Thermodynamics of phospholipid self-assembly

Area of Science:

• Biochemistry
• Physical Chemistry
• Membrane Biophysics

Background:

• Negatively charged phospholipids are essential components of biological membranes.
• Understanding their self-assembly is key to comprehending membrane structure and function.

Purpose of the Study:

• To determine the thermodynamic parameters governing the self-assembly of anionic phospholipids.
• To elucidate the contributions of hydrophobic and electrostatic interactions to this process.

Main Methods:

• Isothermal titration calorimetry was employed to measure heats of demicellization.
• Experiments were conducted on dioctanoyl phosphatidylglycerol (PG), phosphatidylserine (PS), and dioctanoyl phosphatidic acid under varying ionic strengths and pH levels.

Main Results:

• A large heat capacity (ΔC°(P) ∼ -400 J.mol(-1) K(-1)) and enthalpy close to zero at physiological temperatures (T(∗) ~ 300 K) indicate the hydrophobic effect as the primary driver for self-assembly.
• The pH and ionic-strength dependencies revealed that electrostatic contributions are mainly entropic, stemming from the electrostatic double layer.

Conclusions:

• The hydrophobic effect is the dominant force in anionic phospholipid self-assembly.
• Electrostatic interactions contribute significantly through entropy, impacting biomembrane stability.
• These findings provide critical thermodynamic insights into the behavior of anionic lipids in biological systems.