Pure Dipalmitoylphosphatidylcholine Liposomes Research Articles

Utilizing DPPC Liposomes to Capture Persistent Organic Pollutants

Polychlorinated biphenyls (PCBs) are a class of man-made persistent organic pollutants that saw wide-spread use in commercial and industrial infrastructure as both an insulator and coolant in electrical transformers and capacitors. 2,2’,3,3′,4,4’-hexachlorobiphenyl (HCBP) was one of the most widely produced PCBs. As these mechanical structures failed or are discarded, PCBs are released into the soil, migrate to the water table, and eventually spread to nearby ecosystems by rain and wind. The remarkable chemical stability of PCBs leave few options for environmental waste and water removal, yet they are becoming increasingly concerning as they pose potential health risks to individuals who become exposed to them. Conventionally, liposomes have been used for their promising drug delivery capabilities. Here, we investigated their potential for the entrapment and removal of HCBP. Liposomes are small, nonpolar lipid bi-layered aggregates capable of capturing a wide variety of both polar and nonpolar compounds. Dipalmitoylphosphatidylcholine (DPPC) is a well-characterized lipid that can be derived from natural sources. It is a phospholipid typically found as a major component of pulmonary surfactant mixtures. To assess the utility of liposomes prepared with pure DPPC in capturing PCBs, they were prepared using probe-tip sonication for both direct and passive incorporation of HCBP. Incorporation was assessed using a combination of differential scanning calorimetry and UV-Vis spectroscopy. For direct incorporation liposome stability generally decreased compared to pure DPPC liposomes based on a corresponding decrease in the phase transition temperature, Tm, from 40.8 °C to 37.4 °C. An analysis of passive incorporation using UV-Vis spectroscopy showed an increase in the incorporation of HCBP proportionate to the length of exposure time up to 24 hours. Together our calorimetry and spectroscopic measurements are indicative of HCBP incorporation into liposomes and shows promising potential for use in sustainable environmental cleanup and water treatment technologies. Future studies aim to further explore their technological capabilities by optimizing their thermal and mechanical stability.

Investigating DPPC Liposomes and Their Capacity to Assimilate 2,2’,3,3’,4,4’-Hexachlorobiphenyl, an Emerging Environmental Pollutant

Persistent organic pollutants (POPs) are organic compounds that bioaccumulate in a wide variety of biological and ecological environments due to their resistive nature to chemical, thermal and photodegradation. The stability of POPs and specifically PCBs (a subclass of POPs) leave few options for environmental waste and water removal. Conventionally, liposomes have been used for their drug delivery capabilities, but here we investigated their potential for the entrapment and removal of this class of emerging environmental pollutants. Liposomes are small, nonpolar lipid bi-layered aggregates capable of capturing a wide variety of both polar and nonpolar compounds. Dipalmitoylphosphatidylcholine (DPPC)is a well-characterized lipid that can be derived from natural sources. It is a phospholipid typically found as a major component of pulmonary surfactant mixtures. To assess the utility of liposomes prepared with pure DPPC in capturing PCBs, they were prepared using probe-tip sonication for both direct and passive incorporation of a representative PCB compound, 2,2’,3,3’,4,4’-hexachlorobiphenyl (HCBP). Incorporation of HCBP was assessed using a combination of differential scanning calorimetry and UV-Vis spectroscopy. For direct incorporation in the presence of HCBP, it was apparent that liposome stability generally decreased compared to pure DPPC liposomes based on a corresponding decrease in the phase transition temperature, Tm, from 40.8 °C to 37.4 °C. Additionally, an analysis of passive incorporation by UV-Vis spectroscopy showed an increase in the incorporation of HCBP proportionate to the length of exposure time up to 24 hours. Both the decrease in Tm and substantial UV absorbance signal produced after the compound was extracted from DPPC liposomes are indicative of its incorporation and demonstrates the potential for the use of liposomes in sustainable environmental cleanup and water treatment technologies.

Cholecalciferol has strong effect on the order and dynamics of DPPC membranes: A combined Fourier transform infrared spectroscopy and differential scanning calorimetry study

The effect of cholecalciferol on dipalmitoyl phosphatidylcholine (DPPC) multilamellar vesicles (MLVs) as a function of temperature and cholecalciferol concentration (1–40 mol%) was studied by using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The investigation of the CH, C = O, and PO2− antisymmetric double stretching modes in FTIR spectra and DSC studies reveal that addition of cholecalciferol into pure DPPC liposomes shifts the phase transition to lower temperature, disorders and increases the dynamics of the acyl chains in both phases and induces hydrogen bond formation in the interfacial region.

The effects of Levothyroxine on the structure and dynamics of DPPC liposome: FTIR and DSC studies

Levothyroxine (3,5,3′,5′-tetraiodothyronine), which is a L-isomer of thyroxine T4 (L-T4), is a synthetic thyroid hormone that is biochemically and physiologically indistinguishable from endogenous T4. It is used as a thyroid hormone replacement drug to treat an underactive thyroid gland. The interaction of L-T4, with zwitterionic dipalmitoyl phosphatidylcholine (DPPC) multilamellar liposomes (MLVs) was studied in the presence (1 mol%, 3 mol%, 6 mol%, 9 mol%, 15 mol%, 24 mol% and 30 mol%) and absence of L-T4 by using two different non-invasive techniques; Fourier Transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC). The results show that L-T4 does perturb the phase transition profile by either decreasing the main transition temperature (Tm) and enthalpy (ΔH) or increasing the width at half height (ΔT½). That means; it changes the physical properties of DPPC bilayers. Addition of L-T4 into pure DPPC liposomes shifts the phase transition to lower temperature, disorder the system in gel phase with opposite effect in liquid-crystalline phase and increases the dynamics of the system in both phases and also causes dehydration of the groups of lipids and the water molecules around.

The pore forming capacity of Sticholysin I in dipalmitoyl phosphatidyl vesicles is tuned by osmotic stress

The osmotic condition modulates the properties of liposomes, particularly those related to their stability and response to external agents such as membrane-active proteins or peptides. In a previous work, we have demonstrated that an osmotic shock can increase, per se, water influx/efflux and the exit of the fluorophore calcein entrapped in the aqueous pool of dipalmitoylphosphatidylcholine (DPPC) and DPPC:sphingomyelin (SM) large unilamellar vesicles (LUVs), suggesting a loss of integrity of the liposome bilayer. In the present work, we have extended our study in order to assess how an osmotic imbalance prior to or synchronous with the addition of a recombinant variant of the pore-forming toxin sticholysin I (rSt I) modifies its pore forming capacity in DPPC and DPPC:SM (1:1) LUVs. Our results conclusively show the capacity of hypotonic gradients to improve the pore forming capacity of rSt I molecules, even in pure DPPC liposomes, rendering pore-formation less dependent on the presence of sphyngomyelin. In fact, non-active toxins in DPPC liposomes become active by a hypotonic imbalance in a similar way to those containing SM as a second component.

Temperature-dependent bifurcation of cooperative interactions in pure and enriched in β-carotene DPPC liposomes

We examined the influence of temperature on lipid intermolecular interactions and the organization of bilayers within multilamellar dipalmitoylphosphatidylcholine (DPPC) liposomes. We also investigated the effect of 0.5 mol% β-carotene, a non-polar carotenoid, on the adhesive properties of these liposomes. Atomic force microscopy (AFM) and differential scanning calorimetry (DSC) were used to correlate the changes in the physical properties of the liposomal systems with their thermotropic behaviour. Using DSC we detected two transitions in pure DPPC vesicles and in those containing 0.5 mol% β-carotene. In both systems the pretransition occurred at 34.5(1)°C and the main phase transition at 41.4 °C during heating. Upon cooling, the temperatures of the pretransition and the main transition decreased by about 6 °C and 1 °C, respectively. Changes in enthalpy and entropy were also similar in the two investigated systems. Data obtained in parallel AFM force experiments show that the adhesive forces between the liposomal systems and AFM probe strongly depend on the loading rate. Moreover, their characteristic monotonic changes and discontinuities are sensitive to temperature. In the range of temperatures from 27 °C to 31 °C, i.e. below the temperature of phase transition from gel to ripple phase, the adhesive forces measured in a water environment are about an order of magnitude higher in the presence of β-carotene than in pure DPPC liposomes. The observed variable dependence of adhesion on the loading rate suggests that there are changes in the long- and short-range interactions between lipids, and that these may be related to the occurrence of some clustering effects. In addition, the simultaneous existence of different subphases was found in the gel phase of DPPC liposomes. The presence of β-carotene at a level of 0.5 mol% stimulates the structural reorganization of DPPC multilamellar vesicles and enhances the bifurcation phenomenon detected in these systems.

Semi-solid fluorinated-DPPC liposomes: Morphological, rheological and thermic properties as well as examination of the influence of a model drug on their skin permeation

The goal of this study was to investigate the influence of an incorporated model drug on the skin permeation of the vehicle itself as it may affect the microstructure and properties of the applied formulation via molecular interactions. For this purpose, we performed skin permeation studies using liposomes prepared with F-DPPC, a monofluorinated analog of dipalmitoylphosphatidylcholine (DPPC), with and without sodium fluorescein (SoFl) serving as model drug. Interestingly, the liposome preparation with F-DPPC yielded semi-solid opalescent systems. Hence, a thorough characterization was accomplished beforehand by electron microscopy imaging, rheological and thermoanalytical experiments. Freeze-fracture electron microscopy images confirmed the existence of globular shaped vesicles in the F-DPPC preparations and oscillatory rheological measurements proved the viscoelastic properties of F-DPPC and F-DPPC+SoFl liposomes in contrast to the viscous characteristics of DPPC liposomes. Thermoanalytical measurements revealed an increased phase transition temperature Tm of about 50°C for F-DPPC and F-DPPC+SoFl liposomes compared to pure DPPC liposomes with a Tm of about 43°C. The similar Tm of F-DPPC+SoFl and F-DPPC liposomes as well as the similar skin permeation of the vehicle compound F-DPPC compared to its drug-free counterpart suggest an incorporation of sodium fluorescein into the aqueous core of F-DPPC liposomes.

Liposomal stabilization of ascorbic acid in model systems and in food matrices

Ascorbic acid (AA) was encapsulated into dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylcholine/cholesterol (DPPC/chol) liposomes. We have shown that co-encapsulation of citric and ascorbic acids into liposomes results in considerable stabilization of the latter in model systems containing catalytic concentrations of copper ions. The rate of AA oxidation was decreased by up to 300-fold that of free AA. Incorporation of cholesterol into membranes resulted in lower stabilization efficacy at room temperature and better thermal stability at higher temperatures than pure DPPC liposomes. Total AA, extra-liposomal AA and leakage of AA out of liposomes during thermal treatment were evaluated using the free radical 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS). A modification of the ABTS based assay for liposome permeability makes it applicable to reducing agents and antioxidants as well as AA. Encapsulated AA was also stabilized in real food matrices. The rate of AA oxidation in apple juice, relative to that of free AA, was decreased by two orders of magnitude when encapsulated in DPPC liposomes and by more than one order of magnitude in DPPC/chol liposomes. Liposomal stabilization of AA in a fermented milk product was less pronounced.

Interactions of Dihydrochloride Fluphenazine with DPPC Liposomes: ATR-IR and 31P NMR Studies

The influence of dihydrochloride fluphenazine (FPh) on the dipalmitoylphosphatidylcholine (DPPC) bilayer structure was investigated using ATR-IR and (31)P NMR methods. The ATR-IR results indicate an increase in conformational disorder in the hydrophobic part compared with pure DPPC liposomes and a decrease in temperature of the chain-melting phase transition in FPh/DPPC liposomes. These effects depended on the concentration of the drug in the DPPC bilayer. The dihydrochloride fluphenazine molecules form H-bonds with the proton-acceptor carbonyl groups of DPPC molecules. At a higher concentration of the drug, the lipid bilayer structure is destroyed, and an isotropic phase is observed using (31)P NMR spectroscopy. The interactions between FPh and the lipid bilayer have a crucial role in MDR (multidrug-resistant) activity of this drug. These results improve one possible strategy of cancer chemoprevention with FPh accompanied by fluidization and destabilization of the model lipid bilayer structure.

Interaction between vitamin D2 and magnesium in liposomes: Differential scanning calorimetry and FTIR spectroscopy studies

Abstract Magnesium (Mg2+) ion is of great importance in physiology by its intervention in 300 enzymatic systems, its role in membrane structure, its function in neuromuscular excitability and vitamin D metabolism and/or action. In the present study, the interaction of Mg2+, at low (1 mole %) and high (7 mole %) concentrations with dipalmitoyl phosphatidylcholine (DPPC) liposomes has been studied in the presence and absence of vitamin D2 (1 mole %) by using two noninvasive techniques, namely differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. DSC studies reveal that the presence of vitamin D2 in the pure or Mg2+ (at both low and high concentrations) containing liposomes diminishes the pretransition. The calorimetric results also reveal that, inclusion of Mg2+ (more significantly at high concentration) into pure or vitamin D2 containing DPPC liposomes increases the main phase transition temperature. The investigation of the CH2 symmetric, the CH3 asymmetric, the C O stretching, and the PO 2 - antisymmetric double bond stretching bands in FTIR spectra with respect to changes occurring in the wavenumber and/or the bandwidth values as a function of temperature reveal that, inclusion of vitamin D2 or Mg2+ into pure DPPC liposomes orders and decreases the dynamics of the acyl chains in both gel and liquid-crystalline phases and does not induce hydrogen bond formation in the interfacial region. Furthermore, the dynamics of the head groups of the liposomes decreases in both phases. Our findings reveal that, simultaneous presence of vitamin D2 and Mg2+ alters the effect of each other, which is reflected as a decrease in the interactions between these two additives within the model membrane.

Competitive effect of vitamin D 2 and Ca 2+ on phospholipid model membranes: an FTIR study

The interaction of Ca 2+, with dipalmitoyl phosphatidylcholine (DPPC) model membranes was studied in the presence and absence of vitamin D 2 by using Fourier transform infrared spectroscopy. Addition of vitamin D 2 and/or Ca 2+ into pure DPPC liposomes shifts the phase transition to higher temperature, orders and decreases the dynamics of the acyl chains in both phases and does not induce hydrogen bond formation in the interfacial region. Moreover, the dynamics of the head group of the phospholipid decreases in both phases. The addition of vitamin D 2 into DPPC liposomes containing Ca 2+, decreases the effect of Ca 2+ at all the functional groups under investigation. Similarly, the effect of vitamin D 2 also decreases in the presence of Ca 2+. This behavior is dominant at high Ca 2+ concentrations. Our results show how simultaneous presence of vitamin D 2 and Ca 2+ alter the behavior of each other, which is reflected as a decrease in the interactions between the ions and vitamin D 2 within the membrane.

The effect of magnesium ions on vitamin D 2-phospholipid model membrane interactions in the presence of different buffer media

Vitamin D plays important roles in the bone formation, in calcium and phosphorus homeostasis and in the treatment and prevention of many diseases. Ions, especially divalent cations like Mg 2+, have indispensable roles in many vital biological events. Mg 2+ is involved in many fundamental processes such as stabilization of membranes and macromolecules, synthesis of nucleic acid and proteins and formation and use of high-energy phosphate bonds. Mg 2+ is also required for synthesis of more than 310 different enzymes of the body and is, therefore, involved in many important activities. The roles of vitamin D and major ions in the body are quite well known. While there are still many unresolved points about the exact molecular mechanism behind such diverse functions, in the present study, the interaction of Mg 2+ with dipalmitoyl phosphatidylcholine (DPPC) model membranes has been studied in the presence and absence of vitamin D 2 by using Fourier transform infrared (FTIR) spectroscopy and turbidity technique at 440 nm. The effect of different buffer media on the system has also been investigated. The temperature dependent investigation of the wavelength of the CH 2 antisymmetric stretching bands revealed that, in the presence of N-[2-hydroxyethyl] piperazine- N′-[2-ethanesulfonic acid] (Hepes) and phosphate buffer, addition of Mg 2+ and/or vitamin D 2 into pure DPPC liposomes does not change the shape of the phase transition profile. Turbidity studies support these results. In the presence of Hepes buffer, the inclusion of Mg 2+ and/or vitamin D 2 into pure DPPC liposomes orders the system. In the presence of phosphate buffer, FTIR study showed that, addition of Mg 2+ into pure DPPC liposomes disorders the system in the gel phase. The precipitation of Mg 2+ with phosphates, which is present in phosphate buffer, may be a reason for this difference in the effect. It is seen that, the binary mixture of Mg 2+–DPPC and the ternary mixture of Mg 2+–vitamin D 2–DPPC behave differently in the presence of two different buffer media.

The potentiality for tm elevation of dipalmitoylphosphatidylcholine liposomes

AbstractIn a previous article from this laboratory [M.S. Fernández and E. Calderón, Ber. Bunsenges. Phys. Chem. 95, 1669 (1991)], the interfacial acid‐base ionization of dipalmitoylphosphatidylcholine (DPPC) was studied by applying the concept of potentiality for tm elevation to experimental results of the increase in the phase transition temperature of DPPC liposomes upon incorporation of cationic stearylamine, at different bulk pH values. In this way, an intrinsic interfacial pK of 4 was determined for the phosphodiester group of DPPC. In this work, a modified form of the theoretical treatment developed previously, is applied to the analysis of the ionization of the same phospholipid using data from the literature for the phase transition temperatures (tm) of lamellar phases of DPPC mixed with cetyl alcohol at neutral and acidic pH, as compared to tm values of pure DPPC liposomes. The intrinsic interfacial pK of lecithin phosphate determined in this way is 3.75, very close to that found using stearylamine as an additive. The similarity between the intrinsic interfacial pK values for lecithin phosphate group estimated from the pH dependent tm shifts induced in DPPC bilayers by additives with quite different polar groups such as stearylamine and cetyl alcohol, gives additional support to the concept of potentiality for tm elevation as correlated with the dissociation degree of DPPC bilayers, that we proposed previously.

Behavior of pure and mixed DPPC liposomes spread or adsorbed at the air-water interface

Liposomes from pure dipalmitoylphosphatidylcholine (DPPC) and mixed DPPC: distearoylphosphatidylcholine (DSPC): soybean lecithin (SL) prepared by the Bangham method with sonication were dispersed into solution or spread at the interface and the kinetics of the surface film formation was studied by measuring and recording the evolution of superficial tension, surface potential, and superficial (14C labeled) DPPC density.

Effects of lipid-soluble substances on the thermotropic properties of liposome filtration

Filtration of dimyristoyl phosphatidylcholine or dipalmitoyl phosphatidylcholine liposomes at various temperatures from 3 to 60°C revealed a discontinuous change in filtration behavior centered about the gel-to-liquid crystal transition temperature. This change was continuous at temperatures immediately above or below the transition temperature. Although pure dipalmitoyl phosphatidylcholine liposomes are in the gel state at 22°C, passage of liposomes composed of dipalmitoyl phosphatidylcholine and cholesterol through the filters at 22°C gave results similar to those obtained with liquid-crystal liposomes. Low cholesterol concentrations were nearly as effective as high concentrations in producing this behavior; this observation is consistent with a shear mechanism for reduction of liposome size, since the stress induced by passage of the otherwise rigid liposome through a small pore would be relieved by fracture at a lattice imperfection. Liposomes composed of egg phosphatidylcholine and cholesterol were retained by the filters to a slightly greater extent than pure egg phosphatidylcholine liposomes; these results are consistent with the known condensing effect of cholesterol on liquid-crystal lipsomes and a shear mechanism occurring with filtration. Liposomes composed of dipalmitoyl phosphatidylcholine and either dipalmitoyl phosphatidic acid or dicetyl phosphate were filtered at 22°C; they showed a filtration characteristic similar to liquid-crystal liposomes. Inclusion of the water-soluble dyes eosin Y or Evans blue in dipalmitoyl phosphatidylcholine liposomes resulted in filtration at 22°C which was similar to that observed for liquid-crystal liposomes. The dyes, sodium fluorescein, 6-carboxyfluorescein and fluoresceinisothiocyanate dextran, did not alter 22°C liposome filtration.