13C-NMR detection of lipid polymorphism in model and biological membranes

1. The application of the 13C-NMR technique to the study of lipid polymorphism is described for various model and biological membranes. 2. The 13-C-NMR line-width of various resonances of the lipid molecule and sensitive to the bilayer in equilibrium hexagonal and the bilayer in equilibrium 'isotropic' phase transition. The latter transition in some cases is accompanied by the occurrence of lipidic particles as detected by freeze-fracturing. Thus, specific 13C-labeling experiments allow the study of the individual phase behaviour of lipids in mixed lipid systems. 3. In diet experiments using rats, the choline group of phosphatidylcholine present in erythrocyte, endoplasmic and sarcoplasmic reticulum membranes could be specifically 13C-labeled. The 13C line-widths of the resonance from the erythrocyte are typical for a lamellar arrangement of the membrane lipids. In strong contrast, the line-width observed at 37 degree C for the endoplasmic and sarcoplasmic reticulum membranes is much smaller, typical of the isotropic phases observed in model membranes. In isolated rat liver microsomes and liver slices, the 13C line-width is strongly temperature dependent. At lower temperatures the line-widths strongly increase towards values typical of lipids in a bilayer structure.