Surfactant assemblies and their various possible roles for the origin(s) of life

New Insights on the Chemical Origin of Life: The Role of Aqueous Polymerization of N-carboxyanhydrides (NCA)

At the origin, the emergence of proteins was based on crucial prebiotic stages in which simple amino acids-based building blocks spontaneously evolved from the prebiotic soup into random proto-polymers called protoproteins. Despite advances in modern peptide synthesis, these prebiotic chemical routes to protoproteins remain puzzling. We discuss in this perspective how polymer science and systems chemistry are reaching a point of convergence in which simple monomers called N-carboxyanhydrides would be able to form such protoproteins via the emergence of a protometabolic cycle involving aqueous polymerization and featuring macromolecular Darwinism behavior.

Hydrolytic nanozymes: Preparation, properties, and applications

Hydrolytic nanozymes, as promising alternatives to hydrolytic enzymes, can efficiently catalyze the hydrolysis reactions and overcome the operating window limitations of natural enzymes. Moreover, they exhibit several merits such as relatively low cost, easier recovery and reuse, improved operating stability, and adjustable catalytic properties. Consequently, they have found relevance in practical applications such as organic synthesis, chemical weapon degradation, and biosensing. In this review, we highlight recent works addressing the broad topic of the development of hydrolytic nanozymes. We review the preparation, properties, and applications of six types of hydrolytic nanozymes, including AuNP-based nanozymes, polymeric nanozymes, surfactant assemblies, peptide assemblies, metal and metal oxide nanoparticles, and MOFs. Last, we discuss the remaining challenges and future directions. This review will stimulate the development and application of hydrolytic nanozymes.

Chirality-Selected Coacervate by Chiral Gemini Surfactant

Chiral surfactants present opportunities to self-assemble into supramolecules with a chiral trait; however, the effects of stereochemistry on the formation of simple coacervates remain unclear. Here, we investigate the chirality-selected phase behavior in mixtures of chiral gemini surfactant 1,4-bis(dodecyl-N,N-dimethylammonium bromide)-2,3-butanediol (12-4(OH)2-12) with an oppositely charged chiral mandelic acid (MA). It demonstrates that altering the chirality of surfactants yields a heightened ability to regulate the phase behavior, leading to the formation of three different network-like structures, i.e., wormlike micelle, coacervate, and hydrogel, in the racemate, enantiomer, and mesomer, respectively. The different aggregate structures arise from the intermolecular and intramolecular hydrogen-bond interactions of the two hydroxyl groups located at stereogenic centers. Intriguingly, although they contain similar microstructures, the solid-like hydrogel and liquid-like wormlike micelle show similar low hydration ability and have no encapsulation capability, whereas only coacervate formed by the enantiomers of 12-4(OH)2-12 displays liquid-like characteristics, strong capacity to sequester diverse solutes, and high affinity for tightly bound water simultaneously. These findings further highlight the unique and advantageous properties of coacervates as a promising model for exploring the biological process and understanding how chirality plays a crucial role in early life scenarios and cell evolution at the molecular level.

Phosphate-Based Amphiphile and Lipidated Lysine Assemble into Superior Protocellular Membranes over Carboxylate and Sulfate-Based Systems: A Potential Missing Link between Prebiotic and the Modern Era?

Amphiphiles are among the most extensively studied building blocks that self-assemble into cell-like compartments. Most literature suggested that the building blocks/amphiphiles of early Earth (fatty acid-based membrane) were much simpler than today's phospholipids. To establish the bridge between the prebiotic fatty acid era and the modern phospholipid era, the investigation and characterization of alternate building blocks that form protocellular membranes are necessary. Herein, we report the potential prebiotic synthesis of alkyl phosphate, alkyl carboxylate, and alkyl sulfate amphiphiles (anionic) using dry-down reactions and demonstrate a more general role of cationic amino acid-based amphiphiles to recruit the anionic amphiphiles via ion-pair, hydrogen bonding, and hydrophobic interactions. The formation and self-assembly of the catanionic (mixed) amphiphilic system to vesicular morphology were characterized by turbidimetric, dynamic light scattering, transmission electron microscopy, fluorescence lifetime imaging microscopy, and glucose encapsulation experiments. Further experiments suggest that the phosphate-based vesicles were more stable than the alkyl sulfate and alkyl carboxylate-based systems. Moreover, the alkyl phosphate system can form vesicles at prebiotically relevant acidic pH (5.0), while alkyl carboxylate mainly forms cluster-type aggregates. An extended supramolecular polymer-type network formation via H-bonding and ion-pair interactions might order the membrane interface and stabilize the phosphate-based vesicles. The results suggest that phosphate-based amphiphiles might be a superior successor to fatty acids as early compartment building blocks. The work highlights the importance of previously unexplored building blocks that participate in protocellular membrane formation to encapsulate important precursors required for the functions of early life.

Sponge-phase Lipid Droplets as Synthetic Organelles: An Ultrafast Study of Hydrogen Bonding and Interfacial Environments

Bottom-up design of biomimetic organelles has gained recent attention as a route towards understanding the transition between non-living matter and life. Despite various artificial lipid membranes being developed, the specific relations between lipid structure, composition, interfacial properties, and morphology are not currently understood. Sponge-phase droplets contain dense, nonlamellar lipid bilayer networks that capture the complexities of the endoplasmic reticulum (ER), making them ideal artificial models of such organelles. Here, we combine ultrafast two-dimensional infrared (2D IR) spectroscopy and molecular dynamics simulations to investigate the interfacial H-bond networks in sponge-phase droplets composed of glycolipid and nonionic detergents. In the sponge phase, the interfacial environments are more hydrated and water molecules confined to the nanometer-scale aqueous channels in the sponge phase exhibit dynamics that are significantly slower compared to bulk water. Surfactant configurations and microscopic phase separation play a dominant role in determining membrane curvature and slow dynamics observed in the sponge phase. The studies suggest that H-bond networks within the nanometer-scale channels are disrupted not only by confinement but also by the interactions of surfactants, which extend 1-2 nm from the bilayer surface. The results provide a molecular-level description for controlling phase and morphology in the design of synthetic lipid organelles.

From autocatalysis to survival of the fittest in self-reproducing lipid systems

Studying autocatalysis - in which molecules catalyse their own formation - might help to explain the emergence of chemical systems that exhibit traits normally associated with biology. When coupled to other processes, autocatalysis can lead to complex systems-level behaviour in apparently simple mixtures. Lipids are an important class of chemicals that appear simple in isolation, but collectively show complex supramolecular and mesoscale dynamics. Here we discuss autocatalytic lipids as a source of extraordinary behaviour such as primitive chemical evolution, chemotaxis, temporally controllable materials and even as supramolecular catalysts for continuous synthesis. We survey the literature since the first examples of lipid autocatalysis and highlight state-of-the-art synthetic systems that emulate life, displaying behaviour such as metabolism and homeostasis, with special consideration for generating structural complexity and out-of-equilibrium models of life. Autocatalytic lipid systems have enormous potential for building complexity from simple components, and connections between physical effects and molecular reactivity are only just beginning to be discovered.

Abiotic synthesis with plausible emergence for primitive phospholipid in aqueous microdroplets

Phospholipids are the protective layer of modern cells, but it is challenging for the formation of phospholipids that require a simple abiotic synthesis before the advent of primitive cells. Here, we reported the abiotic synthesis for lysophosphatidic acids (LPAs) with prebiotically plausible reactants in aqueous microdroplets under ambient conditions. The LPAs formation is carried out by fusing two microdroplets streams: one contains glycerol and pyrophosphate in water and the other one contains fatty acids in acetonitrile. Compared with the bulk solution, LPAs were generated in microdroplets without the addition of catalyst and heating. Conditions of reactant concentrations and microdroplet size varied and suggested that LPAs formation occurred near or at the microdroplet surface. The LPAs formation also showed chemoselective toward on chain-length of fatty acids. Finally, the formation of LPAs underwent two-step reactions with glycerol phosphorylation eliminating one water molecule followed by esterification with fatty acids. These results also implicated that pyrophosphate functioned as both catalysts and precursors in prebiotic LPAs synthesis. The approach using fusion aqueous microdroplets has desirable features in studying the substance exchange and interaction in atmosphere.

Exploring the Lipid World Hypothesis: A Novel Scenario of Self-Sustained Darwinian Evolution of the Liposomes

According to the Lipid World hypothesis, life on Earth originated with the emergence of amphiphilic assemblies in the form of lipid micelles and vesicles (liposomes). However, the mechanism of appearance of the information molecules (ribozymes/RNA) accompanying that process, considered obligatory for Darwinian evolution, is unclear. We propose a novel scenario of self-sustained Darwinian evolution of the liposomes driven by ever-present natural phenomena: solar UV radiation, day/night cycle, gravity, and the formation of liposomes in an aqueous media. The central tenet of this scenario is the liposomes' encapsulation of the heavy solutes, followed by their gravitational submerging in the water. The submerged liposomes, being protected from the damaging UV radiation, acquire the longevity necessary for autocatalytic replication of amphiphiles, their mutation, and the selection of those amphiphilic assemblies that provide the greatest membrane stability. These two sets of adaptive compositional information (heavy content and amphiphilic assemblies design) generate a population of liposomes with self-replication/reproduction properties, which are amendable to mutation, inheritance, and selection, thereby establishing Darwinian progression. Temporary and spatial expansion of this liposomal population will provide the basis for the next evolutionary step-a transition of accidentally entrapped RNA precursor molecules into complex functional molecules, such as ribozymes/RNA.

Lipidated Lysine and Fatty Acids Assemble into Protocellular Membranes to Assist Regioselective Peptide Formation: Correlation to the Natural Selection of Lysine over Nonproteinogenic Lower Analogues

The self-assembly of prebiotically plausible amphiphiles (fatty acids) to form a bilayer membrane for compartmentalization is an important factor during protocellular evolution. Such fatty acid-based membranes assemble at relatively high concentrations, and they lack robust stability. We have demonstrated that a mixture of lipidated lysine (cationic) and prebiotic fatty acids (decanoic acid, anionic) can form protocellular membranes (amino acid-based membranes) at low concentrations via electrostatic, hydrogen bonding, and hydrophobic interactions. The formation of vesicular membranes was characterized by dynamic light scattering (DLS), pyrene and Nile Red partitioning, cryo-transmission electron microscopy (TEM) images, and glucose encapsulation studies. The lipidated nonproteinogenic analogues of lysine (Lys), such as ornithine (Orn) and 2,4-diaminobutyric acid (Dab), also form membranes with decanoate (DA). Time-dependent turbidimetric and ¹H NMR studies suggested that the Lys-based membrane is more stable than the membranes prepared from nonproteinogenic lower analogues. The Lys-based membrane embeds a model acylating agent (aminoacyl-tRNA mimic) and facilitates the colocalization of substrates to support regioselective peptide formation via the α-amine of Lys. These membranes thereby assist peptide formation and control the positioning of the reactants (model acylating agent and -NH₂ of amino acids) to initiate biologically relevant reactions during early evolution.

Origins of Life Research: The Conundrum between Laboratory and Field Simulations of Messy Environments

Most experimental results that guide research related to the origin of life are from laboratory simulations of the early Earth conditions. In the laboratory, emphasis is placed on the purity of reagents and carefully controlled conditions, so there is a natural tendency to reject impurities and lack of control. However, life did not originate in laboratory conditions; therefore, we should take into consideration multiple factors that are likely to have contributed to the environmental complexity of the early Earth. This essay describes eight physical and biophysical factors that spontaneously resolve aqueous dispersions of ionic and organic solutes mixed with mineral particles and thereby promote specific chemical reactions required for life to begin.

Synthesis and Characterization of Amino Acid Decyl Esters as Early Membranes for the Origins of Life

Understanding how membrane forming amphiphiles are synthesized and aggregate in prebiotic settings is required for understanding the origins of life on Earth 4 billion years ago. Amino acids decyl esters were prepared by dehydration of decanol and amino acid as a model for a plausible prebiotic reaction at two temperatures. Fifteen amino acids were tested with a range of side chain chemistries to understand the role of amino acid identity on synthesis and membrane formation. Products were analyzed using LC-MS as well as microscopy. All amino acids tested produced decyl esters, and some of the products formed membranes when rehydrated in ultrapure water. One of the most abundant prebiotic amino acids, alanine, was remarkably easy to get to generate abundant, uniform membranes, indicating that this could be a selection mechanism for both amino acids and their amphiphilic derivatives.

Influence of Surfactant for Stabilization and Pipeline Transportation of Iron Ore Water Slurry: A Review

Iron ore is generally transported using a traditional method that releases significant amounts of dust into the environment. In contrast, the pipeline transportation of slurry is noticeably a sustainable approach for efficiently transporting iron ore by reducing the environmental pollution. The interparticle interaction of the iron ore particles should be mutually repulsive for steady dispersion. Surfactants and polymers adsorb efficiently at the solid/liquid interface due to their amphiphilic character, rendering the surface hydrophilic or hydrophobic to create a stable dispersion. The present review discusses the interaction of surfactants on the stabilization of solid particles for the ease of pipeline transportation using various types of stabilization mechanisms. In addition to the effect of surfactant alone, its combination with some other parameters such as particle size distribution, temperature, solid concentration, etc. has been discussed. The review also describes the detailed classification of iron ore, surfactant, and characteristic properties of surfactants.

Optical tweezer platform for the characterization of pH-triggered colloidal transformations in the oleic acid/water system

HYPOTHESIS

Soft colloidal particles that respond to their environment have innovative potential for many fields ranging from food and health to biotechnology and oil recovery. The in situ characterisation of colloidal transformations that triggers the functional response remain a challenge.

EXPERIMENTS

This study demonstrates the combination of an optical micromanipulation platform, polarized optical video microscopy and microfluidics in a comprehensive approach for the analysis of pH-driven structural transformations in emulsions. The new platform, together with synchrotron small angle X-ray scattering, was then applied to research the food-relevant, pH-responsive, oleic acid in water system.

FINDINGS

The experiments demonstrate structural transformations in individual oleic acid particles from micron-sized onion-type multilamellar oleic acid vesicles at pH 8.6, to nanostructured emulsions at pH < 8.0, and eventually oil droplets at pH < 6.5. The smooth particle-water interface of the onion-type vesicles at pH 8.6 was transformed into a rough particle surface at pH below 7.5. The pH-triggered changes of the interfacial tension at the droplet-water interface together with mass transport owing to structural transformations induced a self-propelled motion of the particle. The results of this study contribute to the fundamental understanding of the structure-property relationship in pH-responsive emulsions for nutrient and drug delivery applications.

From vesicles toward protocells and minimal cells

In contrast to ordinary condensed matter systems, "living systems" are unique. They are based on molecular compartments that reproduce themselves through (i) an uptake of ingredients and energy from the environment, and (ii) spatially and timely coordinated internal chemical transformations. These occur on the basis of instructions encoded in information molecules (DNAs). Life originated on Earth about 4 billion years ago as self-organised systems of inorganic compounds and organic molecules including macromolecules (e.g. nucleic acids and proteins) and low molar mass amphiphiles (lipids). Before the first living systems emerged from non-living forms of matter, functional molecules and dynamic molecular assemblies must have been formed as prebiotic soft matter systems. These hypothetical cell-like compartment systems often are called "protocells". Other systems that are considered as bridging units between non-living and living systems are called "minimal cells". They are synthetic, autonomous and sustainable reproducing compartment systems, but their constituents are not limited to prebiotic substances. In this review, we focus on both membrane-bounded (vesicular) protocells and minimal cells, and provide a membrane physics background which helps to understand how morphological transformations of vesicle systems might have happened and how vesicle reproduction might be coupled with metabolic reactions and information molecules. This research, which bridges matter and life, is a great challenge in which soft matter physics, systems chemistry, and synthetic biology must take joined efforts to better understand how the transformation of protocells into living systems might have occurred at the origin of life.

Synthesis of Phospholipids Under Plausible Prebiotic Conditions and Analogies with Phospholipid Biochemistry for Origin of Life Studies

Phospholipids are essential components of biological membranes and are involved in cell signalization, in several enzymatic reactions, and in energy metabolism. In addition, phospholipids represent an evolutionary and non-negligible step in life emergence. Progress in the past decades has led to a deeper understanding of these unique hydrophobic molecules and their most pertinent functions in cell biology. Today, a growing interest in "prebiotic lipidomics" calls for a new assessment of these relevant biomolecules.

Vesicle-based artificial cells: materials, construction methods and applications

The construction of artificial cells with specific cell-mimicking functions helps to explore complex biological processes and cell functions in natural cell systems and provides an insight into the origins of life. Bottom-up methods are widely used for engineering artificial cells based on vesicles by the in vitro assembly of biomimetic materials. In this review, the design of artificial cells with a specific function is discussed, by considering the selection of synthetic materials and construction technologies. First, a range of biomimetic materials for artificial cells is reviewed, including lipid, polymeric and hybrid lipid/copolymer materials. Biomaterials extracted from natural cells are also covered in this part. Then, the formation of microscale, giant unilamellar vesicles (GUVs) is reviewed based on different technologies, including gentle hydration, electro-formation, phase transfer and microfluidic methods. Subsequently, applications of artificial cells based on single vesicles or vesicle networks are addressed for mimicking cell behaviors and signaling processes. Microreactors for synthetic biology and cell-cell communication are highlighted here as well. Finally, current challenges and future trends for the development and applications of artificial cells are described.

Fatty Acid-Based Coacervates as a Membrane-free Protocell Model

Membrane-less scenarios that involve liquid-liquid phase separation (coacervation) provide clues for how protocells might emerge. Here, we report a versatile approach to construct coacervates by mixing fatty acid with biomolecule dopamine as the protocell model. The coacervate droplets are easily formed over a wide range of concentrations. The solutes with different interaction characteristics, including cationic, anionic, and hydrophobic dyes, can be well concentrated within the coacervates. In addition, reversible self-assemblies of the coacervates can be controlled by concentration, pH, temperature, salinity, and bioreaction realizing cycles between compartmentalization and noncompartmentalization. Through in situ dopamine polymerization, the stability of coacervate droplets is significantly improved, leading to higher resistance toward external factors. Therefore, the coacervates based on fatty acid and dopamine could serve as a bottom-up membrane-less protocell model that provides the links between the simple (small molecule) and complex (macromolecule) systems in the process of cell evolution.

Scum of the Earth: A Hypothesis for Prebiotic Multi-Compartmentalised Environments

Compartmentalisation by bioenergetic membranes is a universal feature of life. The eventual compartmentalisation of prebiotic systems is therefore often argued to comprise a key step during the origin of life. Compartments may have been active participants in prebiotic chemistry, concentrating and spatially organising key reactants. However, most prebiotically plausible compartments are leaky or unstable, limiting their utility. Here, we develop a new hypothesis for an origin of life environment that capitalises upon, and mitigates the limitations of, prebiotic compartments: multi-compartmentalised layers in the near surface environment—a ’scum’. Scum-type environments benefit from many of the same ensemble-based advantages as microbial biofilms. In particular, scum layers mediate diffusion with the wider environments, favouring preservation and sharing of early informational molecules, along with the selective concentration of compatible prebiotic compounds. Biofilms are among the earliest traces imprinted by life in the rock record: we contend that prebiotic equivalents of these environments deserve future experimental investigation.

A systematic assessment of the relationship between synthetic surfactants and mucosal adjuvanticity

Intranasal immunization with surfactants as vaccine adjuvants enhances protective immunity against invasive mucosal pathogens. However, the effects of surfactants and their adjuvanticity on mucosal immune responses remain unclear. Comparison of the mucosal adjuvanticity of 20 water-soluble surfactants from the four classes based upon the polarity composition of the hydrophilic headgroup revealed that the order of mucosal adjuvanticity was as follows: amphoteric > nonionic > cationic > anionic. Within the same class, each surfactant displayed different adjuvanticity values. Analysis of the diameter and ζ-potential of amphoteric surfactant-OVA complexes and their surface physicochemical properties revealed that the diameter was approximately 100 nm, which is considered suitable for immune induction, and that the ζ-potential of the anionic surfactant-OVA complexes was exceedingly negative. The increase in the number of carbon atoms in the hydrophobic tailgroups of the amphoteric surfactant resulted in an increase in the OVA-specific Ab titers. Our findings demonstrate that amphoteric surfactants exhibit potent mucosal adjuvanticity and highlight the importance of the number of carbon atoms in the tailgroups and the diameter and ζ-potential of the complexes when designing mucosal adjuvants.

The Role of Glycerol and Its Derivatives in the Biochemistry of Living Organisms, and Their Prebiotic Origin and Significance in the Evolution of Life

The emergence and evolution of prebiotic biomolecules on the early Earth remain a question that is considered crucial to understanding the chemistry of the origin of life. Amongst prebiotic molecules, glycerol is significant due to its ubiquity in biochemistry. In this review, we discuss the significance of glycerol and its various derivatives in biochemistry, their plausible roles in the origin and evolution of early cell membranes, and significance in the biochemistry of extremophiles, followed by their prebiotic origin on the early Earth and associated catalytic processes that led to the origin of these compounds. We also discuss various scenarios for the prebiotic syntheses of glycerol and its derivates and evaluate these to determine their relevance to early Earth biochemistry and geochemistry, and recapitulate the utilization of various minerals (including clays), condensation agents, and solvents that could have led to the successful prebiotic genesis of these biomolecules. Furthermore, important prebiotic events such as meteoritic delivery and prebiotic synthesis reactions under astrophysical conditions are also discussed. Finally, we have also highlighted some novel features of glycerol, including glycerol nucleic acid (GNA), in the origin and evolution of the life.

Symmetry Breaking of Phospholipids

Either stereo reactants or stereo catalysis from achiral or chiral molecules are a prerequisite to obtain pure enantiomeric lipid derivatives. We reviewed a few plausibly organic syntheses of phospholipids under prebiotic conditions with special attention paid to the starting materials as pro-chiral dihydroxyacetone and dihydroxyacetone phosphate (DHAP), which are the key molecules to break symmetry in phospholipids. The advantages of homochiral membranes compared to those of heterochiral membranes were analysed in terms of specific recognition, optimal functions of enzymes, membrane fluidity and topological packing. All biological membranes contain enantiomerically pure lipids in modern bacteria, eukarya and archaea. The contemporary archaea, comprising of methanogens, halobacteria and thermoacidophiles, are living under extreme conditions reminiscent of primitive environment and may indicate the origin of one ancient evolution path of lipid biosynthesis. The analysis of the known lipid metabolism reveals that all modern cells including archaea synthetize enantiomerically pure lipid precursors from prochiral DHAP. Sn-glycerol-1-phosphate dehydrogenase (G1PDH), usually found in archaea, catalyses the formation of sn-glycerol-1-phosphate (G1P), while sn-glycerol-3-phosphate dehydrogenase (G3PDH) catalyses the formation of sn-glycerol-3-phosphate (G3P) in bacteria and eukarya. The selective enzymatic activity seems to be the main strategy that evolution retained to obtain enantiomerically pure lipids. The occurrence of two genes encoding for G1PDH and G3PDH served to build up an evolutionary tree being the basis of our hypothesis article focusing on the evolution of these two genes. Gene encoding for G3PDH in eukarya may originate from G3PDH gene found in rare archaea indicating that archaea appeared earlier in the evolutionary tree than eukarya. Archaea and bacteria evolved probably separately, due to their distinct respective genes coding for G1PDH and G3PDH. We propose that prochiral DHAP is an essential molecule since it provides a convergent link between G1DPH and G3PDH. The synthesis of enantiopure phospholipids from DHAP appeared probably firstly in the presence of chemical catalysts, before being catalysed by enzymes which were the products of later Darwinian selection. The enzymes were probably selected for their efficient catalytic activities during evolution from large libraries of vesicles containing amino acids, carbohydrates, nucleic acids, lipids, and meteorite components that induced symmetry imbalance.

Turbulent coherent structures and early life below the Kolmogorov scale

Major evolutionary transitions, including the emergence of life, likely occurred in aqueous environments. While the role of water’s chemistry in early life is well studied, the effects of water’s ability to manipulate population structure are less clear. Population structure is known to be critical, as effective replicators must be insulated from parasites. Here, we propose that turbulent coherent structures, long-lasting flow patterns which trap particles, may serve many of the properties associated with compartments — collocalization, division, and merging — which are commonly thought to play a key role in the origins of life and other evolutionary transitions. We substantiate this idea by simulating multiple proposed metabolisms for early life in a simple model of a turbulent flow, and find that balancing the turnover times of biological particles and coherent structures can indeed enhance the likelihood of these metabolisms overcoming extinction either via parasitism or via a lack of metabolic support. Our results suggest that group selection models may be applicable with fewer physical and chemical constraints than previously thought, and apply much more widely in aqueous environments.

Models of the origin of life generally require a mechanism to structure emerging populations. Here, Krieger et al. develop spatial models showing that coherent structures arising in turbulent flows in aquatic environments could have provided compartments that facilitated the origin of life.

Membranes Composed of Lipopeptides and Liponucleobases Inspired Protolife Evolution

Amino acids and peptides have been demonstrated to form lipoamino acids and lipopeptides under presumed prebiotic conditions, and readily form liposomes. Of the common nucleobases, adenine forms a liponucleobase even below 100 °C. Adenine as well as other nucleobases can also be derivatized with ethylene carbonate (and likely other similar compounds) onto which fatty acids can be attached. The fatty acid tails along with appropriately functionalized nucleobases provide some solubility of liponucleobases in membranes. Such membranes would provide a structure in which three of biology's major components are closely associated and available for chemical interactions. Nucleobase-to-nucleobase interactions would ensure that the liponucleobases would have a uniquely different head-group relationship than other amphiphiles within a membrane, likely forming rafts due their π-π interactions and providing surface discontinuities that could serve as catalytic sites. The π-π bond distance in aromatic compounds is typically 0.34 nm, commensurate with that of the amine to carboxylate distance in alpha amino acids. This would have provided opportunity for hydrogen bonding between amino acids and the distal primary amines or tautomeric carbonyl/hydroxyl groups of two π-bonded nucleobases. Such bonding would weaken the covalent linkages within the amino acids, making them susceptible to forming peptide bonds with an adjacent amino acid, likely a lipoamino acid or lipopeptide. Were this second lipoamino acid bound to a third π-bonded nucleobase, it could result in orientation, destabilization and peptide formation. The stacked triplet of nucleobases might constitute the primordial codon triplet from which peptides were synthesized: primordial translation.

Possible Roles of Amphiphilic Molecules in the Origin of Biological Homochirality

A review. The question of homochirality is an intriguing problem in the field of chemistry, and is deeply related to the origin of life. Though amphiphiles and their supramolecular assembly have attracted less attention compared to biomacromolecules such as RNA and proteins, the lipid world hypothesis sheds new light on the origin of life. This review describes how amphiphilic molecules are possibly involved in the scenario of homochirality. Some prebiotic conditions relevant to amphiphilic molecules will also be described. It could be said that the chiral properties of amphiphilic molecules have various interesting features such as compositional information, spontaneous formation, the ability to exchange components, fission and fusion, adsorption, and permeation. This review aims to clarify the roles of amphiphiles regarding homochirality, and to determine what kinds of physical properties of amphiphilic molecules could have played a role in the scenario of homochirality.

Prebiotic Protocell Model Based on Dynamic Protein Membranes Accommodating Anabolic Reactions

The nature of the first prebiotic compartments and their possible minimal molecular composition is of great importance in the origin of life scenarios. Current protocell model membranes are proposed to be lipid-based. This paradigm has several shortcomings such as limited membrane stability of monoacyl lipid-based membranes (e.g., fatty acids), missing pathways to synthesize protocell membrane components (e.g., phospholipids) under early earth conditions, and the requirement for different classes of molecules for the formation of compartments and the catalysis of reactions. Amino acids on the other hand are known to arise and persist with remarkable abundance under early earth conditions since the fundamental Miller-Urey experiments. They were also postulated early to form protocellular structures, for example, proteinoid capsules. Here, we present a protocell model constituted by membranes assembled from amphiphilic proteins based on prebiotic amino acids. Self-assembled dynamic protein membrane-based compartments (PMBCs) are impressively stable and compatible with prevalent cellular membrane constituents forming protein-only or protein-lipid hybrid membranes. They can embed processes essential for extant living cells, such as enclosure of molecules, membrane fusion, phase separation, and complex biosynthetic elements from modern cells demonstrating "upward" compatibility. Our findings suggest that prebiotic PMBCs represent a new type of protocell as a possible ancestor of current lipid-based cells. The presented prebiotic PMBC model can be used to design artificial cells, important for the study of structural, catalytic, and evolutionary pathways related to the emergence of life.

Nanotube-Mediated Path to Protocell Formation

Cellular compartments are membrane-enclosed, spatially distinct microenvironments that confine and protect biochemical reactions in the biological cell. On the early Earth, the autonomous formation of compartments is thought to have led to the encapsulation of nucleotides, thereby satisfying a starting condition for the emergence of life. Recently, surfaces have come into focus as potential platforms for the self-assembly of prebiotic compartments, as significantly enhanced vesicle formation was reported in the presence of solid interfaces. The detailed mechanism of such formation at the mesoscale is still under discussion. We report here on the spontaneous transformation of solid-surface-adhered lipid deposits to unilamellar membrane compartments through a straightforward sequence of topological changes, proceeding via a network of interconnected lipid nanotubes. We show that this transformation is entirely driven by surface-free energy minimization and does not require hydrolysis of organic molecules or external stimuli such as electrical currents or mechanical agitation. The vesicular structures take up and encapsulate their external environment during formation and can subsequently separate and migrate upon exposure to hydrodynamic flow. This may link the self-directed transition from weakly organized bioamphiphile assemblies on solid surfaces to protocells with secluded internal contents.

Emergence of Compartments Formed from Unconventional Surfactants in Dynamic Combinatorial Libraries

Assembly processes can drive the selection of self-assembling molecules in dynamic combinatorial libraries, yielding self-synthesizing materials. We now show how such selection in a dynamic combinatorial library made from an amphiphilic building block which, by itself, assembles into micelles, can yield membranous aggregates ranging from vesicles to sponge phases. These aggregates are made from a mixture of unconventional surfactant molecules, showing the power of dynamic combinatorial selection approaches for the discovery of new, not readily predictable, self-assembly motifs.

Organization and Compartmentalization by Lipid Membranes Promote Reactions Related to the Origin of Cellular Life

Liquid crystals have certain physical properties that promote chemical reactions which cannot occur in bulk phase media. These properties are displayed, among other molecules, by amphiphilic compounds which assemble into membrane structures then concentrate and organize biologically relevant monomers within their confined spaces. When mixtures of lipids and nucleotides are cycled multiple times between hydrated and anhydrous conditions, the monomers polymerize in the dry phase into oligonucleotides. Upon rehydration, mixtures of the polymers are encapsulated in lipid-bounded compartments called protocells. Reactions in liquid crystalline organizing matrices represent a promising approach for future research on how primitive cells could emerge on the early Earth and other habitable planets.

Protection against Neurodegeneration in the Hippocampus Using Sialic Acid- and 5-HT-Moduline-Conjugated Lipopolymer Nanoparticles

Significant involvement of oxidative stress in the brain can develop Alzheimer's disease (AD); however, a great number of clinical trials explains the limited success of antioxidant therapy in dealing with this neurodegenerative disease. Here, we established a lipopolymer system of poly(lactide-co-glycolide) (PLGA) nanoparticles (NPs) incorporated with phosphatidic acid (PA) and modified with sialic acid (SA) and 5-hydroxytryptamine-moduline (5HTM) to improve quercetin (QU) activity against oxidative stress induced by amyloid-β (Aβ) deposits. Morphological studies revealed a uniform exterior of QU-SA-5HTM-PA-PLGA NPs with a spherical structure and enhanced aggregation with inclusion of PA in the formulation. A better brain-targeted delivery of the lipopolymeric NPs was verified from the high blood-brain barrier (BBB) permeability of QU through strong interactions of surface SA and 5HTM with O-linked N-acetylglucosamine and 5-HT1B receptors, respectively. Immunofluorescence staining images also supported QU-SA-5HTM-PA-PLGA NPs to traverse the microvessels of AD rat brain. Western blot analysis showed that QU-loaded PA-PLGA NPs suppressed caspase-3 expression. The ability of the nanocarriers to recognize Aβ fibrils was demonstrated from the reduced senile plaque formation and the attenuated acetylcholinesterase and malondialdehyde activity in the hippocampus. Hence, the medication of QU-SA-5HTM-PA-PLGA NPs can facilitate the BBB penetration and prevent Aβ accumulation, lipid peroxidation, and neuronal apoptosis for the AD management.

The sustainable synthesis of peptidomimetics via chemoenzymatic tandem oxidation-Ugi reaction

A simply and green synthetic protocol based on the selective laccase-oxidation of alcohol to a corresponding aldehyde and a following Ugi reaction in a micellar system made of SDS was developed and is reported herein. Special emphasis was placed on the metal-free chemoenzymatic tandem reaction based on alcohol oxidation strategies using molecular oxygen from air, followed by an Ugi reaction. The reaction was carried out without the use of a transition metal or organic solvents as a reaction medium. The presented protocol offers an efficient and environmentally friendly procedure.

α-Synuclein and Polyunsaturated Fatty Acids: Molecular Basis of the Interaction and Implication in Neurodegeneration

α-Synuclein (α-syn) is a 140-amino acid protein, the physiological function of which has yet to be clarified. It is involved in several neurodegenerative disorders, and the interaction of the protein with brain lipids plays an important role in the pathogenesis of Parkinson’s disease (PD). Polyunsaturated fatty acids (PUFA) are highly abundant in the brain where they play critical roles in neuronal membrane fluidity and permeability, serve as energy reserves and function as second messengers in cell signaling. PUFA concentration and composition in the brain are altered with age when also an increase of lipid peroxidation is observed. Considering that PD is clearly correlated with oxidative stress, PUFA abundance and composition became of great interest in neurodegeneration studies because of PUFA’s high propensity to oxidize. The high levels of the PUFA docosahexaenoic acid (DHA) in brain areas containing α-syn inclusions in patients with PD further support the hypothesis of possible interactions between α-syn and DHA. Additionally, a possible functional role of α-syn in sequestering the early peroxidation products of fatty acids was recently proposed. Here, we provide an overview of the current knowledge regarding the molecular interactions between α-syn and fatty acids and the effect exerted by the protein on their oxidative state. We highlight recent findings supporting a neuroprotective role of the protein, linking α-syn, altered lipid composition in neurodegenerative disorders and PD development.

Adhesion of like-charged lipid vesicles induced by rod-like counterions

Adhesion of electrically charged lipid vesicles and subsequent formation of multi-vesicle aggregates can be induced by multivalent rod-like counterions. Motivated by recent experimental observations we calculate the equilibrium conformation of two identical vesicles that adhere onto each other. The degree of adhesion reflects the competition between predominantly electrostatic attraction and vesicle bending. Our model assumes the enclosed vesicle volume is allowed to freely adjust and the area of the vesicle membrane is fixed and remains constant. We describe the electrostatic attraction, which arises from the bridging of the rod-like counterions between the two like-charged vesicles, using a recently developed mean-field theory. Bending fluctuation-induced entropic repulsion, depletion forces between the apposed vesicle membranes induced by the rod-like counterions, and van der Waals attraction between the vesicles are estimated to induce only minor shifts in the equilibrium vesicle conformation. Our model predicts the dependence of vesicle adhesion (including its onset) exclusively from material or molecular parameters such as vesicle size and charge, bending stiffness of the membrane, effective length and net charge of the added rod-like counterions, as well as concentrations of rod-like counterions and additional salt content. We demonstrate that the demixing of charged lipids between the adhesion region and the uncomplexed parts of the vesicles has only a minor influence on the degree of adhesion. Our predictions are in qualitative agreement with recent experimental findings.

The synthesis of mono-alkyl phosphates and their derivatives: an overview of their nature, preparation and use, including synthesis under plausible prebiotic conditions

Nucleic acids, phospholipids and other organic phosphates play central roles in biological pathways.

Crude phosphorylation mixtures containing racemic lipid amphiphiles self-assemble to give stable primitive compartments

It is an open question how the chemical structure of prebiotic vesicle-forming amphiphiles complexified to produce robust primitive compartments that could safely host foreign molecules. Previous work suggests that comparingly labile vesicles composed of plausibly prebiotic fatty acids were eventually chemically transformed with glycerol and a suitable phosphate source into phospholipids that would form robust vesicles. Here we show that phosphatidic acid (PA) and phosphatidylethanolamine (PE) lipids can be obtained from racemic dioleoyl glycerol under plausibly prebiotic phosphorylation conditions. Upon in situ hydration of the crude phosphorylation mixtures only those that contained rac-DOPA (not rac-DOPE) generated stable giant vesicles that were capable of encapsulating water-soluble probes, as evidenced by confocal microscopy and flow cytometry. Chemical reaction side-products (identified by IR and MS and quantified by 1H NMR) acted as co-surfactants and facilitated vesicle formation. To mimic the compositional variation of such primitive lipid mixtures, self-assembly of a combinatorial set of the above amphiphiles was tested, revealing that too high dioleoyl glycerol contents inhibited vesicle formation. We conclude that a decisive driving force for the gradual transition from unstable fatty acid vesicles to robust diacylglyceryl phosphate vesicles was to avoid the accumulation of unphosphorylated diacylglycerols in primitive vesicle membranes.

Removal of Cr(VI) by surfactant modified Auricularia auricula spent substrate: biosorption condition and mechanism

Auricularia auricula spent substrate (AASS) modified by didodecyldimethylammonium bromide(DDAB) was used as adsorbent to remove Cr(VI) from aqueous solution. Based on a single-factor experiment and response surface methodology, the optimal conditions were adsorbent dosage of 1.5 g/L, pH value of 4.0, initial Cr(VI) concentration of 19 mg/L, temperature of 25 °C, biosorption time of 120 min, rotational speed of 150 r/min, respectively, under which biosorption capacity could reach 12.16 mg/g compared with unmodified AASS (6.058 mg/g). DDAB modification could enlarge the specific surface area and porous diameter of the adsorbents, and supply hydrophilic and hydrophobic groups capable of adsorbing at the interfaces. In addition, DDAB increased ionic exchange and complex formation demonstrated by variations of elemental contents, shifts of carboxyl, amine groups, hydroxyl, alkyl chains, and phosphate groups as well as the crystal structure of the Cr-O compounds. Variations of peaks and energy in XPS analysis also testified the reduction of Cr(VI) to Cr(III).The biosorption behavior of modified AASS was in line with Langmuir and Freundlich isotherm equation. The final regeneration efficiency was 62.33% after three biosorption-desorption cycles. Apparently, DDBA is a eximious modifier and DDBA-modified AASS was very efficient for Cr(VI) removal.

Effect of protic ionic liquid nanostructure on phospholipid vesicle formation

The formation of bilayer-based lyotropic liquid crystals and vesicle dispersions by phospholipids in a range of protic ionic liquids has been investigated by polarizing optical microscopy using isothermal penetration scans, differential scanning calorimetry, and small angle X-ray and neutron scattering. The stability and structure of both lamellar phases and vesicle dispersions is found to depend primarily on the underlying amphiphilic nanostructure of the ionic liquid itself. This finding has significant implications for the use of ionic liquids in soft and biological materials and for biopreservation, and demonstrates how vesicle structure and properties can be controlled through selection of cation and anion. For a given ionic liquid, systematic trends in bilayer thickness, chain-melting temperature and enthalpy increase with phospholipid acyl chain length, paralleling behaviour in aqueous systems.

Vesicles: self-assembly beyond biological lipids

This review presents an overview of synthetic systems that self-assemble to form vesicles.

Efficient Ugi reactions in an aqueous vesicle system

A new, alternative route for the synthesis of a variety of α-aminoacyl amides via the four-component Ugi reaction in the presence of different surfactants in aqueous solution was investigated. High reaction yields were obtained in many cases with vesicles formed from DDAB.

Sequestration of Proteins by Fatty Acid Coacervates for Their Encapsulation within Vesicles

Encapsulating biological materials in lipid vesicles is of interest for mimicking cells; however, except in some particular cases, such processes do not occur spontaneously. Herein, we developed a simple and robust method for encapsulating proteins in fatty acid vesicles in high yields. Fatty acid based, membrane-free coacervates spontaneously sequester proteins and can reversibly form membranous vesicles upon varying the pH value, the precrowding feature in coacervates allowing for protein encapsulation within vesicles. We then produced enzyme-enriched vesicles and show that enzymatic reactions can occur in these micrometric capsules. This work could be of interest in the field of synthetic biology for building microreactors.

Visualization of the spontaneous emergence of a complex, dynamic, and autocatalytic system

Autocatalytic chemical reactions are widely studied as models of biological processes and to better understand the origins of life on Earth. Minimal self-reproducing amphiphiles have been developed in this context and as an approach to de novo "bottom-up" synthetic protocells. How chemicals come together to produce living systems, however, remains poorly understood, despite much experimentation and speculation. Here, we use ultrasensitive label-free optical microscopy to visualize the spontaneous emergence of an autocatalytic system from an aqueous mixture of two chemicals. Quantitative, in situ nanoscale imaging reveals heterogeneous self-reproducing aggregates and enables the real-time visualization of the synthesis of new aggregates at the reactive interface. The aggregates and reactivity patterns observed vary together with differences in the respective environment. This work demonstrates how imaging of chemistry at the nanoscale can provide direct insight into the dynamic evolution of nonequilibrium systems across molecular to microscopic length scales.

Prebiotic Lipidic Amphiphiles and Condensing Agents on the Early Earth

It is still uncertain how the first minimal cellular systems evolved to the complexity required for life to begin, but it is obvious that the role of amphiphilic compounds in the origin of life is one of huge relevance. Over the last four decades a number of studies have demonstrated how amphiphilic molecules can be synthesized under plausibly prebiotic conditions. The majority of these experiments also gave evidence for the ability of so formed amphiphiles to assemble in closed membranes of vesicles that, in principle, could have compartmented first biological processes on early Earth, including the emergence of self-replicating systems. For a competitive selection of the best performing molecular replicators to become operative, some kind of bounded units capable of harboring them are indispensable. Without the competition between dynamic populations of different compartments, life itself could not be distinguished from an otherwise disparate array or network of molecular interactions. In this review, we describe experiments that demonstrate how different prebiotically-available building blocks can become precursors of phospholipids that form vesicles. We discuss the experimental conditions that resemble plausibly those of the early Earth (or elsewhere) and consider the analytical methods that were used to characterize synthetic products. Two brief sections focus on phosphorylating agents, catalysts and coupling agents with particular attention given to their geochemical context. In Section 5, we describe how condensing agents such as cyanamide and urea can promote the abiotic synthesis of phospholipids. We conclude the review by reflecting on future studies of phospholipid compartments, particularly, on evolvable chemical systems that include giant vesicles composed of different lipidic amphiphiles.

The impact of alkanes on the structure of Triton X100 micelles

Here we investigate the structural evolution of TX100 micelles upon loading with several linear and cyclic alkanes by DLS, PGSE-NMR, 2D NOESY NMR, viscosity measurements, and molecular dynamic simulations.

Faceted fatty acid vesicles formed from single-tailed perfluorinated surfactants

The aggregation behavior and rheological properties of two mixtures of perfluorononanoic acid (PFNA)/NaOH and perfluorodecanoic acid (PFDA)/NaOH were investigated in aqueous solutions. Interestingly, pH-sensitive polyhedral fatty acid vesicles were spontaneously formed in both systems, which were determined by freeze-fracture transmission electron microscopy (FF-TEM) measurements. Especially, a phase transition from faceted vesicles to the L3 phase with the increase of pH was observed in the PFNA/NaOH system while it was not observed in the PFDA/NaOH system. Differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS) measurements confirmed that the bilayers of the faceted vesicles were in the crystalline station indicating that the crystallization of fluorocarbon chains was the main driving force for their formation. Besides, the two systems of faceted perfluorofatty acid vesicles exhibit interesting rheological properties, for instance, they showed high viscoelasticity and shear-thinning behaviour, and the elastic modulus (G') and viscous modulus (G'') of PFDA/NaOH vesicles were much higher than those of PFNA/NaOH vesicles. Conversely, the solution of the L3 phase with fluid bilayers did not present viscoelastic properties. Therefore, the viscoelastic properties of vesicles resulted from the crystalline fluorinated alkyl chains with high rigidity at room temperature and the dense packing of vesicles. As far as we know, such faceted fatty acid vesicles formed from single-tailed perfluorinated surfactants have been rarely reported. Our work successfully constructs polyhedral fatty acid vesicles and proposes their formation mechanism, which should be a great advance in the fundamental research of fatty acid vesicles.

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.

Current Ideas about Prebiological Compartmentalization

Contemporary biological cells are highly sophisticated dynamic compartment systems which separate an internal volume from the external medium through a boundary, which controls, in complex ways, the exchange of matter and energy between the cell's interior and the environment. Since such compartmentalization is a fundamental principle of all forms of life, scenarios have been elaborated about the emergence of prebiological compartments on early Earth, in particular about their likely structural characteristics and dynamic features. Chemical systems that consist of potentially prebiological compartments and chemical reaction networks have been designed to model pre-cellular systems. These systems are often referred to as "protocells". Past and current protocell model systems are presented and compared. Since the prebiotic formation of cell-like compartments is directly linked to the prebiotic availability of compartment building blocks, a few aspects on the likely chemical inventory on the early Earth are also summarized.