Chemical exchange between lamellar and non-lamellar lipid phases. A one- and two-dimensional 31P-NMR study

A Versatile Nanocarrier-Cubosomes, Characterization, and Applications

The impact of nanotechnology on the exponential growth of several research areas, particularly nanomedicine, is undeniable. The ability to deliver active molecules to the desired site could significantly improve the efficiency of medical treatments. One of the nanocarriers developed which has drawn researchers' attention are cubosomes, which are nanosized dispersions of lipid bicontinuous cubic phases in water, consisting of a lipidic interior and aqueous domains folded in a cubic lattice. They stand out due to their ability to incorporate hydrophobic, hydrophilic, and amphiphilic compounds, their tortuous internal configuration that provides a sustained release, and the capacity to protect and safely deliver molecules. Several approaches can be taken to prepare this structure, as well as different lipids like monoolein or phytantriol. This review paper describes the different methods to prepare nanocarriers. As it is known, the physicochemical properties of nanocarriers are very important, as they influence their pharmacokinetics and their ability to incorporate and deliver active molecules. Therefore, an extensive characterization is essential to obtain the desired effect. As a result, we have extensively described the most common techniques to characterize cubosomes, particularly nanocarriers. The exceptional properties of the cubosomes make them suitable to be used in several applications in the biomedical field, from cancer therapeutics to imaging, which will be described. Taking in consideration the outstanding properties of cubosomes, their application in several research fields is envisaged.

Liquid Disordered-Liquid Ordered Phase Coexistence in Lipid/Cholesterol Mixtures: A Deuterium 2D NMR Exchange Study

Model membranes composed of two types of long chain phospholipids, one unsaturated and one saturated, along with cholesterol can exhibit two coexisting fluid phases (liquid disordered ([Formula: see text]) and liquid ordered ([Formula: see text])) at various temperatures and compositions. Here we used 1D and 2D ²H NMR to compare the behavior of multilamellar dispersions, magnetically oriented bicelles, and mechanically aligned bilayers on glass plates, all of which contain the same proportions of dipalmitoleoylphosphatidylcholine (DPoPC), dimyristoylphosphatidylcholine (DMPC), and cholesterol. We found that multilamellar dispersions and bilayers aligned on glass plates behave very similarly. These samples were close to a critical composition and exhibit exchange of the lipids between the two fluid phases at temperatures near the [Formula: see text] to [Formula: see text]-[Formula: see text] phase boundary. On the other hand, when a short chain lipid is added to the ternary long chain lipid/cholesterol mixture to form bicelles, the phase behavior is changed significantly and the [Formula: see text] phase occurs at a higher than expected temperature. In addition, there was no evidence of exchange of lipids between the [Formula: see text] and [Formula: see text] phases or critical fluctuations at the temperature where the bulk of the sample enters the two-phase region for these bicelles. It appears that the addition of the short chain lipid results in these samples no longer being near a critical composition.

Solid-state NMR investigation of the selective disruption of lipid membranes by protegrin-1

The interaction of a beta-hairpin antimicrobial peptide, protegrin-1 (PG-1), with various lipid membranes is investigated by (31)P, (2)H, and (13)C solid-state NMR. Mixed lipid bilayers containing anionic lipids and cholesterol are used to mimic the bacterial and mammalian cell membranes, respectively. (31)P and (2)H spectra of macroscopically oriented samples show that PG-1 induces the formation of an isotropic phase in anionic bilayers containing phosphatidylglycerol. Two-dimensional (31)P exchange experiments indicate that these isotropic lipids are significantly separate from the residual oriented lamellar bilayers, ruling out toroidal pores as the cause for the isotropic signal. (1)H spin diffusion experiments show that PG-1 is not exclusively bound to the isotropic phase but is also present in the residual oriented lamellar bilayers. This dynamic and morphological heterogeneity of the anionic membranes induced by PG-1 is supported by the fact that (13)C T(2) relaxation times measured under cross polarization and direct polarization conditions differ significantly. In contrast to the anionic membrane, the zwitterionic phosphatidylcholine (PC) membrane does not form an isotropic phase in the presence of PG-1 but shows significant orientational disorder. The addition of cholesterol to the PC bilayer significantly reduces this orientational disorder. The (13)C T(2) relaxation times of the PC lipids in the presence of both cholesterol and PG-1 suggest that the peptide may decrease the dynamic heterogeneity of the cholesterol-containing membrane. The observed selective interaction of PG-1 with different lipid membranes is consistent with its biological function and may be caused by its strong cationic and amphipathic structure.

A hydroxyethylated cholesterol-based cationic lipid for DNA delivery: effect of conditioning

We have synthesised a novel cholesterol-based cationic lipid to promote DNA transfer in cells. This lipid, dimethyl hydroxyethyl aminopropane carbamoyl cholesterol iodide (DMHAPC-Chol) contains a biodegradable carbamoyl linker and a hydroxyethyl group in the polar amino head moiety and is characterised by NMR. Liposomes prepared from this lipid and dioleoyl phosphatidyl ethanolamine (DOPE) in equimolar proportion showed a weak cytotoxicity as revealed by MTT assays and are efficient to deliver plasmids DNA evaluated by the expression of reporter genes in vitro and in vivo. In this paper, we present an original method to determine the lipid concentration based on the colorimetric detection of the colipid DOPE and the measure of the molar ratio DOPE/cationic lipid in the liposome by FTIR spectroscopy. The liposomes and lipid/DNA complexes structures were characterized by transmission electron microscopy (TEM) and by quasi-elastic light scattering (QLS). TEM indicated that the complexes correspond to aggregates containing globular substructures with liposomes size. The method of immuno-gold labelling was used to detect plasmid in the complex and reveals the presence of DNA inside the aggregates. Transfection results showed efficient DNA transfer depending on the charge ratio and liposomes conditioning. Gel retardation results indicated that at a molar charge ratio between X = 1.5 and X = 2.5 (depending on the liposome conditioning), all DNA was taken by liposomes. We showed that conditioning by freeze-drying (lyophilization) facilitates storage and improves transfection efficiency. When the liposomes were lyophilized prior to DNA addition or when the complexes were subjected to freeze-thawing cycles, the obtained complexes showed a transfection with levels enhanced up to four and five-fold respectively for the lyophilized liposomes and freeze-thawed complexes. NMR was used to characterize the modifications under freezing which showed an effect on 31P spectra.

An innovative procedure using a sublimable solid to align lipid bilayers for solid-state NMR studies

Uniaxially aligned phospholipid bilayers are often used as model membranes to obtain structural details of membrane-associated molecules, such as peptides, proteins, drugs, and cholesterol. Well-aligned bilayer samples can be difficult to prepare and no universal procedure has been reported that orients all combinations of membrane-embedded components. In this study, a new method for producing mechanically aligned phospholipid bilayer samples using naphthalene, a sublimable solid, was developed. Using (31)P-NMR spectroscopy, comparison of a conventional method of preparing mechanically aligned samples with the new naphthalene procedure found that the use of naphthalene significantly enhanced the alignment of 3:1 1-palmitoyl-2-oleoyl-phosphatidylethanolamine to 1-palmitoyl-2-oleoyl-phosphatidylglycerol. The utility of the naphthalene procedure is also demonstrated on bilayers of many different compositions, including bilayers containing peptides such as pardaxin and gramicidin. These results show that the naphthalene procedure is a generally applicable method for producing mechanically aligned samples for use in NMR spectroscopy. The increase in bilayer alignment implies that this procedure will improve the sensitivity of solid-state NMR experiments, in particular those techniques that detect low-sensitivity nuclei, such as 15N and 13C.

Determination of L(alpha)-H(II) phase transition temperature for 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine

The thermodynamic properties of fully-hydrated lipids provide important information about the stability of membranes and the energetic interactions of lipid bilayers with membrane proteins (Nagle and Scott, Physics Today, 2:39, 1978). The lamellar/inverse hexagonal (L(alpha)-H(II)) phase transition of 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) water mixtures is a first-order transition and, therefore, at constant pressure, must have a thermodynamically well-defined equilibrium transition temperature. The observed transition temperature is known to be dependent upon the rate at which the temperature is changed, which accounts for the many different values in the literature. X-ray diffraction was used to study the phase transition of fully-hydrated DOPE to determine the rate-independent transition temperature, T(LH). Samples were heated or cooled for a range of rates, 0.212 < r < 225 degrees C/hr, and the rate-dependent apparent phase transition temperatures, T(A)(r) were determined from the x-ray data. By use of a model-free extrapolation method, the transition temperature was found to be T(LH) = 3.33 +/- 0.16 degrees C. The hysteresis, /T(A)(r) - T(LH)/, was identical for heating and cooling rates, +/-r, and varied as /r/beta for beta approximately 1/4. This unexpected power-law relationship is consistent with a previous study (Tate et al., Biochemistry, 31:1081-1092, 1992) but differs markedly from the exponential behavior of activation barrier kinetics. The methods used in this study are general and provide a simple way to determine the true mesomorphic phase transition temperatures of other lipid and lyotropic systems.

Effect of urea, dimethylurea, and tetramethylurea on the phase behavior of dioleoylphosphatidylethanolamine

The phase behavior of dioleoylphosphatidylethanolamine in aqueous solutions of urea, N,N'-dimethylurea (DMU), and N,N,N',N'-tetramethylurea (TMU) has been characterized by synchrotron X-ray diffraction and differential scanning calorimetry. All three solutes stabilize the lamellar liquid-crystalline phase at the expense of lamellar-gel phase and inverted hexagonal phase of the phospholipid when present in concentrations up to 3 M. X-ray diffraction data demonstrated that the repeat spacing of DOPE increased with increasing urea concentration, but decreased as the DMU and TMU concentrations increased. The repeat spacing of DOPE in the liquid-crystal phase dispersed in the three solutes is d(urea)>d(DMU)>d(TMU). The molecular mechanisms underlying these observations are discussed in terms of either membrane Hofmeister effect, where urea acts as a water structure breaker, or a direct insertion effect of the amphiphilic DMU and TMU molecules into the lipid head groups in the interfacial region of the phospholipid bilayer.

Orientational behavior of phospholipid membranes with mastoparan studied by 31P solid state NMR

Solid state 31P NMR spectroscopy was used to study the perturbing effect of the wasp venom peptide mastoparan (MP) on lipid bilayers composed of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG). The 31P chemical shift anisotropy of multilamellar vesicles decreased with increasing peptide concentration, indicating that MP interacts strongly and selectively with the charged DMPG head group. Macroscopically oriented MP-lipid samples between glass plates were studied by 31P NMR as a function of tilt angle. These spectra showed the coexistence of orientation-dependent lamellar signals as well as an isotropic peak, suggesting that MP can induce non-lamellar phases in DMPC/DMPG membranes.

Cholesterol versus cholesterol sulfate: effects on properties of phospholipid bilayers containing docosahexaenoic acid

The important omega-3 fatty acid docosahexaenoic acid (DHA) is present at high concentration in some membranes that also contain the unusual sterol cholesterol sulfate (CS). The association between these lipids and their effect on membrane structure is presented here. Differential scanning calorimetry (DSC), MC540 fluorescence, erythritol permeability, pressure/area isotherms on lipid monolayers and molecular modeling are used to compare the effect of CS and cholesterol on model phospholipid membranes. By DSC, CS decreases the main phase transition temperature and broadens the transitions of dipalmitolyphosphatidylcholine (DPPC), 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (18:0,18:1 PC) and 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0,22:6 PC) to a much larger extent than does cholesterol. In addition CS produces a three-component transition in 18:0,18:1 PC bilayers that is not seen with cholesterol. In a mixed phospholipid bilayer composed of 18:0,18:1 PC/18:0,22:6 PC (1:1, mol/mol), CS at 2.5 membrane mol% or more induces lateral phase separation while cholesterol does not. CS decreases lipid packing density and increases permeability of 18:0,18:1 PC and 18:0,22:6 PC bilayers to a much larger extent than cholesterol. CS disrupts oleic acid-containing bilayers more than those containing DHA. Molecular modeling confirms that the anionic sulfate moiety on CS renders this sterol more polar than cholesterol with the consequence that CS likely resides higher (extends further into the aqueous environment) in the bilayer. CS can therefore be preferentially accommodated into DHA-enriched bilayers where its tetracyclic ring system may fit into the delta 4 pocket of DHA, a location excluded to cholesterol. It is proposed that CS may in part replace the membrane function of cholesterol in DHA-rich membranes.

Tracking phospholipid populations in polymorphism by sideband analyses of 31P magic angle spinning NMR

A method was developed to track the distributional preferences of phospholipids in polymorphism based on sideband analyses of the 31P magic angle spinning nuclear magnetic resonance spectra. The method was applied to lipid mixtures containing phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and either cholesterol (Chol) or tetradecane, as well as mixtures containing the anionic phosphatidylmethanol, phosphatidylethanolamine, and diolein. The phospholipid composition of coexisting lamellar (Lalpha) and inverted hexagonal (HII) phases remained constant throughout the Lalpha --> HII transition in all mixtures, except those that contained saturated PtdCho and unsaturated PtdEtn in the presence of cholesterol-mixtures that are known to be microimmiscible because of favored associations between Chol and saturated acyl chains. In the latter mixture, saturated PtdCho was enriched in the planar bilayer structure, and unsaturated PtdEtn was enriched in the highly curved HII structure. This enrichment was coincident with an increase in the transition width. When compositional heterogeneity among coexisting phases was observed, it appeared that preexisting lateral microheterogeneities led to compositionally distinct transitional clusters, such that the distributional preferences that resulted were not those of the individual phospholipids.

Measurement of the lateral diffusion of dipalmitoylphosphatidylcholine adsorbed on silica beads in the absence and presence of melittin: a 31P two-dimensional exchange solid-state NMR study

31P two-dimensional exchange solid-state NMR spectroscopy was used to measure the lateral diffusion, D(L), in the fluid phase of dipalmitoylphosphatidylcholine (DPPC) in the presence and absence of melittin. The use of a spherical solid support with a radius of 320 +/- 20 nm, on which lipids and peptides are adsorbed together, and a novel way of analyzing the two-dimensional exchange patterns afforded a narrow distribution of D(L) centered at a value of (8.8 +/- 0.5) x 10(-8) cm2/s for the pure lipid system and a large distribution of D(L) spanning 1 x 10(-8) to 10 x 10(-8) cm2/s for the lipids in the presence of melittin. In addition, the determination of D(L) for nonsupported DPPC multilamellar vesicles (MLVs) suggests that the support does not slow down the lipid diffusion and that the radii of the bilayers vary from 300 to 800 nm. Finally, the DPPC-melittin complex is stabilized at the surface of the silica beads in the gel phase, opening the way to further study of the interaction between melittin and DPPC.

Acyl chain orientational order in large unilamellar vesicles: comparison with multilamellar liposomes: a 2H and 31P nuclear magnetic resonance study

Large unilamellar vesicles (LUVs) composed of 1-[2H31]palmitoyl-2-oleoyl phosphatidylcholine (POPC-d31), with diameters of approximately 117 +/- 31 and 180 +/- 44 nm, were prepared by extrusion through polycarbonate filters with pore sizes of 0.1 and 0.2 microns, respectively. The 2H nuclear magnetic resonance (NMR) spectra obtained at 21 degrees C contain two components: a broad component (approximately 17 kHz linewidth) corresponding to the methylene groups and a narrower component originating from the methyl groups. Spectra with increasing powder pattern characteristics were obtained by reducing the rate of phospholipid reorientations by addition of glycerol (to increase the solvent viscosity) and by lowering the temperature. Full powder spectra, characteristic of liquid-crystalline bilayers, were obtained for both LUV samples at 0 degrees C in the presence of 50 wt% glycerol. Individual quadrupolar splittings were not resolved in these spectra, due to broader linewidths in the LUVs, which have significantly shorter values for spin-spin relaxation time T2 measured from the decay of the quadrupolar echo (90 microseconds) than the multilmellar vesicles (MLVs; 540 microseconds). Smoothed order parameter profiles (OPPs) were obtained for these samples by integration of the dePaked spectra. The OPPs were very similar to the OPP of POPC-d31 MLVs in 50 wt% glycerol at the same temperature, indicating that orientational order in MLVs and LUVs with a diameter of > or = 100 nm is essentially the same. The presence of 80 wt% glycerol was found to have a disordering effect on the vesicles.