Microviscosity, encapsulation, and permeability of 2-ketooctanoic acid vesicle membranes

A primitive cell model involving Vesicles, microtubules and asters

HYPOTHESIS

Simple single-chain amphiphiles (sodium monododecyl phosphate, SDP) and organic small molecules (isopentenol, IPN), both of primitive relevance, are proved to have been the building blocks of protocells on the early Earth. How do SDP-based membrane and coexisting IPN come together in specific ways to produce more complex chemical entities? What kind of cell-like behavior can be endowed with this protocell model? These are important questions in the pre-life chemical origin scenario that have not been answered to date.

EXPERIMENTS

The phase behavior and formation mechanism of the aggregates for SDP/IPN/H2O ternary system were characterized and studied by different electron microscopy, fluorescent probe technology, DLS, IR, ESI-MS, SAXS, etc. The stability (freeze-thaw and wet-dry treatments) and cell-like behavior (chemical signaling communication) were tested via simulating particular scenarios.

FINDINGS

Vesicles, microtubules and asters phases resembling the morphology and structure of modern cells/organelles were obtained. The intermolecular hydrogen bonding is the main driving force for the emergence of the aggregates. The protocell models not only display remarkable stabilities by simulating the primordial Earth's diurnal temperature differences and ocean tides but also are able to exhibit cell-like behavior of chemical signaling transition.

The Spontaneous Vesicle-Micelle Transition in a Catanionic Surfactant System: A Chemical Trapping Study

Typically, the formation of vesicles requires the addition of salts or other additives to surfactant micelles. However, in the case of catanionic surfactants, unilamellar vesicles can spontaneously form upon dilution of the micellar solutions. Our study explores the intriguing spontaneous vesicle-to-micelle transition in catanionic surfactant systems, specifically cetyltrimethyl ammonium bromide (CTAB) and sodium octylsulfonate (SOS). To gain insights into the changes occurring at the interface, we employ a chemical trapping method to characterize variations in the molarities of sulfonate headgroups, water, and bromide ions during the transition. Our findings reveal the formation of ion pairs between the cationic component of CTAB and the anionic component of SOS, leading to tight interfacial packing in CTAB/SOS solutions. This interfacial packing promotes vesicle formation at low surfactant concentrations. Due to the significant difference in critical micelle concentration (cmc) between CTAB and SOS, an increase in the stoichiometric surfactant concentration results in a substantial rise in the SOS-to-CTAB ratio within the interfacial region. This enrichment of SOS in the aggregates triggers the transition from vesicles to micelles. Overall, our study may shed new light on the design of morphologies in catanionic and other surfactant systems.

Vesicle formation of single-tailed amphiphilic alkyltrimethylammonium bromides in water induced by dehydration-rehydration

We recently found that rough glass surfaces (RGSs) can in situ mediate the micelle-to-vesicle transition in single-component solutions of simple single-tailed amphiphiles (STAs), but only result in a relatively small number of vesicles coexisting with a large number of micelles. In the current work, a dehydration-rehydration (DHRH) method was used to induce the formation of vesicles in the single-component aqueous solutions of alkyltrimethylammonium bromides (C_nTABs, n = 12, 14, and 16), a kind of typical cationic STAs. That is, a C_nTAB micelle solution dropped on smooth glass surfaces (SGSs) was first dried, and the dried C_nTAB aggregates were then rehydrated in a monomer solution of C_nTAB. A large population of vesicles and even pure vesicle (or vesicle-dominated) systems were obtained, indicating that the DHRH process could more effectively induce the formation of STA vesicles than RGS in situ mediation. The so-obtained vesicles were characterized using DLS, FF-/cryo-TEM, AFM, SAXS, and fluorescence techniques, and their stability was determined. In addition, the effects of the conditions of DHRH and the chain length of C_nTABs on the vesicle formation were examined. It was demonstrated that the vesicles can be formed as long as the concentrations of C_nTABs in the rehydrated systems are higher than their critical micelle concentrations. The size and wall thickness of vesicles increase with an increase in chain length. A possible mechanism for the DHRH-induced vesicle formation is proposed: bilayer sheets are formed on SGSs during dehydration, and then detached from the SGSs to form vesicles during rehydration. A highly interdigitated structure of alkyl chains between two leaflets was identified in the bilayers, which probably is the origin of the formation and stability of STA vesicles.

Spontaneous vesicle formation and vesicle-to-α-gel transition in aqueous mixtures of sodium monododecylphosphate and guanidinium salts

Monoalkyl phosphates (MAPs) are one kind of important single-chain weak acid/salt type surfactants, but the understanding of their aggregation behavior in water is very limited due to their insolubility at room temperature. In the current work, the effect of guanidinium salts (GuSalts) on the solubility of sodium monododecylphosphate (SDP), a typical MAP, in water was determined at 25.0 °C, and the aggregation behavior of SDP in the GuSalt/water mixtures was investigated. The solubility of SDP is significantly improved by GuSalts including GuCl, GuSO4, GuSO3, GuPO4, and GuCO3 at 25.0 °C, resulting in an isotropic phase. SDP vesicles are spontaneously formed in the isotropic phase, with a critical vesicle concentration of ∼1.0 mM independent of the type of GuSalts. A "bridging dimer" mechanism is proposed to explain the formation of SDP vesicles. The SDP vesicles have a unilamellar structure with a size of ∼80 nm and an alkyl interdigitated degree of ∼25%, and exhibit size-selective permeability. Interestingly, a temperature-induced reversible transition between vesicles and α-gels was observed for the SDP/GuSalt/H2O systems when the SDP content is higher than 20 mM. The α-gels obtained are composed of vesicles and bilayer sheets, showing similar viscoelasticity to conventional gels, although their water content is as high as ∼98 wt%. The microviscosity of SDP vesicle membranes (ca. 35.79-49.34 mPa s at 25.0 °C) and the transition temperature between vesicles and α-gels (ca. 21.0-22.8 °C) are all dependent of the type of GuSalts. This work deepens the understanding of the aggregation behavior of MAPs and also provides valuable information for their practical applications.

Vesicle formation of single-chain amphiphilic 4-dodecylbenzene sulfonic acid in water and micelle-to-vesicle transition induced by wet-dry cycles

Simple single-chain amphiphiles (SCAs) can form vesicular structures in their single-component aqueous solutions, which has attracted great attention, but the understanding of their aggregation behavior is still limited. In this work, the aggregation behavior of 4-dodecylbenzene sulfonic acid (DBSA), a typical simple SCA, in water was investigated. The structure and properties of the aggregates formed were determined. In particular, the effect of wet-dry cycles on the structures of aggregates was examined. The mechanisms of aggregate formation and structural transition were discussed. It was found that the increase of DBSA concentration can drive the occurrence of a micelle-to-vesicle transition, showing a critical micelle concentration and critical vesicle concentration of ∼0.53 and 2.14 mM, respectively. The vesicles formed coexist with micelles in solution, with a unilamellar structure and ∼80 nm size, and exhibit size-selective permeability. In addition, the vesicles show remarkable stability upon long-term storage, exposure to high temperature, and freeze-thaw cycles. The H-bonding interaction between DBSA species and the interdigitated structure of alkyl chains in bilayers play a key role in the formation and stability of DBSA vesicles. Interestingly, it was found that the wet-dry cycle can induce a micelle-to-vesicle transition and an obvious increase in the size of the original vesicles, accompanied by the formation of some multilamellar vesicles. This work provides a better understanding of the aggregation behavior of simple SCAs in their single-component aqueous solutions.

Oil-Water Interfacial-Directed Spontaneous Self-Assembly of Natural Quillaja Saponin for Controlling Interface Permeability in Colloidal Emulsions

Assembly of amphiphiles at the interface of two immiscible fluids is of great scientific and technological interest in offering efficient routes to smart vehicles for functional deliveries. Natural Quillaja saponin (QS) has gathered widespread interest within the scientific community as a result of its unique interfacial properties. Herein, spontaneously interface-driven self-assembly (SIDSA) of QS at the oil-water interface was systematically studied by morphology and spectroscopy. It was found to self-assemble into a micrometer-scale network in helical fibers by combined intermolecular π-π stacking and hydrogen bonding among saponins at the liquid-liquid interface. From SIDSA, multilayer films on the surfaces of dispersed droplets were formed and enhanced emulsion stability. Interfacial QS-based films on droplet surfaces were also shown to confine interfacial diffusion processes by serving as transport barriers. Furthermore, they can be exploited to control the release of volatiles from the dispersed liquid phase by regulating the interface film, which is shown by molecular dynamics to occur through a hydrogen-bonded mechanism. These results provide new insight into the interfacial assembly structure that can enable unique controllable release in a broad range of applications in food, beverages, pharmaceuticals, and cosmetics.

Mass Transport in Coacervate-Based Protocell Coated with Fatty Acid under Nonequilibrium Conditions

Construction of protocell models from prebiotically plausible components to mimic the basic features or functions of living cells is still a challenge. In this work, we prepare a hybrid protocell model by coating sodium oleate on the coacervate droplet constituted by poly(l-lysine) and oligonucleotide and investigate the transport of different molecules under electric field. Results show that sodium oleate forms a layered viscoelastic membrane on the droplet surface, which is selectively permeable to small, polar molecules, such as oligolysine. As the droplet is stimulated at 10 V cm^-1, the oleate membrane slips along the direction of electric field while maintaining its integrity. Most of the molecules are still excluded under such conditions. As repetitive cycles of vacuolization occur at 20 V cm^-1, all molecules are internalized and sequestrated in the droplet through their specific pathways except enzyme, which anchors in the oleate membrane and is immune to electric field. Cascade enzymatic reactions are then carried out, and the product generated from the membrane exhibits a time-dependent concentration gradient across the droplet. Our work makes a step toward the nonequilibrium functionalization of synthetic protocells capable of biomimetic operations.