The physical properties and photopolymerization of diacetylene-containing phospholipid liposomes

Photopolymerized microdomains in both lipid leaflets establish diffusive transport pathways across biomimetic membranes

Controlled transport within a network of aqueous subcompartments provides a foundation for the construction of biologically-inspired materials. These materials are commonly assembled using the droplet interface bilayer (DIB) technique, adhering droplets together into a network of lipid membranes. DIB structures may be functionalized to generate conductive pathways by enhancing the permeability of pre-selected membranes, a strategy inspired by nature. Traditionally these pathways are generated by dissolving pore-forming toxins (PFTs) in the aqueous phase. A downside of this approach when working with larger DIB networks is that transport is enabled in all membranes bordering the droplets containing the PFT, instead of occurring exclusively between selected droplets. To rectify this limitation, photopolymerizable phospholipids (23:2 DiynePC) are incorporated within the aqueous phase of the DIB platform, forming conductive pathways in the lipid membranes post-exposure to UV-C light. Notably these pathways are only formed in the membrane if both adhered droplets contain the photo-responsive lipids. Patterned DIB networks can then be generated by controlling the lipid composition within select droplets which creates conductive routes one droplet thick. We propose that the incorporation of photo-polymerizable phospholipids within the aqueous phase of DIB networks will improve the resolution of the patterned conductive pathways and reduce diffusive loss within the synthetic biological network.

Nanofiber Formation and Polymerization of Bolalipids with Diacetylene-Modified Single Alkyl Chains

The nanofiber formation in aqueous suspension of two classes of symmetric single-chain bolaamphiphiles with different polar headgroups and a diacetylene-modified alkyl chain with a length of 32, 34, and 36 C atoms was investigated by differential scanning calorimetry, transmission electron microscopy, and small-angle neutron scattering. As observed before for other bolalipids with phosphocholine (PC) and dimethyl-phosphoethanolamine (Me₂PE) headgroups, the molecules form fibers when suspended in water at low temperatures but disassemble into micellar-like aggregates upon heating. The introduction of a diacetylene group in the middle of the long chain leads to a perturbation of chain packing so that this fiber-micelle transition occurs at lower temperature compared to the other bolalipids having unmodified alkyl chains. The aim of our project was the introduction of diacetylene groups into alkyl chains to be able to polymerize the fibers at low temperature. This should enhance the fiber stability and prevent the disassembly into micellar aggregates at higher temperature. Polymerization of aggregates containing diacetylene-modified bolaamphiphiles can be easily traced by UV/vis spectroscopy as colored products are formed. We found that polymerization of bolaamphiphiles with PC headgroups leads to a breakdown of most fibers into micellelike aggregates, and only some longer fibers segments are still detectable. In contrast, the use of Me₂PE headgroups improves polymerizability and length of the polymerized fibers. The compound with 36 C atoms in the chain could be polymerized at low temperatures, and the fibers remained stable at least up to a temperature of 60 °C. This shows that the perturbation of the chain packing due to the diacetylene groups in the chains can be overcome by elongation of the chains, so that thermostable fibers with a diameter of the length of the bolalipid molecule can be successfully formed.

Formation of nanopores in DiynePC-DPPC complex lipid bilayers triggered by on-demand photo-polymerization

Vesicles have unique characteristics that enable the release of drugs as well as encapsulation while maintaining biocompatibility. A photo-polymerizable liposome composed of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (23:2 DiynePC) has been investigated as vehicles for triggered delivery of drugs to cells. In this study, we confirmed for the first time that supported lipid bilayers (SLBs) prepared with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/DiynePC mixture generated pores ca. 100-300 nm in size on the membrane after UV polymerization. This direct observation was done by analyzing the SLBs formed with the DPPC/DiynePC mixture by employing atomic force microscopy (AFM) in a liquid environment. However, photo-polymerization did not occur in the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/DiynePC mixed bilayer and pores were not formed. A theoretical study was performed to explore the phase behavior of the lipid mixtures. A coarse-grained model of DiynePC was developed that is comparable with the Martini force field; the parameters were validated against atomistic simulations. Transition from fluidic to gel phase was observed only when DiynePC was mixed with DPPC, whereas the DOPC mixture remained fluidic over the entire domain. This implies a correlation between the formation of DiynePC-rich gel phase domains and the generation of pores after polymerization. The size of the pores were found to be controlled by the amount of polymerizable lipid which results in higher release rate of encapsulated calcein from the vesicles with larger pores.

Triggered release of molecules across droplet interface bilayer lipid membranes using photopolymerizable lipids

A combination of nonpolymerizable phospholipids (DPPC or DPhPC) and a smaller amount of cross-linking photopolymerizable phospholipids (23:2 DiynePC) is incorporated in an unsupported artificial lipid bilayer formed using the droplet interface bilayer (DIB) approach. The DIB is formed by contacting lipid monolayer-coated aqueous droplets against each other in a dodecane-lipid medium. Cross-linking of the photopolymerizable lipids incorporated in the DIB was obtained by exposure to UV-C radiation (254 nm), resulting in pore formation. The effect of cross-linking on the DIB properties was characterized optically by measuring the diffusion of selectively encapsulated dye molecules (calcein) from one droplet of the DIB to the other droplet. Changes in DIB conductivity due to UV-C exposure were investigated using current-voltage (I-V) measurements. The leakage of dye molecules across the DIB and the increase in DIB conductivity after UV-C exposure indicates the formation of membrane pores. The results indicate that the DIB approach offers a simple and flexible platform for studying phototriggered drug delivery systems in vitro.

Treatment of calcium channel blocker-induced cardiovascular toxicity with drug scavenging liposomes

Calcium channel blocker (CCB) overdose is potentially lethal. Verapamil and diltiazem are particularly prone to acute toxicity due to their dual effect on cardiac and vascular tissues. Unfortunately, conventional decontamination measures are ineffective in accelerating blood clearance and, to date, few efforts have been made to develop antidotes. To address the issue, injectable long-circulating liposomes bearing a transmembrane pH-gradient are proposed as efficient detoxifying agents of CCB poisoning. By scavenging the drug in situ, these circulating nanocarriers can restrict its distribution in tissues and hinder its pharmacological effect. In vitro, we showed that liposomes stability in serum and their ability to sequester CCBs could be finely-tuned by modulating their internal pH, surface charge, and lipid bilayer structure. Subsequently, we verified their efficacy in reversing the cardiovascular effects of verapamil in rats implanted with telemetric pressure/biopotential transmitters. In animals orally intoxicated to verapamil, an intravenous injection of the liposomal antidote rapidly attenuated the reduction in blood pressure. Areas under diastolic, systolic, and mean pressures curves were significantly reduced by up to 60% and the time to hemodynamic recovery was shortened from 19 to only 11 h. These findings confirm the protective effect of pH-gradient liposomes against cardiovascular failure after CBB intoxication, and endorse their potential as efficient, versatile antidotes.

Material properties of matrix lipids determine the conformation and intermolecular reactivity of diacetylenic phosphatidylcholine in the lipid bilayer

Photopolymerizable phospholipid DC(8,9)PC (1,2-bis-(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine) exhibits unique assembly characteristics in the lipid bilayer. Because of the presence of the diacetylene groups, DC(8,9)PC undergoes polymerization upon UV (254 nm) exposure and assumes chromogenic properties. DC(8,9)PC photopolymerization in gel-phase matrix lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monitored by UV-vis absorption spectroscopy occurred within 2 min after UV treatment, whereas no spectral shifts were observed when DC(8,9)PC was incorporated into liquid-phase matrix 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Liquid chromatography-tandem mass spectrometry analysis showed a decrease in the amount of DC(8,9)PC monomer in both DPPC and POPC environments without any change in the matrix lipids in UV-treated samples. Molecular dynamics (MD) simulations of DPPC/DC(8,9)PC and POPC/DC(8,9)PC bilayers indicate that the DC(8,9)PC molecules adjust to the thickness of the matrix lipid bilayer. Furthermore, the motions of DC(8,9)PC in the gel-phase bilayer are more restricted than in the fluid bilayer. The restricted motional flexibility of DC(8,9)PC (in the gel phase) enables the reactive diacetylenes in individual molecules to align and undergo polymerization, whereas the unrestricted motions in the fluid bilayer restrict polymerization because of the lack of appropriate alignment of the DC(8,9)PC fatty acyl chains. Fluorescence microscopy data indicates the homogeneous distribution of lipid probe 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-lissamine rhodamine B sulfonyl ammonium salt (N-Rh-PE) in POPC/DC(8,9)PC monolayers but domain formation in DPPC/DC(8,9)PC monolayers. These results show that the DC(8,9)PC molecules cluster and assume the preferred conformation in the gel-phase matrix for the UV-triggered polymerization reaction.

Polymeric lipid assemblies as novel theranostic tools

Polymerizable lipids have been used in research and medical applications such as membrane models, imaging platforms, drug delivery systems, vaccine carriers, biosensors, and coating materials. The polymerization of these lipid molecules forms a covalent bond between lipid moieties, which improves the noncovalent interactions that maintain the lipid lamellar phase architecture and increases the stability of the polymerized system. Because such lipid molecules form nanoassemblies with modifiable structures that acquire the stability of polymers following covalent bond formation, these lipids are of considerable interest in the emerging field of theranostics. In this Account, we summarize the biomedical applications of polymerizable lipids (primarily phospholipids) in the context of various nanoplatforms. We discuss stable nanoplatforms, which have been used in a variety of theranostics applications. In addition, we describe methods for assembling triggerable theranostics by combining appropriate nonpolymerizable lipids with polymerizable lipids. Polymeric lipids hold promise as nanotools in the field of medical imaging, targeting, and on-demand drug delivery. Because of their similarity to biological lipids, long-term toxicity issues from polymerizable lipid nanoplatforms are predicted to be minimal. Although the field of polymeric nanocapsules is still in development, intensive efforts are underway to produce systems which could be applied to disease diagnosis and treatment. We envision that nanoimaging platforms coupled with localized drug delivery technology will have a significant impact on cancer therapy and other related diseases. The existing wealth of clinical knowledge both in the photochemistry of imaging agents and/or drugs and modifications of these agents using light will prove valuable in the further development of polymeric theranostic lipid-based nanoparticles.

Interface of covalently bonded phospholipids with a phosphorylcholine head: characterization, protein nonadsorption, and further functionalization

Surface anchored poly(methylhydrosiloxane) (PMHS) thin films on oxidized silicon wafers or glass substrates were functionalized via the SiH hydrosilylation reaction with the internal double bonds of 1,2-dilinoleoyl-sn-glycero-3-phosphorylcholine (18:2 Cis). The surface was characterized by X-ray photoelectron spectroscopy, contact angle measurements, atomic force microscopy, and scanning electron microscopy. These studies showed that the PMHS top layer could be efficiently modified resulting in an interfacial high density of phospholipids. Grafted phospholipids made the initially hydrophobic surface (θ = 106°) very hydrophilic and repellent toward avidin, bovine serum albumin, bovine fibrinogen, lysozyme, and α-chymotrypsin adsorption in phosphate saline buffer pH 7.4. The surface may constitute a new background-stable support with increased biocompatibility. Further possibilities of functionalization on the surface remain available owing to the formation of interfacial SiOH groups by Karstedt-catalyzed side reactions of SiH groups with water. The presence of interfacial SiOH groups was shown by zeta potential measurements. The reactivity and surface density of SiOH groups were checked by fluorescence after reaction of a monoethoxy silane coupling agent bearing Alexa as fluorescent probe.

Quantitative analysis of phospholipids using nanostructured laser desorption ionization targets

Since its introduction as an ionization technique in mass spectrometry, matrix-assisted laser desorption ionization (MALDI) has been applied to a wide range of applications. Quantitative small molecule analysis by MALDI, however, is limited due to the presence of intense signals from the matrix coupled with non-homogeneous surfaces. The surface used in nano-structured laser desorption ionization (NALDI) eliminates the need for a matrix and the resulting interferences, and allows for quantitative analysis of small molecules. This study was designed to analyze and quantitate phospholipid components of liposomes. Here we have developed an assay to quantitate the DPPC and DC(8,9)PC in liposomes by NALDI following various treatments. To test our method we chose to analyze a liposome system composed of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and DC(8,9)PC (1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine), as DC(8,9)PC is known to undergo cross-linking upon treatment with UV (254 nm) and this reaction converts the monomer into a polymer. First, calibration curves for pure lipids (DPPC and DC(8,9)PC) were created using DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) as an internal standard. The calibration curve for both DPPC and DC(8,9)PC showed an R(2) of 0.992, obtained using the intensity ratio of analyte and internal standard. Next, DPPC:DC(8,9)PC liposomes were treated with UV radiation (254 nm). Following this treatment, lipids were extracted from the liposomes and analyzed. The analysis of the lipids before and after UV exposure confirmed a decrease in the signal of DC(8,9)PC of about 90%. In contrast, there was no reduction in DPPC signal.

Partially polymerized liposomes: stable against leakage yet capable of instantaneous release for remote controlled drug delivery

A critical issue for current liposomal carriers in clinical applications is their leakage of the encapsulated drugs that are cytotoxic to non-target tissues. We have developed partially polymerized liposomes composed of polydiacetylene lipids and saturated lipids. Cross-linking of the diacetylene lipids prevents the drug leakage even at 40 °C for days. These inactivated drug carriers are non-cytotoxic. Significantly, more than 70% of the encapsulated drug can be instantaneously released by a laser that matches the plasmon resonance of the tethered gold nanoparticles on the liposomes, and the therapeutic effect was observed in cancer cells. The remote activation feature of this novel drug delivery system allows for precise temporal and spatial control of drug release.

DESIGN OF LIPOSOMES CONTAINING PHOTOPOLYMERIZABLE PHOSPHOLIPIDS FOR TRIGGERED RELEASE OF CONTENTS

We describe a novel class of light-triggerable liposomes prepared from a photo-polymerizable phospholipid DC(8,9)PC (1,2- bis (tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine) and DPPC (1,2-Dipalmitoyl-sn-Glycero-3-Phosphocholine). Exposure to UV (254 nm) radiation for 0-45 minutes at 25 degrees C resulted in photo-polymerization of DC(8,9)PC in these liposomes and the release of an encapsulated fluorescent dye (calcein). Kinetics and extents of calcein release correlated with mol% of DC(8,9)PC in the liposomes. Photopolymerization and calcein release occurred only from DPPC/DC(8,9)PC but not from Egg PC/DC(8,9)PC liposomes. Our data indicate that phase separation and packing of polymerizable lipids in the liposome bilayer are major determinants of photo-activation and triggered contents release.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Highly stable vesicles composed of a new chain-terminus acetylenic photopolymeric phospholipid

Stability, ease of production, and storage convenience were addressed for polymerized vesicles composed of 1,2-bis(trideca-12-ynoyl)-sn-glycero-3-phosphocholine. The following vesicle properties were investigated before and after polymerization: size, shape, lamellarity, dispersity, degree of polymerization, membrane fluidity, and structural stability. A fairly monodisperse, unilamellar sub-micron vesicle suspension undergoes nearly complete polymerization of the chain-terminus acetylenic to polyacetylenic conversion as monitored by Fourier transform infrared spectroscopy. (1)H nuclear magnetic resonance spectroscopy and thin layer chromatography provide additional evidence for extensive lipid polymerization. Using differential scanning calorimetry, a gel/liquid transition was not observed for either polymerized or non-polymerized vesicles within the temperature range of 5-65 degrees C. These polymerized vesicles remained structurally stable and suspended for months at room temperature. However, vesicle size did decrease with increasing degree of polymerization. Polymerized vesicles remained spherical but decreased in size by 15% when subjected to 52 wt.% aqueous ethanol and did not change significantly in size and dispersity after a freeze-dry/resuspend cycle.

Poly(phthaloyl-L-lysine)-coated multilamellar vesicles for controlled drug delivery: in vitro and in vivo performance evaluation

Nonionic surfactant vesicles were prepared using Span 60, cholesterol and dicetyl phosphate. The prepared multilamellar vesicles (MLVs) were coated by interfacial polymerization technique using p-phthaloyl dichloride and L-lysine. The formation of the polymeric coat was confirmed by optical microscopic and transmission electron microscopic studies. The prepared, plain and polymer-coated MLVs were studied for their size, shape, encapsulation efficiency, in vitro release profile and effect of osmotic shock on vesicle. The results observed showed that the polymer-coated MLVs were stable under various osmotic conditions. In vivo studies were carried out on albino rats. The half-life and area under curve were found to be high in the case of polymer-coated MLVs as compared to plain MLVs and plain drug solution. In vivo studies using inflammed rat model also indicated that the polymer-coated MLVs were more stable and could release the drug in a controlled fashion as compared to plain MLVs.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Paramagnetic polymerized liposomes as new recirculating MR contrast agents

We describe a well-tolerated blood pool contrast agent with extended recirculatory half-life based on paramagnetic polymerized liposomes (PPLs). PPLs were constructed from a new type of polymerizable lipid molecule that has a derivative of gadopentetate dimeglumine as the hydrophilic head group and diacetylene groups in the hydrophobic acyl chains, which cross-link when irradiated with ultraviolet (UV) light. Biodistribution, blood pool half-life, and MR image enhancement were determined for PPLs composed of 10% of the gadopentetate dimeglumine lipid and 90% of ditricosadiynoyl tricosadiynayl phosphatidylcholine (DAPC) at a dose of 0.015 mmol Gd+3/kg in rats. In T1-weighed MR images (TR/TE = 400/18 msec), an average signal enhancement of 34% in the kidneys and 20% in the liver was observed, which persisted for at least 90 minutes after administration of the PPLs. Biodistribution studies using radiolabeled PPLs confirmed that 80% of the injected dose remained in the blood pool after 2 hours. The half-life of elimination from the blood pool was 19 hours. The preparation was well tolerated in rats and produced similar MR contrast enhancement of the blood pool as produced by other liposome contrast agents. However, the half-life of PPL elimination from the blood pool was prolonged relative to other liposome systems.

Phase transitions of polymerizable phospholipids

Polymerizable lipids have received considerable attention in the last ten years as polymerization of lipids in vesicle systems is a possibility to increase the stability of lipid bilayers. Lipids with various polymerizable groups have been synthesized in the last years. This paper is focussed on those lipids which are closely related to natural phospholipids, i.e. molecules which have two hydrophobic chains and a head group containing a phosphate moiety. The phase behaviour of polymerizable phospholipids as lipid monomers and in the polymerized state is reviewed and discussed.

Differential scanning calorimetric study of the thermotropic phase behavior of a polymerizable, tubule-forming lipid

A comparative study of the polymorphism exhibited by the polymerizable, tubule-forming phospholipid 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3- phosphocholine (DC23PC) and its saturated analog 1,2-ditricosanoyl-sn-glycero-3-phosphocholine (DTPC) in aqueous suspension is reported. Differential scanning calorimetry (DSC), as well as freeze-fracture electron microscopy and Raman spectroscopy, have been used to study the influence on phase behavior of rigid diacetylene groups in the fatty acyl chains of a phosphatidylcholine. DTPC large multilamellar vesicle (MLV) and small unilamellar vesicle (SUV) suspensions were found to retain liposome morphology after chain crystallization had occurred. In marked contrast, diacetylenic DC23PC suspensions do not maintain liposomal morphology in converting to the low temperature phase. Large MLVs of DC23PC with outer diameters in excess of 1 micron convert to a gel phase with cylindrical or tubular morphology at 38 degrees C, just a few degrees below the lipid's chain melting temperature (TM(H), i.e. temperature of an endothermic event observed during a heating scan) of 43.1 degrees C. Unlike the large MLVs, small MLVs or SUVs of DC23PC, with diameters of 0.4 +/- 0.3 micron and 0.04 +/- 0.02 micron, respectively, exhibit metastability in the liquid-crystalline state for several tens of degrees below the chain melting temperature prior to converting to a gel phase which, by electron microscopy, manifests itself as extended multilamellar sheets. Raman data collected at TM(H) -40 degrees C demonstrate that the gel state formed by DC23PC is very highly ordered relative to that of DTPC, suggesting that special chain packing requirements are responsible for the novel phase behavior of DC23PC.

Polymeric phospholipids as new biomaterials

Phospholipid polymers form a new class of biomaterials with many potential applications in medicine and research. The development of these compounds is based upon the mimicry of cell surfaces and reflects our current understanding of the properties of membrane lipids. Physicochemical characterization of the monomeric, diacetylenic phospholipids illustrates the similarities to naturally occurring lipids, similarities that are confirmed by the capacity to enrich the membranes of A. laidlawii to the level of 90% diacetylenic lipid. Polymerization of diacetylenic phospholipids is easily attained by irradiation and produces a stable, crystalline array. The ability to link membrane lipids covalently permits the isothermal restriction in their motion, and is useful in basic studies of biomembranes. The thromboresistance of polymeric phosphatidylcholines in vitro may be a consequence of the inability of phosphatidylcholines to participate in coagulation. The restricted lateral diffusion of proteins along a polymeric lattice will also inhibit the formation of coagulation complexes. Existing polymers may be altered by a coating of polymeric lipid obtained by the Langmuir-Blodgett method. Polymerized vesicles display significant reductions in permeability and aggregation. Entrapment of soluble materials and reconstitution of membrane proteins may be exploited in controlled and site-directed drug delivery. Polymerization of cells in situ produces "cellular capsules" with entrapped membrane and cellular components. Polymeric hemosomes are capable of gas transport and may function as red cell surrogates. The hybrid qualities of biomembranes (polar surfaces, nonthrombogenic, low antigenic potential, and low permeability) and synthetic polymers (chemical and physical stability) suggest that polymeric phosphatidylcholines may serve as models for biomaterials design.

Influence of phospholipid peroxidation on the phase behavior of phosphatidylcholine and phosphatidylethanolamine in aqueous dispersions

The influence of oxygen-induced phospholipid peroxidation on the phase behavior of aqueous dispersions of both egg phosphatidylcholine (egg-PC) and egg phosphatidylethanolamine (egg-PE) has been investigated. Phospholipid peroxidation was followed via malondialdehyde formation and analyses of acyl chain compositions. 13C nuclear magnetic resonance spectroscopy (NMR) and the amino-indicating probe trinitrobenzenesulfonic acid were used to study the effect of peroxidation on the chemical structure of hydrated egg-PE. The macroscopic organization of the phospholipids was monitored by 31P NMR and small-angle X-ray diffraction. Differential scanning calorimetry was employed to study the influence of peroxidation on the thermotropic behavior of egg-PE. The results show that egg-PE is more sensitive to the effects of peroxidation than egg-PC. In the latter, no changes in the macromolecular organization were observed. However, peroxidation strongly influenced the polymorphic phase behavior of PE. Initial peroxidation stabilized hydrated egg-PE in a lamellar system up to 70 degrees C, presumably by modification of the head group. Such modifications were confirmed by 13C NMR experiments, which indicated the formation of Schiff bases between PE head groups and aldehydes. Furthermore, quantitative analyses of trinitrobenzenesulfonic acid reactable egg-PE and the corresponding fatty acid compositions revealed the presence of cross-links between the ethanolamine head groups, likely involving the bifunctional malondialdehyde. Prolonged peroxidation of egg-PE resulted in a loss of order in the system, possibly by the formation of intermediate nonbilayer structures.

Biomembrane surfaces as models for polymer design: the potential for haemocompatibility

A major restriction in the application of polymeric biomaterials is the propensity of their surfaces to support thrombosis. Theoretical approaches to the design of thromboresistant polymers have been inadequate because of the complexity of surface thrombosis. We have developed a new, practical approach to this problem--the design of polymers which mimic the thromboresistant surfaces of blood cell membranes. Haemostatic processes are mediated by reactions which occur at membrane-plasma interfaces. The extra-cellular surfaces of the plasma membranes of red blood cells and quiescent platelets are thromboresistant; in contrast, their cytoplasmic surfaces are thrombogenic. The simplest common feature among the blood-compatible cellular and model membranes is the high content of the electrically neutral phospholipids which contain the phosphorylcholine head group. We have developed model systems of biological membranes which utilize polymerizable phosphatidylcholines and which mimic nonreactive cell surfaces. Polymeri phospholipids represent a new class of hybrid biomaterials with characteristics both of biomembranes (polar surfaces, nonthrombogenic, low antigenic potential and low permeability) and of synthetic polymers (chemical and physical stability).