A study of head groups in phosphatidylcholine-cholesterol bilayers by 14N-NMR

Abstract

The 14N nuclear relaxation times of egg phosphatidylglycerol (EPC) in mixed EPC-cholesterol small unilamellar vesicles were measured for the first time. No dependence of longitudinal relaxation time T 1( 14N) on cholesterol content was observed up to a molar ratio of cholesterol to total lipid X ch = 0.4. The 14N nuclear quadruple coupling constant e 2 qQ/ h and the rapid segmental motions at the positively charged group of EPC are not affected by the moderate increase in the area available to each head group by the cholesterol incorporation. The increase in the 14N line width with cholesterol was analyzed using 14N residual quadrupole splittings Δν Q ( 14N) . The line width of the high resolution spectrum of single bilayer vesicles has a considerable contribution from the lateral diffusion of EPC molecules. The diffusion coefficient D decreases abruptly to one half between X ch = 0.1 and 0.2 at 20°C. This result is in good agreement with that obtained for a multilamellar phase by Rubenstein et al. (Proc. Natl. Acad. Sci. U.S.A., 76 (1979) 15–18), showing an essential similarity between unilamellar and multilamellar phases.