Effect of phospholipids and a transmembrane peptide on the stability of the cubic phase of monoolein: implication for protein crystallization from a cubic phase

Self-consistent field modeling of mesomorphic phase changes of monoolein and phospholipids in response to additives

Mapping the topological phase behaviour of lipids in aqueous solution is time consuming and finding the ideal lipid system for a desired application is often a matter of trial and error. Modelling techniques that can accurately predict the mesomorphic phase behaviour of lipid systems are therefore of paramount importance. Here, the self-consistent field theory of Scheutjens and Fleer (SF-SCF) in which a lattice refinement has been implemented, is used to scrutinize how various additives modify the self-assembled phase behaviour of monoolein (MO) and 1,2-dioleoyl-phosphatidylcholine (DOPC) lipids in water. The mesomorphic behaviour is inferred from trends in the mechanical properties of equilibrium lipid bilayers with increasing additive content. More specifically, we focus on the Helfrich parameters, that is, the mean and Gaussian bending rigidities (κ and [small kappa, Greek, macron], respectively) supplemented with the spontaneous curvature of the monolayer (Jm0). We use previously established interaction parameters that position the unperturbed DOPC system in the lamellar Lα phase ([small kappa, Greek, macron] < 0, κ > 0 and Jm0 ≈ 0). Similar interaction parameters position the MO system firmly in a bicontinuous cubic phase ([small kappa, Greek, macron] > 0). In line with experimental data, a mixture of MO and DOPC tends to be in one of these two phases, depending on the mixing ratio. Moreover we find good correlations between predicted trends and experimental data concerning the phase changes of MO in response to a wide range of additives. These correlations give credibility to the use of SF-SCF modelling as a valuable tool to quickly explore the mesomorphic phase space of (phospho)lipid bilayer systems including additives.

Lyotropic liquid crystal engineering moving beyond binary compositional space - ordered nanostructured amphiphile self-assembly materials by design

Ordered amphiphile self-assembly materials with a tunable three-dimensional (3D) nanostructure are of fundamental interest, and crucial for progressing several biological and biomedical applications, including in meso membrane protein crystallization, as drug and medical contrast agent delivery vehicles, and as biosensors and biofuel cells. In binary systems consisting of an amphiphile and a solvent, the ability to tune the 3D cubic phase nanostructure, lipid bilayer properties and the lipid mesophase is limited. A move beyond the binary compositional space is therefore required for efficient engineering of the required material properties. In this critical review, the phase transitions upon encapsulation of more than 130 amphiphilic and soluble additives into the bicontinuous lipidic cubic phase under excess hydration are summarized. The data are interpreted using geometric considerations, interfacial curvature, electrostatic interactions, partition coefficients and miscibility of the alkyl chains. The obtained lyotropic liquid crystal engineering design rules can be used to enhance the formulation of self-assembly materials and provides a large library of these materials for use in biomedical applications (242 references).

Low-pH-Induced Lamellar to Bicontinuous Primitive Cubic Phase Transition in Dioleoylphosphatidylserine/Monoolein Membranes

Electrostatic interactions (EIs) play important roles in the structure and stability of inverse bicontinuous cubic (Q_II) phases of lipid membranes. We examined the effect of pH on the phase of dioleoylphosphatidylserine (DOPS)/monoolein (MO) membranes at low ionic strengths using small-angle X-ray scattering (SAXS). We found that the phase transitions from lamellar liquid-crystalline (L_α) to primitive cubic (Q_II^P) phases in DOPS/MO (2/8 molar ratio) membranes occurred in buffers containing 50 mM NaCl at and below the final pH of 2.75 as the pH of the membrane suspension was decreased from a neutral value. The kinetic pathway of this transition was revealed using time-resolved SAXS with a stopped-flow apparatus. The first step is a rapid transition from the L_α phase to the hexagonal II (H_II) phase, and the second step is a slow transition from the H_II phase to the Q_II^P phase. We determined the rate constants of the first step, k₁, and of the second step, k₂, by analyzing the time course of SAXS intensities quantitatively. The k₁ value increased with temperature. The analysis of this result provided the values of its apparent activation energy, which were constant over temperature but increased with pH. This can be explained by an EI effect on the free energy of the transition state. In contrast, the k₂ value decreased with temperature, indicating that the true activation energy increased with temperature. These experimental results were analyzed using the theory of the activation energy of phase transitions of lipid membranes when the free energy of the transition state depends on temperature. On the basis of these results, we discussed the mechanism of this phase transition.

How Peptide Molecular Structure and Charge Influence the Nanostructure of Lipid Bicontinuous Cubic Mesophases: Model Synthetic WALP Peptides Provide Insights

Nanostructured bicontinuous lipidic cubic phases are used for the encapsulation of proteins in a range of applications such as in meso crystallization of transmembrane proteins and as drug delivery vehicles. The retention of the nanoscale order of the cubic phases subsequent to protein incorporation, as well as retention of the protein structure and function, is essential for all of these applications. Herein synthetic peptides (WALP21, WALPS53, and WALPS73) with a common α-helical hydrophobic domain, but varying hydrophilic loop size, were designed to systematically examine the effect of peptide structure and charge on bicontinuous cubic phases. The effect of the cubic phases on the secondary structure of the peptides was also investigated. The incorporation of the WALP peptides in cubic phases formed by a range of lipids showed that hydrophobic mismatch of the peptides with the lipid bilayers, the hydrophilic domain size, and peptide charge were all significant factors determining the response of the lipid nanomaterial to protein insertion. As charge repulsion had the most significant effect on the phase transitions observed, we suggest that buffer pH and salt concentration must be carefully considered to ensure cubic mesophase retention. Importantly, the WALP peptides were found to have a different conformation depending on the local lipid environment. Such structural changes could potentially affect membrane protein function, which is crucial for both current and prospective applications.

Activation Energy of the Low-pH-Induced Lamellar to Bicontinuous Cubic Phase Transition in Dioleoylphosphatidylserine/Monoolein

Electrostatic interaction is an important factor for phase transitions between lamellar liquid-crystalline (Lα) and inverse bicontinuous cubic (QII) phases. We investigated the effect of temperature on the low-pH-induced Lα to double-diamond cubic (QII(D)) phase transition in dioleoylphosphatidylserine (DOPS)/monoolein (MO) using time-resolved small-angle X-ray scattering with a stopped-flow apparatus. Under all conditions of temperature and pH, the Lα phase was directly transformed into an intermediate inverse hexagonal (HII) phase, and subsequently the HII phase slowly converted to the QII(D) phase. We obtained the rate constants of the initial step (i.e., the Lα to HII phase transition) and of the second step (i.e., the HII to QII(D) phase transition) using the non-negative matrix factorization method. The rate constant of the initial step increased with temperature. By analyzing this result, we obtained the values of its apparent activation energy, Ea (Lα → HII), which did not change with temperature but increased with an increase in pH. In contrast, the rate constant of the second step decreased with temperature at pH 2.6, although it increased with temperature at pH 2.7 and 2.8. These results indicate that the value of Ea (HII → QII(D)) at pH 2.6 increased with temperature, but the values of Ea (HII → QII(D)) at pH 2.7 and 2.8 were constant with temperature. The values of Ea (HII → QII(D)) were smaller than those of Ea (Lα → HII) at the same pH. We analyzed these results using a modified quantitative theory on the activation energy of phase transitions of lipid membranes proposed initially by Squires et al. (Squires, A. M.; Conn, C. E.; Seddon, J. M.; Templer, R. H. Soft Matter 2009, 5, 4773). On the basis of these results, we discuss the mechanism of this phase transition.

Cubosomes: remarkable drug delivery potential

Cubosomes are nanostructured liquid crystalline particles, made of certain amphiphilic lipids in definite proportions, known as biocompatible carriers in drug delivery. Cubosomes comprise curved bicontinuous lipid bilayers that are organized in three dimensions as honeycombed structures and divided into two internal aqueous channels that can be exploited by various bioactive ingredients, such as chemical drugs, peptides and proteins. Owing to unique properties such as thermodynamic stability, bioadhesion, the ability of encapsulating hydrophilic, hydrophobic and amphiphilic substances, and the potential for controlled release through functionalization, cubosomes are regarded as promising vehicles for different routes of administration. Based on the most recent reports, this review introduces cubosomes focusing on their structure, preparation methods, mechanism of release and potential routes of administration.

Initial step of pH-jump-induced lamellar to bicontinuous cubic phase transition in dioleoylphosphatidylserine/monoolein

Electrostatic interactions (EI) are an important factor for phase transitions between lamellar liquid-crystalline (L(α)) and inverse bicontinuous cubic (Q(II)) phases. We investigated the low pH-induced L(α) to double-diamond cubic (Q(II)(D)) phase transition in dioleoylphosphatidylserine (DOPS)/monoolein (MO) using time-resolved small-angle X-ray scattering. Using a stopped-flow apparatus, a suspension of liposomes (multilamellar vesicles (MLVs) or large unilamellar vesicles (LUVs)) of 20%-DOPS/80%-MO membrane at neutral pH was rapidly mixed with a low pH buffer, and then the structural change of the membranes in the resultant suspension was observed as a function of time (i.e., pH-jump experiment). At the initial step, the L(α) phase was directly transformed into the hexagonal II (H(II)) phase, and subsequently, the H(II) phase slowly converted into the Q(II)(D) phase. We obtained the rate constants of the initial step (i.e., the L(α) to H(II) phase transition) and of the second step (i.e., the H(II) to Q(II)(D) phase transition) using the non-negative matrix factorization method. The rate constant of the initial step was independent of the MLV concentration, indicating that single MLVs can convert into the HII phase without any interaction with other MLVs. On the other hand, the rate constant of the initial step increased with a decrease in pH, 0.041 s(-1) at pH 2.6 and 0.013 s(-1) at pH 2.8, and also exhibited a size dependence; for smaller vesicles such as LUVs and smaller MLVs with diameters of ~1 μm, the rate constant was smaller. They were reasonably explained by the classical nucleation theory. These results provide the first experimental evidence of the total kinetics of EI-induced L(α)/Q(II) phase transitions.

Aggregation behavior of aqueous dioctadecyldimethylammonium bromide/monoolein mixtures: a multitechnique investigation on the influence of composition and temperature

A recently described non-viral gene delivery system [dioctadecyldimethylammonium bromide (DODAB)/monoolein (MO)] has been studied in detail to improve knowledge on the interactions between lamellar (DODAB) and non-lamellar-forming (MO) lipids, as a means to enhance their final cell transfection efficiency. Indeed, the morphology, fluidity, and size of these cationic surfactant/neutral lipid mixtures play an important role in the ability of these systems to complex nucleic acids. The different techniques used in this work, namely dynamic light scattering (DLS), fluorescence spectroscopy, differential scanning calorimetry (DSC), cryogenic transmission electron microscopy (cryo-TEM), light microscopy (LM), and surface pressure-area isotherms, allowed fully characterization of the phase behavior and aggregate morphology of DODAB/MO mixtures at different molar ratios. Overall, the results indicate that the final morphology of DODAB/MO aggregates depends on the balance between the tendency of DODAB to form zero-curvature bilayer structures and the propensity of MO to form non-bilayer structures with negative curvature. These results also show that in the MO-rich region, an increase in temperature has a similar effect on aggregate morphology as an increase in MO concentration.

Kinetics of low pH-induced lamellar to bicontinuous cubic phase transition in dioleoylphosphatidylserine∕monoolein

Recently, it has been well recognized that the modulation of electrostatic interactions due to surface charges can induce transitions between lamellar liquid-crystalline (L(α)) and inverse bicontinuous double-diamond cubic (Q(II)(D)) phases in biological lipids. To reveal their kinetic pathway and mechanism, we investigated the low pH-induced L(α) to Q(II)(D) phase transitions in 20%-dioleoylphosphatidylserine (DOPS)/80%-monoolein (MO) using time-resolved small-angle x-ray scattering and a rapid mixing method. At a final pH of 2.6-2.9, the L(α) phase was transformed completely into the hexagonal II (H(II)) phase within 2-10 s after mixing a low pH buffer with a suspension of multilamellar vesicles of 20%-DOPS∕80%-MO (the initial step). Subsequently, the H(II) phase slowly converted into the Q(II)(D) phase and completely disappeared within 15-30 min (the second step). The rate constants of the second step were obtained using the singular value decomposition analysis. On the basis of these data, we discuss the underlying mechanism of the kinetic pathway of the low pH-induced L(α) to Q(II)(D) phase transitions.

Adsorption behaviors of DPPC/MO aggregates on SiO2 surfaces

The adsorption kinetics of extruded 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC)/1-(cis-9-octadecenoyl)- rac-glycerol (monoolein, MO) aggregates on SiO 2 surface at 25 degrees C is investigated in real time, using the dissipative quartz crystal microbalance (QCM) technique. Four adsorption pathways have been identified depending on the molar fraction of MO in the DPPC/MO system: (I) intact vesicle adsorption, (II) vesicle reorganization on a SiO 2 surface, (III) supported lipid bilayer (SLB) formation, and (IV) cubosome adsorption. The results can be understood by the fact that DPPC is a lamellar phase-forming lipid, whereas MO prefers the cubic phase. Therefore, the incorporation of MO in DPPC increases the packing parameter. Equally important, MO also increases the mobility of lipid molecules and lateral pressure in the bilayers as a result of the presence of a unique cis- double bond. Before extrusion, the vesicles size increases with the MO content when X MO <or= 0.7 and cubosomes are formed for X MO >or= 0.8. The extruded DPPC/MO suspensions consist of reformed vesicles for X MO <or= 0.7 and filtered cubosomes for X MO >or= 0.8, all with a uniform diameter of approximately 100 nm. Differential scanning calorimetry (DSC) further indicates that the addition of MO lowers the main phase transition temperature of DPPC and thus makes the hydrophobic interior more fluid.

Low-pH-induced transformation of bilayer membrane into bicontinuous cubic phase in dioleoylphosphatidylserine/monoolein membranes

Cubic biomembranes, nonbilayer membranes with connections in three-dimensional space that have a cubic symmetry, have been observed in various cells. Interconversion between the bilayer liquid-crystalline (L(alpha)) phase and cubic phases attracted much attention in terms of both biological and physicochemical aspects. Herein we report the pH effect on the phase and structure of dioleoylphosphatidylserine (DOPS)/monoolein (MO) membranes under a physiological ion concentration condition, which was revealed by small-angle X-ray scattering (SAXS) measurement. At neutral pH, DOPS/MO membranes containing high concentrations of DOPS were in the L(alpha) phase. First, the pH effect on the phase and structure of the multilamellar vesicles (MLVs) of the DOPS/MO membranes preformed at neutral pH was investigated by adding various low-pH buffers into the MLV suspension. For 20%-DOPS/80%-MO MLVs, at and below pH 2.9, a transition from the L(alpha) to cubic (Q(224)) phase occurred within 1 h. This phase transition was reversible; a subsequent increase in pH to a neutral one in the membrane suspension transformed the cubic phase into the original L(alpha) phase. Second, we found that a decrease in pH transformed large unilamellar vesicles of DOPS/MO membranes into the cubic phase under similar conditions. We have proposed the mechanism of the low-pH-induced phase transition and also made a quantitative analysis on the critical pH of the phase transition. This finding is the first demonstration that a change in pH can induce a reversible phase transition between the L(alpha) and cubic phases of lipid membranes within 1 h.

Tuning curvature and stability of monoolein bilayers by designer lipid-like peptide surfactants

This study reports the effect of loading four different charged designer lipid-like short anionic and cationic peptide surfactants on the fully hydrated monoolein (MO)-based Pn3m phase (Q(224)). The studied peptide surfactants comprise seven amino acid residues, namely A(6)D, DA(6), A(6)K, and KA(6). D (aspartic acid) bears two negative charges, K (lysine) bears one positive charge, and A (alanine) constitutes the hydrophobic tail. To elucidate the impact of these peptide surfactants, the ternary MO/peptide/water system has been investigated using small-angle X-ray scattering (SAXS), within a certain range of peptide concentrations (R Collapse

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MESH Headings

• Anions
• Lipid Bilayers
• Lipids/chemistry
• Models, Molecular
• Peptides/chemistry
• Scattering, Radiation
• Surface-Active Agents/chemistry
• Temperature

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Grants Collapse

Affiliation(s)

Anan Yaghmur Institute of Biophysics and Nanosystems Research (IBN), Austrian Academy of Sciences, Graz, Austria.
Peter Laggner
Shuguang Zhang
Michael Rappolt

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Nanotubes from a Vitamin C-Based Bolaamphiphile

A bolaform surfactant, 1,12-diascorbyl dodecanedioate (BOLA12), with ascorbic acid units as the polar headgroups was synthesized for the first time. Once dispersed in water above 0.5% w/w, BOLA12 forms hollow nanotubes as revealed by cryo-TEM experiments. These nanostructures transform into clear micellar solutions on heating. X-ray diffraction and SAXS experiments were performed both on the pure solid and on its aqueous dispersions. The critical aggregation concentration and the phase behavior were determined by conductivity and DSC experiments. The latter technique provided also the amount of strongly bound, solvating water molecules that surround the polar headgroups. BOLA12 shows the same reducing properties of ascorbic acid, as indicated by the antioxidant activity evaluated with the DPPH method. This feature was used for the reduction of Pd(II) ions on the surface of the nanoassemblies, which lead to the formation of large bundles homogeneously coated with palladium as observed in SEM micrographs.

Formation of cubic phases from large unilamellar vesicles of dioleoylphosphatidylglycerol/monoolein membranes induced by low concentrations of Ca2+

We developed a new method for the transformation of large unilamellar vesicles (LUVs) into the cubic phase. We found that the addition of low concentrations of Ca(2+) to suspensions of multilamellar vesicles (MLVs) of membranes of monoolein (MO) and dioleoylphosphatidylglycerol (DOPG) mixtures (DOPG/MO) changed their L(alpha) phase to the cubic phases. For instance, the addition of 15-25 mM Ca(2+) to 30%-DOPG/70%-MO-MLVs induced the Q(229) phase, whereas the addition of > or =28 mM Ca(2+) induced the Q(224) phase. LUVs of DOPG/MO membranes containing > or =25 mol % DOPG were prepared easily. Low concentrations of Ca(2+) transformed these LUVs in excess buffer into the Q(224) or the Q(229) phase, depending on the Ca(2+) concentration. For example, 15 and 50 mM Ca(2+) induced the Q(224) and Q(229) phase in the 30%-DOPG/70%-MO-LUVs at 25 degrees C, respectively. This finding is the first demonstration of transformation of LUVs of lipid membranes into the cubic phase under excess water condition.

Effect of positively charged short peptides on stability of cubic phases of monoolein/dioleoylphosphatidic acid mixtures

To elucidate the stability and phase transition of cubic phases of biomembranes with infinite periodic minimal surface is indispensable from biological and physicochemical aspects. In this report, we investigated the effect of positively charged peptide-3K (LLKKK) and poly(L-lysine) on the phase stability of monoolein (MO) membranes containing negatively charged dioleoylphosphatidic acid (DOPA) (i.e., DOPA/MO membranes) using small-angle X-ray scattering. At first, the effect of peptide-3K on 10% DOPA/90% MO membrane in excess water, which is in the Q229 phase, was investigated. At 3.4 mM peptide-3K, a Q229 to Q230 phase transition occurred, and at >3.4 mM peptide-3K, the membrane was in the Q230 phase. Poly(L-lysine) (M(w) 1K-4K) also induced the Q230 phase, but peptide-2K (LLKK) could not induce it in the same membrane. We also investigated the effect of peptide-3K on the multilamellar vesicle (MLV) of 25% DOPA/75% MO membrane, which is in L(alpha) phase. In the absence of peptide, the spacing of MLV was very large (11.3 nm), but at > or = 8 mM peptide-3K, it greatly decreased to a constant value (5.2 nm), irrespective of the peptide concentration, indicating that peptide-3K and the membranes form an electrostatically stabilized aggregation with low water content. Poly(L-lysine) also decreased greatly the spacing of the 25% DOPA/75% MO MLV, indicating the formation of a similar aggregation. To compare the effects of peptide-3K and poly(L-lysine) with that of osmotic stress on stability of the cubic phase, we investigated the effect of poly(ethylene glycol) with molecular weight 7500 (PEG-6K) on the phase stability of 10% DOPA/90% MO membrane. With an increase in PEG-6K concentration, i.e., with an increase in osmotic stress, the most stable phase changed as follows; Q229 (Schwartz's P surface) --> Q224 (D) --> Q230 (G). On the basis of these results, we discuss the mechanism of the effects of the positively charged short peptides (peptide-3K) and poly(L-lysine) on the structure and phase stability of DOPA/MO membranes.

Glycerol Derivatives of Cutin and Suberin Monomers: Synthesis and Self-Assembly

Glycerol derivatives of cutin and suberin monomers were synthesized by acid catalysis. Their dispersion in an aqueous solution was examined by phase contrast microscopy, neutron scattering, and solid state NMR. It is shown that the phase behavior strongly depends on the nature of the derivatives forming either lumps of aggregated membranes or well dispersed membranes.