The lipid world

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

A large number of surfactants (surface active molecules) are chemically simple compounds that can be obtained by simple chemical reactions, in some cases even under presumably prebiotic conditions. Surfactant assemblies are self-organized polymolecular aggregates of surfactants, in the simplest case micelles, vesicles, hexagonal and cubic phases. It may be that these different types of surfactant assemblies have played various, so-far underestimated important roles in the processes that led to the formation of the first living systems. Although nucleic acids are key players in the formation of cells as we know them today (RNA world hypothesis), it is still unclear how RNA could have been formed under prebiotic conditions. Surfactants with their self-organizing properties may have assisted, controlled and compartimentalized some of the chemical reactions that eventually led to the formation of molecules like RNA. Therefore, surfactants were possibly very important in prebiotic times in the sense that they may have been involved in different physical and chemical processes that finally led to a transformation of non-living matter to the first cellular form(s) of life. This hypothesis is based on four main experimental observations: (i) Surfactant aggregation can lead to cell-like compartimentation (vesicles). (ii) Surfactant assemblies can provide local reaction conditions that are very different from the bulk medium, which may lead to a dramatic change in the rate of chemical reactions and to a change in reaction product distributions. (iii) The surface properties of surfactant assemblies that may be liquid- or solid-like, charged or neutral, and the elasticity and packing density of surfactant assemblies depend on the chemical structure of the surfactants, on the presence of other molecules, and on the overall environmental conditions (e. g. temperature). This wide range of surface characteristics of surfactant assemblies may allow a control of surface-bound chemical reactions not only by the charge or hydrophobicity of the surface but also by its "softness". (iv) Chiral polymolecular assemblies (helices) may form from chiral surfactants. There are many examples that illustrate the different roles and potential roles of surfactant assemblies in different research areas outside of the field of the origin(s) of life, most importantly in investigations of contemporary living systems, in nanotechnology applications, and in the development of drug delivery systems. Concepts and ideas behind many of these applications may have relevance also in connection to the different unsolved problems in understanding the origin(s) of life.

Self-assembly of fatty acid-alkylboladiamine salts

Long-chain fatty acids are insoluble in aqueous solution and form crystal precipitates. It is then of particular importance to determine the physicochemical parameters allowing their dispersion in water to improve their bioavailability and their utilization as surfactants. Herein, we report a study on salt-free catanionic systems in aqueous solution made of mixtures between palmitic or stearic fatty acids and alkylboladiamines (Abd's) differing by their alkyl chain length. Phase contrast microscopy, solid-state NMR, Fourier transform infrared spectroscopy, and small-angle neutron scattering were used to characterize the phase behavior of these systems at molar ratio of fatty acid to Abd of 1 and 2. Whatever the Abd and the molar ratio, fatty acids were embedded at low temperature in a bilayer gel phase which crystallizes after a period of rest. At an equimolar ratio, the gel phases transited upon raising the temperature to an isotropic phase made of worm-like micelles except in the case of the ethylenediamine chain for which a lamellar fluid phase was observed. At a molar ratio of 2 and high temperature, fatty acids were embedded in a lamellar fluid phase which self-orients with its stacking axis perpendicular to the magnetic field. However, for a long alkylboladiamine such as spermine, worm-like micelles formed. The phase behavior at high temperature is discussed in terms of molecular volume.

Comparison of Phosphatidylcholine Vesicle Properties Related to Geometrical Isomerism†

Glycerophosphatidylcholine containing trans-unsaturated fatty acid residues was prepared by reaction of the corresponding naturally occurring cis lipid with photochemically generated thiyl radicals. This modified lipid was chosen as the simplest model for gaining some insights of the complex scenario of membrane formation, in connection with the role of lipid geometry and the predominance of cis lipids in eukaryotic cells. The critical aggregation concentration for the spontaneous formation of vesicles was determined for cis and trans isomers with cis-parinaric acid used as a fluorescent probe and it was found to be similar for both lipids. Vesicle dimensions were investigated by light scattering and electron microscopy, and the type of fatty acid residues influenced the vesicle diameter, with a decrease along the series cis > trans > saturated. Fluorescence measurement of dye release from trans and cis vesicles showed also a different permeability. A picture emerged of the geometrical isomer preference in cells as a process driven by natural selection during the life evolution of different organisms, both in terms of compartment dimensions and membrane functionality.

DIVERSITY AND EVOLUTION OF PROTEIN TRANSLOCATION

Cells need to translocate proteins into and across hydrophobic membranes in order to interact with the extracellular environment. Although a subset of proteins are thought to spontaneously insert into lipid bilayers, translocation of most transported proteins requires additional cellular components. Such components catalyze efficient lateral transport into or across cellular membranes in prokaryotes and eukaryotes. These include, among others, the conserved YidC/Oxa1/Alb3 proteins as well as components of the Sec and the Tat pathways. Our current knowledge of the function and distribution of these components and their corresponding pathways in organisms of the three domains of life is reviewed. On the basis of this information, the evolution of protein translocation is discussed.

Computational studies on conditions of the emergence of autopoietic protocells

It has been pointed out that the acquisition of self-maintaining protocells was one of the most important evolutionary steps in the earliest stage of life. However, there remains little evidence to show what the components of the protocells were and how they were acquired. A theoretical study to investigate the possible process of the emergence of protocells is therefore required. In this paper, we present a computational model that demonstrates the emergence and evolution of self-maintaining and self-reproducing structures to analyze conditions under which precellular autocatalytic molecules could evolve into self-reproducing protocells. We focused on the supply rates of molecular resources as environmental parameters and explored the pathways of evolution from molecular replication to cellular reproduction under various conditions. The results showed that the spontaneous organization of protocells from a random initial state takes place in a parameter region where the metabolism becomes difficult because of an insufficient supply of resources, but once cell structures are organized, they can survive in a wider range of environments. We investigated the evolution in temporally or spatially changing environments and found that the protocells can take over the precellular metabolic system after the transition. These results suggest a possible scenario for the evolution from precellular to cellular reproduction.

Hypothesis: the origin of life in a hydrogel environment

A hypothesis is proposed that the first cell(s) on the Earth assembled in a hydrogel environment. Gel environments are capable of retaining water, oily hydrocarbons, solutes, and gas bubbles, and are capable of carrying out many functions, even in the absence of a membrane. Thus, the gel-like environment may have conferred distinct advantages for the assembly of the first cell(s).

Did life begin on the beach?

Water is one of the prerequisites of life. Further requirements are the existence of a system of interacting organic molecules capable of capturing and converting the supply of external energy and elaborating the replicating function that is needed for propagation. None of this would be possible without the existence of some means of concentrating, selecting, and then containing these mutually interacting substances in proximity to one another, i.e., a primitive cell. Starting from this hypothesis we propose a model for the development of life on Earth. Our model embodies the following new features: (1) rapid cycles of catalysis and transport of material, (2) desegregation (separation by tidal action and degradation by catalysis) as well as segregation (by chromatography on tidal beaches), (3) cross-catalysis instead of auto-catalysis, as well as (4) compartmentalization, although the latter idea is of course not new. But our "lipid first" model, in contrast to earlier "peptide first" or "RNA first" models, provides for the compartments needed to act as a cradle for the subsequent development of information- rich molecules like peptides and RNA. If anything, the earliest information-rich molecules were probably membrane-spanning peptides/proteins.

Chiral recognition of dipeptides in a biomembrane model

Chiral recognition of the enantiomeric couples of ditryptophan and diphenylalanine was observed by (1)H NMR spectroscopy in micelles formed by sodium N-dodecanoyl-L-prolinate. Ditryptophan showed a selective association with the Z domains of the amidic aggregates, whereas diphenylalanine did not show any selectivity in the association. Partition coefficients between water and aggregates were evaluated by diffusion NMR experiments. Intramolecular distances of ditryptophan isomers associated with chiral aggregates were obtained by ROESY experiments and were used as constraints in molecular mechanics calculations. From these calculations, information on the conformation of the peptides in the chiral aggregates was obtained.

The place of metabolism in the origin of life

Metabolism and replication are generally considered essential features of biological life. Workers in the field of the origin of life are mostly split into two groups, depending on which of these two functions is postulated to have occurred first. Because of difficulties encountered by the replication-first (or genetics-first) approach, some workers have postulated that a highly developed metabolism must have originated before replication and the formation of a genetic apparatus. However, as supporters of a replication-first approach have pointed out, and as is discussed in this article, the alternative metabolism-first approach has fundamental problems that have not been sufficiently addressed.

Polymer GARD: computer simulation of covalent bond formation in reproducing molecular assemblies

The basic Graded Autocatalysis Replication Domain (GARD) model consists of a repertoire of small molecules, typically amphiphiles, which join and leave a non-covalent micelle-like assembly. Its replication behavior is due to occasional fission, followed by a homeostatic growth process governed by the assembly's composition. Limitations of the basic GARD model are its small finite molecular repertoire and the lack of a clear path from a 'monomer world' towards polymer-based living entities. We have now devised an extension of the model (polymer GARD or P-GARD), where a monomer-based GARD serves as a 'scaffold' for oligomer formation, as a result of internal chemical rules. We tested this concept with computer simulations of a simple case of monovalent monomers, whereby more complex molecules (dimers) are formed internally, in a manner resembling biosynthetic metabolism. We have observed events of dimer 'take-over' - the formation of compositionally stable, replication-prone quasi stationary states (composomes) that have appreciable dimer content. The appearance of novel metabolism-like networks obeys a time-dependent power law, reminiscent of evolution under punctuated equilibrium. A simulation under constant population conditions shows the dynamics of takeover and extinction of different composomes, leading to the generation of different population distributions. The P-GARD model offers a scenario whereby biopolymer formation may be a result of rather than a prerequisite for early life-like processes.

A kinetic study of the growth of fatty acid vesicles

Membrane vesicles composed of fatty acids can be made to grow and divide under laboratory conditions, and thus provide a model system relevant to the emergence of cellular life. Fatty acid vesicles grow spontaneously when alkaline micelles are added to buffered vesicles. To investigate the mechanism of this process, we used stopped-flow kinetics to analyze the dilution of non-exchanging FRET probes incorporated into preformed vesicles during growth. Oleate vesicle growth occurs in two phases (fast and slow), indicating two pathways for the incorporation of fatty acid into preformed vesicles. We propose that the fast phase, which is stoichiometrically limited by the preformed vesicles, results from the formation of a "shell" of fatty acid around a vesicle, followed by rapid transfer of this fatty acid into the preformed vesicle. The slower phase may result from incorporation of fatty acid which had been trapped in an intermediate state. We provide independent evidence for the rapid transformation of micelles into an aggregated intermediate form after transfer from high to low pH. Our results show that the most efficient incorporation of added oleate into oleic acid/oleate vesicles occurs under conditions that avoid a large transient increase in the micelle/vesicle ratio.

Pre-biotic stage of life origin under non-photosynthetic conditions

Spontaneous assembling of a simplest bacterial cell even if all necessary molecules are present in a solution seems to be extremely rare event and from the scientific standpoint has to be considered as impossible. Therefore, a predecessor of a living cell has to be very simple for providing its self-assembling and at the same time it should be able of progressive increase in complexity. Now phase-separated particles, first of all micelles, are put forward as possible predecessors of living cell. According to the offered working concept only phase-separated particles possessing autocatalytic properties can be considered as predecessors of living cells. The first stage of evolution of these phase-separated autocatalytic systems is the appearance of pre-biotic metabolism providing synthesis of amphiphiles for formation of capsules of these systems. This synthesis is maintained by the energy of a base reaction being a component of a planet-chemical cycle. Catalytic system providing functioning of pre-biotic metabolism is based on multivariate oligomeric autocatalyst, which reproduces itself from monomers, penetrating the particles from the outside. Since the autocatalyst realizes random polymerization then a collection of other oligomers possessing different catalytic functions is produced. In the paper the functioning of multivariate oligomeric autocatalyst in flow reactor is analyzed.

Interaction between complementary liposomes: a process leading to multicompartment systems formation

Interaction of complementary liposomes induces a series of processes, involving reorganization of their membrane lipids, which lead to the formation of large aggregates. In several cases these aggregates exhibit multicompartment structures and only primitively mimic, in some aspects at least, the multicompartmental features of cells. Similar multicompartment structures were repeatedly obtained following the interaction of a diversity of complementary liposomal pairs. Thus, a working hypothesis is proposed, according to which, molecular recognition of liposomes induces the formation of multicompartment structures.

An overlooked riddle of life's origins: energy-dependent nucleic acid unzipping

The imposing progress in understanding contemporary life forms on Earth and in manipulating them has not been matched by a comparable progress in understanding the origins of life. This paper argues that a crucial problem of unzipping of the double helix molecule of nucleic acid during its replication has been underrated, if not plainly overlooked, in the theories of life's origin and evolution. A model is presented of how evolution may have solved the problem in its early phase. Similar to several previous models, the model envisages the existence of a protocell, in which osmotic disbalance is being created by accumulation of synthetic products resulting in expansion and division of the protocell. Novel in the model is the presence in the protocell of a double-stranded nucleic acid, with each of its two strands being affixed by its 3'-terminus to the opposite sides of the membrane of a protocell. In the course of the protocell expansion, osmotic force is utilized to pull the two strands longitudinally in opposite directions, unzipping the helix and partitioning the strands between the two daughter protocells. The model is also being used as a background for arguments of why life need operate in cycles. Many formal models of life's origin and evolution have not taken into account the fact that logical possibility does not equal thermodynamic feasibility. A system of self-replication has to consist of both replicators and replicants.

Membrane growth can generate a transmembrane pH gradient in fatty acid vesicles

Electrochemical proton gradients are the basis of energy transduction in modern cells, and may have played important roles in even the earliest cell-like structures. We have investigated the conditions under which pH gradients are maintained across the membranes of fatty acid vesicles, a model of early cell membranes. We show that pH gradients across such membranes decay rapidly in the presence of alkali-metal cations, but can be maintained in the absence of permeable cations. Under such conditions, when fatty acid vesicles grow through the incorporation of additional fatty acid, a transmembrane pH gradient is spontaneously generated. The formation of this pH gradient captures some of the energy released during membrane growth, but also opposes and limits further membrane area increase. The coupling of membrane growth to energy storage could have provided a growth advantage to early cells, once the membrane composition had evolved to allow the maintenance of stable pH gradients.

A relationship between membrane properties forms the basis of a selectivity mechanism for vesicle self-reproduction

Self-reproduction and the ability to regulate their composition are two essential properties of terrestrial biotic systems. The identification of non-living systems that possess these properties can therefore contribute not only to our understanding of their functioning but also hint at possible prebiotic processes that led to the emergence of life. Growing lipid vesicles have been previously established as having the capacity to self-reproduce. Here it is demonstrated that vesicle self-reproduction can occur only at selected values of vesicle properties. We treat as an example a simple vesicle with membrane elastic properties defined by a membrane bending modulus kappa and spontaneous curvature C0, whose volume variation depends on the membrane hydraulic permeability Lp and whose membrane area doubles in time Td. Vesicle self-reproduction is described as a process in which a growing vesicle first transforms its shape from a sphere into a budded shape of two spheres connected by a narrow neck, and then splits into two spherical daughter vesicles. We show that budded vesicle shapes can be reached only under the condition that Td Lpkappa C0(4)> or =1.85. Thus, in a growing vesicle population containing vesicles of different composition, only the vesicles for which this condition is fulfilled can increase their number in a self-reproducing manner. The obtained results also suggest that at times much longer than Td the number of vesicles with their properties near the "edge" in the system parameter space defined by the minimum value of the product Td Lpkappa C0(4), will greatly exceed the number of any other vesicles.

Prospects of a computational origin of life endeavor

While the last century brought an exquisite understanding of the molecular basis of life, very little is known about the detailed chemical mechanisms that afforded the emergence of life on early earth. There is a broad agreement that the problem lies in the realm of chemistry, and likely resides in the formation and mutual interactions of carbon-based molecules in aqueous medium. Yet, present-day experimental approaches can only capture the synthesis and behavior of a few molecule types at a time. On the other hand, experimental simulations of prebiotic syntheses, as well as chemical analyses of carbonaceous meteorites, suggest that the early prebiotic hydrosphere contained many thousands of different compounds. The present paper explores the idea that given the limitations of test-tube approaches with regards to such a 'random chemistry' scenario, an alternative mode of analysis should be pursued. It is argued that as computational tools for the reconstruction of molecular interactions improve rapidly, it may soon become possible to perform adequate computer-based simulations of prebiotic evolution. We thus propose to launch a computational origin of life endeavor (http://ool.weizmann.ac.il/CORE), involving computer simulations of realistic complex prebiotic chemical networks. In the present paper we provide specific examples, based on a novel algorithmic approach, which constitutes a hybrid of molecular dynamics and stochastic chemistry. As one potential solution for the immense hardware requirements dictated by this approach, we have begun to implement an idle CPU harvesting scheme, under the title ool@home.

A possible route to prebiotic vesicle reproduction

Spherical bounded structures such as those formed by surfactant aggregates (mostly micelles and vesicles), with an inside that is chemically and physically different from the outside medium, can be seen as primitive cell models. As such, they are fundamental structures for the theory of autopoiesis as originally formulated by Varela and Maturana. In particular, since self-reproduction is a very important feature of minimal cellular life, the study of self-reproduction of micelles and vesicles represents a quite challenging bio-mimetic approach. Our laboratory has put much effort in recent years into implementing self-reproduction of vesicles as models for self-reproduction of cellular bounded structures, and this article is a further contribution in this direction. In particular, we deal with the so-called matrix effect of vesicles, related to the fact that when fresh surfactant is added to an aqueous solution containing preformed vesicles of a very narrow size distribution, the newly formed vesicles (instead of being polydisperse, as is usually the case) have dimensions very close to those of the preformed ones. In practice, this corresponds to a mechanism of reproduction of vesicles of the same size. In this article, the matrix effect is re-elaborated in the perspective of the origin of life, and in particular in terms of the prebiotic mechanisms that might permit the growth and reproduction of vesicles. The data are analyzed by dynamic light scattering with a new program that permits the calculation of the number-weighted size distribution. It is shown that, on adding a stoichiometric amount of oleate micelles to preformed oleate vesicles extruded at 50 and 100 nm, the final distribution contains about twice the initial number of particles, centered around 50 and 100 nm. The same holds when oleate is added to preformed phospholipid liposomes. By contrast, when the same amount of oleate is added to an aqueous solution (as a control experiment), a very broad distribution ranging between 20 and 1000 nm is obtained. The data can then be seen as a kind of reproduction of the same size vesicles, and the argument is advanced that this may correspond to a simple prebiotic mechanism of vesicle multiplication in prebiotic times, when only physical forces might be responsible for the basic mechanisms of early protocell growth and division. Preliminary data also show that repeated addition of oleate maintains the same basic initial features, and that surfactants other than oleate also respect the reproductive mode of the matrix effect.

Spatial models of prebiotic evolution: soup before pizza?

The problem of information integration and resistance to the invasion of parasitic mutants in prebiotic replicator systems is a notorious issue of research on the origin of life. Almost all theoretical studies published so far have demonstrated that some kind of spatial structure is indispensable for the persistence and/or the parasite resistance of any feasible replicator system. Based on a detailed critical survey of spatial models on prebiotic information integration, we suggest a possible scenario for replicator system evolution leading to the emergence of the first protocells capable of independent life. We show that even the spatial versions of the hypercycle model are vulnerable to selfish parasites in heterogeneous habitats. Contrary, the metabolic system remains persistent and coexistent with its parasites both on heterogeneous surfaces and in chaotically mixing flowing media. Persistent metabolic parasites can be converted to metabolic cooperators, or they can gradually obtain replicase activity. Our simulations show that, once replicase activity emerged, a gradual and simultaneous evolutionary improvement of replicase functionality (speed and fidelity) and template efficiency is possible only on a surface that constrains the mobility of macromolecule replicators. Based on the results of the models reviewed, we suggest that open chaotic flows ('soup') and surface dynamics ('pizza') both played key roles in the sequence of evolutionary events ultimately concluding in the appearance of the first living cell on Earth.

Life, death and membrane bilayers

Membrane bilayers are essential elements of life, and the synthesis of the hydrocarbons that make up membrane bilayers may have preceded the appearance of life on Earth. Membrane-associated processes are significant components of metabolism, and the acyl composition of membrane bilayers is associated with metabolic activity in a predictable manner. This has resulted in the "membrane pacemaker" theory of metabolism, which proposes that the relative balance between monounsaturated and long-chain polyunsaturated acyl chains in membrane bilayers is a fundamental determinant of metabolic rate of a species. The omega-3 polyunsaturated docosahexaenoate is an especially important component of membranes in this regard. Whilst it is suggested that the physical properties of membrane polyunsaturates are important with respect to their influence on metabolic rate, it is their chemistry that is important in aging. Membrane acyl composition is related to maximum lifespan in mammals and birds, probably via their role in lipid peroxidation. Calorie restriction modifies acyl composition of membrane bilayers and is associated with decreased membrane lipid peroxidation and lifespan extension. The membrane pacemaker theory of metabolism has given birth to the membrane pacemaker hypothesis of aging, which will require further investigation.

Early assembly of cellular life

Popular hypotheses that attempt to explain the origin of prebiotic molecules and cellular life capable of growth and division are not always agreed upon. In this manuscript, information on early bacterial life on Earth is examined using information from several disciplines. For example, knowledge can be integrated from physics, thermodynamics, planetary sciences, geology, biogeochemistry, lipid chemistry, primordial cell structures, cell and molecular biology, microbiology, metabolism and genetics. The origin of life also required a combination of elements, compounds and environmental physical-chemical conditions that allowed cells to assemble in less than a billion years. This may have been widespread in the subsurface of the early Earth located at microscopic physical domains.

Evolution was chemically constrained

The objective of this paper is to present a systems view of the major features of biological evolution based upon changes in internal chemistry and uses of cellular space, both of which it will be stated were dependent on the changing chemical environment. The account concerns the major developments from prokaryotes to eukaryotes, to multi-cellular organisms, to animals with nervous systems and a brain, and finally to human beings and their uses of chemical elements in space outside themselves. It will be stated that the changes were in an inevitable progression, and were not just due to blind chance, so that "random searching" by a coded system to give species had a fixed overall route. The chemical sequence is from a reducing to an ever-increasingly oxidizing environment, while organisms retained reduced chemicals. The process was furthered recently by human beings who have also increased the range of reduced products trapped on Earth in novel forms. All the developments are brought about from the nature of the chemicals which organisms accumulate using the environment and its changes. The relationship to the manner in which particular species (gene sequences) were coincidentally changed, the molecular view of evolution, is left for additional examination. There is a further issue in that the changes of the chemistry of the environment developed largely at equilibrium due to the relatively fast reactions there of the available inorganic chemicals. Inside cells, some of these same chemicals also came to equilibrium within compounds. All such equilibria reduced the variance (degrees of freedom) of the total environmental/biological system and its possible development. However, the more sophisticated organic chemistry, almost totally inside cells until humans evolved, is kinetically controlled and limited by the demands of cellular reduction necessary to produce essential chemicals and by the availability of certain elements and energy. Hence the variability of reductive cellular organic chemistry and its limitations in cells have to be considered separately. While as a whole they drive the oxidation of the environment, they also allow speciation within the major changes of organisms. Human beings have introduced recently new, virtually irreversible, inorganic and organic chemistry in the environment, much of it new modes of irreversible storage of reduced chemicals, and this is, we state, the last possible step of chemical evolution. We must attempt to evaluate its effect on organisms generally. It must be clear that all the changes and the original life forms are dependent upon energy as well as material capture and flow. We shall have to consider in which forms energy was available over the period of evolution, how it was usefully transformed, and the ways in which its sources changed.

How did cells get their size?

Cells exercise size homeostasis, and the origins of their ability to do so is the topic of this essay. Before there were cells, there were protocells. The most basic questions about protocells as objects are: What were they made of, and how big were they? Asking how big they were implies that the answer to the first part includes a boundary. The best candidate for that boundary is a self-assembling lipid bilayer. Therefore, protocells are defined here as Darwinian liposomes-bilayer vesicles with mutable on-board replicases linked to phenotypes. Because liposomes undergo spontaneous fission and fusion, and are subject to osmotic forces, size regulation in the earliest protocells would essentially have been liposome physics. For successful protocells, averting osmotic lysis would have been the first order of business. However, from the outset size mattered too, because of sex and reproduction (i.e., genome mixing and genome copying in entities with phenotypes). Protocell fission and fusion would have blended seamlessly into protocell sex and reproduction, making any gene product that furnished control over protocell size changes doubly adaptive. A recurrent theme is the feedback role of bilayer tension in protocell size control. Ways in which primitive peptides and their aggregates (e.g., channels) might have allowed liposomes to gain improved volume and surface area homeostasis are suggested. Domain-swapped proteins that polymerize as filaments are discussed as the origin of cytoskeleton structures that diversify and stabilize liposome shapes and sizes. Throughout, attention is paid to the question of set points for cell size.

Membrane self-assembly processes: steps toward the first cellular life

This review addresses the question of the origin of life, with emphasis on plausible boundary structures that may have initially provided cellular compartmentation. Some form of compartmentation is a necessary prerequisite for maintaining the integrity of interdependent molecular systems that are associated with metabolism, and for permitting variations required for speciation. The fact that lipid-bilayer membranes define boundaries of all contemporary living cells suggests that protocellular compartments were likely to have required similar, self-assembled boundaries. Amphiphiles such as short-chain fatty acids, which were presumably available on the early Earth, can self-assemble into stable vesicles that encapsulate hydrophilic solutes with catalytic activity. Their suspensions in aqueous media have therefore been used to investigate nutrient uptake across simple membranes and encapsulated catalyzed reactions, both of which would be essential processes in protocellular life forms.

Chiral amplification of oligopeptides in two-dimensional crystalline self-assemblies on water

Differences in the two-dimensional packing arrangements of racemic and enantiomeric crystalline self-assemblies on the water surface of amphiphilic activated analogs of lysine and glutamic acid have been used to prepare oligopeptides of homochiral sequence and oligopeptides of single handedness from chiral nonracemic mixtures. The crystalline structures on the water surface were determined by grazing incidence x-ray diffraction and the diastereomeric composition of the oligopeptides by matrix-assisted laser desorption time-of-flight mass spectrometry with enantio-labeling. These results suggest that reactivity of ordered clusters at interfaces might have played a role in the generation of early homochiral biopolymers.

Self-assembled vesicles of monocarboxylic acids and alcohols: conditions for stability and for the encapsulation of biopolymers

We tested the ability of saturated n-monocarboxylic acids ranging from eight to 12 carbons in length to self-assemble into vesicles, and determined the minimal concentrations and chain lengths necessary to form stable bilayer membranes. Under defined conditions of pH and concentrations exceeding 150 mM, an unbranched monocarboxylic acid as short as eight carbons in length (n-octanoic acid) assembled into vesicular structures. Nonanoic acid (85 mM) formed stable vesicles at pH 7.0, the pK of the acid in bilayers, and was chosen for further testing. At pH 6 and below, the vesicles were unstable and the acid was present as droplets. At pH ranges of 8 and above clear solutions of micelles formed. However, addition of small amounts of an alcohol (nonanol) markedly stabilized the bilayers, and vesicles were present at significantly lower concentrations (approximately 20 mM) at pH ranges up to 11. The formation of vesicles near the pK(a) of the acids can be explained by the formation of stable RCOO(-)...HOOCR hydrogen bond networks in the presence of both ionized and neutral acid functions. Similarly, the effects of alcohols at high pH suggests the formation of stable RCOO(-)...HOR hydrogen bond networks when neutral RCOOH groups are absent. The vesicles provided a selective permeability barrier, as indicated by osmotic activity and ionic dye capture, and could encapsulate macromolecules such as DNA and a protein. When catalase was encapsulated in vesicles of decanoic acid and decanol, the enzyme was protected from degradation by protease, and could act as a catalyst for its substrate, hydrogen peroxide, which readily diffused across the membrane. We conclude that membranous vesicles produced by mixed short chain monocarboxylic acids and alcohols are useful models for testing the limits of stabilizing hydrophobic effects in membranes and for prebiotic membrane formation.

Prebiotic oligomerization on or inside lipid vesicles in hydrothermal environments

Oligomerization of amino acids proceeded on or inside lipid vesicles as a model of prebiotic cells in a simulated hydrothermal environment. When the suspension of lipid vesicles taking up monomeric glycine underwent a sudden temperature drop by traversing from a hot (180 degrees C) to a cold (0 degree C) region repeatedly while circulating through a closed reaction circuit, oligopeptides up to heptaglycine were formed even in the absence of condensing agents.

The first cell membranes

Organic compounds are synthesized in the interstellar medium and can be delivered to planetary surfaces such as the early Earth, where they mix with endogenous species. Some of these compounds are amphiphilic, having polar and nonpolar groups on the same molecule. Amphiphilic compounds spontaneously self-assemble into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to have been produced from amphiphilic compounds on the prebiotic Earth. Laboratory simulations show that such vesicles readily encapsulate functional macromolecules, including nucleic acids and polymerases. The goal of future investigations will be to fabricate artificial cells as models of the origin of life.

The molecular roots of compositional inheritance

Non-covalent compositional assemblies, made of monomeric mutually catalytic molecules, constitute an alternative to alphabet-based informational biopolymers as a mechanism of primordial inheritance. Such assemblies appear implicitly in many "Metabolism First" origin of life scenarios, and more explicitly in the Graded Autocatalysis Replication Domain (GARD) model [Segréet al. (2000). Proc. Natl Acad. Sci. U.S.A.97, 4112-4117]. In the present work, we provide a detailed analysis of the quantitative molecular roots of such behavior. It is demonstrated that the fidelity of reproduction provided by a newly defined heritability measure eta(*)(s), strongly depends on the values of molecular recognition parameters and on assembly size. We find that if the catalytic rate acceleration coefficients are distributed normally, transfer of compositional information becomes impossible, due to frequent "compositional error catastrophes". In contrast, if the catalytic acceleration rates obey a lognormal distribution, as actually predicted by a statistical formalism for molecular repertoires, high reproduction fidelity is obtained. There is also a clear dependence on assembly size N, whereby maximal eta is seen in a narrow range around N approximately 3.5 N(G)/lambda, where N(G)is the size of the primordial molecular repertoire and lambda is a molecular interaction statistical parameter. Such relationships help define the physicochemical conditions that could underlie the early steps in pre-biotic evolution.

Artificial chemistries--a review

This article reviews the growing body of scientific work in artificial chemistry. First, common motivations and fundamental concepts are introduced. Second, current research activities are discussed along three application dimensions: modeling, information processing, and optimization. Finally, common phenomena among the different systems are summarized. It is argued here that artificial chemistries are "the right stuff" for the study of prebiotic and biochemical evolution, and they provide a productive framework for questions regarding the origin and evolution of organizations in general. Furthermore, artificial chemistries have a broad application range of practical problems, as shown in this review.

Composing life

Textbooks often assert that life began with specialized complex molecules, such as RNA, that are capable of making their own copies. This scenario has serious difficulties, but an alternative has remained elusive. Recent research and computer simulations have suggested that the first steps toward life may not have involved biopolymers. Rather, non-covalent protocellular assemblies, generated by catalyzed recruitment of diverse amphiphilic and hydrophobic compounds, could have constituted the first systems capable of information storage, inheritance and selection. A complex chain of evolutionary events, yet to be deciphered, could then have led to the common ancestors of today's free-living cells, and to the appearance of DNA, RNA and protein enzymes.

Compositional genomes: prebiotic information transfer in mutually catalytic noncovalent assemblies

Mutually catalytic sets of simple organic molecules have been suggested to be capable of self-replication and rudimentary chemical evolution. Previous models for the behavior of such sets have analyzed the global properties of short biopolymer ensembles by using graph theory and a mean field approach. In parallel, experimental studies with the autocatalytic formation of amphiphilic assemblies (e.g., lipid vesicles or micelles) demonstrated self-replication properties resembling those of living cells. Combining these approaches, we analyze here the kinetic behavior of small heterogeneous assemblies of spontaneously aggregating molecules, of the type that could form readily under prebiotic conditions. A statistical formalism for mutual rate enhancement is used to numerically simulate the detailed chemical kinetics within such assemblies. We demonstrate that a straightforward set of assumptions about kinetically enhanced recruitment of simple amphiphilic molecules, as well as about the spontaneous growth and splitting of assemblies, results in a complex population behavior. The assemblies manifest a significant degree of homeostasis, resembling the previously predicted quasi-stationary states of biopolymer ensembles (Dyson, F. J. (1982) J. Mol. Evol. 18, 344-350). Such emergent catalysis-driven, compositionally biased entities may be viewed as having rudimentary "compositional genomes." Our analysis addresses the question of how mutually catalytic metabolic networks, devoid of sequence-based biopolymers, could exhibit transfer of chemical information and might undergo selection and evolution. This computed behavior may constitute a demonstration of natural selection in populations of molecules without genetic apparatus, suggesting a pathway from random molecular assemblies to a minimal protocell.