13C nuclear magnetic resonance studies of egg phosphatidylcholine

Area per lipid and cholesterol interactions in membranes from separated local-field (13)C NMR spectroscopy

Investigations of lipid membranes using NMR spectroscopy generally require isotopic labeling, often precluding structural studies of complex lipid systems. Solid-state (13)C magic-angle spinning NMR spectroscopy at natural isotopic abundance gives site-specific structural information that can aid in the characterization of complex biomembranes. Using the separated local-field experiment DROSS, we resolved (13)C-(1)H residual dipolar couplings that were interpreted with a statistical mean-torque model. Liquid-disordered and liquid-ordered phases were characterized according to membrane thickness and average cross-sectional area per lipid. Knowledge of such structural parameters is vital for molecular dynamics simulations, and provides information about the balance of forces in membrane lipid bilayers. Experiments were conducted with both phosphatidylcholine (dimyristoylphosphatidylcholine (DMPC) and palmitoyloleoylphosphatidylcholine (POPC)) and egg-yolk sphingomyelin (EYSM) lipids, and allowed us to extract segmental order parameters from the (13)C-(1)H residual dipolar couplings. Order parameters were used to calculate membrane structural quantities, including the area per lipid and bilayer thickness. Relative to POPC, EYSM is more ordered in the ld phase and experiences less structural perturbation upon adding 50% cholesterol to form the lo phase. The loss of configurational entropy is smaller for EYSM than for POPC, thus favoring its interaction with cholesterol in raftlike lipid systems. Our studies show that solid-state (13)C NMR spectroscopy is applicable to investigations of complex lipids and makes it possible to obtain structural parameters for biomembrane systems where isotope labeling may be prohibitive.

Two-dimensional 1H/13C heteronuclear chemical shift correlation spectroscopy of lipid bilayers

Using solid-state magic angle spinning nuclear magnetic resonance (NMR) techniques, we have obtained two-dimensional (2D), 1H/13C chemical shift-correlated spectra of liquid crystalline 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayers in 30 wt% PO4/D2O buffer. Linewidths in both the 13C and the 1H dimensions were less than 0.3 ppm wide. The 2D spectrum consists of chemical shift correlations between all resolvable, directly bonded 13C-1H pairs and exhibits considerably greater spectral dispersion than either ID 1H or 13C MAS spectra. This approach promises to be an important tool in structural studies of biological membranes.

High-resolution 13C NMR study of pressure effects on the main phase transition in L-alpha-dipalmitoyl phosphatidylcholine vesicles

The effects of pressure on the liquid-crystalline to gel transition in vesicles of L-alpha-dipalmitoyl phosphatidylcholine were investigated by high-resolution proton-decoupled natural-abundance 13C NMR spectroscopy. The linewidths of several 13C resonances, including the choline methyl groups, carbonyl carbons, and choline methylene groups and the palmitoyl methyl groups are reported as a function of pressure at 52.7 degrees C. These preliminary NMR experiments clearly demonstrate that high-pressure, high-resolution proton-decoupled natural-abundance 13C NMR spectra are a promising tool to study the phase-transition behavior and the dynamics of model membrane systems.

Association of cholesterol with lysophosphatidylcholine

With equimolar cholesterol, lysophosphatidylcholine (lysoPC) or 1-ether-2-deoxylyso-phosphatidylcholine (etherdeoxylysoPC) form unilamellar vesicles of identical dimensions. 13C-NMR spectra of such vesicles are interpreted on the premise that suppression of a signal by broadening (i.e. decrease of T*2 relaxation time) indicates a decrease of motion of the carbon atom relative to its surroundings. The signals for sn-glycerol C-1 and C-2 are completely suppressed in the lysoPC-cholesterol vesicles. In contrast, in the vesicles containing etherdeoxylysoPC, all three glycerol carbon signals make their appearance, with the T*2 of C-2 approaching the T*2 in the monomolecularly dissolved lysolipid. This result argues for lipid-lipid complexing in the "hydrogen belts' of the lysoPC-cholesterol bilayer, specifically, for hydrogen bonding involving the hydroxyl and carbonyl groups of lysoPC and the hydroxyl of cholesterol.

Phosphorus-31 and carbon-13 nuclear magnetic resonance studies of divalent cation binding to phosphatidylserine membranes: use of cobalt as a paramagnetic probe

The paramagnetic divalent cation cobalt has large and well-understood effects on NMR signals from ligands bound in the first coordination sphere, i.e., inner-sphere ligands, and we have used these effects to identify divalent cation binding sites at the surface of phosphatidylserine membranes. 31P NMR results show that 13% of the bound cobalt ions are involved in inner-sphere complexes with the phosphodiester group, while 13C NMR results show that 54% of the bound cobalt ions are involved in unidentate inner sphere complexes with the carboxyl group. No evidence is found for cobalt binding to the carbonyl groups, but proton release studies suggest that 32% of the bound cobalt ions are involved in chelate complexes that contain both the carboxyl and the amine groups. All (i.e., 13% + 54% + 32% = 99%) of the bound cobalt ions can thus be accounted for in terms of inner sphere complexes with the phosphodiester group or the carboxyl group. We suggest that the unidentate inner-sphere complex between cobalt and the carboxyl group of phosphatidylserine and the inner-sphere complex between cobalt and the phosphodiester group of phosphatidylserine provide reasonable models for complexes between alkaline earth cations and phosphatidylserine membranes.

The adsorption of divalent cations to phosphatidylglycerol bilayer membranes

The ability of Stern equation to describe the adsorption of divalent cations to phosphatidylglycerol membranes was tested by combining 31P-NMR and electrophoretic mobility measurements. In 0.1 M sodium chloride both the 31P-NMR and the zeta potential data are well described by the Stern equation. 31P-NMR and 13C-NMR results indicate that cobalt forms inner-sphere complexes only with the phosphate group of phosphatidylglycerol molecules and that a substantial fraction of the adsorbed cobalt ions form outer-sphere complexes. Evidence is presented that suggests the alkaline earth cations also bind to phospholipids mainly by forming outer sphere complexes. Electrophoretic mobility measurements were performed with several different divalent cations. In all cases the zeta potentials in 0.1 M sodium chloride were well described by the Stern equation. The intrinsic 1:1 association constants (M-1) for the phosphatidylglycerol complexes decreased in the sequence: Mn2+, 11.5; Ca2+, 8.5; Ni2+, 7.5; Co2+, 6.5; Mg2+, 6.0; Ba2+, 5.5 and Sr2+, 5.0.

Lipoprotein-X: carbon-13 nuclear magnetic resonance studies on native, reconstituted, and model systems

Lipoprotein-X (LP-X), a lipoprotein isolated from human cholestatic plasma by ethanol--acetate precipitation and zonal ultracentrifugation, has been studied by 13C NMR at 67.9 MHz. Spectra of LP-X and its three subfractions are markedly different from those of normal human high-density lipoprotein3 (HDL3) or low-density lipoprotein (LDL). Spectra of LP-X are characterized by the presence of unusually broad resonance lines, especially those attributable to C6 of unesterified cholesterol (160--260 Hz) and to C beta of phospholipid glyceride (240--290 Hz). In contrast, the CH2O, CH2N, and N(CH3)3 choline resonances have line widths comparable to those of normal LDL and HDL3. For the subfraction LP-X1, spin--lattice relaxation times (T1) of the fatty acyl olefin resonances at 129.8 and 128.0 ppm and of the unesterified cholesterol C6 at 120.1 ppm were measured to be 675, 766, and 162 ms, respectively. These times are comparable to those measured for the corresponding resonances in single bilayer vesicles whose lipid composition approximates that of LP-X. The three LP-X subfractions isolated by zonal ultracentrifugation gave spectra which are identical, within experimental error, as judged qualitatively from their appearance and quantitatively from the line widths of selected resonances. In addition, 13C NMR spectra of sonicated total LP-X lipids are similar to spectra of the intact native lipoprotein. This study suggests (a) that motions of lipids in LP-X as probed by 13C NMR are similar to the motions of lipids found in model vesicular systems, (b) that the motions of the cholesterol rings and phospholipid fatty acyl chains are significantly more restricted in LP-X than in HDL3 and LDL, and (c) that the motions of the phosphoryl moieties in all three systems are similar.

13C-NMR detection of lipid polymorphism in model and biological membranes

1. The application of the 13C-NMR technique to the study of lipid polymorphism is described for various model and biological membranes. 2. The 13-C-NMR line-width of various resonances of the lipid molecule and sensitive to the bilayer in equilibrium hexagonal and the bilayer in equilibrium 'isotropic' phase transition. The latter transition in some cases is accompanied by the occurrence of lipidic particles as detected by freeze-fracturing. Thus, specific 13C-labeling experiments allow the study of the individual phase behaviour of lipids in mixed lipid systems. 3. In diet experiments using rats, the choline group of phosphatidylcholine present in erythrocyte, endoplasmic and sarcoplasmic reticulum membranes could be specifically 13C-labeled. The 13C line-widths of the resonance from the erythrocyte are typical for a lamellar arrangement of the membrane lipids. In strong contrast, the line-width observed at 37 degree C for the endoplasmic and sarcoplasmic reticulum membranes is much smaller, typical of the isotropic phases observed in model membranes. In isolated rat liver microsomes and liver slices, the 13C line-width is strongly temperature dependent. At lower temperatures the line-widths strongly increase towards values typical of lipids in a bilayer structure.

13 C NMR measurements of unsonicated phosphatidylcholine bilayers of different fatty acid and sterol composition

(1) 13C NMR linewidths were measured for various 13 C resonances in unsonicated dispersions of synthetic and natural phosphatidylcholines both in the absence and presence of cholesterol at temperatures where the acyl chains are in the liquid-crystalline state. (2) In the absence of cholesterol the linewidths of the various resolved chain resonances were decreased with increasing unsaturation and decreasing chain length. The motion of the delta9-cis olefinic carbon atoms in dioleoylphosphatidylcholine was more restricted than the motion of the delts9-cis olefinic carbon atoms in 1-stearoyl-2-oleoyl-phosphatidylcholine despite the higher overall fluidity of dioleoylphosphatidylcholine. The polar head-group motion was not dependent upon the fatty acid composition. (3) Incorporation of cholesterol broadens all observed chain resonances of all phosphatidylcholines, thus demonstrating a reduction in chain motion by cholesterol. For both the saturated and unsaturated phosphatidylcholines the reduction of the chain motion is decreased with increasing chain length. (4) The chemical shift of the carbonyl resonances of sonicated dipalmitoylphosphatidylcholine vesicle labelled with 13C in both chains in the 1-position was slightly decreased by the incorporation of 50 mol% of cholesterol. In contrast, 50 mol% of epicholesterol, the 3alpha0OH isomer, produced a large downfield shift and a splitting of the carbonyl resonance.

Biological and model membranes studied by nuclear magnetic resonance of spin one half nuclei

There is an inherent anisotropy in biological and model membrane systems and this anisotropy has a profound influence on the nuclear magnetic resonance spectra of such systems. For nuclei with a quadrupole moment the quadrupole coupling usually dominates the NMR spectrum, while for nuclei with spin quantum numberI= ½ dipolar couplings provides the most important effects. The quadrupole coupling only affects isolated spins and usually gives rise to simple spectra. The dipole interactions on the other hand couple several spins. In a lipid bilayer with a multitude of spin-½ nuclei these couplings can give rise to complicated manybody effects. Thus the interpretation of the NMR spectra of membrane systems poses problems but at the same time there is a lot of information to be gained.

Effect of bilayer curvature on vibrational Raman spectroscopic behavior of phospholipid-water assemblies

In order to clarify the effect of bilayer curvature upon phospholipid conformation, vibrational Raman spectra were recorded for dipalmitoyl and dimyristroyl phosphatidylcholine in the gel state for both multilayer and single-wall vesicle assemblies. An intensity comparison, based upon a nonperturbing internal standard, between the two classes of bilayrer systems reflected a decrease in peak height intensity for the observed hydrocarbon chain transitions in the single shell vesicle form. No intensity change between bilayer form was detected, however, for the two observed head group modes. Trends in the peak height intensity rations for the 1100 cm-1 carbon-carbon stretching vibrations indicated an increase in hydrocarbon chain transgauche isomerization for the vesicle in comparison to the multilayer arrangements. The sensitivity of the methylene carbon-hydrogen stretching modes to interchain interactions was demonstrated by comparisons of the intensity patterns in the 2900 cm-1 region to the intensity characteristics of the carbon-carbon stretching region for polycrystalline, multilayer and vesicle materials. Examination of various carbon-carbon stretching mode intensity ratios for cholesterol doped dipalmitoyl phosphatidylcholine bilayers indicated that while 25 mol% cholesterol increased the transgauche acyl chain isomerization in multilayers, no comparable effect was observed for the vesicle forms. In contrast, the methylene twisting/methylene deformation intensity ratios for the cholesterol containing systems suggested that some further type of interchain perturbation occurs in the vesicle aggregations.

Temperature-dependent 13C nuclear magnetic resonance studies of human serum low density lipoproteins

The natural abundance 13C nuclear magnetic resonance (NMR) spectrum of human serum low density lipoproteins (LDL) shows significant temperature-dependent changes. These temperature-dependent spectra have been used to monitor changes in the organization of cholesterol esters within the LDL particle. Comparison with 13C NMR spectra of both cholesterol linoleate and an aqueous codispersion of cholesterol linoleate and egg phosphatidylcholine suggests that at low temperatures (10 degrees C), the cholesterol esters in LDL are organized in a smectic-like, liquid-crystalline arrangement. At temperatures above the order-disorder transition exhibited by the cholesterol esters of LDL, the cholesterol esters appear to be partially melted but still are motionally restricted compared with liquid cholesterol esters.

Molecular motion and order in single-bilayer vesicles and multilamellar dispersions of egg lecithin and lecithin-cholesterol mixtures. A deuterium nuclear magnetic resonance study of specifically labeled lipids

Deuterium (2H) nuclear magnetic resonance (NMR) quadrupole splittings and relaxation times have been measured for a variety of specifically deuterated lipids intercalated in lamellar-multibilayer dispersions and single-bilayer vesicles of egg lecithin and lecithin-cholesterol mixtures. The deduced order parameters and relaxation times vary with position of deuteration, acyl chain length, unsaturation, and temperature. The order parameters and spinlattice relaxation times T1 indicate rapid intramolecular motions of restricted amplitude in both the choline head group and hydrocarbon chains. The ordering profile for the acyl chains is similar to that predicted by statistical-mechanical theory. The order parameters yield estimates of the bilayer thickness and linear coefficient of expansion in close agreement with the x-ray determinations. A comparison of the deuterium and electron spin resonance spinprobe order parameters demonstrates the perturbation of the bilayer by the bulky nitroxide probe. The transverse relaxation time T2 for single-bilayer vesicles is quantitatively accounted for by a simple modification of classical relaxation theory which takes into account the modulation of the static quadrupole interaction by rapid local molecular motions and the modulation of the residual quadrupole interaction by the slower overall tumbling of the vesicle. It is unambiguously demonstrated that molecular motion and order in single-bilayer vesicles are very similar to those in lamellar multibilayers. Significant differences occur only for a few segments near the terminal methyl groups of the acyl chains, where the order parameters for vesicles are 10-30% smaller than those found for lamellae. The incorporation of cholesterol in lecithin bilayers is shown to increase the degree of orientational order in vesicles and lamellae, and to increase the hydrodynamic radius of vesicles. Thus, single-bilayer vesicles and multilamellar dispersions of phospholipids are equally useful models for biological membranes. They yield equivalent information about the internal organization and mobility of lipid bilayers, when the spectral manifestations of overall vesicle motion are correctly taken into account.