Transformation of Lipid Vesicles into Micelles by Adding Nonionic Surfactants: Elucidating the Structural Pathway and the Intermediate Structures

Cryo-electron tomography study of the evolution of wormlike micelles to saturated networks and perforated vesicles

HYPOTHESIS

The formation of micellar aggregates and the changes in their morphology are crucial for numerous practical applications of surfactants. However, a proper structural characterization of complicated micellar nanostructures remains a challenge. This paper demonstrates the advances of cryo-electron tomography (cryo-ET) in revealing the structural characteristics that accompany the evolution of surfactant aggregates.

EXPERIMENTS

By using cryo-ET in combination with cryo-transmission electron microscopy (cryo-TEM), small-angle neutron scattering (SANS), and rheometry, studies were carried out on a model system composed of zwitterionic and nonionic surfactants. In this system, the molecular packing parameter was increased gradually by increasing the molar fraction of nonionic surfactant.

FINDINGS

A series of structural transformations was observed: linear wormlike micelles (WLMs) → branched WLMs → saturated network of multiconnected WLMs → perforated vesicles (stomatosomes). The transformations occur through an increase in the number of branches at the expense of cylindrical subchains and semispherical endcaps. Exponential distribution of subchains length was confirmed experimentally for multiconnected saturated networks. The stomatosomes were formed when the length of subchains becomes much shorter than the persistence length, causing the three-dimensional (3D) structure to transform into a two-dimensional (2D) membrane. This work identifies the mechanism of the structural changes, which can be further used to design various surfactant self-assemblies.

Tolterodine Tartrate Loaded Cationic Elastic Liposomes for Transdermal Delivery: In Vitro, Ex Vivo, and In Vivo Evaluations

OBJECTIVE

Tolterodine tartrate (TOTA) is a first-line therapy to treat overactive urinary bladder (OAB). Oral delivery causes high hepatic clearance, xerostomia, headache, constipation, and blurred vision. We addressed Hansen solubility parameter (HSP) and Design Expert oriented optimized cationic elastic liposomes for transdermal application.

METHODS

The experimental solubility was conducted in HSPiP predicted excipients to tailor formulations using surfactants, stearylamine, ethanol, and phosphatidylcholine (PC). These were evaluated for formulation characteristics. The optimized OTEL1 and OTEL1-G (gel) were compared against the drug solution (DS) and liposomes. In vitro and ex vivo studies were accomplished to investigate the insights into the mechanistic understanding of TOTA release and permeation ability. Finally, confocal laser scanning microscopy (CLSM) supported ex vivo results.

RESULTS

HSP values of TOTA were closely related to tween-80, stearylamine, and human's skin. The size (153 nm), %EE (87.6%), and PDI (0.25) values of OTEL1 were in good agreement to the predicted values (161 nm, 80.4%, and 0.31) with high desirability (0.963). Spherical and smooth OTEL1 (including OTEL1-G and liposomes) vesicles followed non-Fickian drug release as compared to DS (Fickian) as evidence with n > 0.5 (Korsmeyer and Peppas coefficient). OTEL1 (containing lipid and surfactant as 90 mg and 13.8 mg, respectively) exhibited 2.6 and 1.8-folds higher permeation flux than DS and liposomes, respectively. Biocompatible cationic OTEL1 was safe and non-hemolytic.

CONCLUSIONS

OTEL1 was promised as a lead vesicular approach and an alternative to conventional oral therapy to treat OAB in children and advanced age patients.

Ultrathin Flexible Silica Nanosheets with Surface Chemistry-Modulated Affinity to Mammalian Cells

Flexibility of nanomaterials is challenging but worthy to tune for biomedical applications. Biocompatible silica nanomaterials are under extensive exploration but are rarely observed to exhibit flexibility despite the polymeric nature. Herein, a facile one-step route is reported to ultrathin flexible silica nanosheets (NSs), whose low thickness and high diameter-to-thickness ratio enables folding. Thickness and diameter can be readily tuned to enable controlled flexibility. Mechanism study reveals that beyond the commonly used surfactant, the "uncommon" one bearing two hydrophobic tails play a guiding role in producing sheeted/layered/shelled structures, while addition of ethanol appropriately relieved the strong interfacial tension of the assembled surfactants, which will otherwise produce large curled sheeted structures. With these ultrathin NSs, it is further shown that the cellular preference for particle shape and rigidity is highly dependent on surface chemistry of nanoparticles: under high particle-cell affinity, NSs, and especially the flexible ones will be preferred by mammalian cells for internalization or attachment, while this preference is basically invalid when the affinity is low. Therefore, properties of the ultrathin silica NSs can be effectively expanded and empowered by surface chemistry to realize improved bio-sensing or drug delivery.

The Impact of Phospholipid-Based Liquid Crystals' Microstructure on Stability and Release Profile of Ascorbyl Palmitate and Skin Performance

The drug delivery potential of liquid crystals (LCs) for ascorbyl palmitate (AP) was assessed, with the emphasis on the AP stability and release profile linked to microstructural rearrangement taking place along the dilution line being investigated by a set of complementary techniques. With high AP degradation observed after 56 days, two stabilization approaches, i.e., the addition of vitamin C or increasing AP concentration, were proposed. As a rule, LC samples with the lowest water content resulted in better AP stability (up to 52% of nondegraded AP in LC1 after 28 days) and faster API release (~18% in 8 h) as compared to the most diluted sample (29% of nondegraded AP in LC8 after 28 days, and up to 12% of AP released in 8 h). In addition, LCs exhibited a skin barrier-strengthening effect with up to 1.2-fold lower transepidermal water loss (TEWL) and 1.9-fold higher skin hydration observed in vitro on the porcine skin model. Although the latter cannot be linked to LCs' composition or specific microstructure, the obtained insight into LCs' microstructure contributed greatly to our understanding of AP positioning inside the system and its release profile, also influencing the overall LCs' performance after dermal application.

PLGA-LEC/F127 hybrid nanoparticles loaded with curcumin and their modulatory effect on monocytes

Aim: To investigate the effect of surfactant type on curcumin-loaded (CUR) PLGA nanoparticles (NPs) to modulate monocyte functions. Materials & methods: The nanoprecipitation method was used, and PLGA NPs were designed using Pluronic F127 (F127) and/or lecithin (LEC) as surfactants. Results: The Z-average of the NPs was <200 nm, they had a spherical shape, Derjaguin-Muller-Toporov modulus >0.128 MPa, they were stable during storage at 4°C, ζ-potential ∼-40 mV, polydispersity index <0.26 and % EE of CUR >94%. PLGA-LEC/F127 NPs showed favorable physicochemical and nanomechanical properties. These NPs were bound and internalized mainly by monocytes, suppressed monocyte-induced reactive oxygen species production, and decreased the ability of monocytes to modulate T-cell proliferation. Conclusion: These results demonstrate the potential of these NPs for targeted therapy.

Small-angle X-ray and neutron scattering applied to lipid-based nanoparticles: Recent advancements across different length scales

Lipid-based nanoparticles (LNPs), ranging from nanovesicles to non-lamellar assemblies, have gained significant attention in recent years, as versatile carriers for delivering drugs, vaccines, and nutrients. Small-angle scattering methods, employing X-rays (SAXS) or neutrons (SANS), represent unique tools to unveil structure, dynamics, and interactions of such particles on different length scales, spanning from the nano to the molecular scale. This review explores the state-of-the-art on scattering methods applied to unveil the structure of lipid-based nanoparticles and their interactions with drugs and bioactive molecules, to inform their rational design and formulation for medical applications. We will focus on complementary information accessible with X-rays or neutrons, ranging from insights on the structure and colloidal processes at a nanoscale level (SAXS) to details on the lipid organization and molecular interactions of LNPs (SANS). In addition, we will review new opportunities offered by Time-resolved (TR)-SAXS and -SANS for the investigation of dynamic processes involving LNPs. These span from real-time monitoring of LNPs structural evolution in response to endogenous or external stimuli (TR-SANS), to the investigation of the kinetics of lipid diffusion and exchange upon interaction with biomolecules (TR-SANS). Finally, we will spotlight novel combinations of SAXS and SANS with complementary on-line techniques, recently enabled at Large Scale Facilities for X-rays and neutrons. This emerging technology enables synchronized multi-method investigation, offering exciting opportunities for the simultaneous characterization of the structure and chemical or mechanical properties of LNPs.

Lipid nanodiscs spontaneously formed by an amphiphilic polymethacrylate derivative as an efficient nanocarrier for molecular delivery to intact cells

There is a great demand for the technology of molecular delivery into living cells using nanocarriers to realise molecular therapies such as gene delivery and drug delivery systems. Lipid-based nanocarriers offer several advantages for molecular delivery in biological systems, such as easy preparation, high encapsulation efficiency of water-insoluble drug molecules, and excellent biocompatibility. In this paper, we first report the interaction of lipid nanodiscs spontaneously formed by the complexation of an amphiphilic polymethacrylate derivative and phospholipid with intact cells. We evaluated the internalisation of polymethacrylate-based lipid nanodiscs by intact HeLa cells and applied them to the delivery of paclitaxel (PTX), an anticancer drug. The lipid nanodisc showed excellent uptake efficiency compared to conventional liposomes at a concentration where nanodiscs do not show cytotoxicity. In addition, the nanodisc encapsulating PTX showed significantly higher anticancer activity than PTX-loaded liposomes against HeLa cells, reflecting their excellent activity in delivering payloads to intact cells. This study demonstrated the potential of a polymethacrylate-based lipid nanodisc as a novel nanocarrier for molecular delivery to intact cells.

Propolis-loaded liposomes: characterization and evaluation of the in vitro bioaccessibility of phenolic compounds

Background and purpose

Propolis has low water solubility, poor stability, and limited bioaccessibility of phenolic constituents when subjected to in vitro digestion. To overcome these drawbacks, the liposomal encapsulation method can be employed.

Experimental approach

Soybean phosphatidylcholine lecithin mixed with Tween 80 (T80) and ammonium phosphatides (AMP) was used to produce propolis extract (PE)-loaded liposomes. The mean particle size, zeta potential, encapsulation efficiency values, and transmission electron microscopy analysis were used to characterize liposomes. Individual phenolics were determined for digested and nondigested propolis-loaded liposomes and propolis extract.

Key results

Tween 80 incorporation reduced the size of unloaded liposomes, whereas AMP inclusion yielded larger liposomes. In both formulations, PE loading significantly increased the size and reduced the zeta potential values and homogeneity of the size distribution. In free PE, the most bioaccessible polyphenols were phenolic acids (3.20 to 5.63 %), and flavonoids such as caffeic acid phenethyl ester, galangin, pinobanksin, and pinocembrin (0.03 to 2.12 %) were the least bioaccessible. Both liposomal propolis provided significantly higher bioaccessibility of phenolic compounds. The liposomes with T80 and AMP in their compositions recovered 52.43 and 185.90 % of the total amount of phenolic compounds in the nondigested samples, respectively. The liposomes containing AMP not only exhibited high solubility for PE but also provided protection to the phenolic compounds during in vitro digestion.

Conclusion

Liposomal encapsulation could be a promising approach to improving the solubility and stability of PE in digestive fluids, making it suitable for the delivery of propolis in oral formulations.

Morphological control and modern applications of bicelles

Bicellar systems have become popularized as their rich morphology can be applied in biochemistry, physical chemistry, and drug delivery technology. To the biochemical field, bicelles are powerful model membranes for the study of transmembrane protein behavior, membrane transport, and environmental interactions with the cell. Their morphological responses to environmental changes reveal a profound fundamental understanding of physical chemistry related to the principle of self-assembly. Recently, they have also drawn significant attention as theranostic nanocarriers in biopharmaceutical and diagnostic research due to their superior cellular uptake compared to liposomes. It is evident that applications are becoming broader, demanding to understand how the bicelle will form and behave in various environments. To consolidate current works on the bicelle's modern applications, this review will discuss various effects of composition and environmental conditions on the morphology, phase behavior, and stability. Furthermore, various applications such as payload entrapment and polymerization templating are presented to demonstrate their versatility and chemical nature.

Soy lecithin: A beneficial substance for adjusting the ADC in aqueous solutions to the values of biological tissues

PURPOSE

To test soy lecithin as a substance added to water for the construction of MRI phantoms with tissue-like diffusion coefficients. The performance of soy lecithin was assessed for the useable range of adjustable ADC values, the degree of non-Gaussian diffusion, simultaneous effects on relaxation times, and spectral signal properties.

METHODS

Aqueous soy lecithin solutions of different concentrations (0%, 0.5%, 1%, 2%, 3% …, 10%) and soy lecithin-agar gels were prepared and examined on a 3 Tesla clinical scanner at 18.5° ± 0.5°C. Echoplanar sequences (b values: 0-1000/3000 s/mm² ) were applied for ADC measurements. Quantitative relaxometry and MRS were performed for assessment of T₁ , T₂ , and detectable spectral components.

RESULTS

The presence of soy lecithin significantly restricts the diffusion of water molecules and mimics the nearly Gaussian nature of diffusion observed in tissue (for b values <1000 s/mm² ). ADC values ranged from 2.02 × 10^-3  mm² /s to 0.48 × 10^-3  mm² /s and cover the entire physiological range reported on biological tissue. Measured T₁ /T₂ values of pure lecithin solutions varied from 2685/2013 to 668/133 ms with increasing concentration. No characteristic signals of soy lecithin were observed in the MR spectrum. The addition of agar to the soy lecithin solutions allowed T₂ values to be well adjusted to typical values found in parenchymal tissue without affecting the soy lecithin-controlled ADC value.

CONCLUSION

Soy lecithin is a promising substance for the construction of diffusion phantoms with tissue-like ADC values. It provides several advantages over previously proposed substances, in particular a wide range of adjustable ADC values, the lack of additional ¹ H-signals, and the possibility to adjust ADC and T₂ values (by adding agar) almost independently of each other.

Pathways of Membrane Solubilization: A Structural Study of Model Lipid Vesicles Exposed to Classical Detergents

Understanding the pathways of solubilization of lipid membranes is of high importance for their use in biotechnology and industrial applications. Although lipid vesicle solubilization by classical detergents has been widely investigated, there are few systematic structural and kinetic studies where different detergents are compared under varying conditions. This study used small-angle X-ray scattering to determine the structures of lipid/detergent aggregates at different ratios and temperatures and studied the solubilization in time using the stopped-flow technique. Membranes composed of either of two zwitterionic lipids, DMPC or DPPC, and their interactions with three different detergents, sodium dodecyl sulfate (SDS), n-dodecyl-beta-maltoside (DDM), and Triton X-100 (TX-100), were tested. The detergent TX-100 can cause the formation of collapsed vesicles with a rippled bilayer structure that is highly resistant to TX-100 insertion at low temperatures, while at higher temperatures, it partitions and leads to the restructuring of vesicles. DDM also causes this restructuring into multilamellar structures at subsolubilizing concentrations. In contrast, partitioning of SDS does not alter the vesicle structure below the saturation limit. Solubilization is more efficient in the gel phase for TX-100 but only if the cohesive energy of the bilayer does not prevent sufficient partitioning of the detergent. DDM and SDS show less temperature dependence compared to TX-100. Kinetic measurements reveal that solubilization of DPPC largely occurs through a slow extraction of lipids, whereas DMPC solubilization is dominated by fast and burst-like solubilization of the vesicles. The final structures obtained seem to preferentially be discoidal micelles where the detergent can distribute in excess along the rim of the disc, although we do observe the formation of worm- and rodlike micelles in the case of solubilization of DDM. Our results are in line with the suggested theory that bilayer rigidity is the main factor influencing which aggregate is formed.

Enhancing the stability and homogeneity of non-ionic polymer nanodiscs by tuning electrostatic interactions

The nanodisc technology is increasingly used for structural studies on membrane proteins and drug delivery. The development of synthetic polymer nanodiscs and the recent discovery of non-ionic inulin-based polymers have significantly broadened the scope of nanodiscs. While the lipid exchange and size flexibility properties of the self-assembled polymer-based nanodiscs are valuable for various applications, the non-ionic polymer nanodiscs are remarkably unique in that they enable the reconstitution of any protein, protein-protein complexes, or drugs irrespective of their charge. However, the non-ionic nature of the belt could influence the stability and size homogeneity of inulin-based polymer nanodiscs. In this study, we investigate the size stability and homogeneity of nanodiscs formed by non-ionic lipid-solubilizing polymers using different biophysical methods. Polymer nanodiscs containing zwitterionic DMPC and different ratios of DMPC:DMPG lipids were made using anionic SMA-EA or non-ionic pentyl-inulin polymers. Non-ionic polymer nanodiscs made using zwitterionic DMPC lipids produced a very broad elution profile on SEC due to their instability in the column, thus affecting sample monodispersity which was confirmed by DLS experiments that showed multiple peaks. However, the inclusion of anionic DMPG lipids improved the stability as observed from SEC and DLS profiles, which was further confirmed by TEM images. Whereas, anionic SMA-EA-based DMPC-nanodiscs showed excellent stability and size homogeneity when solubilizing zwitterionic lipids. The stability of DMPC:DMPG non-ionic polymer nanodiscs is attributed to the inter-nanodisc repulsion by the anionic-DMPG that prevents the uncontrolled collision and fusion of nanodiscs. Thus, the reported results demonstrate the use of electrostatic interactions to tune the solubility, stability, and size homogeneity of non-ionic polymer nanodiscs which are important features for enabling functional and atomic-resolution structural studies of membrane proteins, other lipid-binding molecules, and water-soluble biomolecules including cytosolic proteins, nucleic acids and metabolites.

Beyond the standard model of solubilization: Non-ionic surfactants induce collapse of lipid vesicles into rippled bilamellar nanodiscs

HYPOTHESIS

Although solubilization of lipid membranes has been studied extensively, questions remain regarding the structural pathways and metastable structures involved. This study investigated whether the non-ionic detergent Triton X-100 follows the classical solubilization pathway or if intermediate nanostructures are formed.

EXPERIMENTS

Small angle X-ray and neutron scattering (SAXS/SANS) was used in combination with transmission electron cryo-microscopy and cryo-tomography to deduce the structure of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles and Triton X-100. Time-resolved SAXS and dynamic light scattering were used to investigate the kinetics of the process.

FINDINGS

Upon addition of moderate detergent amounts at low temperatures, the lipid vesicles implode into ordered rippled bilamellar disc structures. The bilayers arrange in a ripple phase to accommodate packing constraints caused by inserted TX-100 molecules. The collapse is suggested to occur through a combination of water structure destabilization by detergents flipping across the membrane and osmotic pressure causing interbilayer attraction internally. The subsequently induced ripples then stabilize the aggregates and prevent solubilization, supported by the observation that negatively charged vesicles undergo a different pathway upon TX-100 addition, forming large bicelles. The findings demonstrate the richness in assembly pathways of simple lipids and detergents and stimulate considerations for the use of certain detergents in membrane solubilization.

Small-Angle Neutron Scattering for Studying Lipid Bilayer Membranes

Small-angle neutron scattering (SANS) is a powerful tool for studying biological membranes and model lipid bilayer membranes. The length scales probed by SANS, being from 1 nm to over 100 nm, are well-matched to the relevant length scales of the bilayer, particularly when it is in the form of a vesicle. However, it is the ability of SANS to differentiate between isotopes of hydrogen as well as the availability of deuterium labeled lipids that truly enable SANS to reveal details of membranes that are not accessible with the use of other techniques, such as small-angle X-ray scattering. In this work, an overview of the use of SANS for studying unilamellar lipid bilayer vesicles is presented. The technique is briefly presented, and the power of selective deuteration and contrast variation methods is discussed. Approaches to modeling SANS data from unilamellar lipid bilayer vesicles are presented. Finally, recent examples are discussed. While the emphasis is on studies of unilamellar vesicles, examples of the use of SANS to study intact cells are also presented.

Events at the Interface: How Do Interfaces Modulate the Dynamics and Functionalities of Guest Molecules?

Chemical and biological interfaces are of various types, which could be between two materials of the same and/or different states, two phases of the same material, biological substrates and the outer environment, surfactant or polymeric membranes and the bulk, and so forth. Small-molecule guests frequently interact with such interfaces that decide their functionalities. The structural and behavioral properties undergo considerable characteristic changes, which control their final course of action in the targeted application. This Perspective will discuss mainly the chemical interfaces constituted by the surfactants, polymers, lipids, and nucleic acids and their impacts on the dynamics of small-molecule guests. Some specific and interesting phenomena and future prospects will be elucidated in this Perspective.