Investigation of phosphatidylethanolamine bilayers by deuterium and phosphorus-31 nuclear magnetic resonance

Abstract

The motion of the ethanolamine head group in unsonicated lipid bilayers above and below the phase transition is studied by means of deuterium and phosphorus magnetic resonance. For this purpose, dipalmitoyl-3-sn-phosphatidylethanolamine is selectively deuterated at the two ethanolamine carbon atoms. The deuterium quadrupole splittings of the corresponding bilayer phases are measured at pH 5.5 as a function of temperature. In addition, the phosphorus-31 chemical shift anisotropies of planor-oriented and randomly dispersed samples of dipalmitoyl-3-sn-phosphatidylethanolamine are measured at pH 5.5 and 11 by applying a proton-decoupling field. The knowledge of the static chemical shift tensor (Kohler, S.J., and Klein, M.P. (1976), Biochemistry 15, 967) provides the basis for a quantitive analysis of the head-group motion. The nuclear magnetic resonance data are consistent with a model in which the ethanolamine group is rotating flat on the surface of the bilayer with rapid transitions occurring between two enantiomeric conformations.

Head-group conformation in phospholipids: a 31P nuclear magnetic resonance study of oriented monodomain dipalmitoylphosphatidylcholine bilayers

“…At higher temperatures additional modes of motion are present and the easiest manner to account for these is to employ the C(2)-C(3) order parameter measured by Gally et al (1975) in ZH N M R experiments. This order parameter is 0.66 at T = 48 "C (Seelig and Gally, 1976) and results in Au = 69 X 0.66 = 45.5 ppm, which agrees well with the measured Auof 47 ppm at this temperature. Thus, the PO4 orientation which we determine from our oriented bilayer experiments will explain the breadth and principal values observed in the 3'P spectra of DPPC in excess water.…”

“…For T < -10 "C, the spectrum begins to assume an axially asymmetric shape, so we take this as the limiting breadth. At higher temperatures this spectrum narrows to -47 ppm a t 48 OC, which is slightly above the chain-melting temperatureof41 "C. Weassume that at the low temperature the predominant mode of the head group is rotation around the normal to the lipid bilayers (Stockton et al, 1974;Seelig and Seelig, 1974a), and this motion is fast compared to the size of the PO4 chemical shift tensor. If this is the case, then the tilted conformation which we measure will produce an axially symmetric powder spectrum with UII = -46 ppm, and, since the trace of the tensor must be invariant to molecular motion, ul = +23 ppm and Au = I ull -uII = 69 ppm.…”

NMR Studies of Membrane Structure and Dynamics

“…Direct measures of the order in the polar regions of phospholipid multilayer systems have recently been obtained from analysis of 31p chemical shift anisotropy and deuterium quadrupolar splittings (23,(29)(30)(31)(32)(33)(34). Phosphorus-31 NMR has been previously used to investigate the phospholipid head group (6,8,(103)(104)(105)(106), but most 31p experiments have been performed on sonicated vesicles, under which conditions the chemical shift anisotropy is rotationally averaged to zero, and hence most of the structural information that may be gleaned from the alp shielding tensor is lost.…”

“…Both Kohler & Klein (33,34) and Seelig and co-workers (30)(31)(32) have examined the chemical shift anisotropy of aqueous multilamellar dispersion of phospholipids in some detail. With the addition of water, the chemical shift tensor becomes axially symmetric and is narrowed from its static dispersion of about 230 ppm to approximately 50 ppm.…”

“…For phospholipid systems, the measured ~aC chemical shiR anisotropies are quite small (27) and no detailed interpretations in terms of segmental order have been attempted. However, ~P chemical shift anisotropies are larger, and these chemical shift anisotropy effects have, in fact, been observed for the more ordered head group (28)(29)(30)(31)(32)(33)(34). The observed spectra are characteristic of system with axial symmetry, even though the static shielding tensor for the a~P nucleus is not axially symmetric.…”