Bilayers of phosphatidylglycerol : a deuterium and phosphorus nuclear magnetic resonance study of the head-group region

The structural properties of the phosphoglycerol polar head group in bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol have been studied with deuterium and phosphorus-31 nuclear magnetic resonance. For this purpose, 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol was deuterated chemically or biosynthetically at all three segments of the glycerol head group. Both the D and the L configurations of the glycerol head group have been synthesized, and the correct stereochemical configuration of the polar group was verified by an enzymtic assay, as direct measurement of the optical rotation was insufficiently sensitive to distinguish between these diastereomers. The phosphatidylglycerol sodium salts were dispersed in excess buffer (0.1 M NaCl, pH 7.0), and the bilayer properties were characterized by small-angle X-ray diffraction and differential scanning calorimetry. The deuterium quadrupole splittings, delta vQ, and the phosphorus-31 chemical shielding anisotropy, delta sigma, were measured as a function of temperature in the range 20-60 degrees C, yielding the following results. (1) Well-resolved deuterium signals were obtained for all head-group segments and could be assigned unambiguously. (2) Almost identical spectra were obtained for 1,2-dipalmitoyl-sn-glycero-3-phospho-1'-glycerol (natural L,D configuration) and 1,2-dipalmitoyl-sn-glycero-3-phospho-3'-glycerol (L,L configuration), suggesting very similar head-group motions and orientations for both diastereomers. (3) The spatial anisotropy of motion and the segmental fluctuations of the negatively charged phosphoglycerol are similar to those of the zwitterionic phosphocholine and phosphoethanolamine head groups but differ distinctly from those of phosphoserine which also carries a net negative charge. (4) The motional inequivalence of geminal deuterons in 1,2-dipalmitoyl-sn-glycero-3-phospho-3'-glycerol was demonstrated by synthesis of a stereospecifically monolabeled analogue.