Ratio Of Dipalmitoylphosphatidylcholine Research Articles

Quantification of lung surfactant lipid (dipalmitoylphosphatidylcholine/sphingomyelin) ratio in binary liposomes using Raman spectroscopy

AbstractEarly diagnosis of neonatal respiratory distress syndrome (nRDS) is important in reducing the mortality of preterm babies. Knowledge of the ratio of two components of lung surfactant, dipalmitoylphosphatidylcholine (DPPC), and sphingomyelin (SM) can be used as biomarkers of lung maturity and inform treatment. Raman spectroscopy is a powerful tool to analyze vibrational spectra of organic molecules which requires only limited sample preparation steps and, unlike IR spectroscopy, is not masked by water absorption. In this paper, we explore the potential of using Raman spectroscopy as a tool to estimate the ratio of DPPC and SM from aqueous vesicles of binary mixture of DPPC and SM. We demonstrate that the ratio of DPPC and SM can be estimated by estimating the ratio of intensity of CO stretch of DPPC and CC stretch of SM as well as CO stretch of DPPC and amide I of SM. Further, we employ a partial least squares regression (PLSR) model to automate the estimation and demonstrate that PLSR method can predict the DPPC and SM ratio with an R2 value of 0.968.

Nanoscale Structure and Dynamics of Model Membrane Lipid Raft Systems, Studied by Neutron Scattering Methods

Quasi-elastic neutron scattering (QENS) and small angle neutron scattering (SANS), in combination with isotopic contrast variation, have been used to determine the structure and dynamics of three-component lipid membranes, in the form of vesicles, comprising an unsaturated [palmitoyl-oleoyl-phosphatidylcholine (POPC) or dioleoyl-phosphatidylcholine (DOPC)], a saturated phospholipid (dipalmitoyl-phosphatidylcholine (DPPC)), and cholesterol, as a function temperature and composition. SANS studies showed vesicle membranes composed of a 1:1:1 molar ratio of DPPC:DOPC:cholesterol and a 2:2:1 molar ratio of DPPC:POPC:cholesterol phase separated, forming lipid rafts of ∼18 and ∼7 nm diameter respectively, when decreasing temperature from 308 to 297 K. Phase separation was reversible upon increasing temperature. The larger rafts observed in systems containing DOPC are attributed to the greater mis-match in lipid alkyl chains between DOPC and DPPC, than for POPC and DPPC. QENS studies, over the temperature range 283–323K, showed that the resulting data were best modelled by two Lorentzian functions: a narrow component, describing the “in-plane” lipid diffusion, and a broader component, describing the lipid alkyl chain segmental relaxation. The overall “in-plane” diffusion was found to show a significant reduction upon increasing temperature due to the vesicle membranes transitioning from one containing rafts to one where the component lipids are homogeneously mixed. The use of different isotopic combinations allowed the measured overall reduction of in-plane diffusion to be understood in terms of an increase in diffusion of the saturated DPPC lipid and a corresponding decrease in diffusion of the unsaturated DOPC/POPC lipid. As the rafts are considered to be composed principally of saturated lipid and cholesterol, the breakdown of rafts decreases the exposure of the DPPC to cholesterol whilst increasing the exposure of cholesterol to unsaturated lipid. These results show the sensitivity of lipid diffusion to local cholesterol concentration, and the importance of considering the local, rather that the global composition of a membrane when understanding the diffusion processes of lipids within the membrane. The novel combination of SANS and QENS allows a non-intrusive approach to characterize the structure and dynamics occurring in phase-separated model membranes which are designed to mimic the lateral heterogeneity of lipids seen in cellular membranes–a heterogeneity that can have pathological consequences.

Fabrication of versatile targeted lipopolymersomes for improved camptothecin efficacy against colon adenocarcinoma in vitro and in vivo

ABSTRACT Background Hybrid vesicular systems (lipopolymersomes) are promising platforms for minimizing the liposomes and polymersomes disadvantages in terms of chemotherapeutic transportation. In this regard, lipopolymersome has been designed to integrate the advantage of both polymersomes and liposomes to enable better structural integrity of the bilayer after encapsulation of hydrophobic drugs while maintaining the soft nature of liposomes, superior serum stability, and high encapsulation efficiency of cargos in the bilayer segment. Research design and Methods In the present study, we reported preparation and characterization of five camptothecin (CPT)-loaded lipopolymersomal formulations composed of poly (ethylene glycol)–poly (lactic acid) (PEG-PLA) and dipalmitoylphosphatidylcholine (DPPC) at different molar ratios using film rehydration method. Afterward, the preferred formulation was tagged with AS1411 DNA aptamer in order to evaluate the therapeutic index using nucleolin-positive colon cancer cell lines (HT29 and C26). Results The obtained data indicated that the prepared CPT-loaded lipopolymersome at a PEG-PLA: DPPC ratio of 75:25 exhibited superior stability and high loading capacity compared to other systems. Moreover, high cytotoxicity of the aptamer-targeted lipopolymersome and increased tumor accumulation were observed in comparison with non-targeted one. Conclusions The designed polymer-rich lipopolymersomal platform offers bright future for the development of potent nanomedicine against cancer.

Marriage of phospholipid and block copolymer in lipopolymersome hybrid structure for efficient tumor accumulation.

Despite the noticeable advantages of the liposomes and polymersomes, they also revealed some drawbacks that could be minimized by preparing hybrid vesicular systems and integrating the advantage of both vehicles into one system named lipopolymersome. Lipopolymesome incorporates the biodegradability, stability, adjustability and chemical flexibility of polymersomes with the elasticity, soft nature and biocompatibility of liposomes. In the current study, wereported the development of five nanoscale lipopolymersomal hybrid vesicular systems consisting different molar ratios of dipalmitoylphosphatidylcholine (DPPC) and poly (ethylene glycol)-poly (lactic acid) (PEG-PLA) (PEG-PLA: DPPC ratio of 100:0, 50:50: 25:75, 75:25 and 0:100). Rhod-6G-loaded hybrid vesicles were prepared via film rehydration. Then, the efficacy of five formulations were evaluated in terms of loading capacity, release pattern, cellular uptake, andin vivobiodistribution in ectopic tumor model in mice. The obtained results demonstrated that the self-assembly, loading capacity, cargo release and stability of hybrid nanoscale lipopolymersomes can be tuned by incorporation of amphiphilic lipid-polymers at various ratios. In this regard, the prepared hybrid nanovesicles consisting of DPPC-PEG-PLA (25:75) exhibited great potential through superior loading capacity, stability and tumor accumulation compared with other systems. It could be concluded that the prepared lipopolymersome offers important opportunities for the development of novel hybrid carriers for efficient transportation of therapeutics into tumor site.

The effects of lyophilization on the physico-chemical stability of sirolimus liposomes.

The major limitation in the widespread use of liposome drug delivery system is its instability. Lyophilization is a promising approach to ensure the long-term stability of liposomes. The aim of this study was to prepare sirolimus-loaded liposomes, study their stability and investigate the effect of lyophilization either in the presence or in the absence of lyoprotectant on liposome properties. Two types of multi-lamellar liposomes, conventional and fusogenic, containing sirolimus were prepared by modified thin film hydration method with different ratio of dipalmitoylphosphatidylcholine (DPPC), cholesterol and dioleoylphosphoethanolamine (DOPE), and were lyophilized with or without dextrose as lyoprotectant. Chemical stability investigation was performed at 4°C and 25°C until 6 months using a validated HPLC method. Physical stability was studied with determination of particle size (PS) and encapsulation efficiency (EE %) of formulations through 6 months. Chemical stability test at 4°C and 25°C until 6 months showed that drug content of liposomes decreased 8.4% and 20.2% respectively. Initial mean EE % and PS were 72.8 % and 582 nm respectively. After 6 months mean EE % for suspended form, lyophilized without lyoprotectant and lyophilized with lyoprotectant were 54.8 %, 62.3% and 67.1 % at 4°C and 48.2%, 60.4 % and 66.8 % at 25°C respectively. Corresponding data for mean PS were 8229 nm, 2397 nm and 688nm at 4°C and 9362 nm, 1944 nm and 737 nm at 25°C respectively. It is concluded that lyophilization with and without dextrose could increase shelf life of liposome and dextrose has lyoprotectant effect that stabilized liposomes in the lyophilization process.

Partitioning of ethanol in multi-component membranes: Effects on membrane structure

The molecular mechanism of ethanol and its effects on neurological function is far from clear. In this study, we investigate the effects of ethanol on various structural and dynamical properties of mixed bilayers consisting of different ratios of dipalmitoylphosphatidylcholine (DPPC), sphingomyelin (SM) and cholesterol that are typical constituents of neural cell membranes (Calderon et al., 1995) using molecular dynamics (MD) simulations. The bilayer properties such as thickness, hydrophobic chain order, and diffusive motion of individual lipids as well collective properties like lateral pressure profiles are affected by the presence of ethanol molecules. The simulations show that the percentage of cholesterol present in the bilayers significantly affects the depth of penetration of ethanol molecules. In particular, presence of very high concentration of cholesterol molecules enhances the rigidity of the bilayer and renders them resistant to the penetration of the ethanol molecules, consistent with experiments. Ethanol molecules compete with cholesterol molecules for hydrogen bonding and disrupt cholesterol–lipid interactions, especially those between SM and cholesterol. Ethanol molecules also affect the lateral pressure profiles in the bilayer systems. These results may have implications in understanding the general anesthetic mechanism and role played by cholesterol on partitioning of such anesthetic/alcohol molecules into cell membranes.

Encapsulation stability and temperature-dependent release kinetics from hydrogel-immobilised liposomes

Composite microparticles consisting of a calcium alginate gel matrix with embedded liposomes made from cholesterol:DPPC (dipalmitoylphosphatidylcholine) mixtures were considered. Factors affecting the encapsulation stability of liposomes during the gel formation by ionic cross-linking – namely temperature and the cholesterol:DPPC ratio – were systematically investigated. The liposomes were found to be tolerant to Ca2+ ions during cross-linking of the gel and stable in the hydrogel matrix for extended periods of time when cholesterol was present in the phospholipid bilayer and temperature was kept sufficiently below the phase transition. The temperature-controlled release rate of encapsulated fluorescent dye was quantified. It is shown that a defined quantity of encapsulated substance can be repeatedly released from the embedded liposomes “on-demand” by short temperature pulses of suitably chosen duration and amplitude. This makes the hydrogel–liposome composites potential candidates for applications such as controlled drug delivery or study of reaction–diffusion phenomena in compartmentalised systems.

Ca2+-Induced Effect on Mechanical Properties of Sulfatide-Incorporated Vesicles

The Ca(2+)-induced effect on the nanomechanical properties of vesicles prepared at a different ratio of dipalmitoylphosphatidylcholine (DPPC)/sulfatide was studied using atomic force microscope (AFM) on a mica surface. Vesicles were prepared by extrusion and adsorbed on the mica surface. The forces, measured between an AFM tip and the vesicle, showed that the breakthrough of the tip into the vesicles occurred two times. Force data prior to the first breakthrough were fitted well with the Hertzian model to estimate Young's modulus and bending modulus of the vesicles. Sulfatide incorporation led to a decrease of around 90% in Young's modulus and bending modulus of the vesicles due to the hydration of the headgroups, while the addition of Ca(2+) induced dehydration to recover the properties. The change of the physical properties seems to be attributed to the headgroup packing order of the vesicles, which is determined by the interference with the hydration shell.

A novel class of photo-triggerable liposomes containing DPPC:DC8,9PC as vehicles for delivery of doxorubcin to cells

Success of nanoparticle-mediated drug delivery is subject to development of optimal drug release strategies within defined space and time (triggered release). Recently, we reported a novel class of photo-triggerable liposomes prepared from dipalmitoyl phosphatidylcholine (DPPC) and photopolymerizable diacetylene phospholipid (DC8,9PC), that efficiently released entrapped calcein (a water soluble fluorescent dye) upon UV (254nm) treatment. To develop these formulations for in vivo applications, we have examined phototriggering of these liposomes by visible light, and the effect of released anticancer drugs on cellular toxicity. Sonicated liposomes containing various ratios of DPPC:DC8,9PC and 4mol% DSPE-PEG2000 were loaded with calcein (Ex/Em, 485/517nm) or a chemotherapy drug, Doxorubicin (DOX, Ex/Em 490/590nm). Our initial experiments showed that 514nm laser treatment of liposomes containing 10 or 20mol% DC8,9PC for 1–3min resulted in significant release of calcein. Based on these results, we performed studies with DOX-loaded liposomes. First, biophysical properties (including liposome size and stability) and DOX encapsulation efficiency of the liposomes were determined. Subsequently, the effect of 514nm laser on DOX release, and cellular toxicity by released DOX were examined. Since liposomes using the 86:10:04 mole ratio of DPPC:DC8,9PC:DSPE-PEG2000, showed highest encapsulation of DOX, these formulations were investigated further. We report that (i) liposomes retained about 70% of entrapped DOX at 37°C in the presence of 0–50% serum. (ii) 514nm laser treatment resulted in DOX release from liposomes in a wavelength-specific manner. (iii) Laser treatment of co-cultures containing DOX-loaded liposomes and cells (Raji and MCF-7) resulted in at least 2–3 fold improved cell killing as compared to untreated samples. Taken together, the photo-triggerable liposomes described here may provide a platform for future drug delivery applications. To our knowledge, this is the first report demonstrating improved cell killing following light-triggered release of an encapsulated anticancer agent from photosensitive liposomes.

Cardiolipin packing ability studied by grazing incidence X-ray diffraction

We report a grazing incidence X-ray diffraction (GIXD) study of pure and mixed Langmuir monolayers of tetramyristoyl cardiolipin (TMCL) and dipalmitoylphosphatidylcholine (DPPC) at 22 °C. The mixing behavior of the two components was investigated at two different surface pressures, 4 and 25 mN m −1. Cardiolipins are found to be in a liquid-condensed (LC) phase at 4 mN m −1 whereas the DPPC molecules appear disordered. At 25 mN m −1, cardiolipins are in a solid phase with their aliphatic chains perpendicular to the interface whereas the DPPC molecules are in the LC phase. At this surface pressure, increasing the amounts of TMCL to DPPC leads to a reduction in tilt angle of the aliphatic chains from nearly 30° for pure DPPC to almost 0° in a 1:1 molar ratio of DPPC and TMCL. At this composition, we also found the closest packing of the aliphatic chains. Further increase of the amount of TMCL does not change the lattice or the tilt and the thermodynamic analysis confirms a partial phase separation. Such a behavior was not observed at 4 mN m −1 where the two phospholipids are miscible at all the compositions studied. Addition of TMCL clearly induces a structuring of the mixed monolayers and increases order by a tight packing in the lipid acyl chains.

Effect of sphingomyelin versus dipalmitoylphosphatidylcholine on the extent of lipid oxidation

Phospholipids with sites of unsaturation are targets of peroxidation. We investigated the effect of two types of lipids with identical headgroups, sphingomyelins (SMs) and dipalmitoylphosphatidylcholine (DPPC), on the extent of oxidation of stearoyl-arachidonoyl phosphatidylcholine (SAPC) with four double bonds. Peroxidation was induced with tert-butylhydroperoxide and FeCl 2 at 35 °C. The decrease of SAPC versus DPPC, or N-palmitoyl SM, or N-stearoyl SM, was monitored by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) at various reaction times. For corresponding molar ratios of DPPC:SAPC and SM:SAPC, SAPC oxidized faster and to a greater extent when DPPC, rather than N-palmitoyl SM or bovine brain SMs, was present. However, at 35 °C the hydrophobic tails in SM mixtures were more disordered than in those of DPPC. The slower oxidation of SAPC in SM-rich vesicles may result from the presence of a tight network of H-bonds that bridge neighboring SM molecules and poses a stronger interfacial barrier to the passage of oxidants.

Adsorption of Surfactant to Bronchial Epithelium: Possible Role of Receptor ‘Unmasking’ in Asthma

Background. In a recent study in animals it has been shown how surface-active phospholipid (SAPL) in the form of a commercially available micronized (5 µmφ) dry powder (ALECT/PumactantT) was able to reduce afferent neural feedback to the brainstem in response to a methacholine challenge by the same order of magnitude as drugs commonly prescribed for asthma. The underlying theory assumed that adsorption of SAPL to bronchial epithelium masked irritant receptors eliciting the bronchoconstrictor reflex, thus providing a barrier to noxious stimuli entering the lungs. Objective. To test the underlying assumption that SAPL was actually adsorbed (i.e., bound to bronchial epithelium), especially the major and most surface-active component of lung surfactant, namely dipalmitoyl phosphatidylcholine (DPPC). A secondary objective was to investigate any role of phosphatidylglycerol (PG) in promoting the adsorption of DPPC. Methods. Radiolabeled DPPC dispersed ultrasonically in saline was used to incubate excised sections of porcine bronchial epithelium. The adsorbed DPPC was then quantified by rigorously rinsing the tissue of adhering fluid and then digesting it for β-scintillation counting. Each test (n = 8 runs) was repeated for ratios of DPPC:PG of 9:1, 7:3 (as per ALECT/PumactantT) and 1:1 for both dipalmitoyl PG (DPPG) and EggPG (as incorporated in ALECT/PumactantT). Results. Despite rigorous rinsing postincubation, bronchial epithelium was found to adsorb DPPC at a level roughly equivalent to one close-packed monolayer; whereas both DPPG and EggPG promoted the adsorption of DPPC in a dose-dependent manner, reaching an approximate threefold increase for 7:3 DPPC:PG. Conclusion. DPPC adsorbs to bronchial epithelium in amounts necessary for the masking of receptors, and this adsorption (probably chemisorption) is quite strongly promoted by PG either in its indigenous state (DPPG) or in the form (EggPG) used in ALECT to suppress the sensitivity of bronchial irritant receptors in our previous study and in clinical trials just completed.

Influence of palmitic acid and hexadecanol on the phase transition temperature and molecular packing of dipalmitoylphosphatidyl-choline monolayers at the air–water interface

Palmitic acid (PA) and 1-hexadecanol (HD) strongly affect the phase transition temperature and molecular packing of dipalmitoylphosphatidylcholine (DPPC) monolayers at the air–water interface. The phase behavior and morphology of mixed DPPC/PA as well as DPPC/HD monolayers were determined by pressure-area-isotherms and fluorescence microscopy. The molecular organization was probed by synchrotron grazing incidence x-ray diffraction using a liquid surface diffractometer. Addition of PA or HD to DPPC monolayers increases the temperature of the liquid-expanded to condensed phase transition. X-ray diffraction shows that DPPC forms mixed crystals both with PA and HD over a wide range of mixing ratios. At a surface pressure (π) of 40 mN/m, increasing the amount of the single chain surfactant leads to a reduction in tilt angle of the aliphatic chains from nearly 30° for pure DPPC to almost 0° in a 1:1 molar ratio of DPPC and PA or HD. At this composition we also find closest packing of the aliphatic chains. Further increase of the amount of PA or HD does not change the lattice or the tilt.

Metabolism of phosphatidylglycerol by alveolar macrophages in vitro.

In whole animal studies, it has been shown that turnover of surfactant dipalmitoylphosphatidylglycerol (DPPG) is faster than that of dipalmitoylphosphatidylcholine (DPPC). The goal of this investigation was to characterize the metabolism of DPPG by alveolar macrophages and to determine whether they contribute to the faster alveolar clearance of DPPG. Isolated rat alveolar macrophages were incubated with liposomes colabeled with [(3)H]DPPG and [(14)C]DPPC. Macrophages internalized both lipids in a time- and temperature-dependent manner. The uptake of both lipids was increased by surfactant protein (SP) A and by adherence of the macrophages to plastic slides. The isotope ratio of DPPC to DPPG internalized by macrophages in suspension in the absence of SP-A was significantly lower than the isotope ratio in liposomes, suggesting that macrophages preferentially internalize DPPG when SP-A is absent. Phospholipase activity in macrophage homogenate was higher toward sn-2-labeled DPPG than toward sn-2-labeled DPPC. These studies show that alveolar macrophages play an important role in catabolizing surfactant lipids and may be partially responsible for the relatively faster clearance of DPPG from the lung.

Properties of mixed monolayers of DPPC and viscoelasticity-giving substances

Static surface pressures and dynamic surface pressures for the mixed monolayers consisting of dipalmitoylphosphatidylcholine (DPPC), cholesterol linoleate (Cho-Linoleate), stearyl alcohol (StOH) and oleic acid (Ole-Ac) on the buffer were measured by using a modified Wilhelmy balance to obtain the surface rheological data such as surface elasticity, surface viscosity, and hysteresis loops of dynamic surface tension. These surface rheorogical data for the mixed monolayers with the molar ratios of DPPC:Cho-Linoleate:StOH:Ole-Ac=11:1:7:1 showed similar to those obtained for a lung surfactant, TA, developed by Fujiwara et al. (T. Fujiwara et al., Elsevier Science, Amsterdam, 1984, pp. 479) which is known to be effective on the treatment of respiratory distress syndrome (RDS).

Stabilizing effect of an S-layer on liposomes towards thermal or mechanical stress

Isolated subunits of the crystalline cell surface layer (S-layer) protein of Bacillus stearothermophilus PV72/p2 were recrystallized on positively charged unilamellar liposomes. Liposomes were composed of dipalmitoylphosphatidylcholine (DPPC), cholesterol and hexadecylamine (HDA) in a molar ratio of 10:5:4 and they were prepared by the dehydration–rehydration method followed by an extrusion procedure. The S-layer protein to DPPC ratio was 5.7 nmol/μmol which approximately corresponds to the theoretical value estimated by using the areas occupied by the S-layer lattice and the lipid membrane. Coating of the positively charged liposomes with S-layer protein resulted in inversion of the ζ-potential from +29.1 mV to −27.1 mV. Covalent crosslinking of the recrystallized S-layer protein was achieved with glutaraldehyde. Chemical analysis revealed that almost all amino groups (>95%) from HDA in the liposomal membrane were involved in the reaction. To study the influence of an S-layer lattice on the stability of the liposomes, the hydrophilic marker carboxyfluoresceine (CF) was encapsulated and its release was determined for plain and S-layer-coated liposomes in the course of mechanical and thermal challenges. In comparison to plain liposomes, S-layer-coated liposomes released only half the amount of enclosed CF upon exposure to shear forces or ultrasonication as mechanical stress factors. Furthermore, temperature shifts from 25°C to 55°C and vice versa induced considerably less CF release from S-layer-coated than from plain liposomes. A similar stabilizing effect of the S-layer lattice was observed after glutaraldehyde treatment of plain and S-layer-coated liposomes.

Surfactant phosphatidylcholine composition during dexamethasone treatment in chronic lung disease

To determine whether dexamethasone 'matures' the phosphatidylcholine (PC) composition of broncheoalveolar fluid in infants at high risk of neonatal chronic lung disease (CLD), either by increasing the proportion of dipalmitoylphosphatidylcholine (DPPC), expressed as a percentage of total PC (%DPPC), or by increasing the ratio of DPPC to palmitoyloleoylphosphatidylcholine (DPPC:POPC ratio). Double blind, placebo controlled. Sixteen infants < 32 weeks' gestation, < 1250 g birth weight who were dependent on mechanical ventilation and requiring a fractional inspired oxygen of > 0.30 at 12 days of chronological age. Randomisation to receive a two week reducing course of dexamethasone base at an initial dose of 0.2 mg/kg three times a day, or equivalent volumes of normal saline, starting at 14 days. Eight infants were randomised into each group. Broncheoalveolar lavage was performed serially throughout the study period or until extubation. PC composition of the fluid was analysed by high performance liquid chromatography. The %DPPC and the DPPC:POPC ratios were calculated for individual infants for days -1 and 0 combined, days 1 and 3 combined, and days 5 and 7 combined. Analysis of covariance was used to analyse the results. The DPPC:POPC ratio was significantly less in the treated group than the placebo group on days 1 and 3, and not greater as the hypothesis stated. Three out of five infants treated with dexamethasone and for whom data were available showed a substantial rise in DPPC:POPC ratio on days 5/7, compared with the placebo group, but overall these changes were not statistically significant. The data do not support the hypothesis that dexamethasone's action in producing a clinical improvement within the first 72 hours of treatment for neonatal CLD is by the 'maturation' of pulmonary surfactant PC.

Phosphatidylcholine composition of endotracheal tube aspirates of neonates and subsequent respiratory disease.

The phosphatidylcholine (PC) content of the initial endotracheal tube aspirate was measured in 105 infants intubated for resuscitation or for ventilation for respiratory distress syndrome, using high performance liquid chromatography and postcolumn fluorescence derivitization with diphenyl-1,3,5-hexatriene. Sixty eight had measurable PC. Of the infants who developed respiratory distress syndrome, with or without subsequent chronic lung disease, neither the percentage of dipalmitoylphosphatidylcholine (DPPC) nor the ratio of DPPC to palmitoyloleoylphosphatidylcholine (POPC), showed any correlation with gestational age. However, both parameters were significantly lower overall in this group than in the group of infants who did not develop respiratory distress syndrome. Infants with a ratio of DPPC:POPC less than 3.0 developed respiratory distress syndrome irrespective of gestational age, but there was considerable overlap between groups for values greater than this. The infants with respiratory distress syndrome who went on to develop chronic lung disease had the same initial PC profile as those with respiratory distress syndrome who did not develop chronic lung disease, but differed as a group by being lighter and more premature. The development of chronic lung disease was not associated with a particular initial PC composition. Other factors related to increasing prematurity must therefore be involved in rendering infants vulnerable to developing chronic lung disease.

Chemical transduction with fluorescent lipid membranes using selective interactions of acetylcholine receptor with agonist/antagonist and acetylcholinesterase with substrate

Alterations in the physical structure of vesicles and monolayers of phospholipids and soybean lecithin were monitored by measurement on the average fluorescence intensity changes from N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)dipalmitoyl-L-a-phosphatidyl ethanolamine (NBD-PE) located in the lipid matrices. This probe was intimately dispersed at a concentration of 1-2 mol-% in lipid membranes and had an emission sensitive to local environmental structure. Alterations in the structure of soybean lecithin vesicles were induced by the selective interaction of acetylcholine receptor with the agonist carbamylcholine and the antagonist alpha-bungarotoxin. Structural changes in vesicles with a 7:3 mole ratio of dipalmitoylphosphatidyl choline to dipalmitoylphosphatidic acid were observed for selective interactions between acetylcholinesterase and acetylcholine. Enhancement of fluorescence emission from the lipid membranes provided transduction of the selective binding events of the receptor and enzyme. A maximum sensitivity of about a 30% enhancement per micromole of carbamylcholine and a detection limit for the toxin of 10 nM were observed for the receptor. Fluorescence microscopy was used to establish that protein could be incorporated in monolayer lipid membranes and to provide information about potential mechanisms of fluorescence enhancement. These studies show that lipid membranes containing NBD-PE can be used as generic transducers of protein-ligand interactions.

A time-resolved study of phase transitions in phosphatidylcholine-phosphatidylethanolamine mixtures

Phase transitions in a 4:1 molar ratio mixture of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE) in excess water have been investigated using time resolved X-ray diffraction at the Daresbury Synchrotron Laboratory, United Kingdom. Phospholipids subjected to temperature scans of 10°C/min or temperature jumps of approx 5°C/s were found to undergo order-disorder phase transitions at previously reported temperatures. A fully hydrated sample with a 4:1 molar ratio of DPPC:DPPE produced a single phase transition, characteristic of the gel to liquid crystal bilayer transition, at approx 40°C when heated at a scan rate of 10°C/min. Upon cooling at 10°C/min, L β′-phase stability occurred at ∼35°C. The L α to L β, transition was characterized by the coexistence of broad acyl chain peaks diagnostic for each phase with the eventual formation of a sharp peak at 0.41 nm. The transition from the L α to the L β′ form in the lipid mixture requires that this miscibility be unaffected by the rate of the cooling process and proceeds by the nucleation of small domains of L β′-phase lipids and the eventual growth to larger domains before the transition is complete. Under rapid heating at approx 5°C/s the gel to liquid crystal bilayer phase transition of this mixture was observed to require approx 7.5 s after ∼40°C was observed.