Synthesis of phospholipid analogues. Variation of the P—N distance

The synthesis of mono-alkyl phosphates and their derivatives: an overview of their nature, preparation and use, including synthesis under plausible prebiotic conditions

Nucleic acids, phospholipids and other organic phosphates play central roles in biological pathways.

Calcium-dependent binding of rabbit C-reactive protein to supported lipid monolayers containing exposed phosphorylcholine group

The interaction of rabbit C-reactive protein (rCRP) with a supported monolayer containing a phosphorylcholine moiety was studied. Three types of phospholipids were synthesized, each containing a insertion spacer of eight, six, or three atoms between the phosphorylcholine group and hydrophobic tail. By varying the length of the insertion spacer, we can vary the extension of the phosphorylcholine group from the membrane surface. By varying the monolayer composition, we can control the lateral distance between the exposed phosphorylcholine groups. Using the surface plasmon resonance technique (SPR), we demonstrated that the calcium-dependent binding of rCRP to the model membrane is governed not only by the ability of the ligand to access the binding pocket fully (spacer length), but also by lateral hindrance within the two-dimensional plane of the membrane. The value of the apparent binding constant was estimated by theoretical analysis, which is obviously dependent on the composition of the lipid mixture, and a maximum of (9.9 +/- 1.5) x 10(6) M-1 was obtained.

Synthesis of DL-alpha-tocopherol-ligated phospholipids

A three-step synthesis of DL-alpha-tocopheryl-6-O-phosphorylcholine and its -6-O-phosphorylethanolamine homologue is described. The reaction pathway involves the conversion of DL-alpha-tocopherol to the corresponding 2-bromoethyl phosphate, hydrolytic cleavage of the remaining phosphorochloridate bond and treatment of the diester intermediate formed with either aqueous trimethylamine or ammonia to give the aminated target derivatives in good overall yields (61-67%). The compounds represent new models of ligated lipid mediators in which a vitamin E-unit is covalently bonded to a naturally occurring phospholipid fragment.

Two-dimensional crystallization of rabbit C-reactive protein on lipid monolayers

Two-dimensional (2D) crystals of rabbit C-reactive protein (CRP) have been obtained by protein binding on lipid monolayers at the air/water interface. Two different types of crystalline arrays of CRP were obtained, by specific binding and non-specific adsorption to the lipids. Electron crystallographic analysis of the negatively stained specimens showed that the unit cell parameters of the CRP 2D crystals formed by specific binding were a=81 angstroms, b=78 angstroms, gamma=118.35 degrees, and those formed by nonspecific adsorption were a=74 angstroms, b=67 angstroms, gamma=95.5 degrees, both with the layer group p1. Projection maps were obtained at a resolution of 26 angstroms and 22 angstroms respectively. They showed that only the monomers of the CRP were packed in the 2D arrays and the orientations of the monomers on the lipid monolayers were different in the two types of crystals. By comparing the two projection maps, a preliminary shape of the CRP monomer has been derived. A model of the pentameric structure of the oligomeric CRP has been proposed.

Analogs of alkyllysophospholipids: chemistry, effects on the molecular level and their consequences for normal and malignant cells

In the search for new approaches to cancer therapy, the first alkyllysophospholipid (ALP) analogs were designed and studied about two decades ago, either as potential immunomodulators or as antimetabolites of phospholipid metabolism. In the meantime, it has been demonstrated that they really act in this way. However, their special importance is based on the fact that, in addition, they interfere with key events of signal transduction, such as hormone (or cytokine)-receptor binding or processing, protein kinase C or phospholipase C function and phosphatidylinositol and calcium metabolism. There are no strict structural requirements for their activity. Differences in the cellular uptake or the state of cellular differentiation seem to be mainly responsible for higher or lower sensitivities of cells towards ALP analogs. Consequences of the molecular effects mentioned on the cellular level are cytostasis, induction of differentiation (while in contrast the effects of known inducers of differentiation such as 12-O-tetradecanoylphorbol-13-acetate are inhibited, probably as a consequence of protein kinase C inhibition) and loss of invasive properties. Already in sublytic concentrations, alterations in the membrane structure were observed, and lysis may begin at concentrations not much higher than those causing the other effects described. Few ALP analogs have already entered clinical studies or are in clinical use. ALP analogs are the only antineoplastic agents that do not act directly on the formation and function of the cellular replication machinery. Therefore, their effects are independent of the proliferative state of the target cells. Because of their interference with cellular regulatory events, including those failing in cancer cells, ALP analogs, beyond their clinical importance, are interesting model compounds for the development of new, more selective drugs for cancer therapy.

Interaction of phospholipase A2 with thioether amide containing phospholipid analogues

Transferred NOE experiments have been carried out on cobra venom (Naja naja naja) phospholipase A2 (PLA2) with substrate analogues which serve as potent inhibitors. 1-(Hexylthio)-2-(hexanoylamino)-1,2-dideoxy-sn-glycero-3-pho sphoethanolamine (PE) and the corresponding phosphocholine analogue (PC) are water-soluble, short-chain, nonhydrolyzable substrate analogues which bind tightly to the enzyme. Because they are small compounds and monomeric in solution, NOEs develop inefficiently in the absence of enzyme. Thus, the PLA2/inhibitor system is ideal for analyzing transferred NOEs. The experiments are carried out under conditions that are optimal for catalysis, pH 7.5 in the presence of 2 mM CaCl2. The data show the inhibitor conformation in the catalytic site of cobra PLA2 in solution. The effect of the thioether in the sn-1 chain on the chemical shift dispersion of the methylene protons allowed for chain-specific assignments and detailed conformational analysis. Both inhibitors adopt a PLA2-bound conformation in which the end of the sn-2 chain is within 5 A of the alpha-methylene of the sn-1 chain. In addition, intermolecular contact points between the inhibitor and the enzyme were identified by NOEs.

Chemical synthesis and surface activity of lung surfactant phospholipid analogs. II. Racemic N-substituted diether phosphonolipids

A series of racemic 16:0 disaturated N-substituted diether phosphonolipid analogs of glycerophospholipids have been synthesized and purified. Isosteric methylene substitution at three of the four ester sites (carboxyl, phosphate) of conventional glycerophospholipids enhanced the hydrophobicity of analog compounds compared with dipalmitoyl phosphatidylcholine (DPPC), the major glycerophospholipid component of lung surfactant. Further substitutions at the nitrogen headgroup also contributed to hydrophobicity/hydrophilicity characteristics, as well as allowing graded variations in headgroup size among the members of the diether phosphonolipid analog series. Interfacial property studies showed that these compounds had significant differences in surface activity characteristics compared with DPPC, including increased adsorption and respreading facility, plus an enhanced ability to generate low surface tension (less than 1 to 4 mN/m) on an oscillating bubble apparatus at 37 degrees C. In addition, pressure-volume mechanical studies in surfactant-deficient excised rat lungs showed that the diether phosphonate analog of DPPC could partially restore pressure-volume characteristics toward normal, both as a pure component and in binary mixtures with palmitoyl-oleoyl phosphatidylglycerol. These findings suggest that selected analog compounds, synthesized with relatively small structural modifications from biologic glycerophospholipids, may have eventual applications as components of synthetic exogenous lung surfactants. Of more immediate importance, analog molecules with defined structural variations are convenient molecular probes for developing structure-surface activity correlates for phospholipid-like surfactants and for investigating the specificity of interactions between glycerophospholipids and other compounds such as proteins.

Chemical synthesis and surface activity of lung surfactant phospholipid analogs. III. Chiral N-substituted ether-amide phosphonolipids

A homologous series of chiral (R) ether-amide phosphonolipid analogs of naturally occurring (R) glycerophospholipids were synthesized and characterized for their interfacial behaviors. The phosphonolipids possess isoteric ether, amide, and phosphonate functions at positions corresponding to the sn-1, sn-2, and sn-3 ester functions, respectively, of naturally occurring glycerophospholipids. All compounds were synthesized with disaturated C16:0 alkyl/acyl moieties to give structural analogy with dipalmitoyl phosphatidylcholine (DPPC), the major glycerophospholipid component of lung surfactant. Further substitutions at the headgroup nitrogen were also used to generate differences in headgroup size and polarity in the synthetic compounds. The surface activity of the ether-amide phospholipids was investigated in terms of adsorption to the air-water interface, together with studies of dynamic respreading after monolayer collapse and surface tension lowering in dynamically compressed spread films and dispersions. Results showed that several ether-amide phosphonolipids had more rapid adsorption and improved dynamic respreading behavior compared to DPPC, plus the ability to lower surface tension into the range of less than 1 to 4 mN/m in spread films and in dispersions under dynamic conditions. In combination with a series of diether phosphonolipids synthetized in a companion study [1], these ether-amide compounds are useful in the development of molecular structure-surface activity correlates for lung surfactant-related materials, and should assist in investigating the specificity of interactions between phospholipids and other pulmonary biological molecules.

Metabolism of ether phospholipids and analogs in neoplastic cells

The ether phospholipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (OM-GPC) is known to be a potent inhibitor of cell growth. Metabolic studies in both Raji and L1210 leukemic cells on OM-GPC, 3H-labeled in the methyl groups of the choline moiety, showed a (diacyl)-phosphatidylcholine as the only labeled metabolite. Since the formation of radiolabeled (diacyl)-phosphatidylcholine showed a direct correlation with cell death, we tested other lipid analogs. One of these compounds, hexadecylphosphocholine (He-PC), which was 3H-labeled in the methyl-choline groups, showed a formation of labeled (diacyl)-phosphatidylcholine similar to that found with OM-GPC. Again, there was a direct linear correlation between the formation of the labeled product and cell death. He-PC was found to be a potent cell toxin in in vitro experiments on cell cultures. However, analogs with an elongated phosphor to trimethylammonium distance showed no toxicity towards the cells in in vitro experiments. From the data, we conclude that the ether phospholipids are substrates for a phospholipase C or related enzyme. This substrate property may be responsible for the toxicity of the compounds in neoplastic cells.

Synthesis of covalently immobilized phospholipid analogues

Cyclic 1-O-acyl-2-O-alkyl-glycero-3-phosphotriesters and 1-O-acyl-2-O-alkyl-glycero-3-bromoethylphosphate with a free acyl moiety in position 1 of the glycerol backbone were synthesized. These phospholipid intermediates were covalently bound to AH-Sepharose via the carbodiimide method. After immobilization the corresponding phosphatidylethanolamine analogues were obtained by acid hydrolysis of the cyclic phosphotriesters and by direct amination of the bromoethylphosphate. Thus, in a short, stepwise synthesis including minimum use of protecting groups, a variety of immobilized phospholipid analogues are available as affinity adsorbents for the purification of enzymes related to phospholipid metabolism.

Properties of canine myocardial phosphatidylethanolamine N-acyltransferase

Dog heart contains a membrane bound N-acyltransferase (transacylase) which transfers acyl groups from the sn-1 position of membrane phospholipids to the amino group of ethanolamine phospholipids in the presence of millimolar Ca2+ concentrations. Using crude membrane preparations, we found this N-acyltransferase activity to be heat sensitive and inhibited by sulfhydryl reagents. Pretreatment of a membrane fraction with trypsin reduced N-acyltransferase activity to 60% while pretreatment with trypsin and Triton X-100 together reduced it to 30% of the control value. At pH 8.0 both Sr2+ and Mn2+ could fully substitute for Ca2+ with respect to optimum ion concentration and molecular species of the product formed in dog heart membranes from endogenous substrates. Ba2+ was equally effective in achieving N-acylation of ethanolamine phospholipids while other divalent cations were less effective or ineffective. The reaction exhibited a pH optimum of 8.5 to 9.0 with both Ca2+ and Sr2+ while Mn2+ precipitated above pH 8.0 resulting in decreased N-acylation activity. Both phosphatidylcholine and 1-acyl lysophosphatidylcholine could serve as acyl donors. Triton X-100 at a concentration of 0.1% stimulated acyl transfer from exogenous phosphatidylcholine but inhibited acyl transfer from lysophosphatidylcholine.

Interaction of hopanoids with phosphatidylcholines containing oleic and omega-cyclohexyldodecanoic acid in lipid bilayer membranes

Lipid bilayer membranes were made from hopanoid phosphatidylcholine mixtures dissolved in decane. The specific capacity of the mixed membranes was found to increase with increasing hopanoid content. This indicates an interaction between hopanoids and lipids which leads to a reduction of the chemical potential of the solvent in the membranes. The structural properties of mixtures of hopanoids and phosphatidylcholines were investigated using charged probe molecules, the negatively charged lipophilic ions dipicrylamine (DPA) and tetraphenylborate (TphiB) and the positively charged potassium complex PV-K+ (PV, cyclo (D-Val-L-Pro-L-Val-D-Pro)3). The transport properties of the lipophilic ions in the mixed membranes indicate that the electrical properties like dipolar potential and surface potentials of phosphatidylcholine membranes are not changed by the insertion of the hopanoids. The translocation rate constant K of the PV-K+ complex is drastically reduced in the hopanoid phosphatidylcholine membranes with increasing hopanoid content. This effect is discussed on the basis of an alteration of the microviscosity in the mixed membranes. There exists a close analogy between the action of cholesterol and hopanoids in bilayer membranes from phosphatidylcholines. A bilayer membrane composed of di-omega-cyclohexyldodecanoyl-phosphatidylcholine (DCPC) was found to possess a higher specific capacity as compared to other phosphatidylcholines. Also a lower translocation rate constant for PV-K+ was observed which may be caused by the relative high microviscosity of this lipid even above the phase transition temperature.

A comparison of the headgroup conformation and dynamics in synthetic analogs of dipalmitoylphosphatidylcholine

14N-NMR spectra and relaxation times for dipalmitoylphosphatidylcholine and three analogs were obtained in both the liquid crystal and gel phases. The analogs either changed the PO-4 to N+ (CH3)3 distance (P-N) within the headgroup by increasing the number of CH2 groups from two in the phosphocholine headgroup (PN-2) to six in the phospho-(N',N',N'-trimethyl)hexanolamine headgroup (PN-6), or replaced the ester linkages to the hydrocarbon chains with either linkages. 31P-NMR spectra were obtained for the four samples in the liquid-crystal phase. (1) The 14N- and 31P-NMR spectra and 14N relaxation times all indicate that increasing the P-N distance within the headgroup causes changes in both the average orientation of the C-N bond and its dynamics. (2) The 14N-NMR spectra provide evidence for a change in orientational order of the headgroup as a result of changing the linkage to the acyl chains. On the other hand, the relaxation time measurements indicate that the molecular motion for the headgroup is independent of the type of linkage. (3) The thermal behaviour of the four samples is clearly reflected in the 14N-NMR spectra. The second moments of the spectra show distinct changes at each of the phase transitions. (4) The 14N-NMR spectra show that the average conformation of the headgroups is not significantly altered by the main phase transition. For the PN-2 samples, T2e, the decay of the quadrupolar echo, decreases discontinuously in the P beta, phase, which is evidence for a possible exchange process between two molecular states within this phase.

Synthesis of the diastereoisomers of 1,2-dipalmitoyl-sn-glycero-3-thiophosphorylethanolamine and their stereospecific hydrolysis by phospholipases A2 and C

A convenient three-step synthesis of the phosphorothioate analogue of phosphatidylethanolamine is described. The reaction pathway involves the conversion of a 1,2-diacyl-sn-glycerol to its corresponding thiophosphoric acid dichloride by using PSCl3 in the presence of a tertiary base. Treatment of the dichloride with ethanolamine results in the formation of a cyclic thiophosphoramidate which, upon acidification, undergoes P--N cleavage, giving rise to 1,2-diacyl-sn-glycero-3-thiophosphorylethanolamine. 31P NMR reveals that both diastereoisomers are present in equivalent amounts. It is not possible, however, to separate the two isomers by high-pressure liquid chromatography. 31P NMR amd high-pressure liquid chromatography are used to show that phospholipases A2 and C exhibit absolute and opposite stereoselectivity in the hydrolysis of the pair of diastereoisomers.

New total synthesis and high resolution 1H NMR spectrum of platelet-activating factor, its enantiomer and racemic mixtures

A new method of synthesis of octadecyl platelet-activating factor (PAF; 1-O-octadecyl 2-O-acetyl sn-glycero-3-phosphorylcholine) is described. Its advantage is to proceed by way of the 'lyso-PAF' which may be substituted by various groups, while avoiding the inconveniences of the total synthesis already described. Moreover, the intermediates in synthesis are easier to purify, with better yields. The platelet-aggregating activity of synthetic PAF, is enantiomer (3-O-octadecyl) and racemic mixtures were 2.7 . 10(-10), 1.7 . 10(-7) and 2.2 . 10(-10) M, respectively. These results indicate the stereospecificity of platelet activation induced by PAF.

Charge-induced pretransition in phosphatidylethanolamine multilayers. The occurrence of ripple structures

The influence of pH and ionic strength on the phase transition behavior of 1,2-dihexadecylphosphatidylethanolamine was studied calorimetrically. In the range of ionic strength from 0.75 to 1.5 M NaCl at pH larger than 13, where the amino group of the phosphatidylethanolamine is in the deprotonated state, resulting in one negative charge per lipid molecule, the calorimetric scan shows a pretransition before the main transition. Accompanying freeze-fracture electron microscopic studies on these preparations in the temperature range between the pre- and main transitions show a regular suface, the so-called ripple structure. These are comparable with the structures seen in phosphatidylcholine-water systems at temperatures between the pre- and main transition.

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.

Perturbations of membrane structure by optical probes: II. Differential scanning calorimetry of dipalmitoyllecithin and its analogs interacting with Merocyanine 540

Differential scanning calorimetry of multilamellar liposomes, interacting with the optical probe Merocyanine 540, yields quantitative information about perturbances of the bilayer structure induced by this dye. At low dye: lipid ratios, the dye perturbs primarily its own microenvironment, which is laterally separated from the unmodified lipid domain and exhibits modified thermotropic properties. A further increase in the dye concentration results in a perturbance of the whole lipid bilayer. The degree of perturbance is sensitive to structural modifications in the head-group region of the lipids. It is concluded that Merocyanine 540 reports in every case, even at infinite dilution, on localized events originating from a perturbed microenvironment.

Motion of the lengthened zwitterionic head groups of C16-lecithin analogues in aqueous solutions as studied by dielectric relaxation measurements

Phosphatidyl choline analogues with increased phosphate-trimethylammonium distance were synthesized and aqueous solutions of these bilayer forming phospholipids were prepared. Dielectric spectra of the solutions were measured at several temperatures around the crystalline/liquid-crystalline phase transition temperature of the samples. The observed data are treated in terms of a Debye relaxation function and also of a relaxation function based on a theoretical model of the aqueous solutions of multibilayer vesicles. As a noteworthy result, a pronounced cooperativity effect in the diffusive motions of the zwitterionic head groups emerges. The degree of cooperativity depends on the radius of curvature of the multibilayer vesicles and also on the length of the phospholipid zwitterions. The values for the mobility of the trimethylammonium group are of the same order of magnitude as those for the mobility of whole phospholipid molecules in its lateral diffusive motion. Indications for a phase transition at a temperature above the main transition temperature are found with solutions of C16- lecithin analogues with 9 and 10 methylene groups between the phosphate and the trimethylammonium group.