Two distinct mechanisms of vesicle-to-micelle and micelle-to-vesicle transition are mediated by the packing parameter of phospholipid-detergent systems

NH125 kills methicillin-resistant Staphylococcus aureus persisters by lipid bilayer disruption

BACKGROUND

NH125, a known WalK inhibitor kills MRSA persisters. However, its precise mode of action is still unknown.

METHODS & RESULTS

The mode of action of NH125 was investigated by comparing its spectrum of antimicrobial activity and its effects on membrane permeability and giant unilamellar vesicles (GUVs) with walrycin B, a WalR inhibitor and benzyldimethylhexadecylammonium chloride (16-BAC), a cationic surfactant. NH125 killed persister cells of a variety of Staphylococcus aureus strains. Similar to 16-BAC, NH125 killed MRSA persisters by inducing rapid membrane permeabilization and caused the rupture of GUVs, whereas walrycin B did not kill MRSA persisters or induce membrane permeabilization and did not affect GUVs.

CONCLUSION

NH125 kills MRSA persisters by interacting with and disrupting membranes in a detergent-like manner.

Changes in Interfacial Tension of a Lipid Membrane Formed at the Water/Chloroform Interface upon DNA Complex Formation

Changes in the interfacial tension of a lipid monolayer membrane formed at the water/chloroform interface upon DNA addition were measured using the quasi-elastic laser scattering (QELS) method. A cationic lipid, N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTAP), as well as zwitterionic lipids, 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), were used to form lipid monolayer membranes at different calcium ion concentrations. A rapid decrease of the interfacial tension resulting from electrostatic interactions between DOTAP and DNA was observed within 10 s. However, such rapid decreases were not observed for DOPE or DOPC. A decrease in the interfacial tension was exhibited by DOPE after 1000 s from the addition of DNA, which may be due to an overall structural change in the DOPE membrane. A DOTAP/DOPE complex system showed behaviors attributable to both DOTAP and DOPE, whereas the behavior of the DOTAP/DOPC system resembled that of DOPC alone. The current results provide a model for the so-called lipoplex carriers used in gene therapy.

Steroidal Surfactants: Detection of Premicellar Aggregation, Secondary Aggregation Changes in Micelles, and Hosting of a Highly Charged Negative Substance

CHAPSO and CHAPS are zwitterionic surfactants derived from bile salts which are usually employed in protein purification and for the preparation of liposomes and bicelles. Despite their spread use, there are significant discrepancies on the critical concentrations that determine their aggregation behavior. In this work, we study the interaction between these surfactants with the negative fluorescent dye pyranine (HPTS) by absorbance, fluorescence, and infrared spectrometry to establish their concentration-dependent aggregation. For the studied surfactants, we detect three critical concentrations showing their concentration-dependent presence as a monomeric form, premicellar aggregates, micelles, and a second type of micelle in aqueous medium. The nature of the interaction of HPTS with the surfactants was studied using analogues of their tails and the negative bile salt taurocholate (TC) as reference for the sterol ring. The results indicate that the chemical groups involved are the hydroxyl groups of the polar face of the sterol ring and the sulfonate groups of the dye. This interaction causes not only the incorporation of the negative dye in CHAPSO and CHAPS micelles but also its association with their premicellar aggregates. Surprisingly, this hosting behavior for a negative charged molecule was also detected for the negative bile salt TC, bypassing, in this way, the electrostatic repulsion between the guest and the host.

Ultrasensitive ROS-Responsive Coassemblies of Tellurium-Containing Molecules and Phospholipids

Reactive oxygen species (ROS) play crucial roles in cell signaling and redox homeostasis and are strongly related to metabolic activities. The increase of the ROS concentration in organisms can result in several diseases, such as cardiovascular diseases and cancer. The concentration of ROS in biologically relevant conditions is typically as low as around tens of micromolars to 100 μM H2O2, which makes it necessary to develop ultrasensitive ROS-responsive systems. A general approach is reported here to fabricate an ultrasensitive ROS-responsive system via coassembly between tellurium-containing molecules and phospholipids, combining the ROS-responsiveness of tellurium and the biocompatibility of phospholipids. By using dynamic light scattering, transmission electron microscopy, scanning electron microscopy, and NMR spectra, coassembly behaviors and the responsiveness of the coassemblies have been investigated. These coassemblies can respond to 100 μM H2O2, which is a biologically relevant ROS concentration, and demonstrate reversible redox properties.

Aqueous worm gels can be reconstituted from freeze-dried diblock copolymer powder

Worm-like diblock copolymer nanoparticles comprising poly(glycerol monomethacrylate) (PGMA) as a stabilizer block and poly(2-hydroxypropyl methacrylate) (PHPMA) as a core-forming block were readily synthesized at 10% w/w solids via aqueous dispersion polymerization at 70 °C using Reversible Addition-Fragmentation chain Transfer (RAFT) chemistry. On cooling to 20 °C, soft transparent free-standing gels are formed due to multiple inter-worm interactions. These aqueous PGMA-PHPMA diblock copolymer worms were freeze-dried, then redispersed in water with cooling to 3-5 °C before warming up to 20 °C; this protocol ensures molecular dissolution of the copolymer chains, which aids formation of a transparent aqueous gel. Rheology, SAXS and TEM studies confirm that such reconstituted gels comprise formed PGMA-PHPMA copolymer worms and they possess essentially the same physical properties determined for the original worm gels prior to freeze-drying. Such worm gel reconstitution is expected to be highly beneficial in the context of various biomedical applications, since it enables worm gels to be readily prepared using a wide range of cell growth media as the continuous aqueous phase.

Membrane remodeling processes induced by phospholipase action

Important cellular events such as division require drastic changes in the shape of the membrane. These remodeling processes can be triggered by the binding of specific proteins or by changes in membrane composition and are linked to phospholipid metabolism for which dedicated enzymes, named phospholipases, are responsible. Here wide-field fluorescence microscopy is used to visualize shape changes induced by the action of phospholipase A1 on dye-labeled supported membranes of POPC (1-palmitoyl-2-oleoly-sn-glycero-3-phosphocholine). Time-lapse imaging demonstrates that layers either shrink and disappear or fold and collapse into vesicles. These vesicles can undergo further transformations such as budding, tubulation, and pearling within 5 min of formation. Using dye-labeled phospholipases, we can monitor the presence of the enzyme at specific positions on the membrane as the shape transformations occur. Furthermore, incorporating the products of hydrolysis into POPC membranes is shown to induce transformations similar to those observed for enzyme action. The results suggest that phospholipase-mediated hydrolysis plays an important role in membrane transformations by altering the membrane composition, and a model is proposed for membrane curvature based on the presence and shape of hydrolysis products.

Surface dilution kinetics of phospholipase A(2) catalyzed lipid-bilayer hydrolysis

Phospholipase A2 (PLA2) enzymes catalyze hydrolysis of phospholipids in membranes. Elucidation of the kinetics of interfacial enzymatic activity is best accomplished by investigating the interface substrate concentration dependence of the activity for which appropriate diluents are required. PLA2 is stereoselective toward the L_enantiomers of phospholipids. A novel approach employing D_phospholipids as diluents to perform surface dilution kinetic studies of PLA2 is presented. Activity of bee venom PLA2 at mixed L+D_DPPC (dipalmitoylphosphatidylcholine) bilayer interfaces was measured as a function of substrate L_DPPC mole fraction and vesicle concentration using a sensitive fluorescence assay. A model for interface enzymatic activity based on the three-step kinetic scheme of (i) binding of PLA2 to the bilayer interface, (ii) binding of a lipid to PLA2 at the interface, and (iii) hydrolysis was applied to the hydrolysis data. Activity profiles showed that D_enantiomers also bind to the enzyme but resist hydrolysis. Activity dependences on vesicle and substrate concentrations could be disentangled, bringing resolution to an outstanding problem in membrane hydrolysis of separating the effects of the three steps. Individual values of the kinetic parameters of the model, including the vesicle-PLA2 equilibrium dissociation constant of step (i), interface Michaelis-Menten-Henri constant for L and D_DPPC of step (ii), and the rate constant for interface hydrolysis, step (iii), were obtained as solutions to equations resulting from fitting the model to the data.

The mechanism of detergent solubilization of lipid bilayers

Multiple data are available on the self-assembly of mixtures of bilayer-forming amphiphiles, particularly phospholipids and micelle-forming amphiphiles, commonly denoted detergents. The structure of such mixed assemblies has been thoroughly investigated, described in phase diagrams, and theoretically rationalized in terms of the balance between the large spontaneous curvature of the curvophilic detergent and the curvophobic phospholipids. In this critical review, we discuss the mechanism of this process and try to explain the actual mechanism involved in solubilization. Interestingly, membrane solubilization by some detergents is relatively slow and the common attribute of these detergents is that their trans-bilayer movement, commonly denoted flip-flop, is very slow. Only detergents that can flip into the inner monolayer cause relatively rapid solubilization of detergent-saturated bilayers. This occurs via the following sequence of events: 1), relatively rapid penetration of detergent monomers into the outer monolayer; 2), trans-membrane equilibration of detergent monomers between the two monolayers; 3), saturation of the bilayer by detergents and consequent permeabilization of the membrane; and 4), transition of the whole bilayer to thread-like mixed micelles. When the detergent cannot flip to the inner monolayer, the outer monolayer becomes unstable due to mass imbalance between the monolayers and inclusion of the curvophilic detergent molecules in a flat surface. Consequently, the outer monolayer forms mixed micellar structures within the outer monolayer. Shedding of these micelles into the aqueous solution results in partial solubilization. The consequent leakage of detergent into the liposome results in trans-membrane equilibration of detergent and subsequent micellization through the rapid bilayer-saturation mechanism.

Quatsomes: vesicles formed by self-assembly of sterols and quaternary ammonium surfactants

Thermodynamically stable nanovesicle structures are of high interest for academia and industry in a wide variety of application fields, ranging from preparation of nanomaterials to nanomedicine. Here, we show the ability of quaternary ammonium surfactants and sterols to self-assemble, forming stable amphiphilic bimolecular building-blocks with the appropriate structural characteristics to form in aqueous phases, closed bilayers, named quatsomes, with outstanding stability, with time and temperature. The molecular self-assembling of cholesterol and surfactant cetyltrimethylammonium bromide (CTAB) was studied by quasi-elastic light scattering, cryogenic transmission electron microscopy, turbidity (optical density) measurements, and molecular dynamic simulations with atomistic detail, upon varying the cholesterol-to-surfactant molar ratio. As pure species, CTAB forms micelles and insoluble cholesterol forms crystals in water. However, our molecular dynamic simulations reveal that the synergy between CTAB and cholesterol molecules makes them self-assemble into bimolecular amphiphiles and then into bilayers in the presence of water. These bilayers have the same structure of those formed by double-tailed unimolecular amphiphiles.

Formation of self-aggregated structures of different types in water of chiral polymerizable amphiphiles from L-tyrosine and L-phenylalanine

Sodium salts of maleamic acid derivatives from lauryl esters of L-tyrosine (MTNa) and L-phenylalanine (MPNa) were synthesized and characterized. The aggregated structures of MTNa and MPNa in water were investigated, employing several independent methods. MPNa showed secondary aggregated structures in contrast to MTNa at concentrations of >1 × 10(-3) M. The results from dynamic light scattering, transmittance, conductivity, and viscosity measurements suggested the formation of aggregated structures of different types in MTNa and MPNa solutions. The measured fluorescence anisotropy (r) at 0.180 of the fluoroprobe, 1,6-diphenyl-1,3,5-hexatriene (DPH), and the d spacing of 38 Å from small-angle X-ray diffraction (SAXD) experiments confirmed the bilayer structures in MPNa. Scanning electron microscope (SEM) images provided the morphological features. The emulsion produced using MPNa solution was more stable. The confocal fluorescence microscopy image of the emulsion from MPNa confirmed the entrapment of water-soluble dye, rhodamine. The models of MTNa and MPNa molecules and the aggregated structures are presented.

Light-induced disassembly of self-assembled vesicle-capped nanotubes observed in real time

Molecular self-assembly is the basis for the formation of numerous artificial nanostructures. The self-organization of peptides, amphiphilic molecules composed of fused benzene rings and other functional molecules into nanotubes is of particular interest. However, the design of dynamic, complex self-organized systems that are responsive to external stimuli remains a significant challenge. Here, we report self-assembled, vesicle-capped nanotubes that can be selectively disassembled by irradiation. The walls of the nanotubes are 3-nm-thick bilayers and are made from amphiphilic molecules with two hydrophobic legs that interdigitate when the molecules self-assemble into bilayers. In the presence of phospholipids, a phase separation between the phospholipids and the amphiphilic molecules creates nanotubes, which are end-capped by vesicles that can be chemically altered or removed and reattached without affecting the nanotubes. The presence of a photoswitchable and fluorescent core in the amphiphilic molecules allows fast and highly controlled disassembly of the nanotubes on irradiation, and distinct disassembly processes can be observed in real time using fluorescence microscopy.

Use of simple kinetic and reaction-order measurements for the evaluation of the mechanism of surfactant-liposome interactions

Surfactant-liposome interactions have been previously studied through different methods and techniques. We present here a classical physical chemistry study on liposome solutions added to destabilizing agents at concentrations well above the solubilization concentration, which enable us to draw useful and interesting conclusions about the mechanism of surfactant-induced liposomal breakdown by simply exploiting the kinetics and the reaction order of the liposomal content release. In such excess of surfactant, the mechanism of surfactant-induced rupture of the liposomes has been demonstrated to be different from that proposed for low surfactant concentrations. Thus, depending on the surfactant concentration, two prevailing processes have been evidenced: (i) a cooperative mechanism that implies the assembly of a critical number of surfactant molecules to trigger the formation of a channel and therefore the release of the liposomal content and (ii) a mechanism driven by direct interaction of the surfactant molecules with the lipids that causes the complete solubilization of the liposomes. The former mechanism occurs at low surfactant concentrations, whereas the latter occurs at higher concentrations and above the CMC of the surfactants. The effect of different guests embedded into the liposomal bilayer on the mechanism of surfactant-induced liposomal breakdown has been compared by using the second-order rate constants measured for the liposome breakdown process.