Chemically induced lipid phase separation in model membranes containing charged lipids: a spin label study

Area of Science:

• Membrane biophysics
• Lipid organization and phase behavior

Background:

• Understanding lipid distribution in mixed membranes is crucial for cell function.
• Charged and uncharged lipids interact differently within bilayers.
• The influence of divalent cations and polycations on membrane structure is a key area of research.

Purpose of the Study:

• To investigate lipid distribution in binary mixed membranes of charged (dipalmitoyl phosphatidic acid) and uncharged (dipalmitoyl lecithin) lipids.
• To determine the effects of Ca2+ and polylysine on lipid organization and phase separation.
• To elucidate the mechanisms underlying induced phase separation in lipid bilayers.

Main Methods:

• Spin label technique utilizing electron spin resonance (ESR) spectroscopy.
• Analysis of ESR spectra via spin exchange frequency (Wex) to assess lipid distribution.
• Comparative studies on bilayer and monolayer membranes.
• Investigation of Ca2+ and polylysine binding to phosphatidic acid.

Main Results:

• Mixed lecithin-phosphatidic acid membranes show significant lipid segregation (miscibility gap) in the fluid state.
• Ca2+ addition dramatically increases phase separation by forming Ca2+-bound phosphatidic acid patches, excluding lecithin.
• Polylysine induces similar segregation effects as Ca2+, shifting the phase transition temperature of bound phosphatidic acid.
• A two-cluster model quantitatively explains the dependence of Wex on Ca2+ concentration.

Conclusions:

• Binary lipid membranes exhibit lateral lipid segregation, particularly in the presence of charged lipids.
• Divalent cations like Ca2+ and polycations like polylysine induce pronounced phase separation through specific interactions with charged lipids.
• These findings suggest potential cooperative conformational changes in biological membranes involving both lipid and protein components.