Physical studies of cell surface and cell membrane structure. Determination of phospholipid head group organization by deuterium and phosphorus nuclear magnetic resonance spectroscopy

Phospholipid head group conformations in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), DPPC-cholesterol, and DPPE-cholesterol dispersions, in excess water above the pure lipid gel to liquid-crystal phase transition temperature, have been calculated by using comparisons between experimental 2H and 31 P NMR spectral parameters and theoretical results obtained from a plausible model of head group motions. The new calculations are compared with results obtained in previous studies [Seelig, J., Gally, H. U., & Wohlgemuth, R. (1977) Biochem, Biophys. Acta 467, 109--117; Brown, M. F. & Seelig, J. (1978) Biochemistry 17, 381--384; Seelig, J., & Gally, H. U. (1976) Biochemistry 15, 5199--5204] and are shown to agree qualitatively under certain highly restrictive conditions. Under more general conditions, it is shown that many possible solutions are generated but that these may often be separated into a small number of likely conformations in which the head group torsion angles are restricted to specific ranges rather than to a discrete set of values. There is no NMR evidence, however, to support the notion that there are only single conformational solutions to the NMR measurements for the above phospholipid systems.