Direct spectroscopic observation of inner and outer hydrocarbon chains of lipid bilayer vesicles

Distearoylphosphatidylcholines labeled with gem-difluoro groups at selected positions of the fatty acid chains have been prepared. These lipids were incorporated into sonicated vesicles and were examined by fluorine nuclear magnetic resonance spectroscopy. All spectra are characterized by two well-separated fluorine resonances. Paramagnetic ions added to the exterior solution of vesicles of fluorinated lipid broaden the upfield resonances, whereas ions trapped in the vesicles interior broaden the downfield resonances. This demonstrates that the fluorine chemical shift of these compounds is sensitive to differences between the inner and outer monolayers of sonicated vesicles.