Determining ethanol distribution in phospholipid multilayers with MAS-NOESY spectra

Area of Science:

• Biophysics
• Membrane Biophysics
• Structural Biology

Background:

• The precise location of ethanol within biological membranes is a debated topic.
• Understanding ethanol's interaction with membranes is crucial for pharmacology and toxicology.

Purpose of the Study:

• To directly investigate and map the distribution of ethanol molecules within lipid bilayers.
• To clarify the controversial issue of ethanol's location in membranes.

Main Methods:

• Utilized two-dimensional Nuclear Overhauser Effect Spectroscopy (2D NOESY) with magic-angle spinning (MAS) NMR.
• Analyzed proton NMR resonances of both lipids and ethanol in multilamellar liposomes.
• Interpreted crosspeak intensity to derive an ethanol distribution function.

Main Results:

• Observed strong proton lipid-ethanol crosspeaks across various lipid types (saturated, monounsaturated, polyunsaturated).
• Ethanol preferentially localizes at the lipid-water interface, near the glycerol backbone and upper hydrocarbon chains.
• Bilayer core ethanol concentration was significantly lower; chain unsaturation had minimal impact.
• Crosspeak intensity decreased with higher water content and temperature, indicating shorter molecular contact times (~1 ns).

Conclusions:

• Ethanol predominantly resides at the membrane's lipid-water interface.
• Lipid and ethanol mobility within the membrane is high, with a dynamic and somewhat arbitrary interface.
• Findings provide direct evidence for ethanol's preferential binding sites within lipid bilayers.