.

Main Methods:

• Measurement of lipid multilayer swelling in solutions of varying bromide (Br) and chloride (Cl) salt concentrations.
• Analysis of electrostatic repulsion and van der Waals (vdW) attraction forces.
• Comparison of experimental results with theoretical predictions, including Debye screening length.

Main Results:

• Lipid lamellar phases swell monotonically with increasing salt concentration for both Br and Cl salts.
• Stronger anion binding (Br > Cl) leads to increased electrostatic repulsion.
• Both electrostatic repulsion and vdW attraction are screened by increasing salt concentration.
• Measured vdW forces decay slower than predicted, but recover expected scaling when salt deficit near bilayers is considered.

Conclusions:

• Ion-specific binding and nonspecific ionic screening of fluctuations explain salt effects on lipid membrane interactions.
• The interplay between screened electrostatic repulsion and vdW attraction governs membrane swelling.
• This model extends to explain Hofmeister effects at macromolecular interfaces.