Spontaneous transformation from micelles to vesicles associated with sequential conversions of comprising amphiphiles within assemblies

Evolution of Proliferative Model Protocells Highly Responsive to the Environment

In this review, we discuss various methods of reproducing life dynamics using a constructive approach. An increase in the structural complexity of a model protocell is accompanied by an increase in the stage of reproduction of a compartment (giant vesicle; GV) from simple reproduction to linked reproduction with the replication of information molecules (DNA), and eventually to recursive proliferation of a model protocell. An encounter between a plural protic catalyst (C) and DNA within a GV membrane containing a plural cationic lipid (V) spontaneously forms a supramolecular catalyst (C@DNA) that catalyzes the production of cationic membrane lipid V. The local formation of V causes budding deformation of the GV and equivolume divisions. The length of the DNA strand influences the frequency of proliferation, associated with the emergence of a primitive information flow that induces phenotypic plasticity in response to environmental conditions. A predominant protocell appears from the competitive proliferation of protocells containing DNA with different strand lengths, leading to an evolvable model protocell. Recently, peptides of amino acid thioesters have been used to construct peptide droplets through liquid-liquid phase separation. These droplets grew, owing to the supply of nutrients, and were divided repeatedly under a physical stimulus. This proposed chemical system demonstrates a new perspective of the origins of membraneless protocells, i.e., the "droplet world" hypothesis. Proliferative model protocells can be regarded as autonomous supramolecular machines. This concept of this review may open new horizons of "evolution" for intelligent supramolecular machines and robotics.

Rapid and Sequential Dual Oxime Ligation Enables De Novo Formation of Functional Synthetic Membranes from Water-Soluble Precursors

Cell membranes define the boundaries of life and primarily consist of phospholipids. Living organisms assemble phospholipids by enzymatically coupling two hydrophobic tails to a soluble polar head group. Previous studies have taken advantage of micellar assembly to couple single-chain precursors, forming non-canonical phospholipids. However, biomimetic nonenzymatic coupling of two alkyl tails to a polar head-group remains challenging, likely due to the sluggish reaction kinetics of the initial coupling step. Here we demonstrate rapid de novo formation of biomimetic liposomes in water using dual oxime bond formation between two alkyl chains and a phosphocholine head group. Membranes can be generated from non-amphiphilic, water-soluble precursors at physiological conditions using micromolar concentrations of precursors. We demonstrate that functional membrane proteins can be reconstituted into synthetic oxime liposomes from bacterial extracts in the absence of detergent-like molecules.

Demolish and Rebuild: Controlling Lipid Self-Assembly toward Triggered Release and Artificial Cells

The ability to modulate the structures of lipid membranes, predicated on our nuanced understanding of the properties that drive and alter lipid self-assembly, has opened up many exciting biological applications. In this Perspective, we focus on two endeavors in which the same principles are invoked to achieve completely opposite results. On one hand, controlled liposome decomposition enables triggered release of encapsulated cargo through the development of synthetic lipid switches that perturb lipid packing in the presence of disease-associated stimuli. In particular, recent approaches have utilized artificial lipid switches designed to undergo major conformational changes in response to a range of target conditions. On the other end of the spectrum, the ability to drive the in situ formation of lipid bilayer membranes from soluble precursors is an important component in the establishment of artificial cells. This work has culminated in chemoenzymatic strategies that enable lipid manufacturing from simple components. Herein, we describe recent advancements in these two unique undertakings that are linked by their reliance on common principles of lipid self-assembly.

Rational design of dynamic imine surfactants for oil-water emulsions: Learning from oil-induced reversible dynamic imine bond formation

HYPOTHESIS

Dynamic imine surfactants (DIS) can be constructed by the formation of dynamic imine bonds (Dibs) between aromatic aldehydes and aliphatic amines in water. Because of the nature of Dibs in water, a thermodynamic equilibrium state was achieved between the DIS and aldehyde and amine precursors to form a dynamic combinatorial library (DCL). When the DIS served as sole emulsifier to form oil-H₂O emulsions, the precursors migrated between the H₂O phase and the oil phase, which altered the DCL equilibrium. The DIS concentration and emulsion stability also changed.

EXPERIMENTS

By mixing 4-(2-sulfobetaine-ethoxy)-benzaldehyde (SBBA) and aliphatic amines of C_nH_2n+1NH₂ (n = 4, BA; n = 6, HA; n = 8, OA; n = 10, DA) in water, four amphoteric DIS (SBBA-BA/HA/OA/DA) were prepared. Dib formation was characterized using ¹H NMR. The DIS surface activity was studied by surface tension and fluorescence probe methods. The reversible switching of DIS and its wormlike micelles were explored.

FINDINGS

SBBA-OA (or SBBA-DA) DIS was not a suitable emulsifier for stable hydrocarbon (HC)-H₂O emulsions. OA and DA were more soluble in the HC phase than the H₂O phase. The precursors of OA and DA migrated from the H₂O to the HC phase, and the thermodynamic equilibrium state of DCL shifted towards Dib dissociation. The Dib could be regenerated by HC phase removal. A novel strategy where volatile HC (such as pentane) was used as a trigger was developed to switch the DIS reversibly and its self-assemblies (such as wormlike micelles) in water without inorganic salt accumulation. The SBBA-HA (or SBBA-BA) DIS was a suitable emulsifier for stable emulsions because HA and BA were more soluble in the H₂O phase.

Pathway-Dependent Phase Transitions of Supramolecular Self-assemblies Containing Cationic Amphiphiles with Azobenzene and Disulfide Groups

There have been several attempts to construct supramolecular chemical systems that mimic the phase transitions in living systems. However, most of these phase transitions are one-to-one and induced by one stimulus or chemical; there have been few reports on the pathway-dependent phase transition of supramolecular self-assemblies in multi-step. To induce multistep phase transitions, molecular crystals were prepared that contained a cationic amphiphile bearing azobenzene and disulfide groups. A reducing agent caused the crystals to become vesicles, and adjacent, non-touching vesicles fused under UV and subsequent visible light. Adding a reducing agent to the worm-like aggregates that were generated after UV irradiation of the original crystals resulted in the growth of sheet-like aggregates. ¹ H NMR and fluorescence anisotropy measurements showed that a series of phase transitions was induced by changes in the phase structures from molecular conversions of the reactive amphiphiles. The multiple pathway-dependent phase transitions of supramolecular self-assemblies can provide a methodology for developing new stimuli-responsive materials that exhibit the desirable properties under specific circumstances from a systems chemistry viewpoint.

Viscoelastic Fluid Formed by Ultralong-Chain Erucic Acid-Base Ionic Liquid Surfactant Responds to Acid/Alkaline, CO2, and Light

As a leftover of grease processing, the efficient utilization of erucic acid is still a challenge. An alternative strategy is to develop erucic acid-derived surfactants. However, erucic acid-based ionic liquid surfactants were barely involved. Here, a novel ionic liquid surfactant, benzyltrimethylammonium erucate (ErBTA), was developed by a simple neutralization reaction, and its aggregations in the diluted and concentrated solution were systematically studied by surface tension, conductivity, rheology, and cryo-TEM techniques. The results showed that ErBTA has a very low metaling point (-7.03 °C) and possesses excellent water solubility (Krafft temperature <4 °C). ErBTA alone starts to form micelles at a very low concentration (0.028 mmol/L) and then to form worm-based viscoelastic fluid at 4.07 mmol/L without any additives, exhibiting excellent self-assembly ability and thickening ability. This viscoelastic fluid formed by ErBTA can simultaneously respond to three stimuli: common acid/alkaline, CO₂ gas, and light, accompanied by an interesting gel-sol conversion, reflecting microstructure transition from wormlike micelles to spherical micelles. Although in essence CO₂ and light also act as pH regulators in the current system, they provide more sophisticated approaches to tune pH. Such a viscoelastic fluid with the characteristics of easy availability, renewability of raw materials, the simplicity of fabrication, good water-solubility, and excellent thickening ability may be an attractive candidate for clean fracturing in oil/gas recovery and fluid drag reduction.

Oxidation-Induced Breakage of the Imine Bond and Aggregate Transition in a Se-Containing Dynamic Covalent Surfactant

Controlling the dynamic imine bonds upon a novel trigger except for pH and temperature is still a significant challenge. Here, a Se-containing imine-based dynamic covalent surfactant (HOBAB-BSeEA) was developed for the first time by mixing two precursors in situ: an asymmetric double-chain cationic surfactant bearing a formyl group at the terminal of one hydrophobic tail and a Se-containing amine (2-(benzylselanyl)ethan-1-amine) in order to confirm the effect of redox on the imine bonds. The imine bond in HOBAB-BSeEA can be regulated not only upon changing the pH as well as other common imine-based surfactants but also by oxidation. The conversion efficiency of imine bonds is closely related with the degree of oxidation and pH. Complete oxidation can decrease the conversion efficiency from ∼87 to 48%, which is comparable to the result of changing the pH from 10.0 to 7.0. With the formation and breaking of imine bonds, the surfactant can be reversibly switched between symmetric and asymmetric structures, accompanied by a morphological transition from vesicles to spherical micelles. Although oxidation cannot demolish all imine bonds, it can completely convert vesicles to spherical micelles, which is mainly ascribed to an increase in the polarity of the micellar microenvironment stemming from the oxidation of Se. However, this transition can only be achieved by reducing the pH to 5.0 instead of 7.0. Nile red loaded in HOBAB-BSeEA vesicles can be quickly, controllably, and step-by-step released upon oxidation stimulus but not pH. Understanding the mechanism of oxidation-induced breakage of imine bonds and disruption of vesicles would be useful in designing redox-responsive imine-based carriers that can unload cargoes according to the level of the local reactive oxygen species.

Lipids: chemical tools for their synthesis, modification, and analysis

Lipids remain one of the most enigmatic classes of biological molecules. Whereas lipids are well known to form basic units of membrane structure and energy storage, deciphering the exact roles and biological interactions of distinct lipid species has proven elusive. How these building blocks are synthesized, trafficked, and stored are also questions that require closer inspection. This tutorial review covers recent advances on the preparation, derivatization, and analysis of lipids. In particular, we describe several chemical approaches that form part of a powerful toolbox for controlling and characterizing lipid structure. We believe these tools will be helpful in numerous applications, including the study of lipid-protein interactions and the development of novel drug delivery systems.

Continuous expansion of a synthetic minimal cellular membrane

A critical aspect of a synthetic minimal cell is expansion of the surrounding boundary layer. This layer should consist of phospholipids (mimics) as these molecules assemble into a bilayer, creating a functional barrier with specific phospholipid species that are essential for membrane related processes. As a first step towards synthetic cells, an in vitro phospholipid biosynthesis pathway has been constructed that utilizes fatty acids as precursors to produce a wide variety of phospholipid species, thereby driving membrane growth. This now needs to be developed further into a sustainable expanding system, meanwhile keeping simplicity in mind. The non-enzymatic synthesis of phospholipid-like molecules forms a realistic alternative for natural enzymatic-based pathways, that nowadays can even support functional membrane proteins. Eventually, coupling to in vitro transcription/translation is required, for which efficient mechanisms of insertion and folding of the involved membrane proteins need to be developed. Such an integrated system will form a suitable foundation of a synthetic minimal cell that eventually can be coupled to other cellular processes such as division.

Synthetic Minimal Cell: Self-Reproduction of the Boundary Layer

A critical aspect in the bottom-up construction of a synthetic minimal cell is to develop an entity that is capable of self-reproduction. A key role in this process is the expansion and division of the boundary layer that surrounds the compartment, a process in which content loss has to be avoided and the barrier function maintained. Here, we describe the latest developments regarding self-reproduction of a boundary layer with a focus on the growth and division of phospholipid-based membranes in the context of a synthetic minimal cell.

Budding and Division of Giant Vesicles Linked to Phospholipid Production

The self-reproduction of supramolecular assemblies based on the synthesis and self-assembly of building blocks is a critical step towards the construction of chemical systems with autonomous, adaptive, and propagation properties. In this report, we demonstrate that giant vesicles can grow and produce daughter vesicles by synthesizing and incorporating phospholipids in situ from ad-hoc precursors. Our model involves acyl chain elongation via copper(I)-catalyzed azide-alkyne [3 + 2] cycloaddition reaction and the ensuing production of synthetic phospholipids to induce budding and division. In addition, the growth and budding of giant vesicles were compatible with the encapsulation and transfer of macromolecules as large as lambda phage DNA to the buds. This chemical system provides a useful model towards the implementation of cell-like compartments capable of propagation and transport of biological materials.

The hallmarks of living systems: towards creating artificial cells

Despite the astonishing diversity and complexity of living systems, they all share five common hallmarks: compartmentalization, growth and division, information processing, energy transduction and adaptability. In this review, we give not only examples of how cells satisfy these requirements for life and the ways in which it is possible to emulate these characteristics in engineered platforms, but also the gaps that remain to be bridged. The bottom-up synthesis of life-like systems continues to be driven forward by the advent of new technologies, by the discovery of biological phenomena through their transplantation to experimentally simpler constructs and by providing insights into one of the oldest questions posed by mankind, the origin of life on Earth.

Biomimetic Generation and Remodeling of Phospholipid Membranes by Dynamic Imine Chemistry

Biomimetic liposomes have a wide array of applications in several areas, ranging from medicinal chemistry to synthetic biology. Due to their biocompatibility and biological relevance, there is particular interest in the formation of synthetic phospholipid vesicles and the development of methods to tune their properties in a controlled manner. However, while true biological membranes are capable of responding to environmental stimuli by enzymatically remodeling their composition, synthetic liposomes are typically static once formed. Herein we report the chemoselective reaction of the natural amine-containing lysosphingomyelin with a series of long-chain aldehydes to form imines. This transformation results in the formation of phospholipid liposomes that are in dynamic equilibrium with the aldehyde-amine form. The reversibility of the imine linkage is exploited in the synthesis of vesicles that are capable of responding to external stimuli such as temperature or the addition of small molecules.

Growing Membranes In Vitro by Continuous Phospholipid Biosynthesis from Free Fatty Acids

One of the key aspects that defines a cell as a living entity is its ability to self-reproduce. In this process, membrane biogenesis is an essential element. Here, we developed an in vitro phospholipid biosynthesis pathway based on a cascade of eight enzymes, starting from simple fatty acid building blocks and glycerol 3-phosphate. The reconstituted system yields multiple phospholipid species that vary in acyl-chain and polar headgroup compositions. Due to the high fidelity and versatility, complete conversion of the fatty acid substrates into multiple phospholipid species is achieved simultaneously, leading to membrane expansion as a first step toward a synthetic minimal cell.

Production of dynamic lipid bilayers using the reversible thiol–thioester exchange reaction

Coupling of phospholipid precursors using the reversible thiol–thioester exchange reaction enables downstream remodeling and functionalization.

De novo vesicle formation and growth: an integrative approach to artificial cells

The assembly of synthetic membranes provides a powerful tool to reconstruct the structure and function of living cells.

The assembly of artificial cells provides a novel strategy to reconstruct life's functions and shed light on how life emerged on Earth and possibly elsewhere. A major challenge to the development of artificial cells is the establishment of simple methodologies to mimic native membrane generation. An ambitious strategy is the bottom-up approach, which aims to systematically control the assembly of highly ordered membrane architectures with defined functionality. This perspective will cover recent advances and the current state-of-the-art of minimal lipid architectures that can faithfully reconstruct the structure and function of living cells. Specifically, we will overview work related to the de novo formation and growth of biomimetic membranes. These studies give us a deeper understanding of the nature of living systems and bring new insights into the origin of cellular life.

Sequential dynamic structuralisation by in situ production of supramolecular building blocks

We herein report an example of sequential dynamic higher structuralisation from disordered droplets into higher-ordered structures, resulting in the formation of a hydrogel. This is a potential model for understanding complicated biosystems in which lower-ordered structures evolve into higher-ordered structures that interact with specific molecules.

Aqueous dispersions of nanostructures formed through the self-assembly of iminolipids with exchangeable hydrophobic termini

The addition of amines to an aldehyde surfactant, which was designed to be analogous to didodecyldimethylammonium bromide, gave exchangeable "iminolipids" that self-assembled to give stable aqueous dispersions of nano-sized vesicles. For example, sonication of suspensions of the n-hexylamine-derived iminolipid gave vesicles 50 to 200 nm in diameter that could encapsulate a water-soluble dye. The iminolipids could undergo dynamic exchange with added amines, and the resulting equilibrium constants (K_rel) were quantified by ¹H NMR spectroscopy. In the absence of lipid self-assembly, in CDCl₃, the assayed primary amines gave very similar K_rel values. However in D₂O the value of K_rel generally increased with increasing amine hydrophobicity, consistent with partitioning into a self-assembled bilayer. Amines with aromatic groups showed significantly higher values of K_rel in D₂O compared to similarly hydrophobic alkylamines, suggesting that π-π interactions favor lipid self-assembly. Given this synergistic relationship, π-rich pyrenyliminolipids were created and used to exfoliate graphite, leading to aqueous dispersions of graphene flakes that were stable over several months.

A Novel Approach to Making the Gas-Filled Liposome Real: Based on the Interaction of Lipid with Free Nanobubble within the Solution

Nanobubbles with a size less than 1 μm could make a promising application in ultrasound molecular imaging and drug delivery. However, the fabrication of stable gas encapsulation nanobubbles is still challenging. In this study, a novel method for preparation of lipid- encapsulated nanobubbles was reported. The dispersed phospholipid molecules in the prefabricated free nanobubbles solution can be assembled to form controllable stable lipid encapsulation gas-filled ultrasound-sensitive liposome (GU-Liposome). The optimized preparation parameters and formation mechanism of GU-Liposome were investigated in detail. Results showed that this type of GU-Liposome had mean diameter of 194.4 ± 6.6 nm and zeta potential of -25.2 ± 1.9 mV with layer by layer self-assembled lipid structure. The acoustic imaging analysis in vitro indicated that ultrasound imaging enhancement could be acquired by both perfusion imaging and accumulation imaging. The imaging enhancement level and duration time was related with the ratios of lipid to gas in the GU-Liposome structure. All in all, by this novel and controllable nanobubble construction technique, it will broaden the future theranostic applications of nanobubbles.

Towards self-assembled hybrid artificial cells: novel bottom-up approaches to functional synthetic membranes

There has been increasing interest in utilizing bottom-up approaches to develop synthetic cells. A popular methodology is the integration of functionalized synthetic membranes with biological systems, producing "hybrid" artificial cells. This Concept article covers recent advances and the current state-of-the-art of such hybrid systems. Specifically, we describe minimal supramolecular constructs that faithfully mimic the structure and/or function of living cells, often by controlling the assembly of highly ordered membrane architectures with defined functionality. These studies give us a deeper understanding of the nature of living systems, bring new insights into the origin of cellular life, and provide novel synthetic chassis for advancing synthetic biology.

Emergent properties arising from the assembly of amphiphiles. Artificial vesicle membranes as reaction promoters and regulators

This article deals with artificial vesicles and their membranes as reaction promoters and regulators. Among the various molecular assemblies which can form in an aqueous medium from amphiphilic molecules, vesicle systems are unique. Vesicles compartmentalize the aqueous solution in which they exist, independent on whether the vesicles are biological vesicles (existing in living systems) or whether they are artificial vesicles (formed in vitro from natural or synthetic amphiphiles). After the formation of artificial vesicles, their aqueous interior (the endovesicular volume) may become - or may be made - chemically different from the external medium (the exovesicular solution), depending on how the vesicles are prepared. The existence of differences between endo- and exovesicular composition is one of the features on the basis of which biological vesicles contribute to the complex functioning of living organisms. Furthermore, artificial vesicles can be formed from mixtures of amphiphiles in such a way that the vesicle membranes become molecularly, compositionally and organizationally highly complex, similarly to the lipidic matrix of biological membranes. All the various properties of artificial vesicles as membranous compartment systems emerge from molecular assembly as these properties are not present in the individual molecules the system is composed of. One particular emergent property of vesicle membranes is their possible functioning as promoters and regulators of chemical reactions caused by the localization of reaction components, and possibly catalysts, within or on the surface of the membranes. This specific feature is reviewed and highlighted with a few selected examples which range from the promotion of decarboxylation reactions, the selective binding of DNA or RNA to suitable vesicle membranes, and the reactivation of fragmented enzymes to the regulation of the enzymatic synthesis of polymers. Such type of emergent properties of vesicle membranes may have been important for the prebiological evolution of protocells, the hypothetical compartment systems preceding the first cells in those chemical and physico-chemical processes that led to the origin of life.

In situ vesicle formation by native chemical ligation

Phospholipid vesicles are of intense fundamental and practical interest, yet methods for their de novo generation from reactive precursors are limited. A non-enzymatic and chemoselective method to spontaneously generate phospholipid membranes from water-soluble starting materials would be a powerful tool for generating vesicles and studying lipid membranes. Here we describe the use of native chemical ligation (NCL) to rapidly prepare phospholipids spontaneously from thioesters. While NCL is one of the most popular tools for synthesizing proteins and nucleic acids, to our knowledge this is the first example of using NCL to generate phospholipids de novo. The lipids are capable of in situ synthesis and self-assembly into vesicles that can grow to several microns in diameter. The selectivity of the NCL reaction makes in situ membrane formation compatible with biological materials such as proteins. This work expands the application of NCL to the formation of phospholipid membranes.