Surface areas of lipid membranes

Area of Science:

• Biochemistry
• Physical Chemistry
• Materials Science

Background:

• Liposomes are crucial in drug delivery and biomembrane research.
• Understanding lipid asymmetry in liposomes is vital for controlling their properties.
• Existing methods for assessing lipid distribution can be limited.

Purpose of the Study:

• To quantify the proportion of external lipids in liposomes using a novel radical probe method.
• To investigate if liposome composition affects the proportion of surface lipids.

Main Methods:

• Photolysis of alkyl pentacyanocobaltate complexes to generate alkyl radicals.
• Reaction of alkyl radicals with spin-labeled lipids on liposome surfaces.
• Measurement of paramagnetic resonance signal loss to determine external lipid proportion.

Main Results:

• Alkyl radicals selectively react with external nitroxide radicals without penetrating phospholipid bilayers.
• Liposomes composed of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) showed consistent external lipid proportions.
• Binary mixtures of DPPC and cholesterol also exhibited the same proportion of external lipid.

Conclusions:

• The employed photolabile radical method is effective for determining external lipid proportions in liposomes.
• Liposome surface lipid distribution is independent of lipid type (DMPC, DPPC) and cholesterol content under the studied conditions.
• This finding has implications for liposome formulation and stability in various applications.