Cationic liposomes formulated with DMPC and a gemini surfactant traverse the cell membrane without causing a significant bio-damage

Cationic liposomes have been intensively studied both in basic and applied research because of their promising potential as non-viral molecular vehicles. This work was aimed to gain more information on the interactions between the plasmamembrane and liposomes formed by a natural phospholipid and a cationic surfactant of the gemini family. The present work was conducted with the synergistic use of diverse experimental approaches: electro-rotation measurements, atomic force microscopy, ζ-potential measurements, laser scanning confocal microscopy and biomolecular/cellular techniques. Electro-rotation measurements pointed out that the interaction of cationic liposomes with the cell membrane alters significantly its dielectric and geometric parameters. This alteration, being accompanied by significant changes of the membrane surface roughness as measured by atomic force microscopy, suggests that the interaction with the liposomes causes locally substantial modifications to the structure and morphology of the cell membrane. However, the results of electrophoretic mobility (ζ-potential) experiments show that upon the interaction the electric charge exposed on the cell surface does not vary significantly, pointing out that the simple adhesion on the cell surface of the cationic liposomes or their fusion with the membrane is to be ruled out. As a matter of fact, confocal microscopy images directly demonstrated the penetration of the liposomes inside the cell and their diffusion within the cytoplasm. Electro-rotation experiments performed in the presence of endocytosis inhibitors suggest that the internalization is mediated by, at least, one specific pathway. Noteworthy, the liposome uptake by the cell does not cause a significant biological damage.

Sugar-bile acid-based bolaamphiphiles: from scrolls to monodisperse single-walled tubules

The introduction of a mannose residue on carbon 3 of lithocholic acid gives rise to an asymmetric and rigid bolaamphiphilic molecule, which self-assembles in water to form elongated tubular aggregates with an outer diameter of about 20 nm. These tubular structures display a temporal evolution, where the average tube diameter decreases with time, which can be followed by time-resolved small-angle X-ray scattering experiments. Cryogenic transmission electron microscopy images collected as a function of time show that at short times after preparation tubular scrolls are formed via the rolling of layers, after which a complex transformation of the scrolls into single-walled tubules takes place. At long time scales, a further evolution occurs where the tubules both elongate and become narrower. The observed self-assembly confirms the tendency of bile acids and their derivatives to form supramolecular aggregates with an ordered packing of the constituent molecules. It also demonstrates that scrolls can be formed as intermediate structures in the self-assembly process of monodisperse single-walled tubules.

Ultrasound well below the intensity threshold of cavitation can promote efficient uptake of small drug model molecules in fibroblast cells

Ultrasound (US) induced enhancement of plasma membrane permeability is a hugely promising tool for delivering exogenous vectors at the specific biological site in a safe and efficient way. In this respect, here we report effects of membrane permeability alteration on fibroblast-like cells undergoing very low-intensity of US. The change in permeability was pointed out in terms of high uptake efficiency of the fluoroprobe calcein, thus resembling internalization of small cell-impermeable model drugs, as measured by fluorescence microscopy and flow cytometry. Fluorescence evidences moreover suggests that the higher the time of exposure, the larger will be the size of molecules can be internalized. The uptake events were related to the cell viability and also with structural changes occurring at membrane level as revealed by infrared spectroscopy and preliminary membrane fluidity and atomic force microscopy (AFM) investigation. Thus, the question of whether the uptake of cell-impermeable molecules is consistent with the presence of disruptions on the cell membrane (sonopore formation) has been addressed. In this framework, our findings may constitute experimental evidence in support of sub-cavitation sonoporation models recently proposed, and they may also provide some hints towards the actual working condition of medical US dealing with the optimum risk to benefit therapeutic ratio.

Designing unconventional Fmoc-peptide-based biomaterials: structure and related properties

We have recently employed L-amino acids in the lipase-catalyzed biofabrication of a class of self-assembling Fmoc-peptides that form 3-dimensional nanofiber scaffolds. Here we report that using d-amino acids, the homochiral self-assembling peptide Fmoc-D-Phe3 (Fmoc-F*F*F*) also forms a 3-dimensional nanofiber scaffold that is substantially distinguishable from its L-peptide and heterochiral peptide (F*FF and FF*F*) counterparts on the basis of their physico-chemical properties. Such chiral peptides self-assemble into ordered nanofibers with well defined fibrillar motifs. Circular dichroism and atomic force microscopy have been employed to study in depth such fibrillar peptide structures. Dexamethasone release kinetics from PLGA and CS-PLGA nanoparticles entrapped within the peptidic hydrogel matrix encourage its use for applications in drug controlled release.

Characterization of carbon nanotube dispersions in solutions of bile salts and derivatives containing aromatic substituents

Bile salts (BS) are known to solubilize high weight fractions of carbon nanotubes (CNTs) in aqueous solutions. Here, the efficiency of derivatives of bile salts (BSDs) containing aromatic substituents in dispersing single-wall CNTs (SWCNTs) has been investigated in order to check whether the presence of aromatic residues, because of their affinity toward carbon nanotube surfaces, determines improvements of the BS dispersion efficiency (DE). Electric arc and CoMoCAT SWCNTs were analyzed. The results, reported for the two surfactant concentrations of 0.06 and 1.0 wt %, show that the DE of BSDs depends on the position, orientation, and structure of the introduced aromatic residues. In the case of the CoMoCAT SWCNTs, at low surfactant concentration a DE improvement is observed in BSDs where the aromatic residue is linked either to carbon 3, located on the rigid four-ring system, or to the side chain. For the latter, this improvement is also enhanced in double-charge derivatives and kept at high surfactant concentration. It was also observed that at low concentrations of surfactant, the DE values of BSs and BSDs are usually larger than those of the more conventional detergent sodium dodecylsulfate.

Chitosan-DNA complexes: charge inversion and DNA condensation

The design of biocompatible polyelectrolyte complexes is a promising strategy for in vivo delivery of biologically active macromolecules. Particularly, the condensation of DNA by polycations received considerable attention for its potential in gene delivery applications, where the development of safe and effective non-viral vectors remains a central challenge. Among polymeric polycations, Chitosan has recently emerged as a very interesting material for these applications. In this study, we compare the observed aggregation behavior of Chitosan-DNA complexes with the predictions of existing models for the complexation of oppositely charged polyelectrolytes. By using different and complementary microscopy approaches (AFM, FESEM and TEM), light scattering and electrophoretic mobility techniques, we characterized the structures of the complexes formed at different charge ratios and Chitosan molecular weight. In good agreement with theoretical predictions, a reentrant condensation, accompanied by charge inversion, is clearly observed as the polycation/DNA charge ratio is increased. In fact, the aggregates reach their maximum size in correspondence of a value of the charge ratio where their measured net charge inverts its sign. This value does not correspond to the stoichiometric 1:1 charge ratio, but is inversely correlated with the polycation length. Distinctive "tadpole-like" aggregates are observed in excess polycation, while only globular aggregates are found in excess DNA. Close to the isoelectric point, elongated fiber-like structures appear. Within the framework of the models discussed, different apparently uncorrelated observations reported in the literature find a systematic interpretation. These results suggest that these models are useful tools to guide the design of new and more efficient polycation-based vectors for a more effective delivery of genetic material.

Between peptides and bile acids: self-assembly of phenylalanine substituted cholic acids

Biocompatible molecules that undergo self-assembly are of high importance in biological and medical applications of nanoscience. Peptides and bile acids are among the most investigated due to their ability to self-organize into many different, often stimuli-sensitive, supramolecular structures. With the aim of preparing molecules mixing the aggregation properties of bile acid and amino acid-based molecules, we report on the synthesis and self-association behavior of two diastereomers obtained by substituting a hydroxyl group of cholic acid with a l-phenylalanine residue. The obtained molecules are amphoteric, and we demonstrate that they show a pH-dependent self-assembly. Both molecules aggregate in globular micelles at high pH, whereas they form tubular superstructures under acid conditions. Unusual narrow nanotubes with outer and inner cross-section diameters of about 6 and 3 nm are formed by the derivatives. The diasteroisomer with α orientation of the substituent forms in addition a wider tubule (17 nm cross-section diameter). The ability to pack in supramolecular tubules is explained in terms of a wedge-shaped bola-form structure of the derivatives. Parallel or antiparallel face-to-face dimers are hypothesized as fundamental building blocks for the formation of the narrow and wide nanotubes, respectively.

Amino acid-bile acid based molecules: extremely narrow surfactant nanotubes formed by a phenylalanine-substituted cholic acid

An amino acid-substituted bile acid forms tubular aggregates with inner and outer diameters of about 3 and 6 nm. The diameters are unusually small for surfactant self-assembled tubes. The results enhance the spectrum of applications of supramolecular tubules and open up possibilities for investigating a novel class of biological amphiphiles.

Cluster phases of decorated micellar solutions with macrocyclic ligands

An aqueous self-assembled micellar system (sodium dodecyl sulfate, SDS, decorated with various adhesive sites, cryptand Kryptofix 222 and crown ether 18-Crown-6 molecules) has been investigated by dynamic light scattering (DLS) and small angle x-ray scattering (SAXS) to have insights into the micellar structure, the micellar interactions, and the aggregation properties of the system. DLS demonstrates the existence of populations of aggregates in the submicrometer/micrometer range, while the Guinier analysis of the SAXS curves helps in detailing objects smaller than 30 nm. The aggregates of micelles are here named cluster phases of micelles (CPMs). Considering that SDS micelles in water do not aggregate at low concentration, it is shown that macrocyclic ligands induce the SDS micelle aggregation as a function of the concentration (i.e., investigated ligand/SDS molar ratios are 5.0, 1.5, 1.0, and 0.5) and hydrophobicity of the adhesive sites. The sizes and the percentages of the micelles and the CPMs have been monitored to test the stability and reversibility of the system. DLS results clearly show that the aggregation processes of the decorated micelles are reproducible at time intervals of the order of 1 month, while the stability may not be entirely maintained after a year. As an issue of particular relevance, the higher the ligand/surfactant molar ratio, the larger are the CPMs induced. The K222 ligand results in being more effective in promoting the micellar aggregation than 18C6 as a consequence of the different hydrophobicity.

Role of macrophage activation in the lipid metabolism of postprandial triacylglycerol-rich lipoproteins

The potential link between the inflammatory effects of postprandial lipemia and the induction of macrophage foam cell formation by triacylglycerol-rich lipoproteins (TGRL) was studied using postprandial triacylglycerol-rich lipoproteins (ppTGRL) derived from human volunteers and primary human monocyte-derived macrophages (HMDM). Subjects were fed a test meal high in dairy fat, followed three hours later by isolation of serum ppTGRL. Pro-inflammatory (M1) and anti-inflammatory (M2) phenotypes were induced in HMDM by treatment with lipopolysaccharide (LPS) or dexamethasone (DEX), respectively. ppTGRL caused a dose-dependent increase in both triacylglycerol (TG) and cholesterol (CH) accumulation in the cells. TG accumulation was unaffected by LPS or DEX treatment, but LPS as compared with DEX-treated HMDM were found to accumulate more CH, and this effect was greater than that induced by ppTGRL in untreated cells. LPS-treatment had no effect on lipid uptake from ppTGRL (via the LDLr, scavenger receptors or SR-B1) or on CH efflux, but the CH synthesis inhibitor mevinolin abolished the difference between CH accumulation in LPS-and DEX-treated cells, suggesting that CH synthesis is enhanced in the inflammatory state. Phospholipid (PL) synthesis was increased in inflammatory M1 as compared with anti-inflammatory M2 HMDM. Moreover, TG synthesis was decreased by ppTGRL in DEX-treated as compared with untreated cells. We conclude, therefore, inflammation causes a greater increase in the accumulation of neutral lipids than ppTGRL in macrophages, and that this effect is related to modulation of PL metabolism and possibly also CH synthesis. Thus, the inflammatory phenotype of macrophages influences their lipid metabolism, and is, therefore, likely to modulate the induction of macrophage lipid accumulation by lipoproteins associated with foam cell formation.

Synthesis and physicochemical characterization of new twin-tailed N-oxide based gemini surfactants

New gemini surfactants (GSs) constituted by two double alkyl chain (from 7 to 17 methylenic units) N-oxide monovalent surfactants joined by a PEG spacer of different length (from 3 to 21 ethylene glycol units), thus combining the properties of both N-oxide and GS surfactants, were synthetized and characterized. The different hydrophilic/hydrophobic balance of the molecular structure strongly influences the morphology and the electrical features of the aggregates. Despite the zwitterionic nature of the polar head groups, all the aggregates are characterized by positive potential thus suggesting protonation at the interface; however, the extent of protonation was shown to strongly depend on the length of the alkyl chain and of the spacer.

Polyelectrolyte-induced aggregation of liposomes: a new cluster phase with interesting applications

Different charged colloidal particles have been shown to be able to self-assemble, when mixed in an aqueous solvent with oppositely charged linear polyelectrolytes, forming long-lived finite-size mesoscopic aggregates. On increasing the polyelectrolyte content, with the progressive reduction of the net charge of the primary polyelectrolyte-decorated particles, larger and larger clusters are observed. Close to the isoelectric point, where the charge of the adsorbed polyelectrolytes neutralizes the original charge of the particles' surface, the aggregates reach their maximum size, while beyond this point any further increase of the polyelectrolyte-particle charge ratio causes the formation of aggregates whose size is progressively reduced. This re-entrant condensation behavior is accompanied by a significant overcharging. Overcharging, or charge inversion, occurs when more polyelectrolyte chains adsorb on a particle than are needed to neutralize its original charge so that, eventually, the sign of the net charge of the polymer-decorated particle is inverted. The stability of the finite-size long-lived clusters that this aggregation process yields results from a fine balance between long-range repulsive and short-range attractive interactions, both of electrostatic nature. For the latter, besides the ubiquitous dispersion forces, whose supply becomes relevant only at high ionic strength, the main contribution appears due to the non-uniform correlated distribution of the charge on the surface of the polyelectrolyte-decorated particles ('charge-patch' attraction). The interesting phenomenology shown by these system has a high potential for biotechnological applications, particularly when the primary colloidal particles are bio-compatible lipid vesicles. Possible applications of these systems as multi-compartment vectors for the simultaneous intra-cellular delivery of different pharmacologically active substances will be briefly discussed.

Polyion-induced cluster formation in different colloidal polyparticle aqueous suspensions

The formation of aggregates in polyion-induced charged colloidal particles in aqueous suspension is characterized, under appropriate conditions, by two complementary effects, known as re-entrant condensation and charge inversion, which are considered as proof for the existence of a cluster phase. In this paper, we extend our previous investigation to a set of aqueous colloidal particle suspensions, such as polystyrene spheres, colloidal gold particles, and polylactic acid particles. These systems are characterized by the evolution of the average size of the aggregates and their surface electrical charge (charge inversion) by means of dynamic light-scattering measurements and laser Doppler electrophoretic techniques. The results, together with the previous ones concerning liposome particles, support the notion of a common behavior of this group of complex colloids characterized by short-ranged attractive interactions. The study provides some insights into these structures, which are potentially useful in biotechnological applications, such as multicompartmental carriers in nonviral drug delivery.

Dielectric properties of differently flexible polyions: a scaling approach

The dielectric relaxations associated with counterion polarization along some typical polyion lengths have been measured in an extended frequency range (from 10 kHz to 2 GHz) for four different polyelectrolyte solutions, differing for the polyion molecular weight and the backbone stiffness. Here, we deal with the so-called intermediate dielectric relaxation, falling between the polarization process concerning the whole polyion chain and the polarization process associated with the field-induced re-orientation of the water molecule dipoles. These observed intermediate relaxations have been characterized by means of two parameters, i.e., the dielectric strength Deltaepsilon and the relaxation frequency nu0, and their dependence on the polyion concentration has been described according to the scaling model of a polyelectrolyte solution. These dependencies follow the expected exponents of the scaling laws, both for the dilute and semidilute (unentangled and entangled) regimes. The different concentration regimes evidenced from dielectric relaxation measurements are in very good agreement with the ones determined by means of the zero-shear viscosity measurements. Our results confirm that the intermediate dielectric relaxation in polyelectrolyte solutions should be attributed to counterion fluctuations along some segments (e.g. the subunits of the Mandel model) of the polyion chain, independently of its overall stiffness. This counterion polarization effect is rather confined to the local structure of the polyion chain instead of the whole chain and it is largely independent of the polyion conformation.

Counterion condensation of differently flexible polyelectrolytes in aqueous solutions in the dilute and semidilute regime

The low-frequency limit of the electrical conductivity (dc conductivity) of differently flexible polyions in aqueous solutions has been measured over an extended polyion concentration range, covering both the dilute and semidilute (entangled and unentangled) regime, up to the concentrated regime. The data have been analyzed taking into account the different flexibility of the polymer chains according to the scaling theory of polyion solutions, in the case of flexible polyions, and according to the Manning model, in the case of rigid polyions. In both cases, the fraction f of free counterions, released into the aqueous phase from the ionizable polyion groups, has been evaluated and its dependence on the polyion concentration determined. Our results show that the counterion condensation follows at least three different regimes in dependence on the polyion concentration. The fraction f of free counterions remains constant only in the semidilute regime (a region that we have named the Manning regime), while there is a marked dependence on the polyion concentration both in the dilute and in the concentrated regime. These results are briefly discussed in the light of the scaling theory of polyelectrolyte aqueous solutions.

Effect of temperature on the reentrant condensation in polyelectrolyte-liposome complexation

Interactions of oppositely charged macroions in aqueous solution give rise to intriguing aggregation phenomena, resulting in finite-size, long-lived clusters, characterized by a quite narrow size distribution. Particularly, the adsorption of highly charged linear polyelectrolytes on oppositely charged colloidal particles is strongly correlated and some short-range order arises from competing electrostatic interactions between like-charged polymer chains (repulsion) and between polymer chains and particle surface (attraction). In these systems, in an interval of concentrations around the isoelectric point, relatively large clusters of polyelectrolyte-decorated particles form. However, the mechanisms that drive the aggregation and stabilize, at the different polymer/particle ratios, a well-defined size of the aggregates are not completely understood. Nor is clear the role that the correlated polyion adsorption plays in the aggregation, although the importance of "patchy interactions" has been stressed as the possible source of attractive interaction term between colloidal particles. Different models have been proposed to explain the formation of the observed cluster phase. However, a central question still remains unanswered, i.e., whether the clusters are true equilibrium or metastable aggregates. To elucidate this point, in this work, we have investigated the effect of the temperature on the cluster formation. We employed liposomes built up by DOTAP lipids interacting with a simple anionic polyion, polyacrylate sodium salt, over an extended concentration range below and above the isoelectric condition. Our results show that the aggregation process can be described by a thermally activated mechanism.

Hybrid niosome complexation in the presence of oppositely charged polyions

We have investigated the formation of complexes between negatively charged niosomal vesicles (hybrid niosomes), built up by dicethylphosphate [DCP], Tween 20 and Cholesterol, and three linear differently charged cationic polyions, such as alpha-polylysine, epsilon-polylysine, and polyethylvinylpyridinium bromide [PEVP], with two different substitution degrees. Our aim is to investigate the interaction mechanism between anionic-nonionic vesicles (hybrid niosomes) and linear polycations, characterizing the resulting aggregates in view of possible applications of these composite colloidal particles as vectors for multidrug delivery. In order to explore the aggregation behavior of the complexes and to gain information on the stability of the single niosomal vesicles within the aggregates, we employed dynamic light scattering (DLS), laser Doppler electrophoretic measurements, and fluorescence measurement techniques. The overall phenomenology is well described in terms of the re-entrant condensation and charge inversion behavior, observed in different colloidal systems. The aggregate size and overall charge depend on the charge ratio between vesicles and polyions, and the aggregates reach their maximum size at the point of charge inversion (re-entrant condensation). While the overall phenomenology is similar for all three polycations investigated, the stability and the integrity of the hybrid niosomal vesicles forming the aggregates strongly depend on the chemical structure of the polycations. The role of the polycations in the aggregation process is discussed by identifying specific interactions with the niosomal membrane, pointing out their importance for possible applications as drug delivery vectors.

Strong repulsive interactions in polyelectrolyte-liposome clusters close to the isoelectric point: a sign of an arrested state

Charged colloidal particles whose interacting potential is governed by a short-range attraction and a long-range screened electrostatic repulsion contributions form aggregates whose shape, size, and overall charge are sensitively dependent on the balance between attraction and repulsion. In some cases, this class of colloidal systems shows an equilibrium cluster phase, where particles associate and dissociate reversibly into clusters. When the aggregation of the charged particles is induced by adding an oppositely charged polyion, very close to the isoelectric condition, the interaggregate interactions become very strong and a dynamical arrested state seems to occur. We provide some experimental evidences of this structural arrest in a colloid system composed by vesicles built up by a cationic lipid stuck together by an oppositely charged linear polyion, by means of the combined use of static and dynamic light scattering technique complemented by laser Doppler electrophoretic measurements. Our results show that the second virial coefficient, which is related to the potential of mean force between two adjacent aggregates, markedly increases in the vicinity of the isoelectric point. We interpret this increase as a print of strong interparticle interactions, yielding to a dynamical arrested state via cluster growth.

Properties of Mixed DOTAP−DPPC Bilayer Membranes as Reported by Differential Scanning Calorimetry and Dynamic Light Scattering Measurements

We have investigated the effect of a cationic lipid [DOTAP] on both the thermotropic phase behavior and the structural organization of aqueous dispersions of dipalmitoyl-phosphatidylcholine [DPPC] by means of high-sensitivity differential scanning calorimetry and dynamic light scattering measurements. We find that the incorporation of increasing quantities of DOTAP progressively reduces the temperature and the enthalpy of the gel-to-liquid crystalline transition. We are further showing that, in mixed DOTAP-DPPC systems, the reduction of the phase transition temperature is accompanied by a reduction of the average size of the structures present in the aqueous mixtures, whatever the DOTAP concentration is. These results, which extend a previous investigation by Campbell et al. (Campbell, R. B.; Balasubramanian, S. V.; Straubinger, R. M.; Biochim. Biosphys. Acta 2001, 27, 1512.) limited to a DOTAP concentration below 20 mol %, confirm that the insertion of cationic head groups in zwitterionic phosphatidylcholine bilayers facilitates the formation of stable, relatively small, unilamellar vesicles. This self-assembling restructuring from an aqueous multilamellar structure toward a liposomal phase is favored by decreasing the phospholipid phase transition temperature and by increasing the temperature of the system. This reduction of the average size and the appearance of a stable liposomal phase is also promoted by a heating and cooling thermal treatment.

Evidence of domain formation in cardiolipin-glycerophospholipid mixed monolayers. A thermodynamic and AFM study

The interaction of the three main components of the mitochondrial membrane, namely cardiolipin, phosphatidylcholine, and phosphatidylethanolamine, has been studied investigating mixed cardiolipin-phosphatidylcholine and cardiolipin-phosphatidylethanolamine monolayers at different cardiolipin molar fractions. The thermodynamic behavior of the mixed monolayers was investigated by means of surface pressure and surface potential measurements, and atomic force microscopy was employed to characterize the morphology of the monolayers. Langmuir isotherms and surface potential curves show a regular behavior with a progressive transition toward the isotherm of the pure component. Positive deviations from ideality in the excess Gibbs energies of mixing suggest the presence of repulsive interactions in both systems. Analysis of partial molecular dipole moment indicates a discontinuity at a definite cardiolipin/phosphatidylethanolamine molar fraction, suggesting the formation of a stoichiometric complex; as a consequence, in mixed cardiolipin-phosphatidylethanolamine monolayers, a phase separation is observed at phosphatidylethanolamine excess. AFM measurements indicate the presence of two domains: one made by phosphatidylethanolamine and the other by a regular arrangement of phosphatidylethanolamine and cardiolipin at a fixed molecular ratio.

Liposome-induced DNA compaction and reentrant condensation investigated by dielectric relaxation spectroscopy and dynamic light scattering techniques

Interaction of DNA with oppositely charged objects, such as multivalent ions, cationic surfactants, cationic liposomes, basic proteins, and alcohols, up to nano- or mesoscopic particles, gives rise to a very interesting and fascinating phenomenology, where the shape, size, and stability of the resulting aggregates depend on a delicate balance between different driving forces, mainly of electrostatic origin. We have studied the cationic liposome-DNA complexes during the whole complexation process, below, close to, and above the isoelectric condition, where the number of cationic lipids equals the number of phosphate groups on the DNA chain. We took advantage of the combined use of dynamic light scattering, laser Doppler electrophoretic mobility, and radio-wave dielectric relaxation measurements in order to characterize both the structural parameters (hydrodynamic radius) and the electrical parameters (charge and counterion concentration) of the resulting structures. These structures are fundamentally of two types, clusters of liposomes stuck together by DNA chains (cluster phase in low-density colloidal suspension) and coexisting DNA coils and DNA globules, according to the procedure through which interactions occur (liposomes in excess DNA solution or DNA in excess liposome suspension).

Radiofrequency dielectric loss relaxation in polyion-induced liposome aggregates

In this note, we present a set of dielectric loss relaxation measurements of aqueous charged liposome suspensions during the whole aggregation process induced by oppositely charged adsorbing polyions. The system experiences two concomitant effects known as "reentrant condensation" and "charge inversion," resulting in the formation of liposome aggregates whose average size reaches a maximum in the vicinity of the electroneutrality condition, accompanied to a progressive reduction of their overall electrical charge. Far from the neutrality, from both sides, polyion-coated liposomes exist with a charge of opposite sign. The dielectric loss relaxation in these complex aggregating systems has never been measured so far and we report here, for the first time, the dielectric loss behavior of liposomes built up by a cationic lipid and stuck together by poly(acrylate), which is a flexible oppositely charged polyion. The data are analyzed in the framework of standard electrokinetic model theory. The evolution of the aggregation process as a function of the polyion content is mainly characterized by a counterion polarization effect, governed by the surface charge density of the aggregates and hence by the zeta-potential.

Polyion-induced liposomal vesicle aggregation: A radiowave dielectric relaxation study

The radiowave dielectric properties of aqueous heterogeneous systems during the complexation of charged polyions and oppositely charged liposomal particles have been measured in a wide frequency range, between 100 Hz and 2 GHz. The formation of a polyion-liposome complex driven by the correlated polyion adsorption at the particle surface implies two concomitant effects referred to as reentrant condensation and charge inversion. Both of them are governed by electrostatic interactions and there is now strong evidence, based on experiments and simulations, that counterion release is the driving force of the aggregation process. From this point of view, dielectric technique may offer a suitable tool in the investigation of the structural properties of these aggregates. In spite of the fact that interaction of polyions with oppositely charged surfaces was extensively experimentally investigated, there are no papers concerning the dielectric properties during the polyion-induced aggregation. To get an insight into this important topic, the authors present here an extensive set of radiowave dielectric measurements of liposomal vesicle aqueous suspensions where the liposome aggregation was induced by an oppositely charged polyion. The aggregation was followed from the beginning, when most of the isolated liposomes predominate, up to the formation of polyion-coated liposomes of inverted charge, crossing the isoelectric condition, where large, almost neutral, aggregates appear. The authors describe the observed dielectric dispersions as due to counterion polarization in the adjacency of the liposome and liposome aggregate surface, primarily governed by the zeta potential, according to the standard electrokinetic model.

Conductometric evidence for intact polyion-induced liposome clusters

In this note, we present a set of electrical conductivity measurements of polyion-induced liposome aggregate aqueous suspensions that supports evidence for the existence of a cluster phase in low-density colloidal systems. Heavily NaCl-loaded liposomes, dispersed in a low-conductivity aqueous solution, are forced by electrostatic interactions with oppositely charged polyions to build up into individual aggregates, where the single vesicles maintain their integrity and, upon an external force, are able to release their ionic content. The conductivity data, within the effective medium approximation theory for heterogeneous systems, are in agreement with the picture of a suspension built up by clusters of vesicles which are able to preserve their content from the external medium. This finding opens new possibilities in multicompartment drug delivery techniques.

Role of Cholesterol, DOTAP, and DPPC in Prostasome/Spermatozoa Interaction and Fusion

Prostasomes are membranous vesicles present in ejaculated human semen. They are very rich in cholesterol and can interact with spermatozoa. Their physiological roles are still under study. Prostasomes were mixed with liposomes prepared from various lipids, such as N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium (DOTAP), DOTAP/1,2-dipalmytoyl-sn-glycero-3-phosphorylcholine (DPPC, 4:1 molar ratio) and DOTAP/cholesterol (4:1, molar ratio) at different pH values (5-8). The mixing of the lipid phases (fusion) was determined by the relief of octadecyl rhodamine B chloride (R(18)) self-quenching and the radii of the vesicles, by light scattering measurements. The mixing of lipids and the radii of prostasomes were both influenced by the addition of liposome, although in a different manner. The ability of prostasomes (modified by previous treatment with liposomes) to transfer lipid to spermatozoa was also measured. Pretreatment with DOTAP decreased the phenomenon and addition of DPPC abolished it. On the other hand, pretreatment of prostasomes with DOTAP/cholesterol liposomes did not affect the transfer of lipid between prostasome and spermatozoa. Therefore, the ability of vesicles to fuse (or, at least, to exchange the lipid component) was affected by the enrichment in either natural or artificial lipid. This may open new possibilities for the modulation of spermatozoa capacitation and acrosome reaction.

Counterion release in overcharging of polyion-liposome complexes

We present a set of electrical conductivity measurements of a mesoscopic equilibrium cluster phase in the aggregation process of charged particles induced by oppositely charged polyions. These measurements supply strong experimental evidence that correlated adsorption of polyions is driven by the counterion release. This phenomenon, known to occur in DNA-liposome mixtures in lamellar phase, i.e., when liposomes fuse together to form a sandwichlike structure encompassing DNA chains, was not previously observed in aqueous suspension of clusters of intact liposomes stuck together by polyions to form reversible aggregates. A simple statistical model of the lateral correlation of polyions at the particle surface justifies quantitatively the observed behavior of the counterion release, as shown by electrical conductivity measurements.

Effect of Gd3+ on the colloidal stability of liposomes

Lanthanide ions such as La3+ and Gd3+ are well known to have large effects on the structure of phospholipid membranes. Unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) were prepared by sonication method and confirmed by transmission electron microscopy. The effects of concentration of gadolinium ions Gd3+ on DPPC unilamellar vesicles in aqueous media were studied by different techniques. As physical techniques, photon correlation spectroscopy, electrophoretic mobility, and differential scanning calorimetry were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersities values were observed as Gd3+ concentration increases, suggesting that this cation induces the aggregation of vesicles. Electrophoretic mobility measurements on unilamellar vesicles as a function of Gd3+ ion concentration show that the vesicles adsorb Gd3+ ions. Above Gd3+ concentrations of 0.1 mol dm-3, the zeta potential and light scattering measurements indicate the beginning of aggregation process. For comparison with similar phospholipids, the zeta potential of phosphatidylcholine interacting with Gd3+ was measured, showing an analogous behavior. Differential scanning calorimetry has been used to determine the effect of Gd3+ on the transition temperature (Tc) and on the enthalpy (DeltaHc) associated with the process.

Direct evidence of multicompartment aggregates in polyelectrolyte-charged liposome complexes

By means of the combined use of dynamic light scattering and transmission electron microscopy measurements, we provide a direct evidence for the existence of an equilibrium cluster phase in the polyion-induced liposome aggregation, where the liposomes maintain their integrity, with the ability of preserving the aqueous core content from the external medium. We prepared single liposomes containing, in their interior, different CsCl electrolyte solutions at different concentrations (0.1 and 0.01 M, respectively). During the polyion-induced complexation of a mixture of these two differently loaded liposomes, reversible aggregates form, whose multicompartmental structure reveals the simultaneous presence of nonfused liposomes. Clusters composed by mesoscopic-sized vesicles and realized by charged lipids coupled to oppositely charged polyions are playing an increasingly important role as model systems in a variety of phenomena in soft matter and for their potential use in biomedical applications as drug delivery systems. Aggregates of liposomes such as those described in this article, where the electrostatic interactions are the primary driving forces promoting aggregation, may represent a new and interesting class of colloids which give rise to a rich phenomenology with several unusual colloidal behaviors that deserve to be further investigated.

Charge Renormalization in Planar and Spherical Charged Lipidic Aqueous Interfaces

The charge renormalization in planar and spherical charged lipidic aqueous interfaces has been investigated by means of thermodynamic and electrokinetic measurements. We analyzed the behavior of mixed DOTAP/DOPE monolayers at the air-electrolyte solution interface and DOTAP/DOPE liposomes 100 nm in size dispersed in an aqueous phase of varying ionic strength. For the two systems, we have compared the "effective" surface charge derived from the measurements of surface potential and zeta-potential to the "bare" charge based on the stoichiometry of the lipid mixture investigated. The results confirm that a strong charge renormalization occurs, whose strength depends on the geometry of the mesoscopic system. The dependence of the "effective" charge on the "bare" charge is discussed in light of an analytical approximation based on the Poisson-Boltzmann equation recently proposed.

Dielectric scaling in polyelectrolyte solutions with different solvent quality in the dilute concentration regime

In this note, we present a set of radiowave dielectric spectroscopy measurements of two dilute, differently-charged polyelectrolyte solutions, under different solvent conditions. We have found that both the dielectric strength, Delta epsilon, and the relaxation time, tau(ion), of the dielectric relaxation process associated with the counterion polarization along a length scale of the order of the correlation length obey the scaling laws with the polyion concentration, according to the Ito model. This is verified with good accuracy independently of the quality of the solvent, which has been varied from poor to good solvent conditions. This finding supports evidence to the fact that, in dilute solutions, the counterion polarization is independent of the polyion concentration, in spite of what occurs at the semi-dilute concentrations.

Solvent quality influence on the dielectric properties of polyelectrolyte solutions: a scaling approach

The dielectric properties of polyelectrolytes in solvent of different quality have been measured in an extended frequency range and the dielectric parameters associated with the polarization induced by counterion fluctuation over some peculiar polyion lengths have been evaluated. Following the scaling theory of polyelectrolyte solutions and the recent models developed by Dobrynin and Rubinstein that explicitly take into account the quality of the solvent on the polyion chain conformation, we have reviewed and summarized a set of scaling laws that describe the dielectric behavior of these systems in the dilute and semidilute regime. Moreover, for poorer solvents, where theory of hydrophobic polyelectrolytes predicts, and computer simulation confirms, a particular chain structure consisting of partially collapsed monomers (beads) connected by monomer strings, we derived a scaling law. These predictions are compared with the results obtained from the dielectric parameters (the dielectric increment delta epsilon and the relaxation time tau(ion)) of the "intermediate" frequency relaxation of two partially charged polymers, which possess a carbon-based backbone for which water is a poor solvent and ethylene glycol is a good solvent. By varying the solvent composition (a water-ethylene glycol mixture), we have tuned the quality of the solvent, passing from poor to good condition and have observed the predicted scaling for all the systems investigated. These findings give a further support to the scaling theory of polyelectrolyte solutions and to the necklace model for hydrophobic polyelectrolytes in poor solvents.

Charge patch attraction and reentrant condensation in DNA–liposome complexes

We investigated the formation of complexes between cationic liposomes built up by DOTAP and three linear anionic polyions, with different charge density and flexibility, such as a single-stranded ssDNA, a double-stranded dsDNA and the polyacrylate sodium salt [NaPAA] of three different molecular weights. Our aim is to gain further insight into the formation mechanism of polyion-liposome aggregates of different sizes (lipoplexes), by comparing the behavior of DNA with a model polyelectrolyte, such as NaPAA, with approximately the same charge density but with a higher flexibility. We employed dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements, in order to explore both the hydrodynamic and structural properties of the aggregates resulting from polyion-liposome interaction and to present a comprehensive picture of the complexation process. The phenomenology can be summarized in a charge ratio-dependent scenario, where the main feature is the formation of large equilibrium clusters due to the aggregation of intact polyion-coated vesicles. At increasing polyion-liposome ratio, the size of the clusters continuously increases, reaching a maximum at a well-defined value of this ratio, and then decreases ("reentrant" condensation). The aggregation mechanism and the role of the polyion charge density in the complex formation are discussed in the light of the recent theories on the correlated adsorption of polyelectrolytes at charged interfaces. Within this framework, the phenomena of charge inversion and the reentrant condensation, peaked at the isoelectric point, finds a simple explanation.

Conductometric properties of linear polyelectrolytes in poor-solvent condition: The necklace model

We present a set of low-frequency electrical conductivity measurements of solutions of differently charged, salt-free polyelectrolytes in poor- and in good-solvent conditions, in the semidilute concentration regime. The data have been analyzed and discussed in light of the necklace model for hydrophobic polyelectrolytes recently proposed by Dobrynin et al. [Macromolecules 29, 2974 (1996)] that predicts the chains to collapse into spheroidal cores connected by narrow strings. By varying the quality of the solvent, we have measured the polyion equivalent conductance lambda(p) in an extended concentration range in the semidilute regime and have demonstrated that this parameter is influenced by the polyion chain conformation, giving further support, when the poor-solvent condition prevails, to the picture of a string of electrostatic blobs. On the contrary, in good-solvent condition, the electrical conductivity data are in reasonable good agreement with the picture of an extended chain consisting of a collection of electrostatic blobs. These electrical conductivity measurements, in light of scaling theory, furnish new experimental support for the necklace model for hydrophobic polyions in poor solvents.

Large equilibrium clusters in low-density aqueous suspensions of polyelectrolyte-liposome complexes: a phenomenological model

In this paper, we revisit some of our previous results together with further experimental evidences for the existence of large equilibrium clusters in low-density aqueous colloidal suspensions, resulting from the screening of charged spherical macroions by oppositely charged linear polyelectrolytes. The aggregation process is described by a simple phenomenological model where aggregates interact via a long-range repulsion Yukawa potential and some supports to the equilibrium properties of the resulting aggregates, on the basis of dynamic light scattering, zeta potential, and transmission electron microscopy techniques, have been provided.

On the phase diagram of reentrant condensation in polyelectrolyte-liposome complexation

Complexation of polyions with oppositely charged spherical liposomes has been investigated by means of dynamic light scattering measurements and a well-defined reentrant condensation has been observed. The phase diagram of charge inversion, recently derived [T. T. Nguyen and B. I. Shklovskii, J. Chem. Phys. 115, 7298 (2001)] for the complexation of DNA with charged spherical macroions, has been employed in order to define the boundaries of the region where polyion-liposome complexes begin to condense, forming larger aggregates, and where aggregates dissolve again, towards isolated polyion-coated-liposome complexes. A reasonable good agreement is observed in the case of complexes formed by negatively charged polyacrylate sodium salt polyions and liposomes built up by cationic lipids (dioleoyltrimethylammoniumpropane), in an extended liposome concentration range.

Complexation of anionic polyelectrolytes with cationic liposomes: evidence of reentrant condensation and lipoplex formation

We have studied the complexation process taking place in cationic liposomes in the presence of anionic polyelectrolytes, in the polyion concentration range from the dilute to the concentrated regime, by combining dynamic light scattering and transmission electron microscopy techniques. We employed as the cationic lipid a two-chained amphiphile (Dioleoyltrimethylammoniumpropane) and sodium polyacrylate salt as the flexible anionic polyelectrolyte. The results evidence a variety of different structures, mainly depending on the liposome-polyion charge ratio, whose peculiar dynamical and structural features are briefly described. In particular, three different polyion concentration regions are found, within which a monomodal or bimodal distribution of aggregates, with a well-defined time evolution, is present. At low polyion content, close to the isoelectric point, large aggregates are formed, deriving from the collapse of the liposomal bilayers into extended charged surfaces, where adsorbed polyions form a two-dimensional strongly correlated array and organize into a two-dimensional Wigner liquid. At high polyion content, above a critical concentration, the size distributions of the complexes are clearly bimodal and a large-component aggregate, continuously increasing with time, coexists with a population of smaller-size aggregates. At an intermediate polyion concentration, spherical, small-size vesicular structures are reformed, connected in a network by polymer chains. A brief discussion tries to summarize our results into a consistent picture.

Time evolution of the formation of different size cationic liposome-polyelectrolyte complexes

We report on the time evolution of the aggregation behaviour of cationic liposome-polyelectrolyte complexes studied by means of dynamic light scattering technique. Pure dioleoyltrimethilammoniumpropane (DOTAP) and mixed DOTAP-dipalmitoylphosphatidylcholine (DPPC) liposomes in polyacrylate sodium salt aqueous solutions in a wide concentration range have been investigated and the size and size distributions of the resulting aggregates evaluated from the intensity autocorrelation function of the scattered light. Under appropriate conditions, we found two discrete aggregation regimes, resulting in two different structural arrangements, whose time evolution depends on the charge ratio and the polyelectrolyte molecular weight. A first small component of average size in the 100-500 range nm coexists with a larger component, whose typical size increases with time, up to some micrometers. The cluster growth from a single liposome, 70 nm in diameter, to the formation of polymer-coated liposome aggregates has been briefly discussed in the light of steric stabilization of colloids. Moreover, it has been found that the kinetics of aggregation of the larger, time-dependent, component follows a dynamical scaling within the diffusion-limited cluster aggregation (DLCA) regime. The understanding of structures resulting from interactions between polyelectrolytes with oppositely charged liposomes may help towards formulation of "lipoplexes" (cationic lipid-DNA complexes) to use as non-viral gene carriers.