Abstract
Mouse fibroblast L-M cells were grown in tissue culture medium containing selectively deuterated choline or ethanolamine. Both compounds were incorporated into the corresponding phospholipids at levels greater than 50% thus leading to a selective deuteration of these phospholipid head groups. Choline and ethanolamine were labeled at either the alpha- or the beta-carbon atom and well-resolved deuterium and phosphorus n.m.r. spectra were obtained from intact cells, crude plasma membranes and lipid extracts, leading to the following conclusions. (i) A large fraction, if not all, of the phospholipids in the intact L-M cell membranes were organized in a liquid crystalline bilayer. (ii) The phosphoethanolamine and the phosphocholine head group conformation were found to be remarkably similar in pure lipid bilayers and in intact L-M cell membranes with the head group dipoles being oriented parallel to the membrane surface. (iii) The deuterium T1 spin lattice relaxation times fell in the range of 7-25 ms and were similar in intact L-M cells and in pure lipid model membranes, suggesting that the two head groups are not involved in strong interactions with membrane proteins. The rotational diffusion rate of the two head groups was reduced by at least a factor of 10 compared to molecules of the same size in aqueous solution. (iv) The phosphocholine head group was sensitive to the size and sign of membrane surface charges as verified in mixing experiments with charged lipids. In L-M cell membranes the phosphocholine appeared to sense an electrically neutral environment in spite of the fact that L-M cell membranes contain 10-20% negatively charged lipids.