A general synthesis of glycerophospholipids

Phosphoinositide-dependent activation of the ADP-ribosylation factor GTPase-activating protein ASAP1. Evidence for the pleckstrin homology domain functioning as an allosteric site

The ADP-ribosylation factor (Arf) family of GTP-binding proteins are regulators of membrane traffic and the actin cytoskeleton. Both negative and positive regulators of Arf, the centaurin beta family of Arf GTPase-activating proteins (GAPs) and Arf guanine nucleotide exchange factors, contain pleckstrin homology (PH) domains and are activated by phosphoinositides. To understand how the activities are coordinated, we have examined the role of phosphoinositide binding for Arf GAP function using ASAP1/centaurin beta4 as a model. In contrast to Arf exchange factors, phosphatidylinositol 4, 5-bisphosphate (PtdIns-4,5-P(2)) specifically activated Arf GAP. D3 phosphorylated phosphoinositides were less effective. Activation involved PtdIns-4,5-P(2) binding to the PH domain; however, in contrast to the Arf exchange factors and contrary to predictions based on the current paradigm for PH domains as independently functioning recruitment signals, we found the following: (i) the PH domain was dispensable for targeting to PDGF-induced ruffles; (ii) activation and recruitment could be uncoupled; (iii) the PH domain was necessary for activity even in the absence of phospholipids; and (iv) the Arf GAP domain influenced localization and lipid binding of the PH domain. Furthermore, PtdIns-4,5-P(2) binding to the PH domain caused a conformational change in the Arf GAP domain detected by limited proteolysis. Thus, these data demonstrate that PH domains can function as allosteric sites. In addition, differences from the published properties of the Arf exchange factors suggest a model in which feedforward and feedback loops involving lipid metabolites coordinate GTP binding and hydrolysis by Arf.

Synthesis of 2-deoxy-2-fluoro-phosphatidylinositol-4,5-bisphosphate and analogues: probes and modulators of the mammalian PI-PLCS

An approach to synthesis of 2-modified phosphatidylinositol-4,5-bisphosphates, which are substrate analogues useful as probes and modulators of the PI-PLC enzyme family, is described and illustrated for the dibutyl-2-deoxy-2-fluoro analogue, a probe designed for delineating substrate and PI-PLC interactions by X-ray crystallography.

Domains in cationic lipid plus polyelectrolyte bilayer membranes: detection and characterization via 2H nuclear magnetic resonance

2H nuclear magnetic resonance (NMR) spectroscopy of choline-deuterolabeled 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-alpha-d2 and POPC-beta-d2) has been used to detect and quantify domain formation induced in cationic lipid-containing bilayers upon the addition of anionic polyelectrolytes. Three different polyelectrolytes, poly(sodium 4-styrenesulfonate) or PSSS, poly(sodium acrylate) or PACA, and poly(sodium glutamate) or PGLU, were added to POPC lipid bilayers containing 1,2-dioleoyl-3-(dimethylamino)propane (DODAP) as the cationic amphiphile. All three polyelectrolytes produced two-component 2H NMR spectra, consistent with two populations of POPC, one polyelectrolyte-bound and another polyelectrolyte-free. The relative intensities of the two spectral components provided the relative amounts of the two POPC populations. The 2H NMR quadrupolar splitting from either spectral component provided the DODAP content of the particular POPC population. The two POPC populations differed in that the polyelectrolyte-bound population contained a stoichiometric polyelectrolyte anion:DODAP cation ratio leading to enrichment with respect to DODAP, while the polyelectrolyte-free population was depleted of DODAP. Estimates of the size of a polyelectrolyte-defined domain revealed a constant number of bound DODAP but a flexible number of bound POPC, which increased in proportion to the global POPC content. The most compact domains were formed by the most hydrophobic polyelectrolyte, PSSS, while the most expansive domains were formed by the most hydrophilic polyelectrolyte, PGLU.

Syntheses of 1,2-di-O-palmitoyl-sn-glycero-3-phosphocholine (DPPC) and analogs with 13C- and 2H-labeled choline head groups

The syntheses of four head group labeled analogs of 1,2-di-O-palmitoyl-sn-glycero-3-phosphocholine (DPPC) (6) by a general method from 1,2-di-O-palmitoyl-sn-glycero-3-phosphatidic acid (5) have been performed. The syntheses of 1,2-di-O-palmitoyl-sn-glycero-3-phospho[alpha-13C]choline (6a) and 1,2-di-O-palmitoyl-sn-glycero-3-phospho[beta-13C]choline (6b) were performed from labeled [1-13C]glycine (1a) in 52% overall yield and from [2-13C]glycine (1b) in 56% overall yield, respectively. 1,2-Di-O-palmitoyl-sn-glycero-3-phospho[N(C2H3)3]choline (9) was prepared from 2-aminoethanol in 39% overall yield. 1,2-Di-O-palmitoyl-sn-glycero-3-phospho[alpha-C2H2]choline (12) was prepared from N,N-dimethylglycine ethyl ester in 50% overall yield.

High-performance liquid chromatography of phosphatidic acid

This paper reviews existing high-performance liquid chromatographic (HPLC) methods for the analysis of phosphatidic acid (PA) in various sample matrices. In addition to the introductory background discussion on important aspects of PA in lipid biochemistry, the review provides comprehensive coverage in the areas of derivatization techniques, detection methods, and HPLC separation techniques. Conversions of PA to suitable derivatives enhance the detection sensitivity and improve the chromatographic behavior of the analytes. Detection methods include the use of state-of-the-art detectors and are discussed in terms of sensitivity, specificity, and compatibility with analytical systems. Pertinent normal-phase and reversed-phase HPLC data for PA are compiled from published methods.

Resolving the two monolayers of a lipid bilayer in giant unilamellar vesicles using deuterium nuclear magnetic resonance

Giant unilamellar vesicles (GUVs) composed of mixtures of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) plus DMPG (1,2-dimyristoyl-sn-glycero-3-phosphoglycerol) and/or CHOL (cholesterol) were prepared using detergent dialysis. Vesicles containing at least 30 mol % CHOL had diameters exceeding 450 nm. POPC in such GUVs, deuterium-labeled at either the choline alpha or beta segments, yielded deuterium (2H) and phosphorus (31P) nuclear magnetic resonance (NMR) Pake pattern line shapes, quadrupole splittings and chemical shift anisotropies identical to those obtained with multilamellar vesicles (MLVs) of identical composition. Exposing exclusively the vesicle exterior to either calcium or perchlorate ions, both of which are known to influence lipid head-group conformation through surface charge effects, caused the appearance of two overlapping 2H Pake patterns of equal intensity. The quadrupole splitting of one component remained unchanged while that of the second component was altered in the manner expected for choline alpha or beta deuterons in the presence of a cationic (calcium) or anionic (perchlorate) surface charge. Freeze-thawing the GUVs to equilibrate the exterior and interior vesicular contents eliminated the initially unchanged spectral component. It was likewise possible to resolve two quadrupole splittings when Staphylococcus aureus delta-toxin, a surface-active peptide known to influence lipid head-group orientational ordering, was added to the exterior vesicular solution only. This indicates that delta-toxin upon binding remains confined to one monolayer of the lipid bilayer and does not traverse the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of lipid lateral distribution on the surface charge response of the phosphatidylcholine headgroup as detected using 2H nuclear magnetic resonance

The effect of lipid lateral distribution on the surface charge response of the phosphatidylcholine headgroup, in bilayers composed of binary mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA), was investigated by monitoring the deuterium nuclear magnetic resonance (2H-NMR) spectrum of choline-deuterated phosphatidylcholine as a function of temperature and DMPA concentration. Addition of DMPA at temperatures corresponding to fully liquid-crystalline membranes caused a progressive increase (decrease) in the 2H-NMR quadrupole splitting from POPC-alpha-d2 (POPC-beta-d2), in agreement with the known response of phosphatidylcholine to negative membrane surface charge (Seelig, J., Macdonald, P.M. and Scherer, P.G. (1987) Biochemistry 26, 7535-7541). Lateral phase separation of DMPA-rich domains was induced in these mixtures by lowering the temperature in the range from 60 degrees C to -15 degrees C, and was accompanied by a reversal of the original effects of DMPA on the quadrupole splitting. Analysis of the 2H-NMR spectral response allows one to generate a temperature/composition phase diagram for the POPC/DMPA system. We conclude that 2H-NMR of headgroup-deuterated phosphatidylcholine can be employed to sense and to quantify inhomogeneities in the lateral distribution of charged membrane components.

Influence of staphylococcal delta-toxin on the phosphatidylcholine headgroup as observed using 2H-NMR

The interaction of the 8-toxin peptide isolated from Staphylococcus aureus with the headgroup region of lipid bilayer membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was investigated using deuterium (2H) and phosphorus (31P) nuclear magnetic resonance (NMR) spectroscopy. At relatively low peptide/lipid ratios (P/L < 0.10), all 2H- and 31P-NMR spectral lineshapes at 25 degrees C were indicative of a single population of liquid-crystalline lipids in a bilayer arrangement. At these P/L ratios, delta-toxin had only marginal effects on the size of the quadrupole splitting measured from POPC labelled at either the alpha-methylene (POPC-alpha-d2) or the beta-methylene segment (POPC-beta-d2) of the choline headgroup and, similarly small effects on the magnitude of the chemical shift anisotropy (CSA) of the 31P-NMR spectrum. With increasing amounts of delta-toxin (0.10 < P/L < 0.15) the size of the 2H quadrupole splitting from POPC-alpha-d2, as well as the magnitude of the 31P-CSA, decreased progressively and rapidly. The quadrupole splitting from POPC-beta-d2, however, remained relatively unaffected. At yet higher levels of delta-toxin (P/L > 0.15), all 2H- and 31P-NMR spectra indicated the presence of multiple lipid populations experiencing varying degrees of increased conformational disordering. The spectral lineshapes of these apparently nonbilayer spectral components reverted to bilayer-type lineshapes upon lowering the measuring temperature to 5 degrees C. At the utmost highest level of delta-toxin measured here (P/L = 0.20), all 2H- and 31P-NMR spectra consisted of a single, broad, apparently nonbilayer-type component, indicative of hindered but virtual isotropic motional averaging of the POPC headgroups. In this case no reversion to bilayer-type spectra could be obtained by decreasing the temperature. We could obtain no evidence that the conformation of the choline headgroup of POPC was responding to any specific influence of delta-toxin on bilayer surface electrostatics.

Synthesis of alkylphosphonates, a new class of antineoplastic agents

Three different phosphonate analogues of hexadecylphosphocholine, a representative of a new class of antineoplastic agents, were synthesized. The structures of the newly synthesized compounds have been devised to contain only one cleavage point for either phospholipase C or phospholipase D. These structural features should allow an examination of the importance of these enzymes for the antineoplastic activities of alkylphosphocholines.

Response of the headgroup of phosphatidylglycerol to membrane surface charge as studied by deuterium and phosphorus-31 nuclear magnetic resonance

The response to membrane surface charge of the glycerol headgroup of dimyristoyl-phosphatidylglycerol (DMPG) was investigated via deuterium and phosphorus-31 nuclear magnetic resonance spectroscopy. The membrane surface charge was manipulated by adding various amounts of neutral dimyristoylphosphatidylcholine (DMPC) and/or positively charged didodecyldimethylammonium bromide (DDAB) to the negatively charged DMPG, selectively deuterated at the alpha and beta segments of its glycerol headgroup. The deuterium and phosphorus-31 nuclear magnetic resonance spectra were all characteristic of random dispersions of liquid-crystalline lipids in a bilayer configuration. Differential scanning calorimetry showed that all mixtures investigated exhibited gel to liquid-crystalline phase transitions below 35 degrees C. Measurements of the deuterium quadrupole splitting and of the phosphorus-31 chemical shift anisotropy lead to the following observations. (1) Dilution of the negative surface charge density by the addition of DMPC had little effect on the quadrupole splitting from either alpha- or beta-deuterated DMPG. (2) Direct cancellation of the negative surface charge density by addition of DDAB led to a progressive decrease in the quadrupole splitting measured from alpha-deuterated DMPG, while the quadrupole splitting measured from beta-deuterated DMPG increased. For alpha-deuterated DMPG addition of 0.3 mole fraction of DDAB resulted in the appearance of two distinct quadrupole splittings. No such effect was observed for beta-deuterated DMPG.

Halo lipids. 10. Synthesis and cytostatic activity of O-alkylglycerophospho-L-serine analogs

The synthesis of O-alkylglycerophospho-L-serine analogs is described, which represent a new class of cytostatically active agents based on phospholipids. The new compounds were obtained by conversion of O-alkylglycerophosphoric ester analogs by means of phospholipase D and by condensing O-alkylglycerophosphoric acid analogs with protected L-serine followed by the removal of protective groups of the resulting intermediates. The structure of the O-alkylglycerophospho-L-serines was confirmed by fast atom bombardment mass spectrometry. The compounds were found to inhibit the growth of Ehrlich ascites tumor cells in vitro. Half maximum inhibition was observed at concentrations between 7 and 15 microM. For the 1-O-alkyl-2-methoxy glycerophosphoserine only a higher value (30 microM) was found. With most of the substances tested growth was completely inhibited at a concentration of 40 microM.

The pH dependence of headgroup and acyl chain structure and dynamics of phosphatidylserine, studied by 2H-NMR

By varying the pH, the influence of the ionization degree on the structure and dynamics of aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS) was studied, using 2H-NMR methods. For this purpose DOPS was synthesized with deuterium labels incorporated either stereospecifically at the beta-position of the serine headgroup ([2-2H]DOPS) or at the 11-position of both acyl chains ([11,11-2H2]DOPS), allowing the effects of pH on headgroup and acyl chains to be measured in parallel. A large scale synthesis procedure of stereospecific 1,2-dioleoyl-sn-glycero-3-phospho-[2-2H]-L- serine is described. The quadrupolar splitting (delta nu q) of [2-2H]DOPS is shown to be a sensitive sensor for the degree of protonation of the molecule. Whereas the delta nu q of [2-2H]DOPS decreases upon lowering the pH, that of [11,11-2H2]DOPS gradually increases, indicating an increase in acyl chain ordering. In the pH range below the pKa value, DOPS exhibits a temperature-dependent bilayer to hexagonal HII phase transition, apparent from the 31P-NMR spectra and the occurrence of a second component in the [11,11-2H2]DOPS 2H-NMR spectrum, with a much smaller delta nu q. The HII phase component in spectra from [2-2H]DOPS coincides with the isotropic position and has no defined delta nu q. In the bilayer organization delta nu q and spin-lattice relaxation time (T1) values for the acyl chain deuterated DOPS are similar to those obtained for other lipid systems. In contrast the PS headgroup region displays a relatively rigid structure as evidenced by a large delta nu q and very small T1 values. Upon adopting the HII phase the T1 values of the acyl chain deuterons are hardly affected. The uniqueness of the PS headgroup with respect to structure and motional properties is reinforced by the occurrence of a T1 minimum at 45 degrees C in the measurement of the temperature dependence of T1 for [2-2H]DOPS in the hexagonal HII configuration. Quantitative analysis yields a correlation time (tau c) for the motions determining T1 under these conditions, of 3.45 ns.

Synthesis and characterization of deoxy analogues of diphytanylglycerol phospholipids

Novel analogues of diphytanyl phospholipids, 2,3-diphytanyl sn-1-glycerol-1-phosphoryl-1'-(1',3'-propanediol) (dPG), 2,3-diphytanyl-sn-glycerol-1-phosphoryl-1'-propanol (ddPG) and 2,3-diphytanyl-sn-glycerol-1-phosphoryl-1'-(1',3'-propanediol-3'-p hosphate) (dPGP), were synthesized according to modifications of previously published procedures. The samples were TLC and analytically pure and were characterized by 13C- and 1H-NMR and negative FAB/MS. The pK values of dPGP in aqueous dispersions or in methanol/water (1:1, v/v) were determined by potentiometric titration and compared with those of 2,3-diphytanyl-sn-glycerol-1-phosphoryl-3'-sn-glycerol-1'-phosphat e (PGP). The dissociation constant of the third ionizable POH group of dPGP was more than 2 pK units higher than that of PGP, indicating that the free glycerol hydroxyl group plays an important role in headgroup conformation and stabilization, perhaps through hydrogen bonding with the phosphate group(s).

Specific interaction of arabinose residue in ginsenoside with egg phosphatidylcholine vesicles

The interaction of the specific sugar residue in ginsenosides with egg phosphatidylcholine vesicles was investigated by ESR spectrometry using phosphatidic acid spin-labeled at the polar head groups. Ginsenoside-Rc, which has an alpha-L-arabinofuranose residue and agglutinability toward egg yolk phosphatidylcholine vesicles (Fukuda, K. et al. (1985) Biochim. Biophys. Acta 820, 199-206), caused the restriction of the segmental motion of spin-labeled phosphatidic acid in egg phosphatidylcholine vesicles, indicating that the saponin interacted with the polar head groups of vesicles. Other ginsenosides-Rb2, Rb1, Rd and p-nitrophenyl glycoside derivatives which have less or no agglutinability were also investigated in the same manner. Only ginsenoside-Rb2 and p-nitrophenyl alpha-L-arabinofuranoside which have the specific sugar residue (arabinose) showed a strong interaction with the polar head groups of vesicles. To gain an insight into the mechanism of agglutination by ginsenoside-Rc, the interaction with the fatty acyl groups was also studied by using phosphatidylcholine spin-labeled at the fatty acyl groups. Ginsenoside-Rc increased the order parameter of the spin-labeled phosphatidylcholine, indicating that the saponin was inserted into lipid bilayers. In other saponins investigated, only ginsenoside-Rb2 interacted with the fatty acyl part of vesicles. The process of expression of agglutination by ginsenoside-Rc was discussed on the basis of the ESR studies.

Modification of the polar head group of platelet-activating factor: influence on the biological activity

Racemic analogues of platelet-activating factor and its lyso derivatives have been prepared in which one methyl of the trimethylammonium group has been replaced by ethyl, propyl, allyl, or carboxymethylene. The influence of chemical modification on the biological activity was assessed by measuring platelet aggregation and desensitization. The results point to a crucial role of a positively charged polar head group for the expression of biological activity of platelet-activating factor. There are also some indications of a more non-specific interaction of the polar head group of platelet-activating factor with its platelet binding sites.

Deuterium NMR study of head-group deuterated phosphatidylserine in pure and binary phospholipid bilayers. Interactions with monovalent cations Na+ and Li+

Head-group deuterated 1,2-dimyristoyl-sn-glycero-3-phosphorylserine (DMPS) was synthesized. 2H NMR spectra reflect the ionic strength-dependent polymorphism of DMPS aqueous dispersions. Results obtained with pure DMPS and mixed bilayers with phosphatidylcholine or phosphatidylethanolamine at various NaCl or LiCl concentrations indicate that interactions with Na+ and Li+ have very different effects upon the head-group quadrupole splittings.

Comparative structural aspects of cation binding to phosphatidylserine bilayers

X-ray diffraction data recorded for monovalent and divalent cation complexes of a series of phosphatidylserines (PS) varying in chain length reveal a simple structural pattern. Only two bilayer structural types differing in hydrocarbon chain tilt but with similar polar group conformations are observed for (i) anhydrous acidic PS, (ii) anhydrous K+-PS, and (iii) Li+, Mg2+, Ca2+, Sr2+, Ba2+, and Pr3+ complexes of 'hydrated' PS. The X-ray diffraction data suggest that PS becomes dehydrated on complexing with Li+, Mg2+, Ca2+, and other divalent cations and adopts either the chain untilted (form I) or tilted (form II) bilayer structure.

Liposomes from polymerizable phospholipids

The synthesis and characterization of a great variety of single and double chain phospholipids containing the diacetylene and butadiene moiety is described. These substances can be dispersed in water by ultrasonication and the resulting vesicles can be photopolymerized with the retention of their original structure. Absorption spectra of the polymerized diacetylenic lipids show significant differences depending on the molecular structure of the monomers. By the polymerization reaction, the gel to liquid crystalline phase transition is suppressed, which does not correspond to the properties of biological membranes. Evidence for enhanced stability of polymerized vesicles is given by treatment with ethanol and detergents showing that trapped markers are released to a much smaller extent than in the case of unpolymerized vesicles. Diacetylenic lipids show a pronounced hysteresis of the phase transition. If the membrane of supercooled vesicles crystallizes, all trapped marker is released within seconds. Possibilities for overcoming this extreme rigidity of the membranes are discussed.

Simple preparation of 1,2-dipalmitoyl-sn-glycero-3-phosphoric acid and deuterated choline derivatives

Analytically pure 1,2-dipalmitoyl-sn-glycero-3-phosphoric acid was prepared in gram amounts, using a simplified version of a previous procedure. The main step, enzymatic cleavage of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine with freshly extracted phospholipase D, was performed in the presence of chloroform and the crude phosphatidic acid was purified by silica gel column chromatography. The interest of the method was illustrated by the synthesis of two dipalmitoyl phosphatidylcholines selectively deuterated on the polar headgroup.

A new synthesis of bis(diacylglycero)phosphate

The chemical synthesis of bis(diacylglycero) phosphate previously named bisphosphatidic acid, starting with a diacylglycerol and phosphatidic acid, is described. The phosphodiester bond formation is catalyzed by triisopropylbenzenesulfonylchloride. This simple approach allows the preparation of saturated as well as unsaturated bis(diacylglycero)phosphate species in one step without the use of any protecting group. The methods used until now yield only mono-acid species, or mixed-acid unsaturated species after many steps involving the introduction and the removal of protecting groups. The synthetic products have been characterized by component analysis and NMR-techniques.

Interaction of enantiomeric alkylacyl and diacylglycero-phosphocholines with cholesterol in bilayer membranes

The sn-1 and sn-3 isomers of dioleoylglycerophosphocholine form vesicles of the same size as the racemic lipid. Identical permeability coefficients were found for the diffusion of glucose and chloride across bilayer membranes of vesicles consisting of these lipids. Vesicles made of mixtures of enantiomeric or racemic dioleoyllecithin with 30 mol% cholesterol have identical radii. Cholesterol reduces the permeability of bilayers for glucose and chloride irrespective of the steric configuration of the constituent phospholipid. Increasing concentrations of cholesterol (17, 33 and 50 mol%, respectively) broaden the (CH2)n signal in the 1H-NMR-spectra (90 MHz) of unilamellar vesicles containing sn-1, sn-3 or rac alkyloleoylglycerophosphocholine to the same extent. These results indicate that the steric configuration of phospholipids has no gross effect on the arrangement of phospholipids and cholesterol in bilayer membranes.

Structure and thermotropic behavior of phosphatidylserine bilayer membranes

The structure and thermotropic properties of a homologous series of diacylphosphatidylserines (PS) in the anhydrous and hydrated states have been examined with low-angle X-ray diffraction and differential scanning calorimetry. In the anhydrous state at low temperatures both acidic PS and its NH4+ salts exhibit lamellar bilayer crystal forms that transform to liquid-crystalline hexagonal (type II) structures at higher temperatures. The crystal leads to liquid-crystal transition temperature increases with increasing chain length, the transition temperature of an NH4+-PS being higher than that of its corresponding acidic form. In contrast, the transition enthalpies of the acidic PS are higher than those of the NH4+ salt forms. Hydrated cyclic PS and NH4+-PS exhibit reversible lamellar gel leads to liquid-crystal transitions. In this case the acidic form undergoes this chain length dependent transition at a higher temperature, but with a lower enthalpy change, than the NH4+-PS. Both below and above the hydrocarbon chain melting transition, hydrated lamellar bilayer structures are present. The temperature-composition phase diagram of the NH4+-dimyristoyl-PS/H2O system has been studied in detail. The chain melting transition decreases with increasing hydration, reaching a limiting value of 39 degrees C. X-ray diffraction shows that both the bilayer gel structure and the bilayer liquid-crystal form take up water continuously (i.e., no hydration limit), a characteristic of lipid bilayers with a net charge. Electron-density profiles a NH4+-dimyristoyl-PS at different hydration levels permit detailed analysis of the structural parameters of the PS bilayer.

Motions and interactions of phospholipid head groups at the membrane surface. 2. Simple alkyl head groups

As a reference point for comparison with more complex head groups, a set of phospholipids with simple alkyl head groups has been studied. These analogues resemble the naturally occurring phospholipids, except they have phosphomethanol, -ethanol, -1-propanol, and -1-butanol as head groups. The gel-to-liquid-crystalline phase transition temperatures were measured with differential scanning calorimetry, and the phase properties of multilamellar dispersions were examined with phosphorus-31 NMR. The effect of the head-group size was found to be rather small. These lipids were synthesized with deuterium labels incorporated into the alcohol portion at all positions in the head groups except butanol, which was labeled only in the C-1 position. Determination of the 2H residual quadrupole splittings led to an analysis of the head-group ordering properties. Specifically, these data showed that increasing the length of the head group leads to a more perpendicular orientation of the head group relative to the bilayer surface. Phosphorus-31 chemical shift anisotropy data were also compatible with this result. Measurement of surface pressure-area diagrams of monolayers of these compounds revealed that at high pressures (30 dyn/cm) all four lipids occupied similar areas (40-44 A2/molecule), yet at lower pressures, the larger the head group, the larger the occupied surface area. This result suggests that in a bilayer the fatty acyl chains occupy similar areas independent of the alkyl head-group size, and the larger head groups cannot pack properly without some conformational adjustment. The addition of phosphatidylcholine with its relatively bulky head group decreases the area available to the alkyl head groups, pushing the alkyl head groups out of the plane of the bilayer surface. Cholesterol, on the other hand, acts as a "spacer", increasing the area available to the head group, and leads to the opposite effect; i.e., the head groups can relax into a conformation more parallel to the bilayer surface. These data illustrate the types of steric effects which can be expected at the membrane surface. Dynamic properties were investigated by measurement of the 2H spin-lattice (T1) NMR relaxation times. These relaxation times could be compared with those from other parts of a phospholipid molecule, namely, the glycerol backbone and the fatty acyl chains. The rates of segmental motion in these head groups were similar to the first C-2 to C-8 segments of the fatty acyl chains, indicating considerable head-group flexibility.

Motions and interactions of phospholipid head groups at the membrane surface. 3. Dynamic properties of amine-containing head groups

The dynamic properties of the amine-containing head groups of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and a new phospholipid, phosphatidylserine methyl ester, were studied. Deuterium NMR spin-lattice (T1) relaxation times of deuterium labels specifically incorporated in the head groups were measured in multilamellar dispersions of these phospholipids. As a reference point, the data were compared with T1 values from a phospholipid with a simple phosphopropanol head group. In the liquid-crystalline state at both the alpha (P--O--CD2--CH) and beta (P--O--CH2--CD) head-group segments, the values of the T1 relaxation times decreased in the order propyl greater than choline greater than ethanolamine greater than serine = serine methyl ester. In the propyl and choline head groups, the beta-segment T1 values were longer than those of the alpha segment, indicating increasing flexibility as one progressed toward the free end of the head group. The ethanolamine and serine head groups had essentially identical T1 values at all positions. Phosphorus-31 NMR spin-lattice relaxation times were found to parallel the deuterium results for the neighboring alpha segment. These data indicate that the phosphatidylserine head group is less flexible than that of phosphatidylethanolamine which in turn was more rigid than the phosphatidylcholine head group. The phosphatidylserine head-group T1 values were as short as those of the glycerol backbone moiety of phosphatidylcholine which is known to be a relatively rigid section of the phospholipid molecule. The relaxation time data for liquid-crystalline phase phosphatidylserine and gel phase phosphatidylglycerol were quantitatively similar, indicating that for those motions which contribute to T1 relaxation, the two head groups are in similar states. These differences in head-group rigidity are discussed in terms of the capacity of the various head groups to bind noncovalently to their neighbors in the plane of the membrane surface.

Synthesis of glycerophospholipids

Recent advances in the synthesis of phosphatidic acids, phosphatidylethanolamines and phosphatidylcholines are described. Methods for the synthesis of some alkylacyl and alk-1-enylacyl analogues of the common diacylglycerophospholipids are also discussed. In addition, synthetic routes are described, that lead to unusual phospholipids such as compounds containing the polar group at position 2 of the glycerol moiety, glycerophospholipids containing alkanolamines of different chain lengths, and glycolphospholipids. All of the common glycerophospholipids can be prepared without the use of protecting groups. Major advances in phospholipid synthesis involve the application of novel phosphorylating agents and the use of cyclic intermediates. Although phosphatidylserines and phosphatidylthreonines as well as phosphatidylglycerols and cardiolipins can be prepared by chemical synthesis, further systematic studies are required to work out procedures that lead to these compounds in high yields.