Dipalmitoylphosphatidylcholine Liposomes Research Articles

Liposome-encapsulated alginate: controlled hydrogel particle formation and release.

Large unilamellar liposomes of dipalmitoyl phosphatidylcholine have been employed as reaction sites for calcium alginate gelation. Encapsulation of sodium alginate was accomplished by extrusion of phospholipid dispersions through polycarbonate filter of uniform pore size followed by incubation with high concentrations of calcium chloride. The diffusion of calcium into the liposome interior resulted in alginate gelatin within the liposome. Detergent treatment of the liposomes resulted in solubilization of the lipid bilayer with subsequent release of the alginate beads which were measured by laser light scattering and electron microscopy. The release profiles of both the liposomes with entrapped alginate beads and the alginate beads (released by detergent treatment of the liposomes) were determined using cytochrome-c as a marker for release. The release profiles show a rapid release of cytochrome-c over the first 2 for both preparations with slower subsequent release rates for the liposomes with encapsulated alginate beads. The similar early profile may be due to the release of cytochrome-c from bound calcium alginate adsorbed to the outer leaflet of the liposome. This is also supported by calorimetric results which indicate a marked reduction in the enthalpy of the main gel to liquid crystalline phase transition of multilamellar and large unilamellar dispersions of the lipid. This method results in the fabrication of easily controlled, unimodal submicron hydrogel particles which may be used for controlled-release applications.

Effect of ?-carotene, lutein and violaxanthin on structural properties of dipalmitoyl-phosphatidylcholine liposomes as studied by ultrasound absorption technique

The effect of three carotenoid pigments, β-carotene, lutein, and violaxanthin, on structural properties of dipalmitoylphosphatidylcholine liposomes was studied by means of ultrasound absorption technique. It was found that the polar carotenoid-lutein enhances drastically ultrasound absorption related to energy consumption during phase transition of a lipid component. The effect of apolar β-carotene was not so much evident. No differences between the sample and control were found in the case of violaxanthin presence in liposomes. The effect of a polar carotenoid is discussed in terms of the reinforcement of the lipid matrix. Physiological aspects of carotenoid action in membranes are also briefly discussed.

Anesthetic-Membrane Interactions Evaluated by ET(30) Solvatochromism and Taft's Polarity Parameters

Interactions between volatile anesthetics and dipalmitoyl phosphatidylcholine liposome were investigated spectrophotometrically. Four volatile anesthetics were studied. The anesthetics increased an empirical polarity scale, E T(30), at the liposomal interface. The change of E T(30) was correlated with Taft's generalized polarity parameters. All anesthetics increased the generalized proton donor scale (α) and decreased the generalized polarity scale (π). The decrease in π was attributed to the access of anesthetics with a smaller π value to the liposomal membrane, and the perturbation of hydrogen bonds at the water-liposome interface. The increase in α value was attributed also to the perturbation of hydrogen bonds. We previously reported that anesthetics decreased π and increased β (generalized proton acceptor scale) at the membrane-water interface. In this study, we reported the effect of anesthetics on the remaining Taft's parameter, α The present results support our previous conclusion that anesthetics release bound water molecules from the water-liposome interface.

Chimeras of surfactant proteins A and D identify the carbohydrate recognition domains as essential for phospholipid interaction.

Pulmonary surfactant proteins A (SP-A) and D (SP-D) possess similar structure as members of the mammalian C-type lectin superfamily. Both proteins are composed of four characteristic domains which are: 1) an NH2-terminal domain involved in interchain disulfide formation (denoted A1 domain for SP-A or D1 for SP-D); 2) a collagenous domain (denoted A2 or D2); 3) a neck domain (denoted A3 or D3); and 4) a carbohydrate recognition domain (denoted A4 or D4). SP-A specifically binds to dipalmitoylphosphatidylcholine, the major lipid component of surfactant, and can regulate the secretion and recycling of this lipid by alveolar type II cells. SP-D binds to phosphatidylinositol (PI) and glucosylceramide (GlcCer), and its role in alveolar lipid metabolism remains to be clarified. To understand the relationship between the structure and the function of both proteins with respect to their interaction with lipids, we expressed recombinant wild type rat SP-D (rSP-D) and chimeric molecules of SP-A and SP-D (A1A2A3D4, A1A2D3D4, and D1D2A3A4) using a baculovirus expression system, and performed lipid binding and aggregation assays. The rSP-D effectively competed with 125I-labeled native rat SP-D in a solid phase binding assay to PI and GlcCer in a manner nearly identical to native SP-D. The rSP-D also bound to PI liposomes with approximately half the affinity of native rat SP-D. Chimera A1A2D3D4 competed with iodinated SP-D in the solid phase binding assay to both PI and GlcCer. This chimera did not bind to dipalmitoylphosphatidylcholine (DPPC) liposomes or induce their aggregation. Chimera A1A2A3D4 did not bind solid phase PI or GlcCer but was equivalent to rSP-D in binding to PI liposomes. This chimera exhibited weak binding to DPPC but failed to aggregate DPPC liposomes. Chimera D1D2A3A4 failed to bind PI and GlcCer and bound weakly to DPPC liposomes but was quite effective at inducing aggregation of DPPC liposomes. These findings demonstrate that the D3 plus D4 domains of SP-D play a role in lipid binding and that the D4 domain is essential for PI binding. Furthermore, the A3 domain of SP-A cannot account for all the lipid binding activity of this protein. In addition, the results implicate the A4 domain of SP-A as an important structural domain in lipid aggregation phenomena.

Homogeneous immunoassay of antibody by use of liposomes made of a model lipid of archaebacteria

Liposomes made of 1,2-di(3 RS,7 R,11 R-phytanyl)- sn-glycero-3-phosphocholine(DPhyPC), which was synthesized as one of the model lipids existing in archaebacterial halophiles, showed excellent stability. Because of this high stability, DPhyPC liposomes could be constituted high ratios (50%) of N-[4-( p-maleimidophenyl) butyryl] dipalmitoyl phosphatidylethanolamine (MPB-DPPE), and consequently could bind large amounts of antigen (α-chymotrypsinogen A) on the liposome surface in comparison with those made of ordinary lipids, such as dipalmitoylphosphatidylcholine (DPPC). Though the characteristics of the DPhyPC liposomal membranes in lysis by the classical complement pathway were similar to those of DPPC liposomes, a high sensitivity and a low detection limit in the liposome immune lysis assay (LILA) of antibodies were attained by binding large amounts of the antigen. Further, by coupling sufficient amounts of antigen, almost all the DPhyPC liposome surface was covered with the antigen, and such liposomes showed higher resistance against non-specific lysis caused by complement activity in serum samples, which may be effective in reducing positive-false errors in LILA.

Comparison of different separative techniques in the quantitative determination of active compound enclosed in liposomal systems

Synopsis The suitability of three different separative techniques, dialysis, gel filtration and centrifugation, for determining the percentage of active compound included (PAI) in liposomal systems was assessed. Two model compounds, glucose and vitamin E acetate were encapsulated in dipalmitoylphosphatidylcholine (DPPC), soybean lecithin (SL) and hydrogenated soybean lecithin (HSL) multilamellar vesicles (MLV). Vitamin E acetate PAI values from DPPC MLV liposomes obtained by dialysis, gel filtration and centrifugation, were compared with those determined by differential scanning calorimetry. Glucose PAI values from DPPC MLV liposomes, obtained using the same separative techniques, were compared with that calculated by taking into account the glucose content of the liposome internal aqueous phase on the basis of liposome mean size determined by light scattering. Vitamin E acetate and glucose PAI values from SL and HSL liposomes were compared with those obtained for DPPC liposomes. Dialysis proved suitable for PAI determination for both lipophilic and hydrophilic compounds, centrifugation was found to be suitable only for the determination of lipophilic compound PAI values while gel filtration using Sephadex G-25M proved inadequate for the determination of PAI values for both lipophilic and hydrophilic compounds in the experimental conditions used in this study.

Taxol-lipid interactions: taxol-dependent effects on the physical properties of model membranes.

Taxol (paclitaxel) is a diterpenoid anticancer agent undergoing intensive human clinical evaluation. The poor aqueous solubility of taxol necessitates administration in excipients causing a variety of adverse effects, including anaphylactoid hypersensitivity reactions. Recently, taxol has been formulated in better-tolerated drug carriers such as liposomes. We investigated the conformation of taxol and the interaction of taxol with dipalmitoylphosphatidylcholine (DPPC) liposomes using fluorescence, circular dichroism, differential scanning calorimetry, fluorescence polarization, and X-ray diffraction. The conformation of taxol in DPPC membranes was similar to that observed in nonpolar solvents such as chloroform. Taxol was found to partition into the bilayer, perturbing the hydrocarbon chain conformation. The taxol C13 side chain was found to be fluorescent, and it displays an environment-sensitive shift in emission spectrum; taxol fluorescence was used to confirm the insertion of the drug into the bilayer. Taxol induces a broadening of the DPPC phase transition, and the location of the drug in the bilayer depends on drug concentration. Incorporation of taxol affects other physical properties of the bilayer such as the lipid order parameter, and this fluidizing effect was also observed upon incorporation of taxol in biological membranes isolated from basolateral plasma membranes of rat liver. These studies demonstrate that taxol incorporated into liposomes penetrates into the acyl chain domain of the bilayer and alters the physical properties of both artificial and biological membranes.

The Low-Frequency Dielectric Response of Liposomes

The dielectric response of dipalmitoylphosphatidylcholine (DPPC) liposomes has been examined over a frequency range from 105 to 10-2 Hz under a variety of conditions in order to investigate the low-frequency dispersion noted in previous studies. The data have been interpreted in terms of the model proposed by Hill and Pickup (J. Mater. Sci. 20, 4431 (1985)), which predicts that the response over this frequency range will be a function of the presence of an electrode barrier layer in series with a bulk layer. The barrier response was found to be related to the nature of the liposomal bilayer, as a linear relationship was observed between the low-frequency capacitance and the mole fraction of cholesterol in the phospholipid bilayer. The size of the vesicles, the use of different lipid isomers, and changing the head group of the phospholipid were all found to alter the bilayer response. It is suggested that the low-frequency response is associated with the presence of a layer of liposomes absorbed onto the electrode surface, hence the low-frequency technique has potential as a means of characterizing liposomal suspensions.

Liposomal Entrapment of Suramin

The liposomal entrapment of suramin and similar compounds in phospholipid vesicles was examined. For dipalmi‐toylphosphatidylcholine (DPPC) liposomes, entrapment percentages ranged from 25 to 65% with 3‐25 mM phospholipid for aqueous solutions containing 0.07 mM of suramin. Incorporation of 30‐50 mol % cholesterol (CHL) into DPPC liposomes reduced the percentage suramin entrapment. Addition of positively‐charged stearylamine (5 mol %) to DPPC/CHL liposomes increased the entrapment from 2.3% to 30.3%. Entrapment was not affected by the incorporation of negatively‐charged phosphatidylglycerol into DPPC/CHL liposomes. When the amount of suramin was increased from 0.07 to 0.7 mM, the entrapment percentage decreased from 37% to 11% when DPPC was held constant at 6 mM. The entrapment of 0.07 mM Evans blue, a molecule similar in structure to suramin, was 51.6% in DPPC liposomes for 6 mM phospholipid. The entrapment percentage, however, decreased by about 50% when incorporated into 7:3 (DPPC/CHL) liposomes. The liposomal entrapment of disodium 1,5‐naphthalenedisulfonic acid (5.5%) and sodium 3‐amino‐2,7‐naphthalene‐disulfonic acid (1.2%) was very low compared to that of suramin or Evans blue. Differential scanning calorimetry studies of suramin and an aqueous dispersion of DPPC showed an apparent interaction between them. These observations suggest that a significant portion of the entrapped suramin results from binding of suramin to the surface of or intercalation into the liposomal bilayer. Surface binding or intercalation into the phospholipid bilayer may be attributed to both ionic and hydrophobic interactions. The ionic interaction would arise from the suramin sulfonate groups associating with the cationic choline portion of the phospholipid. The hydrophobic interaction would arise from the central portion of the suramin molecule associating with the phospholipid fatty acid chains.

Partitioning and differential scanning calorimetry studies of N-alkyllactame ester dermal prodrugs of indomethacin

The mechanism of indomethacin percutaneous absorption enhancement by N-alkyllactam ester prodrugs (esters I-III) was investigated by assessing stratum corneum partitioning and solubility together with diffusion parameters of both indomethacin and its prodrugs. Skin diffusional characteristics of indomethacin and its esters were evaluated on the basis of their apparent diffusion coefficients and their interaction, assessed by differential scanning calorimetry, with multilamellar liposomes of dipalmitoylphosphatidylcholine. From the results obtained, the enhancement ability of esters I and II could be attributed to their greater stratum corneum solubility associated with better diffusion characteristics compared to the parent drug, while the enhancement effect of ester III could be due only to its higher diffusion coefficient.

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.

Temperature-sensitive liposomes: liposomes bearing poly ( N-isopropylacrylamide)

In order to develop a novel temperature-sensitive liposome, liposomes bearing poly ( N-isopropylacrylamide), which shows a lower critical solution temperature (LCST), were prepared and release behaviors of these liposomes were investigated. A copolymer of N-isopropylacrylamide and octadecyl acrylate was synthesized by radical copolymerization in dioxane using azobisisobutyronitrile as the initiator. The copolymer synthesized revealed the LCST at 27°C. Calcein- or carboxyfluorescein-loaded liposome of egg yolk phosphatidylcholine (EYPC) or dipalmitoylphosphatidylcholine (DPPC) bearing the copolymer was prepared. While release of calcein from DPPC liposome bearing the copolymer was negligible below 20°C, enhancement of the release was observed above 25°C compared with parent DPPC liposome. Fast and extensive release of the fluorophore from the liposome occurred at the phase transition temperature of DPPC. The enhancement of release was also observed in the case of carboxyfluorescein-loaded EYPC liposome bearing the copolymer. These results indicate that increase in hydrophobicity of the copolymer with raising temperature induces stronger interaction of the copolymer with lipid membranes, resulting in enhancement of release of the fluorophores in the inner aqueous phase.

Hydrophobic Zn(II)-naphthalocyanines as photodynamic therapy agents for Lewis lung carcinoma

Four Zn(II) 2,3-naphthalocyanines (unsubstituted ZnNc1, tetracetylamido substituted ZnNc2, tetramino substituted ZnNc3 and tetramethoxy substituted ZnNc4) incorporated into unilamellar liposomes of dipalmitoylphosphati-dylcholine have been injected intra-peritoneally (i.p.) (0.25-0.3 mg kg −1) to male C57/Black mice bearing a transplanted Lewis lung carcinoma. The pharmacokinetic investigations show that three of the four studied ZnNcs, 1, 2 and 4, are good tumor-localizers in Lewis lung carcinoma. The highest concentration is detected after ZnNc1 administration. The lowest tumor concentration as well as the lowest phototherapeutic effect were established with ZnNc3. In previous work it was shown that this ZnNc did not differ from the other three studied ZnNcs regarding the quantum yield of 1O 2-formation and the photoinduced electron transfer. Obviously not only the good photochemical properties but also the tumor drug uptake can be an important factor of effective PDT. The biodistribution investigations also show that 72 h after drug injection, the skin concentration of the studied ZnNcs returns to the original base line. Indeed, we can expect that the skin photosensitivity will last for no longer than three days after PDT. The established higher drug concentration in the tumor rather than in the liver tissue (20 h after injection) shows again the tumor targeting selectivity of the applied liposome-sensitiser delivered procedure. Evaluating the PDT effect as reflected in the dynamics of the mean tumor diameter, we obtained unambiguous data on the potential capacity of ZnNcs 1,2,4 as PDT-photosensitisers. The data obtained from the assessment of the cytotoxic effect of PDT on the basis of the degree of induced necrosis, gave an adequate characterization of the tumor tissue destruction. The results from the morphological analysis show the presence of direct photocytotoxic changes of neoplastic cell targets ie. membrane, mitochondria and rough endoplasmic reticulum, as well as delayed damage in the endothelial cells. The lack of haemorrhagic necrosis suggests a different mechanism of photonecrosis in comparison with the mechanism of photoinduced tumor necrosis after HpD photosensitization, where heavy haemorrhagic changes are observed. We consider that the observed different mechanism of tumor cell photodamage is not a unique property of the studied ZnNcs because similar changes have been observed from other authors after Zn-phthalocyanine photosensitization. We also suppose that the nature of the induced necrosis is different from that of the spontaneous one (untreated tumors), where there was no established endothelial cell damage. In conclusion we consider that ZnNc 1, 2 and 4 can be effective sensitizers for PDT of cancer owing to their selective targeting and slow clearance from tumor tissue, fast clearance from skin and pronounced phototherapeutic effect evaluated by a large number of parameters.

Correlation between Monoamino Oxidase Inhibitor Activity of Some Thiazol-2-ylhydrazines and Their Interaction with Dipalmitoylphosphatidylcholine Liposomes

A calorimetric investigation has been carried out on the influence exerted by some 1-(alkoxybenzoyl)-2-{4-substituted thiazolyl- 2-yl)hydrazines, possessing monoamine oxidase inhibitory (MAOI) activity, on the thermotropic behavior of model membranes constituted by dipalmitoylphosphatidylcholine (DPPC) vesicles. Attention was paid to evaluate how structural variations of drugs may influence drug-lipid interaction. The examined drugs were found to modify the gel to liquid-crystal phase transition of DPPC liposomes, by causing a shift of the transition temperature (Tm) toward lower values and a negligible variation in the enthalpy changes (△H). The different effects on DPPC thermotropic behavior of these MAOI drugs could be considered in terms of different substituents on the molecule’s backbone. The calorimetric results were related to drug’s MAO inhibitory activity measured by fluorescence techniques and the apparent distribution coefficient of the compounds in water/n-octanol. A hypothesis on a correlation between a drug’s structure, Inhibitory activity, and membrane interaction has been suggested.

Partition coefficients of ?-phenylalkanols between water and liposome membranes of phospholipids

For the liposome of two types of phospholipids (zwitterionic dipalmitoyl-phosphatidylcholine (DPPC) and anionic dimyristoylphosphatidic acid (DMPA)), the partition coefficient KX for partition of homologous ω-phenylalkanols (C6H5(CH2)mpOH; mp=0–8) between bulk water and the liposome membrane was determined on the basis of the gel to liquid crystalline phase transition temperature Tm of the liposome membrane. The plot of log KX vs. mp gave a break at mp=7 for both phospholipids, and a second break at mp=4 was observed for DPPC. The local polarity of the surface region and the orderliness of phospholipid molecules in the liposome membrane were estimated from ESR spectra of two spin probes solubilized in the membrane. The results suggest that the hydration of DPPC liposome membrane is relatively restricted to its surface region, but for DMPA the hydration spreads not only along the surface but also to the inside of the membrane. The main factor controlling the partitioning of the alkanols is the local polarity. The higher alkanols (mp=7, 8) are solubilized not only in the liquid crystalline phase but also in the gel phase, although the other lower alkanols are solubilized in the liquid phase only.

Liposome-delivered Zn(II)-2,3-naphthalocyanines as potential sensitizers for PDT: synthesis, photochemical, pharmacokinetic and phototherapeutic studies

The aim of this investigation is to report the synthesis and fundamental photochemical properties of naphthalocyanines with potential interest for photodynamic therapy (PDT), as well as their pharmacokinetics and phototherapeutic effects in a tumor model. Four zinc naphthalocyanines (ZnNc), unsubstituted ZnNc 1, tetraacetylamido-substituted ZnNc 2, tetraamino-substituted ZnNc 3 and tetramethoxy-substituted ZnNc 4 absorbing around 760–770 nm, were synthesized. The dye-sensitized photo-oxidation of 1,3-diphenylisobenzofuran via 1O 2 was studied in dimethylsulfoxide (DMSO). Quantum yields for this photoreaction are 0.135–0.164 and are relatively independent of the kind of substituent. In addition, the photoinduced electron transfer studied in N, N-dimethylformamide—water in the presence of methylviologen and mercaptoethanol is only slightly influenced by the kind of substituent. The pharmacokinetic properties of ZnNc 1 in hamsters bearing a transplanted rhabdomyosarcoma were studied using dipalmitoylphosphatidylcholine liposomes. Experimental PDT of rhabdomyosarcoma was carried out using liposome-delivered ZnNc 1–4. The phototherapeutic effect was evaluated by tumor photonecrosis, the mean tumor diameter during the observation period and the percentage of cured animals. The best effect was found after PDT with ZnNc 2 (50% of the treated animals were cured). A slightly lower effect was observed after application of ZnNc 4 (40% cured animals). No effect at all was noted after PDT with ZnNc 3 and a very low efficiency was found after treatment with ZnNc 1 as photosensitizer. Obviously, the photodynamic effect depends on the biological characteristics as well as on the nature of the substituents.

Further studies on the antioxidant role of pyrophosphate in model membranes

Our previous studies (G. Cervato et al., Chem. Phys. Lipids 56 (1990), 91–99) demonstrated that 100 μM pyrophosphate (PPi) inhibits Fe 2+/ascorbate-induced peroxidation of arachidonic acid (AA) inserted in unilamellar liposomes (SUVs) of dipalmitoylphosphatidylcholine (DPPC), by chelating ferrous ions in the aqueous phase. In this work we demonstrate that the kinetics of AA peroxidation in DPPC SUVs are strongly affected by PPi, also at very low concentration (1 μM). In fact at low PPi concentration there is a longer lag-phase, while the maximum of thiobarbituric acid reactive substances (TBARS) is similar in both the presence and absence of 1 μM PPi. The lag-phase of peroxidation of AA in lysophosphatidylcholine (palmitoyl) (LysoPC) micelles is also prolonged. The AA peroxidation in membrane models without choline as the polar headgroup (dipalmitoylphosphatidylethanolamine (DPPE) SUVs, or Triton X-100 micelles), or in which AA is not free but esterified with glycerol (stearoylarachidonylphosphatidylcholine (SAPC)), is unaffected by the presence of 1 μM PPi.

Calorimetric studies of freeze-induced dehydration of phospholipids

Differential scanning calorimetry (DSC) was used to determine the amount of water that freezes in an aqueous suspension of multilamellar dipalmitoylphosphatidylcholine (DPPC) liposomes. The studies were performed with dehydrated suspensions (12-20 wt% water) and suspensions containing an excess of water (30-70 wt% water). For suspensions that contained > or = 18 wt% water, two ice-formation events were observed during cooling. The first was attributed to heterogeneous nucleation of extraliposomal ice; the second was attributed to homogeneous nucleation of ice within the liposomes. In suspensions with an initial water concentration between 13 and 16 wt%, ice formation occurred only after homogeneous nucleation at temperatures below -40 degrees C. In suspensions containing < 13 wt% water, ice formation during cooling was undetectable by DSC, however, an endotherm resulting from ice melting during warming was observed in suspensions containing > or = 12 wt% water. In suspensions containing < 12 wt% water, an endotherm corresponding to the melting of ice was not observed during warming. The amount of ice that formed in the suspensions was determined by using an improved procedure to calculate the partial area of the endotherm resulting from the melting of ice during warming. The results show that a substantial proportion of water associated with the polar headgroup of phosphatidylcholine can be removed by freeze-induced dehydration, but the amount of ice depends on the thermal history of the samples. For example, after cooling to -100 degrees C at rates > or = 10 degrees C/min, a portion of water in the suspension remains supercooled because of a decrease in the diffusion rate of water with decreasing temperature. A portion of this supercooled water can be frozen during subsequent freeze-induced dehydration of the liposomes under isothermal conditions at subfreezing storage temperature Ts. During isothermal storage at Ts > or = -40 degrees C, the amount of unfrozen water decreased with decreasing Ts and increasing time of storage. After 30 min of storage at Ts = -40 degrees C and subsequent cooling to -100 degrees C, the amount of water associated with the polar headgroups was < 0.1 g/g of DPPC. At temperatures > -50 degrees C, the amount of unfrozen water associated with the polar headgroups of DPPC decreased with decreasing temperature in a manner predicted from the desorption isotherm of DPPC. However, at lower temperatures, the amount of unfrozen water remained constant, in large part, because the unfrozen water underwent a liquid-to-glass transformation at a temperature between -50 degrees and -140 degrees C.

Spectroscopic studies of tin ethyl etiopurpurin in homogeneous and heterogeneous systems

Tin ethyl etiopurpurin is a promising second generation photosensitizer for photodynamic treatment of cancer. This compound is only poorly soluble in aqueous media and, therefore, needs a delivery system for administration to animals. Successful tumor eradication has previously been reported, following light exposure of rats previously administered with the purpurin formulated as a Cremophor El emulsion, in dipalmitoyl—phosphatidyl—choline liposomes or with gamma cyclodextrins. In this paper, we describe some absorbance and fluorescence studies of tin ethyl etiopurpurin in homogeneous and heterogeneous systems. In general, absorption and emission maxima were found to be red shifted as the environment changed from polar to non-polar. The viscosity and dielectric constant of the medium affected the purpurin fluorescence intensity. The liposome preparations were characterized by particle size determination, differential scanning calorimetry and by sensitizer fluorescence quenching studies. Photobleaching studies also showed variation owing to changes in the environment in which the dye was located.

Effect of some potassium selective crown ethers on the permeability and structure of a phospholipid membrane.

The effect of some new crown ethers on the cation efflux and phase transition parameters of dipalmitoyl phosphatidylcholine liposomes was studied. The effects were correlated with the lipophilicity of the crown ethers. The results indicate that the presence of two crown ring structures in one crown either molecule is a prerequisite for the increase of ion permeability of liposomes. The effective crown ethers decrease the temperature, enthalpy and cooperativity of the gel-to-liquid crystalline phase transition. The crown ethers increase membrane permeability for potassium and, to a lesser extent, for rubidium and sodium. The ratio of permeability increase for potassium/rubidium significantly correlates with the lipophilicity of the crown ethers.