Effect of inorganic cations on phase transitions

Area of Science:

• Biochemistry
• Physical Chemistry
• Materials Science

Background:

• Phospholipids undergo a crystal (gel)-to-liquid crystal transition (Tm) influenced by their environment.
• The ionization state and hydration of phospholipids are critical factors affecting Tm.

Purpose of the Study:

• To summarize the effects of protons and cations on the Tm of isoelectric and negatively charged phospholipids.
• To elucidate the mechanisms behind these effects, distinguishing between non-specific and specific interactions.

Main Methods:

• Review and synthesis of existing data on phospholipid Tm.
• Analysis of trends in Tm as a function of pH and cation type/concentration.
• Application of theories like Gouy-Chapman to explain electrostatic effects.

Main Results:

• Tm correlates with phospholipid ionization state, increasing with protonation and decreasing with deprotonation.
• Cation effects on Tm are categorized as non-specific (charge screening, hydration competition) and specific (direct binding).
• Specific cation binding, particularly to negatively charged phospholipids, can induce isothermal crystallization and drastically increase Tm (e.g., Ca2+-phosphatidylserine complexes).

Conclusions:

• Protonation and cation charge screening generally increase Tm, consistent with electrostatic theories.
• Specific cation binding can override electrostatic effects, leading to significant Tm alterations due to structural changes and hydration loss.
• Understanding these ion-lipid interactions is crucial for predicting phospholipid behavior in various biological and material contexts.