Preferred conformation and dynamics of the glycerol backbone in phospholipids. An NMR and X-ray single-crystal...

The conformation of the glycerol group of a number of diacyl and monoacyl (lyso) phospholipids differing in the chemical nature of the head group was studied by 1H high-resolution NMR and X-ray crystallography. The NMR measurements were carried out with solutions or micellar dispersions of the lipids in deuteriated organic solvents or 2H2O. Both solutions, in which the lipid is present as monomers, and lipid micelles give rise to good high-resolution NMR spectra exhibiting spin coupling hyperfine interactions. From 1H spin coupling it is concluded that there are two stable conformations about the glycerol C(2)-C(3) bond of phospholipids. One of these (rotamer A) is characterized by torsion angles theta 3 = antiperiplanar, theta 4 = +synclinal, and the other (rotamer B) by theta 3 = +synclinal, theta 4 = -synclinal. In both rotamers A and B the ester oxygens on the glycerol carbon atoms C(2) and C(3) are synclinal, and hence both types of rotamers readily allow the parallel alignment of the two hydrocarbon chains. By comparison of NMR and single-crystal X-ray data it is obvious that both conformations are minimum free energy conformations. Rotamer A is the conformation prevailing in phospholipid single-crystal structures. The conformation of rotamer B is also found in phospholipid single-crystal structures though to a lesser extent, e.g., in 2,3-dilauroyl-DL-glycero-1-phospho-N,N-dimethylethanolamine and 2,3-dimyristoyl-D-glycero-phospho-DL-glycerol. NMR measurements indicate that in liquid crystals the diacylglycerol part of phospholipids fluctuates between the two stable staggered conformations of rotamers A and B.(ABSTRACT TRUNCATED AT 250 WORDS)