The Hofmeister effect in relation to membrane lipid phase stability

Area of Science:

• Biophysical Chemistry
• Materials Science
• Physical Chemistry

Background:

• The phase behavior of phospholipids is crucial for biological membranes.
• Understanding how external agents affect lipid phase transitions is vital for various applications.
• The Hofmeister series describes ion-specific effects on protein and macromolecule solution properties.

Purpose of the Study:

• To investigate the influence of sodium salts and non-ionic co-solutes on the phase behavior of L-alpha-1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE).
• To correlate the observed lipid phase property changes with the Hofmeister series and water structure perturbation.
• To elucidate the mechanism by which co-solutes affect the lipid/water interface.

Main Methods:

• Aqueous dispersions of POPE were prepared.
• Phase transition temperatures (Tm and Th) were measured in the presence of various sodium salts and non-ionic co-solutes.
• X-ray diffraction was employed to analyze structural changes.

Main Results:

• Anions exhibited a graded response mirroring the Hofmeister series, affecting POPE phase transition temperatures.
• Salts early in the Hofmeister series (e.g., Na2SO4, NaCl) decreased Th and increased Tm.
• Salts late in the series (e.g., NaI, NaSCN) increased Th and decreased Tm, similar effects observed with non-ionic co-solutes.
• The extent of perturbation correlated with the co-solute's ability to alter bulk water structure.
• X-ray diffraction suggested changes in the lipid/water interface mediate these effects.

Conclusions:

• The phase behavior of POPE is sensitive to the Hofmeister series of anions and non-ionic co-solutes.
• Co-solute-induced alterations in water structure are a primary driver of changes in POPE phase transitions.
• Modulation of the lipid/water interface plays a key role in the observed effects.