Divergent prebiotic synthesis of pyrimidine and 8-oxo-purine ribonucleotides

The impact of organic matter and iron/calcium coupling on phosphorus retention in the hyporheic zone of the Danjiangkou area tributary: Evidence from bonding recognition

The coupling between organic matter (OM) and minerals considerably influences the phosphorus (P) cycle within the hyporheic zone, but the role of different geological mineral-organic complexes (MOCs) on P burial during hyporheic exchange remains under-explored. This study investigates the effects of OM and iron (Fe)/calcium (Ca) coupling on P migration within the hyporheic zone of an agricultural tributary to the Danjiangkou Reservoir. These relationships were explored by measuring hyporheic flow (q), organic and inorganic P forms, and sediment PO4-P adsorption capacity [following treatment with fulvic acid (FA), Fe-OM, or Ca-OM]. Multivariate statistical analysis, X-Ray Diffraction, Fourier-transform Infrared Spectroscopy, and X-ray Photoelectron Spectroscopy were employed to elucidate the underlying mechanisms. Results indicate that upward hyporheic flow transports dissolved porewater P into surface water, contributing 11.27-12.13 % of the total P flux. MOCs associated with Fe(III)/Ca silicate minerals, along with FA and labile OM, were identified as key OM fractions influencing P migration, contributing 5-24 %, 10-11.7 %, and 6-14.9 % to the overall flux, respectively. FA and labile OM facilitate P release, whereas MOCs enhance P retention. Ca-OM is the most efficient PO4-P adsorption [adsorption capacity (AC): 0.8980-0.9524 mg/g], followed by Fe-OM (AC: 0.5120-0.7020 mg/g), original sediment (AC: 0.4368-0.5596 mg/g), and FA (AC: 0.2657-0.2769 mg/g). Cation bridges, primarily formed by -OH and -NH2 groups within Ca-OM (outer-sphere complexes), promote greater P adsorption than Fe-OM (inner-sphere complexes, mainly associated with -COOH). However, Fe-OM-P exhibits a more stable structure. In high P environments, P adsorption onto Ca-OM may induce the release of labile OM, temporarily retaining P through resorption onto labile OM. Hyporheic flow with higher pH and Eh values promotes MOC formation, underscoring their significant P retention capacity. Therefore, strategic MOC use within the hyporheic zone is crucial for mitigating surface water eutrophication.

On the aqueous origins of the condensation polymers of life

Water is essential for life as we know it, but it has paradoxically been considered inimical to the emergence of life. Proteins and nucleic acids have sustained evolution and life for billions of years, but both are condensation polymers, suggesting that their formation requires the elimination of water. This presents intrinsic challenges at the origins of life, including how condensation polymer synthesis can overcome the thermodynamic pressure of hydrolysis in water and how nucleophiles can kinetically outcompete water to yield condensation products. The answers to these questions lie in balancing thermodynamic activation and kinetic stability. For peptides, an effective strategy is to directly harness the energy trapped in prebiotic molecules, such as nitriles, and avoid the formation of fully hydrolysed monomers. In this Review, we discuss how chemical energy can be built into precursors, retained, and released selectively for polymer synthesis. Looking to the future, the outstanding goals include how nucleic acids can be synthesized, avoiding the formation of fully hydrolysed monomers and what caused information to flow from nucleic acids to proteins.

Multicomponent Synthesis of C(8)-Substituted Purine Building Blocks of Peptide Nucleic Acids from Prebiotic Compounds

We have explored the reaction of a three-components mixture of aminomalononitrile, urea and α-amino acid methyl esters for the multicomponent synthesis substituted purines resembling PNA's building blocks. 2,6-diamino-purines, 6-amino-3,9-dihydro-2H-purin-2-one (iso-guanines), and 3,9-dihydro-6H-purin-6-one derivatives, selectively decorated at C(8)-position of the purine ring with different amino acid residues, were obtained from acceptable to good yields. The regio-selectivity of the transformation was controlled by the use of urea in the ternary mixture and by the annulation agent involved in the ring-closure of amino-imidazole carbonitrile intermediates. Solvent free conditions, microwave irradiation and simple one-carbon containing reagents further satisfied the major requirement of atom economy and sustainable chemistry. Due to the prebiotic nature of the three-components mixture and of annulation agents, it also embodies the possibility for the synthesis of novel PNAs bearing purine nucleobases decorated at C(8)-position of the imidazole ring as alternative RNA analogues in molecular evolution.

Weak effects of prebiotically plausible peptides on self-triphosphorylation ribozyme function

Catalytic RNAs (ribozymes) were central to early stages of life on earth. The first ribozymes probably emerged in the presence of prebiotically generated peptides because amino acids can be generated under abiotic conditions, and amino acids can oligomerize into peptides under prebiotically plausible conditions. Here we tested whether the presence of prebiotically plausible peptides could have aided the emergence of ribozymes, by an in vitro selection of self-triphosphorylation ribozymes from random sequence in the presence of ten different octapeptides. These peptides were composed of ten different, prebiotically plausible amino acids, each as mixture of d- and l-stereoisomers. After five rounds of selection and high throughput sequencing analysis, ten ribozymes that appeared most promising for peptide benefits were tested biochemically for possible benefits from each of the ten peptides. The strongest peptide benefit enhanced ribozyme activity by 2.6-fold, similar to the effect from an increase in the pH by one-half unit. Four arbitrarily chosen ribozymes from a previous selection without peptides showed no significant change in their activity in the presence of the ten peptides. Therefore, the used prebiotically plausible peptides - peptides without evolutionarily optimized sequence, without cationic or aromatic side chains - did not provide a strong benefit for the emergence of ribozyme activity. This finding stands in contrast to previously identified polycationic peptides, conjugates between peptides and polyaromatic hydrocarbons, and modern mRNA encoded proteins, all of which can strongly increase ribozyme function. The results are discussed in the context of origins of life.

Spontaneous Peptide Ligation Mediated by Cysteamine

The fundamental and universal nature of life's exploitation of peptides suggests they must have played a vital role during the onset of life, but their spontaneous chemoselective synthesis in water remains unknown. Aminonitriles (1) are widely accepted as prebiotic precursors of both amino acids and peptides, but they do not spontaneously polymerize in water to yield peptides. Here, we demonstrate that the simple prebiotically plausible aminothiol, cysteamine (5), participates in Strecker chemistry to furnish β-mercaptoethyl-α-aminonitriles (8) and β-mercaptoethyl-amino acids (16), which are predisposed to spontaneously form peptides in water. Intramolecular thiol catalyzed ligation is faster, higher-yielding, and more α-selective than previously reported prebiotic peptide ligation chemistries, enabling, for example, the highly regioselective α-ligation of Asp- and Glu-dinitriles in quantitative yields. Our findings suggest that cysteamine (5), the thiol bearing moiety of the universal thiol cofactor coenzyme A, may have played an important role in the selective chemical synthesis of prebiotic α-peptides.

Prebiotically plausible chemoselective pantetheine synthesis in water

Coenzyme A (CoA) is essential to all life on Earth, and its functional subunit, pantetheine, is important in many origin-of-life scenarios, but how pantetheine emerged on the early Earth remains a mystery. Earlier attempts to selectively synthesize pantetheine failed, leading to suggestions that "simpler" thiols must have preceded pantetheine at the origin of life. In this work, we report high-yielding and selective prebiotic syntheses of pantetheine in water. Chemoselective multicomponent aldol, iminolactone, and aminonitrile reactions delivered spontaneous differentiation of pantoic acid and proteinogenic amino acid syntheses, as well as the dihydroxyl, gem-dimethyl, and β-alanine-amide moieties of pantetheine in dilute water. Our results are consistent with a role for canonical pantetheine at the outset of life on Earth.

Biological Catalysis and Information Storage Have Relied on N-Glycosyl Derivatives of β-D-Ribofuranose since the Origins of Life

Most naturally occurring nucleotides and nucleosides are N-glycosyl derivatives of β-d-ribose. These N-ribosides are involved in most metabolic processes that occur in cells. They are essential components of nucleic acids, forming the basis for genetic information storage and flow. Moreover, these compounds are involved in numerous catalytic processes, including chemical energy production and storage, in which they serve as cofactors or coribozymes. From a chemical point of view, the overall structure of nucleotides and nucleosides is very similar and simple. However, their unique chemical and structural features render these compounds versatile building blocks that are crucial for life processes in all known organisms. Notably, the universal function of these compounds in encoding genetic information and cellular catalysis strongly suggests their essential role in the origins of life. In this review, we summarize major issues related to the role of N-ribosides in biological systems, especially in the context of the origin of life and its further evolution, through the RNA-based World(s), toward the life we observe today. We also discuss possible reasons why life has arisen from derivatives of β-d-ribofuranose instead of compounds based on other sugar moieties.

Prebiotic Synthesis of ATP: A Terrestrial Volcanism-Dependent Pathway

Adenosine triphosphate (ATP) is a multifunctional small molecule, necessary for all modern Earth life, which must be a component of the last universal common ancestor (LUCA). However, the relatively complex structure of ATP causes doubts about its accessibility on prebiotic Earth. In this paper, based on previous studies on the synthesis of ATP components, a plausible prebiotic pathway yielding this key molecule is constructed, which relies on terrestrial volcanism to provide the required materials and suitable conditions.

Discovery and properties of a monoclonal antibody targeting 8-oxoA, an oxidized adenine lesion in DNA and RNA

8-oxoA, a major oxidation product of adenosine, is a mispairing, mutagenic lesion that arises in DNA and RNA when ^•OH radicals or one-electron oxidants attack the C8 adenine atom or polymerases misincorporate 8-oxo(d)ATP. The danger of 8-oxoA is underscored by the existence of dedicated cellular repair machinery that explicitly excise it from DNA, the attenuation of translation induced by 8-oxoA-mRNA or damaged ribosomes, and its potency as a TLR7 agonist. Here we present the discovery, purification, and biochemical characterization of a new mouse IgGk1 monoclonal antibody (6E4) that specifically targets 8-oxoA. Utilizing an AchE-based competitive ELISA assay, we demonstrate the selectivity of 6E4 for 8-oxoA over a plethora of canonical and chemically modified nucleosides including 8-oxoG, A, m6A, 2-oxoA, and 5-hoU. We further show the ability of 6E4 to exclusively recognize 8-oxoA in nucleoside triphosphates (8-oxoATP) and DNA/RNA oligonucleotides containing a single 8-oxoA. 6E4 also binds 8-oxoA in duplex DNA/RNA antigens where the lesion is either paired correctly or base mismatched. Our findings define the 8-oxoAde nucleobase as the critical epitope and indicate mAb 6E4 is ideally suited for a broad range of immunological applications in nucleic acid detection and quality control.

Selective Synthesis of Lysine Peptides and the Prebiotically Plausible Synthesis of Catalytically Active Diaminopropionic Acid Peptide Nitriles in Water

Why life encodes specific proteinogenic amino acids remains an unsolved problem, but a non-enzymatic synthesis that recapitulates biology's universal strategy of stepwise N-to-C terminal peptide growth may hold the key to this selection. Lysine is an important proteinogenic amino acid that, despite its essential structural, catalytic, and functional roles in biochemistry, has widely been assumed to be a late addition to the genetic code. Here, we demonstrate that lysine thioacids undergo coupling with aminonitriles in neutral water to afford peptides in near-quantitative yield, whereas non-proteinogenic lysine homologues, ornithine, and diaminobutyric acid cannot form peptides due to rapid and quantitative cyclization that irreversibly blocks peptide synthesis. We demonstrate for the first time that ornithine lactamization provides an absolute differentiation of lysine and ornithine during (non-enzymatic) N-to-C-terminal peptide ligation. We additionally demonstrate that the shortest lysine homologue, diaminopropionic acid, undergoes effective peptide ligation. This prompted us to discover a high-yielding prebiotically plausible synthesis of the diaminopropionic acid residue, by peptide nitrile modification, through the addition of ammonia to a dehydroalanine nitrile. With this synthesis in hand, we then discovered that the low basicity of diaminopropionyl residues promotes effective, biomimetic, imine catalysis in neutral water. Our results suggest diaminopropionic acid, synthesized by peptide nitrile modification, can replace or augment lysine residues during early evolution but that lysine's electronically isolated sidechain amine likely provides an evolutionary advantage for coupling and coding as a preformed monomer in monomer-by-monomer peptide translation.

Photochemistry of 2-thiooxazole: a plausible prebiotic precursor to RNA nucleotides

Potentially prebiotic chemical reactions leading to RNA nucleotides involve periods of UV irradiation, which are necessary to promote selectivity and destroy biologially irrelevant side products. Nevertheless, UV light has only been applied to promote specific stages of prebiotic reactions and its effect on complete prebiotic reaction sequences has not been extensively studied. Here, we report on an experimental and computational investigation of the photostability of 2-thiooxazole (2-TO), a potential precursor of pyrimidine and 8-oxopurine nucleotides on early Earth. Our UV-irradiation experiments resulted in rapid decomposition of 2-TO into unidentified small molecule photoproducts. We further clarify the underlying photochemistry by means of accurate ab initio calculations and surface hopping molecular dynamics simulations. Overall, the computational results show efficient rupture of the aromatic ring upon the photoexcitation of 2-TO via breaking of the C-O bond. Consequently, the initial stage of the divergent prebiotic synthesis of pyrimidine and 8-oxopurine nucleotides would require periodic shielding from UV light either with sun screening chromophores or through a planetary scenario that would protect 2-TO until it is transformed into a more stable intermediate compound, e.g. oxazolidinone thione.

Borate-guided ribose phosphorylation for prebiotic nucleotide synthesis

Polymers of ribonucleotides (RNAs) are considered to store genetic information and promote biocatalytic reactions for the proto life on chemical evolution. Abiotic synthesis of ribonucleotide was successful in past experiments; nucleoside synthesis occurred first, followed by phosphorylation. These abiotic syntheses are far from biotic reactions and have difficulties as a prebiotic reaction in reacting chemicals in a specific order and purifying intermediates from other molecules in multi-steps of reactions. Another reaction, ribose phosphorylation followed by nucleobase synthesis or nucleobase addition, is close to the biotic reactions of nucleotide synthesis. However, the synthesis of ribose 5′-phosphate under prebiotically plausible conditions remains unclear. Here, we report a high-yield regioselective one-pot synthesis of ribose 5′-phosphate from an aqueous solution containing ribose, phosphate, urea, and borate by simple thermal evaporation. Of note, phosphorylation of ribose before the nucleoside formation differs from the traditional prebiotic nucleotide syntheses and is also consistent with biological nucleotide synthesis. Phosphorylation occurred to the greatest extent in ribose compared to other aldopentoses, only in the presence of borate. Borate is known to improve the stability of ribose preferentially. Geological evidence suggests the presence of borate-rich settings on the early Earth. Therefore, borate-rich evaporitic environments could have facilitated preferential synthesis of ribonucleotide coupled with enhanced stability of ribose on the early Earth.

Protometabolism as out-of-equilibrium chemistry

It is common to compare life with machines. Both consume fuel and release waste to run. In biology, the engine that drives the living system is referred to as metabolism. However, attempts at deciphering the origins of metabolism do not focus on this energetic relationship that sustains life but rather concentrate on nonenzymatic reactions that produce all the intermediates of an extant metabolic pathway. Such an approach is akin to studying the molecules produced from the burning of coal instead of deciphering how the released energy drives the movement of pistons and ultimately the train when investigating the mechanisms behind locomotion. Theories that do explicitly invoke geological chemical gradients to drive metabolism most frequently feature hydrothermal vent conditions, but hydrothermal vents are not the only regions of the early Earth that could have provided the fuel necessary to sustain the Earth's first (proto)cells. Here, we give examples of prior reports on protometabolism and highlight how more recent investigations of out-of-equilibrium systems may point to alternative scenarios more consistent with the majority of prebiotic chemistry data accumulated thus far. This article is part of the theme issue 'Emergent phenomena in complex physical and socio-technical systems: from cells to societies'.

Prebiotic synthesis and triphosphorylation of 3'-amino-TNA nucleosides

Nucleosides are essential to the emergence of life, and so their synthesis is a key challenge for prebiotic chemistry. Although amino-nucleosides have enhanced reactivity in water compared with ribonucleosides, they are assumed to be prebiotically irrelevant due to perceived difficulties with their selective formation. Here we demonstrate that 3'-amino-TNA nucleosides (TNA, threose nucleic acid) are formed diastereoselectively and regiospecifically from prebiotic feedstocks in four high-yielding steps. Phosphate provides an unexpected resolution, leading to spontaneous purification of the genetically relevant threo-isomer. Furthermore, 3'-amino-TNA nucleosides are shown to be phosphorylated directly in water, under mild conditions with cyclic trimetaphosphate, forming a nucleoside triphosphate (NTP) in a manner not feasible for canonical nucleosides. Our results suggest 3'-amino-TNA nucleosides may have been present on the early Earth, and the ease with which these NTPs form, alongside the inherent selectivity for the Watson-Crick base-pairing threo-monomer, warrants further study of the role they could play during the emergence of life.

A Nonenzymatic Analog of Pyrimidine Nucleobase Biosynthesis

Metabolic theories for the origin of life posit that inorganic catalysts enabled self-organized chemical precursors to the pathways of metabolism, including those that make genetic molecules. Recently, experiments showing nonenzymatic versions of a number of core metabolic pathways have started to support this idea. However, experimental demonstrations of nonenzymatic reaction sequences along the de novo ribonucleotide biosynthesis pathways are limited. Here we show that all three reactions of pyrimidine nucleobase biosynthesis that convert aspartate to orotate proceed at 60 °C without photochemistry under aqueous conditions in the presence of metals such as Cu2+ and Mn4+ . Combining reactions into one-pot variants is also possible. Life may not have invented pyrimidine nucleobase biosynthesis from scratch, but simply refined existing nonenzymatic reaction channels. This work is a first step towards uniting metabolic theories of life's origin with those centered around genetic molecules.

Surface Roughening Strategy for Highly Efficient Bifunctional Electrocatalyst: Combination of Atomic Layer Deposition and Anion Exchange Reaction

Electrocatalytic water splitting, which is an interface-dominated process, can be significantly accelerated by increasing the number of front-line surface active sites (N_A ) of the electrocatalyst. In this study, a unique method is used for increasing the N_A by converting the smooth ultrathin atomic-layer-deposited nanoshells of the electrocatalysts into nano-roughened active shell layers using a controlled anion-exchange reaction (AER). The coarse thin nanoshells present abundant surface active sites, which are generated owing to the inherent unit-cell volume mismatch induced during the AER. Consequently, the nano-roughened electrodes accelerate the sluggish water reaction kinetics and lower the overpotentials required for the hydrogen and oxygen evolution reactions. In addition, the electronic modulation induced by the nanoshell layer at the core-nanoshell interface amplifies the local electron density, as confirmed using electrochemical analysis data and density functional theory calculations. Because of the integrity of the composite electrodes during water-splitting half-cell reactions, their durability for industrial seawater electrolysis is evaluated. The results indicate that their electrochemical activity does not change significantly after 10 days of continuous overall water splitting.

A New Perspective on the Maillard Reaction and the Origin of Life

The Maillard reaction, a spontaneous 'one pot' reaction between amino acids and reducing sugars that occurs at low reactant concentrations and low temperatures, is a good candidate for having played a role in the origin of life on the Earth. In view of the probability that RNA and DNA were preceded by an evolutionary forerunner with a more straightforward prebiotic synthesis, it is a testament to the prescience of Oró and colleagues that, in 1975, they drew attention to the Maillard reaction, in particular evidence that melanoidin polymers (the end-product of the reaction) contain '…heterocyclic nitrogen compounds similar to the nitrogenous bases' (Nissenbaum in J Mol Evol 6:253-270, 1975). Indeed, reports of the Maillard reaction product, 2-Acetyl-6-(Hydroxymethyl)-5,6-Dihydro-4H-Pyridinone (AHDP), with a structure reminiscent of the pyrimidine nucleobase uracil, suggest the Maillard reaction might have played a key role in the synthesis of components of a proto-RNA polymer, with AHDP and two structurally related products predicted to be similar to uracil in the latter's ability to form non-standard base pair interactions. It is possible that the primary function of these interactions was to allow molecules such as AHDP to separate out of the prebiotic chemical clutter. If this were the case, catalysis, and coding-made possible by the polymerization of proto-nucleoside monomers into linear sequence strings-would have been evolving properties.

When Is a Reaction Network a Metabolism? Criteria for Simple Metabolisms That Support Growth and Division of Protocells

With the aim of better understanding the nature of metabolism in the first cells and the relationship between the origin of life and the origin of metabolism, we propose three criteria that a chemical reaction system must satisfy in order to constitute a metabolism that would be capable of sustaining growth and division of a protocell. (1) Biomolecules produced by the reaction system must be maintained at high concentration inside the cell while they remain at low or zero concentration outside. (2) The total solute concentration inside the cell must be higher than outside, so there is a positive osmotic pressure that drives cell growth. (3) The metabolic rate (i.e., the rate of mass throughput) must be higher inside the cell than outside. We give examples of small-molecule reaction systems that satisfy these criteria, and others which do not, firstly considering fixed-volume compartments, and secondly, lipid vesicles that can grow and divide. If the criteria are satisfied, and if a supply of lipid is available outside the cell, then continued growth of membrane surface area occurs alongside the increase in volume of the cell. If the metabolism synthesizes more lipid inside the cell, then the membrane surface area can increase proportionately faster than the cell volume, in which case cell division is possible. The three criteria can be satisfied if the reaction system is bistable, because different concentrations can exist inside and out while the rate constants of all the reactions are the same. If the reaction system is monostable, the criteria can only be satisfied if there is a reason why the rate constants are different inside and out (for example, the decay rates of biomolecules are faster outside, or the formation rates of biomolecules are slower outside). If this difference between inside and outside does not exist, a monostable reaction system cannot sustain cell growth and division. We show that a reaction system for template-directed RNA polymerization can satisfy the requirements for a metabolism, even if the small-molecule reactions that make the single nucleotides do not.

Timescales for Prebiotic Photochemistry Under Realistic Surface Ultraviolet Conditions

Ultraviolet (UV) light has long been invoked as a source of energy for prebiotic chemical synthesis, but experimental support does not involve sources of UV light that look like the young Sun. Here we experimentally investigate whether the UV flux available on the surface of early Earth, given a favorable atmosphere, can facilitate a variety of prebiotic chemical syntheses. We construct a solar simulator for the UV light of the faint young Sun on the surface of early Earth, called StarLab. We then attempt a series of reactions testing different aspects of a prebiotic chemical scenario involving hydrogen cyanide (HCN), sulfites, and sulfides under the UV light of StarLab, including hypophosphite oxidation by UV light and hydrogen sulfide, photoreduction of HCN with bisulfite, the photoanomerization of α-thiocytidine, the production of a chemical precursor of a potentially prebiotic activating agent (nitroprusside), the photoreduction of thioanhydrouridine and thioanhydroadenosine, and the oxidation of ethanol (EtOH) by photochemically generated hydroxyl radicals. We compare the output of StarLab to the light of the faint young Sun to constrain the timescales over which these reactions would occur on the surface of early Earth. We predict that hypophosphite oxidation, HCN reduction, and photoproduction of nitroprusside would all operate on the surface of early Earth in a matter of days to weeks. The photoanomerization of α-thiocytidine would take months to complete, and the production of oxidation products from hydroxyl radicals would take years. The photoreduction of thioanhydrouridine with hydrogen sulfide did not succeed even after a long period of irradiation, providing a lower limit on the timescale of several years. The photoreduction of thioanhydroadenosine with bisulfite produced 2'-deoxyriboadenosine (dA) on the timescale of days. This suggests the plausibility of the photoproduction of purine deoxyribonucleotides, such as the photoproduction of simple sugars, proceeds more efficiently in the presence of bisulfite.

Depsipeptide Nucleic Acids: Prebiotic Formation, Oligomerization, and Self-Assembly of a New Proto-Nucleic Acid Candidate

The mechanism by which informational polymers first formed on the early earth is currently unknown. The RNA world hypothesis implies that RNA oligomers were produced prebiotically, before the emergence of enzymes, but the demonstration of such a process remains challenging. Alternatively, RNA may have been preceded by an earlier ancestral polymer, or proto-RNA, that had a greater propensity for self-assembly than RNA, with the eventual transition to functionally superior RNA being the result of chemical or biological evolution. We report a new class of nucleic acid analog, depsipeptide nucleic acid (DepsiPNA), which displays several properties that are attractive as a candidate for proto-RNA. The monomers of depsipeptide nucleic acids can form under plausibly prebiotic conditions. These monomers oligomerize spontaneously when dried from aqueous solutions to form nucleobase-functionalized depsipeptides. Once formed, these DepsiPNA oligomers are capable of complementary self-assembly and are resistant to hydrolysis in the assembled state. These results suggest that the initial formation of primitive, self-assembling, informational polymers on the early earth may have been relatively facile if the constraints of an RNA-first scenario are relaxed.

Selective Prebiotic Synthesis of α-Threofuranosyl Cytidine by Photochemical Anomerization

The structure of life's first genetic polymer is a question of intense ongoing debate. The "RNA world theory" suggests RNA was life's first nucleic acid. However, ribonucleotides are complex chemical structures, and simpler nucleic acids, such as threose nucleic acid (TNA), can carry genetic information. In principle, nucleic acids like TNA could have played a vital role in the origins of life. The advent of any genetic polymer in life requires synthesis of its monomers. Here we demonstrate a high-yielding, stereo-, regio- and furanosyl-selective prebiotic synthesis of threo-cytidine 3, an essential component of TNA. Our synthesis uses key intermediates and reactions previously exploited in the prebiotic synthesis of the canonical pyrimidine ribonucleoside cytidine 1. Furthermore, we demonstrate that erythro-specific 2',3'-cyclic phosphate synthesis provides a mechanism to photochemically select TNA cytidine. These results suggest that TNA may have coexisted with RNA during the emergence of life.

On the origin of the genetic code: a 27-codon hypothetical precursor of an intricate 64-codon intermediate shaped the modern code

The modern genetic code reveals numerous traces of specific relationships between the early codons which, together with its internal asymmetries, suggest a sequential appearance of the nucleobases in primitive RNA molecules. Keeping the hypothesis of triplet pairings between primitive RNA molecules at the origin of the code, this work systematically examines complete codon-anticodon interaction matrices assuming distinct pairing options at each position of the triplet duplexes. Application of these principles suggests that a 27-codon precursor having a reasonable coding capacity for short peptide synthesis could have started with primitive RNA molecules able to form two distinct pairs with different free energies between a single purine and two pyrimidines (such as G with C and U). Conservation of the same pairing options at positions 1 and 2 of codons at the arrival of a second purine with distinct pairing preferences (such as A) generated a 64-codon intermediate code made of interrelated pairs or groups of codons (designated here as intricacy). The numerous traces of this hypothetical scheme that are visible in the standard and variant forms of the modern code demonstrate without ambiguity that the ancestral codon-anticodon duplexes required high energetic pairings at their central position (Watson-Crick) but tolerated less energetic pairings at the first codon position (G • U type). Combined with the sequential appearance of the nucleobases, the predicted codon intricacy allows a stepwise reconstruction of the evolution of the coding repertoire, by simple a posteriori comparison to the modern code. This reconstruction reveals a remarkable internal coherence in terms of amino acids and tRNA synthetases recruitment. The code started with a group of amino acids (Ala, Gly, Pro, Ser and Thr) that are now all activated by class II tRNA synthetases before reaching an intermediate period during which up to 14 distinct amino acids could be encoded by a full set of intricated codons. The perfect coincidence between the last 6 amino acids predicted in this reconstruction and the speculated action of the arrival of free atmospheric oxygen on proteins is spectacular, and suggests that the code has only reached its present form after the great oxidation event.

Inosine in Biology and Disease

The nucleoside inosine plays an important role in purine biosynthesis, gene translation, and modulation of the fate of RNAs. The editing of adenosine to inosine is a widespread post-transcriptional modification in transfer RNAs (tRNAs) and messenger RNAs (mRNAs). At the wobble position of tRNA anticodons, inosine profoundly modifies codon recognition, while in mRNA, inosines can modify the sequence of the translated polypeptide or modulate the stability, localization, and splicing of transcripts. Inosine is also found in non-coding and exogenous RNAs, where it plays key structural and functional roles. In addition, molecular inosine is an important secondary metabolite in purine metabolism that also acts as a molecular messenger in cell signaling pathways. Here, we review the functional roles of inosine in biology and their connections to human health.

The Emergence of RNA from the Heterogeneous Products of Prebiotic Nucleotide Synthesis

Recent advances in prebiotic chemistry are beginning to outline plausible pathways for the synthesis of the canonical ribonucleotides and their assembly into oligoribonucleotides. However, these reaction pathways suggest that many noncanonical nucleotides are likely to have been generated alongside the standard ribonucleotides. Thus, the oligomerization of prebiotically synthesized nucleotides is likely to have led to a highly heterogeneous collection of oligonucleotides comprised of a wide range of types of nucleotides connected by a variety of backbone linkages. How then did relatively homogeneous RNA emerge from this primordial heterogeneity? Here we focus on nonenzymatic template-directed primer extension as a process that would have strongly enriched for homogeneous RNA over the course of multiple cycles of replication. We review the effects on copying the kinetics of nucleotides with altered nucleobase and sugar moieties, when they are present as activated monomers and when they are incorporated into primer and template oligonucleotides. We also discuss three variations in backbone connectivity, all of which are nonheritable and regenerate native RNA upon being copied. The kinetic superiority of RNA synthesis suggests that nonenzymatic copying served as a chemical selection mechanism that allowed relatively homogeneous RNA to emerge from a complex mixture of prebiotically synthesized nucleotides and oligonucleotides.

Ultrafast nonradiative deactivation of photoexcited 8-oxo-hypoxanthine: a nonadiabatic molecular dynamics study

In the scientific endeavor to understand the chemical origins of life, the photochemistry of the smallest life building blocks, nucleobases, has been a constant object of focus and intense research. Here, we report the results of the first theoretical study on the photo-properties of an 8-oxo-hypoxanthine molecule, the chromophore of 8-oxo-inosine, which is relevant to the recently proposed, prebiotically plausible synthetic routes to the formation of purine- and pyrimidine-nucleotides. With ab initio and semi-empirical OM2/MRCI quantum-chemistry calculations, we predict a strong photostability of the 8-oxo-hypoxanthine system and see the origin of this effect in ultrafast nonradiative relaxation through puckering of the 6-membered heterocyclic ring.

Cofactors are Remnants of Life's Origin and Early Evolution

The RNA World is one of the most widely accepted hypotheses explaining the origin of the genetic system used by all organisms today. It proposes that the tripartite system of DNA, RNA, and proteins was preceded by one consisting solely of RNA, which both stored genetic information and performed the molecular functions encoded by that genetic information. Current research into a potential RNA World revolves around the catalytic properties of RNA-based enzymes, or ribozymes. Well before the discovery of ribozymes, Harold White proposed that evidence for a precursor RNA world could be found within modern proteins in the form of coenzymes, the majority of which contain nucleobases or nucleoside moieties, such as Coenzyme A and S-adenosyl methionine, or are themselves nucleotides, such as ATP and NADH (a dinucleotide). These coenzymes, White suggested, had been the catalytic active sites of ancient ribozymes, which transitioned to their current forms after the surrounding ribozyme scaffolds had been replaced by protein apoenzymes during the evolution of translation. Since its proposal four decades ago, this groundbreaking hypothesis has garnered support from several different research disciplines and motivated similar hypotheses about other classes of cofactors, most notably iron-sulfur cluster cofactors as remnants of the geochemical setting of the origin of life. Evidence from prebiotic geochemistry, ribozyme biochemistry, and evolutionary biology, increasingly supports these hypotheses. Certain coenzymes and cofactors may bridge modern biology with the past and can thus provide insights into the elusive and poorly-recorded period of the origin and early evolution of life.

Prebiotic Nucleoside Synthesis: The Selectivity of Simplicity

Ever since the discovery of nucleic acids 150 years ago,[1] major achievements have been made in understanding and decrypting the fascinating scientific questions of the genetic code.[2] However, the most fundamental question about the origin and the evolution of the genetic code remains a mystery. How did nature manage to build up such intriguingly complex molecules able to encode structure and function from simple building blocks? What conditions were required? How could the precursors survive the unhostile environment of early Earth? Over the past decades, promising synthetic concepts were proposed providing clarity in the field of prebiotic nucleic acid research. In this Minireview, we show the current status and various approaches to answer these fascinating questions.

Ultraviolet-Driven Deamination of Cytidine Ribonucleotides Under Planetary Conditions

A previously proposed synthesis of pyrimidine ribonucleotides makes use of ultraviolet (UV) light to convert β-d-ribocytidine-2',3'-cyclic phosphate to β-d-ribouridine-2',3'-cyclic phosphate, while simultaneously selectively degrading synthetic byproducts. Past studies of the photochemical reactions of pyrimidines have employed mercury arc lamps, characterized by narrowband emission centered at 254 nm, which is not representative of the UV environment of the early Earth. To further assess this process under more realistic circumstances, we investigated the wavelength dependence of the UV-driven conversion of β-d-ribocytidine-2',3'-cyclic phosphate to β-d-ribouridine-2',3'-cyclic phosphate. We used constraints provided by planetary environments to assess the implications for pyrimidine nucleotides on the early Earth. We found that the wavelengths of light (255-285 nm) that most efficiently drive the deamination of β-d-ribocytidine-2',3'-cyclic phosphate to β-d-ribouridine-2',3'-cyclic phosphate are accessible on planetary surfaces such as those of the Hadean-Archaean Earth for CO₂-N₂-dominated atmospheres. However, continued irradiation could eventually lead to low levels of ribocytidine in a low-temperature, highly irradiated environment, if production rates are slow.

A continuous reaction network that produces RNA precursors

Continuous reaction networks, which do not rely on purification or timely additions of reagents, serve as models for chemical evolution and have been demonstrated for compounds thought to have played important roles for the origins of life such as amino acids, hydroxy acids, and sugars. Step-by-step chemical protocols for ribonucleotide synthesis are known, but demonstrating their synthesis in the context of continuous reaction networks remains a major challenge. Herein, compounds proposed to be important for prebiotic RNA synthesis, including glycolaldehyde, cyanamide, 2-aminooxazole, and 2-aminoimidazole, are generated from a continuous reaction network, starting from an aqueous mixture of NaCl, NH4Cl, phosphate, and HCN as the only carbon source. No well-timed addition of any other reagents is required. The reaction network is driven by a combination of γ radiolysis and dry-down. γ Radiolysis results in a complex mixture of organics, including the glycolaldehyde-derived glyceronitrile and cyanamide. This mixture is then dried down, generating free glycolaldehyde that then reacts with cyanamide/NH3 to furnish a combination of 2-aminooxazole and 2-aminoimidazole. This continuous reaction network models how precursors for generating RNA and other classes of compounds may arise spontaneously from a complex mixture that originates from simple reagents.

Prebiotic Syntheses of Noncanonical Nucleosides and Nucleotides

The origin of nucleotides is a major question in origins-of-life research. Given the central importance of RNA in biology and the influential RNA World hypothesis, a great deal of this research has focused on finding possible prebiotic syntheses of the four canonical nucleotides of coding RNA. However, the use of nucleotides in other roles across the tree of life might be evidence that nucleotides have been used in noncoding roles for even longer than RNA has been used as a genetic polymer. Likewise, it is possible that early life utilized nucleotides other than the extant nucleotides as the monomers of informational polymers. Therefore, finding plausible prebiotic syntheses of potentially ancestral noncanonical nucleotides may be of great importance for understanding the origins and early evolution of life. Experimental investigations into abiotic noncanonical nucleotide synthesis reveal that many noncanonical nucleotides and related glycosides are formed much more easily than the canonical nucleotides. An analysis of the mechanisms by which nucleosides and nucleotides form in the solution phase or in drying-heating reactions from pre-existing sugars and heterocycles suggests that a wide variety of noncanonical nucleotides and related glycosides would have been present on the prebiotic Earth, if any such molecules were present.

Unified prebiotically plausible synthesis of pyrimidine and purine RNA ribonucleotides

Theories about the origin of life require chemical pathways that allow formation of life's key building blocks under prebiotically plausible conditions. Complex molecules like RNA must have originated from small molecules whose reactivity was guided by physico-chemical processes. RNA is constructed from purine and pyrimidine nucleosides, both of which are required for accurate information transfer, and thus Darwinian evolution. Separate pathways to purines and pyrimidines have been reported, but their concurrent syntheses remain a challenge. We report the synthesis of the pyrimidine nucleosides from small molecules and ribose, driven solely by wet-dry cycles. In the presence of phosphate-containing minerals, 5'-mono- and diphosphates also form selectively in one-pot reactions. The pathway is compatible with purine synthesis, allowing the concurrent formation of all Watson-Crick bases.

The evolution of metabolism: How to test evolutionary hypotheses at the genomic level

The origin of primordial metabolism and its expansion to form the metabolic networks extant today represent excellent systems to study the impact of natural selection and the potential adaptive role of novel compounds. Here we present the current hypotheses made on the origin of life and ancestral metabolism and present the theories and mechanisms by which the large chemical diversity of plants might have emerged along evolution. In particular, we provide a survey of statistical methods that can be used to detect signatures of selection at the gene and population level, and discuss potential and limits of these methods for investigating patterns of molecular adaptation in plant metabolism.

Interplay of Guanine Oxidation and G-Quadruplex Folding in Gene Promoters

Living in an oxygen atmosphere demands an ability to thrive in the presence of reactive oxygen species (ROS). Aerobic organisms have successfully found solutions to the oxidative threats imposed by ROS by evolving an elaborate detoxification system, upregulating ROS during inflammation, and utilizing ROS as messenger molecules. In this Perspective, recent studies are discussed that demonstrate ROS as signaling molecules for gene regulation by combining two emergent properties of the guanine (G) heterocycle in DNA, namely, oxidation sensitivity and a propensity for G-quadruplex (G4) folding, both of which depend upon sequence context. In human gene promoters, this results from an elevated 5'-GG-3' dinucleotide frequency and GC enrichment near transcription start sites. Oxidation of DNA by ROS drives conversion of G to 8-oxo-7,8-dihydroguanine (OG) to mark target promoters for base excision repair initiated by OG-glycosylase I (OGG1). Sequence-dependent mechanisms for gene activation are available to OGG1 to induce transcription. Either OGG1 releases OG to yield an abasic site driving formation of a non-canonical fold, such as a G4, to be displayed to apurinic/apyrimidinic 1 (APE1) and stalling on the fold to recruit activating factors, or OGG1 binds OG and facilitates activator protein recruitment. The mechanisms described drive induction of stress response, DNA repair, or estrogen-induced genes, and these pathways are novel potential anticancer targets for therapeutic intervention. Chemical concepts provide a framework to discuss the regulatory or possible epigenetic potential of the OG modification in DNA, in which DNA "damage" and non-canonical folds collaborate to turn on or off gene expression. The next steps for scientific discovery in this growing field are discussed.

A Model for the Emergence of RNA from a Prebiotically Plausible Mixture of Ribonucleotides, Arabinonucleotides, and 2'-Deoxynucleotides

The abiotic synthesis of ribonucleotides is thought to have been an essential step toward the emergence of the RNA world. However, it is likely that the prebiotic synthesis of ribonucleotides was accompanied by the simultaneous synthesis of arabinonucleotides, 2′-deoxyribonucleotides, and other variations on the canonical nucleotides. In order to understand how relatively homogeneous RNA could have emerged from such complex mixtures, we have examined the properties of arabinonucleotides and 2′-deoxyribonucleotides in nonenzymatic template-directed primer extension reactions. We show that nonenzymatic primer extension with activated arabinonucleotides is much less efficient than with activated ribonucleotides, and furthermore that once an arabinonucleotide is incorporated, continued primer extension is strongly inhibited. As previously shown, 2′-deoxyribonucleotides are also less efficiently incorporated in primer extension reactions, but the difference is more modest. Experiments with mixtures of nucleotides suggest that the coexistence of ribo- and arabinonucleotides does not impede the copying of RNA templates. Moreover, chimeric oligoribonucleotides containing 2′-deoxy- or arabinonucleotides are effective templates for RNA synthesis. We propose that the initial genetic polymers were random sequence chimeric oligonucleotides formed by untemplated polymerization, but that template copying chemistry favored RNA synthesis; multiple rounds of replication may have led to pools of oligomers composed mainly of RNA.

Chemistry of Abiotic Nucleotide Synthesis

The chemistry of abiotic nucleotide synthesis of RNA and DNA in the context of their prebiotic origins on early earth is a continuing challenge. How did (or how can) the nucleotides form and assemble from the small molecule inventories and under conditions that prevailed on early earth 3.5-4 billion years ago? This review provides a background and up-to-date progress that will allow the reader to judge where the field stands currently and what remains to be achieved. We start with a brief primer on the biological synthesis of nucleotides, followed by an extensive focus on the prebiotic formation of the components of nucleotides-either via the synthesis of ribose and the canonical nucleobases and then joining them together or by building both the conjoined sugar and nucleobase, part-by-part-toward the ultimate goal of forming RNA and DNA by polymerization. The review will emphasize that there are-and will continue to be-many more questions than answers from the synthetic, mechanistic, and analytical perspectives. We wrap up the review with a cautionary note in this context about coming to conclusions as to whether the problem of chemistry of prebiotic nucleotide synthesis has been solved.

Nucleic Acid Extraction and Sequencing from Low-Biomass Synthetic Mars Analog Soils for In Situ Life Detection

Recent studies regarding the origins of life and Mars-Earth meteorite transfer simulations suggest that biological informational polymers, such as nucleic acids (DNA and RNA), have the potential to provide unambiguous evidence of life on Mars. To this end, we are developing a metagenomics-based life-detection instrument which integrates nucleic acid extraction and nanopore sequencing: the Search for Extra-Terrestrial Genomes (SETG). Our goal is to isolate and sequence nucleic acids from extant or preserved life on Mars in order to determine if a particular genetic sequence (1) is distantly related to life on Earth, indicating a shared ancestry due to lithological exchange, or (2) is unrelated to life on Earth, suggesting convergent origins of life on Mars. In this study, we validate prior work on nucleic acid extraction from cells deposited in Mars analog soils down to microbial concentrations (i.e., 10⁴ cells in 50 mg of soil) observed in the driest and coldest regions on Earth. In addition, we report low-input nanopore sequencing results from 2 pg of purified Bacillus subtilis spore DNA simulating ideal extraction yields equivalent to 1 ppb life-detection sensitivity. We achieve this by employing carrier sequencing, a method of sequencing sub-nanogram DNA in the background of a genomic carrier. After filtering of carrier, low-quality, and low-complexity reads we detected 5 B. subtilis reads, 18 contamination reads (including Homo sapiens), and 6 high-quality noise reads believed to be sequencing artifacts.

In search of a primitive signaling code

Cells must have preceded by simpler chemical systems (protocells) that had the capacity of a spontaneous self-assembly process and the ability to confine chemical reaction networks together with a form of information. The presence of lipid molecules in the early Earth conditions is sufficient to ensure the occurrence of spontaneous self-assembly processes, not defined by genetic information, but related to their chemical amphiphilic nature. Ribozymes are plausible molecules for early life, being the first small polynucleotides made up of random oligomers or formed by non-enzymatic template copying. Compartmentalization represents a strategy for the evolution of ribozymes; the attachment of ribozymes to surfaces, such as formed by lipid micellar aggregates may be particular relevant if the surface itself catalyzes RNA polymerization.It is conceivable that the transition from pre-biotic molecular aggregates to cellular life required the coevolution of the RNA world, capable of synthesizing specific, instead of statistical proteins, and of the Lipid world, with a transition from micellar aggregates to semipermeable vesicles. Small molecules available in the prebiotic inventory might promote RNA stability and the evolution of hydrophobic micellar aggregates into membrane-delimited vesicles. The transition from ribozymes catalyzing the assembly of statistical polypeptides to the synthesis of proteins, required the appearance of the genetic code; the transition from hydrophobic platforms favoring the stability of ribozymes and of nascent polypeptides to the selective transport of reagents through a membrane, required the appearance of the signal transduction code.A further integration between the RNA and Lipid worlds can be advanced, taking into account the emerging roles of phospholipid aggregates not only in ensuring stability to ribozymes by compartmentalization, but also in a crucial step of evolution through natural selection mechanisms, based on signal transduction pathways that convert environmental changes into biochemical responses that could vary according to the context. Here I present evidences on the presence of traces of the evolution of a signal transduction system in extant cells, which utilize a phosphoinositide signaling system located both at nucleoplasmic level as well as at the plasma membrane, based on the very same molecules but responding to different rules. The model herewith proposed is based on the following assumptions on the biomolecules of extant organisms: i) amphiphils can be converted into structured aggregates by hydrophobic forces thus giving rise to functional platforms for the interaction of other biomolecules and to their compartmentalization; ii) fundamental biochemical pathways, including protein synthesis, can be sustained by natural ribozymes of ancient origin; iii) ribozymes and nucleotide-derived coenzymes could have existed long before protein enzymes emerged; iv) signaling molecules, both derived from phospholipids and from RNAs could have guided the evolution of complex metabolic processes before the emergence of proteins.

Investigating Prebiotic Protocells for A Comprehensive Understanding of the Origins of Life: A Prebiotic Systems Chemistry Perspective

Protocells are supramolecular systems commonly used for numerous applications, such as the formation of self-evolvable systems, in systems chemistry and synthetic biology. Certain types of protocells imitate plausible prebiotic compartments, such as giant vesicles, that are formed with the hydration of thin films of amphiphiles. These constructs can be studied to address the emergence of life from a non-living chemical network. They are useful tools since they offer the possibility to understand the mechanisms underlying any living cellular system: Its formation, its metabolism, its replication and its evolution. Protocells allow the investigation of the synergies occurring in a web of chemical compounds. This cooperation can explain the transition between chemical (inanimate) and biological systems (living) due to the discoveries of emerging properties. The aim of this review is to provide an overview of relevant concept in prebiotic protocell research.

Prebiotic Soup Components Trapped in Montmorillonite Nanoclay Form New Molecules: Car-Parrinello Ab Initio Simulations

The catalytic effects of complex minerals or meteorites are often mentioned as important factors for the origins of life. To assess the possible role of nanoconfinement within a catalyst consisting of montmorillonite (MMT) and the impact of local electric field on the formation efficiency of the simple hypothetical precursors of nucleic acid bases or amino acids, we performed ab initio Car–Parrinello molecular dynamics simulations. We prepared four condensed-phase systems corresponding to previously suggested prototypes of a primordial soup. We monitored possible chemical reactions occurring within gas-like bulk and MMT-confined four simulation boxes on a 20-ps time scale at 1 atm and 300 K, 400 K, and 600 K. Elevated temperatures did not affect the reactivity of the elementary components of the gas-like boxes considerably; however, the presence of the MMT nanoclay substantially increased the formation probability of new molecules. Approximately 20 different new compounds were found in boxes containing carbon monoxide or formaldehyde molecules. This observation and an analysis of the atom–atom radial distribution functions indicated that the presence of Ca²⁺ ions at the surface of the internal MMT cavities may be an important factor in the initial steps of the formation of complex molecules at the early stages of the Earth’s history.

A Prebiotic Synthesis of Canonical Pyrimidine and Purine Ribonucleotides

The "RNA first" model for the origin of life holds that RNA emerged spontaneously on early Earth and developed into life through its dual capabilities for genetics and catalysis. The model's central weakness is the difficulty of making its building blocks, in particular, the glycosidic bond joining nucleobases to ribose. Thus, the focus of much of the modern literature on the topic is directed toward solving this difficulty and includes elegant, though indirect, methods for making this bond. Here, we report that the glycosidic bond in canonical pyrimidine and purine ribonucleotides can be formed by direct coupling of cyclic carbohydrate phosphates with free nucleobases, all reported to be available by experimentally supported pathways that might have operated on early Earth.

Making Molecules with Clay: Layered Double Hydroxides, Pentopyranose Nucleic Acids and the Origin of Life

A mixture of sugar diphosphates is produced in reactions between small aldehyde phosphates catalysed by layered double hydroxide (LDH) clays under plausibly prebiotic conditions. A subset of these, pentose diphosphates, constitute the backbone subunits of nucleic acids capable of base pairing, which is not the case for the other products of these LDH-catalysed reactions. Not only that, but to date no other polymer found capable of base pairing—and therefore information transfer—has a backbone for which its monomer subunits have a plausible prebiotic synthesis, including the ribose-5-phosphate backbone subunit of RNA. Pentose diphosphates comprise the backbone monomers of pentopyranose nucleic acids, some of the strongest base pairing systems so far discovered. We have previously proposed that the first base pairing interactions were between purine nucleobase precursors, and that these were weaker and less specific than standard purine-pyrimidine interactions. We now propose that the inherently stronger pairing of pentopyranose nucleic acids would have compensated for these weaker interactions, and produced an informational polymer capable of undergoing nonenzymatic replication. LDH clays might also have catalysed the synthesis of the purine nucleobase precursors, and the polymerization of pentopyranose nucleotide monomers into oligonucleotides, as well as the formation of the first lipid bilayers.

A one-pot, water compatible synthesis of pyrimidine nucleobases under plausible prebiotic conditions

A prebiotically plausible one-pot formation pathway of 4-substituted pyrimidine nucleobases in water, starting from cyanoacetylene.

Prebiotic nucleic acids need space to grow

What were the conditions on early Earth when nucleotides were formed, and what are the most plausible nucleoside candidates? Answering these questions will require mechanistic chemistry and planetary science to work together, enhancing not limiting each other’s scope of investigation.