Conformation of fatty acyl chains in alpha- and beta-phosphatidylcholine and phosphatidylethanolamine derivatives in sonicated vesicles.

Mono- and dimethylated derivatives constitute important intermediates in the conversion of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) in eucaryote membranes. 1H-NMR techniques were utilized to examine the conformation of the region of the fatty acyl chains that is close to the polar group in the series of alpha-phospholipids: PE, N-methyl-PE, N,N-dimethyl-PE, and PC. The same series of polar groups, but on phospholipid containing sn-1 and/or sn-3 fatty acyl chains (beta-phospholipids) were also examined. All of the phospholipids were in the form of small sonicated vesicles which are widely utilized as membrane models. The alpha-methylene group of the sn-1 and sn-2 fatty acyl chains of the alpha-phospholipids give rise to separate signals due to the non-equivalency of these chains with respect to the glycerol phosphate backbone on all alpha-phospholipids tested. Additionally, differences in the environment of the PC molecules as well as N-methyl-PE, and N,N-dimethyl-PE, but not PE itself on the inside and outside of the vesicles are reflected in the chemical shift of the alpha-methylene protons. On the other hand, all of the beta-phospholipids (including beta-PE) were found to reflect the inside/outside packing differences in their alpha-methylene groups. The bilayer packing does not induce any nonequivalence in the chemically equivalent acyl chains. In mixed micelles with detergents, beta-phospholipids showed one alpha-CH2 signal for all phospholipids. These results are consistent with a common conformational arrangement for the fatty acyl chains in all alpha-phospholipids that have been investigated no matter what aggregated form. The conformational arrangement in the beta-phospholipids is different, but again is similar for all of the compounds tested in various aggregated forms.