Pathways to Meteoritic Glycine and Methylamine

Collisional excitation of propargylimine by helium: new ab initio 3D-potential energy surfaces and scattering calculations

We present quantum coupled-state calculations for the rotational excitation of interstellar propargylimine due to collisions with helium. The calculations are based on new high-accurate three-dimensional potential energy surfaces (3D-PESs) adapted for rigid-rotor scattering computations. The two PESs (Z/E-PGIM-He) were determined using the explicitly correlated coupled-cluster approach with single, double and perturbative triple excitation [CCSCD(T)-F12] and the standard aug-cc-pVTZ basis set. These PESs present many minima with a global minimum of -47.61 cm-1 for Z-PGIM-He and -54.16 cm-1 for E-PGIM-He. While the PESs for both complexes are qualitatively similar, that of E-PGIM-He is more anisotropic. The state-to-state collisional cross-section calculations are performed for all rotational levels J ≤ 12 with energies below Erot = 30 cm-1 and for total energies up to 500 cm-1. The corresponding collisional rate coefficients are derived for kinetic temperatures up to 120 K. A propensity rule is seen, for rotational excitation cross sections and de-excitation rate coefficients, that favors even ΔJ transitions but with different orders of magnitude. We expect that the retrieved results will contribute to improving atmospheric and astrophysics models.

Identification of HOC•HC(O)H, HOCH2C•O, and HOCH2CH2O• Intermediates in the Reaction of H + Glycolaldehyde in Solid Para-Hydrogen and Its Implication to the Interstellar Formation of Complex Sugars

Glycolaldehyde [HOCH2C(O)H, GA], the primitive sugar-like molecule detected in the interstellar medium (ISM), is a potential precursor for the synthesis of complex sugars. Despite its importance, the mechanism governing the formation of these higher-order sugars from GA under interstellar circumstances remains elusive. Radical intermediates HOCH2CH2O• (1), HOCH2C•HOH (2), HOCH2C•O (3), HOC•HC(O)H (4), and O•CH2C(O)H (5) derived from GA could be potential precursors for the formation of glyceraldehyde (aldose sugar), dihydroxyacetone (ketose sugar), and ethylene glycol (sugar alcohol) in dark regions of ISM. However, the spectral identification of these intermediates and their roles were little investigated. We conducted reactions involving H atoms and the Cis-cis conformer of GA (Cc-GA) in solid p-H2 at 3.2 K and identified IR spectra of radicals Cc-HOCH2C•O (3) and Cc-HOC•HC(O)H (4) produced from H abstraction as well as closed-shell HOCHCO (6) produced via consecutive H abstraction of GA. In addition, Cc-HOCH2CH2O• (1) and C•H2OH + H2CO (7) were produced through the H addition and the H-induced fragmentation channels, respectively. In darkness, when only H-tunneling reactions occurred, the formation of (3) was major and that of (1) was minor. In contrast, during IR irradiation to produce H atoms with higher energy, the formation of (4) and C•H2OH + H2CO (7) became important. We also successfully converted most Cc-GA to the second-lowest-energy conformer Trans-trans-GA (Tt-GA) by prolonged IR irradiation at 2827 nm to investigate H + Tt-GA; Tt-HOCH2C•O (3'), Tt-HOC•HC(O)H (4'), HOCHCO (6), Tt-HOCH2CH2O• (1'), and C•H2OH + H2CO (7) were observed. We discuss possible routes for the formation of higher-order sugars or related compounds involving (7), (1), (3), and (4), but neither (2), which was proposed previously, nor (5) plays a significant role in H + GA. Such previously unreported rich chemistry in the reaction of H + GA, with four channels of three distinct types, indicates the multiple roles that GA might play in astronomical chemistry.

Unlocking the surface chemistry of ionic minerals: a high-throughput pipeline for modeling realistic interfaces

A systematic procedure is introduced for modeling charge-neutral non-polar surfaces of ionic minerals containing polyatomic anions. By integrating distance- and charge-based clustering to identify chemical species within the mineral bulk, our pipeline, PolyCleaver, renders a variety of theoretically viable surface terminations. As a demonstrative example, this approach was applied to forsterite (Mg2SiO4), unveiling a rich interface landscape based on interactions with formaldehyde, a relevant multifaceted molecule, and more particularly in prebiotic chemistry. This high-throughput method, going beyond techniques traditionally applied in the modeling of minerals, offers new insights into the potential catalytic properties of diverse surfaces, enabling a broader exploration of synthetic pathways in complex mineral systems.

Emergence of Order in Origin-of-Life Scenarios on Mineral Surfaces: Polyglycine Chains on Silica

The polymerization of amino acids (AAs) to peptides on oxide surfaces has attracted interest owing to its high importance in biotechnology, prebiotic chemistry, and origin of life theories. However, its mechanism is still poorly understood. We tried to elucidate the reactivity of glycine (Gly) from the vapor phase on the surface of amorphous silica under controlled atmosphere at 160 °C. Infrared (IR) spectroscopy reveals that Gly functionalizes the silica surface through the formation of ester species, which represent, together with the weakly interacting silanols, crucial elements for monomers activation and polymerization. Once activated, β-turns start to form as initiators for the growth of long linear polypeptides (poly-Gly) chains, which elongate into ordered structures containing both β-sheet and helical conformations. The work also points to the role of water vapor in the formation of further self-assembled β-sheet structures that are highly resistant to hydrolysis.

Bacterial Utilisation of Aliphatic Organics: Is the Dwarf Planet Ceres Habitable?

The regolith environment and associated organic material on Ceres is analogous to environments that existed on Earth 3-4 billion years ago. This has implications not only for abiogenesis and the theory of transpermia, but it provides context for developing a framework to contrast the limits of Earth's biosphere with extraterrestrial environments of interest. In this study, substrate utilisation by the ice-associated bacterium Colwellia hornerae was examined with respect to three aliphatic organic hydrocarbons that may be present on Ceres: dodecane, isobutyronitrile, and dioctyl-sulphide. Following inoculation into a phyllosilicate regolith spiked with a hydrocarbon (1% or 20% organic concentration wt%), cell density, electron transport activity, oxygen consumption, and the production of ATP, NADPH, and protein in C. hornerae was monitored for a period of 32 days. Microbial growth kinetics were correlated with changes in bioavailable carbon, nitrogen, and sulphur. We provide compelling evidence that C. hornerae can survive and grow by utilising isobutyronitrile and, in particular, dodecane. Cellular growth, electron transport activity, and oxygen consumption increased significantly in dodecane at 20 wt% compared to only minor growth at 1 wt%. Importantly, the reduction in total carbon, nitrogen, and sulphur observed at 20 wt% is attributed to biotic, rather than abiotic, processes. This study illustrates that short-term bacterial incubation studies using exotic substrates provide a useful indicator of habitability. We suggest that replicating the regolith environment of Ceres warrants further study and that this dwarf planet could be a valid target for future exploratory missions.

Computational studies on the possible formation of glycine via open shell gas-phase chemistry in the interstellar medium

Glycine is the simplest proteinogenic amino acid. It has significant astrobiological implications owing to the ongoing investigation for its detection in the interstellar medium (ISM). Hence, a suitable mechanistic elucidation for its formation in the ISM is of current research interest. In the present work, by employing electronic structure calculations [UCCSD(T) and density functional theory (DFT)], various plausible chemical pathways in the gas phase have been examined for the formation of glycine (whose existence has been indirectly proposed in the ISM) and other simple amino acids (yet to be detected in the ISM) from some simpler molecules present in the ISM. This work suggests that step 1: HO-CO (radical) + CH₂NH → NHCH₂COOH (radical) and step 2a: NHCH₂COOH (radical) + H₂ → glycine + H (radical) have very small barriers of 0.14 kcal mol^-1 and ∼3 kcal mol^-1, respectively (easily surmountable at a temperature of ∼50 K under putative interstellar conditions). Hence this should likely be feasible in interstellar gas-phase chemistry. Therefore, HO-CO (radical), CH₂NH, and H₂ could be the possible precursors for the formation of glycine (subject to the presence of the HO-CO radical). The energetic information related to the interstellar reactions, and how this work takes the putative interstellar conditions into account are presented. This paper also highlights how a reaction found to be unsuitable for interstellar molecular evolution via surface chemistry could nonetheless occur via gas-phase chemistry. Based on our results, this work also recommends detecting three new open-shell molecules, HO-CO radical, NHCH₂COOH radical, and NH₂CHCOOH radical, in the ISM.

Freeze-thaw cycles enable a prebiotically plausible and continuous pathway from nucleotide activation to nonenzymatic RNA copying

The replication of RNA without the aid of evolved enzymes may have enabled the inheritance of useful molecular functions during the origin of life. Template-directed RNA copying is a posited step in RNA replication. Key steps on the path to copying of RNA templates have been studied in isolation, including chemical nucleotide activation, generation of a key reactive intermediate, and template-directed polymerization. Here we report a prebiotically plausible scenario under which these reactions can happen together under mutually compatible conditions. Thus, this pathway could potentially have operated in nature without the complicating requirement for exchange of materials between distinct environments.

Nonenzymatic template-directed RNA copying using chemically activated nucleotides is thought to have played a key role in the emergence of genetic information on the early Earth. A longstanding question concerns the number and nature of different environments that might have been necessary to enable all of the steps from nucleotide synthesis to RNA copying. Here we explore three sequential steps from this overall pathway: nucleotide activation, synthesis of imidazolium-bridged dinucleotides, and template-directed RNA copying. We find that all three steps can take place in one reaction mixture undergoing multiple freeze-thaw cycles. Recent experiments have demonstrated a potentially prebiotic methyl isocyanide-based nucleotide activation chemistry. However, the original version of this approach is incompatible with nonenzymatic RNA copying because the high required concentration of the imidazole activating group prevents the accumulation of the essential imidazolium-bridged dinucleotide. Here we report that ice eutectic phase conditions facilitate not only the methyl isocyanide-based activation of ribonucleotide 5′-monophosphates with stoichiometric 2-aminoimidazole, but also the subsequent conversion of these activated mononucleotides into imidazolium-bridged dinucleotides. Furthermore, this one-pot approach is compatible with template-directed RNA copying in the same reaction mixture. Our results suggest that the simple and common environmental fluctuation of freeze-thaw cycles could have played an important role in prebiotic nucleotide activation and nonenzymatic RNA copying.

Tracing the Primordial Chemical Life of Glycine: A Review from Quantum Chemical Simulations

Glycine (Gly), NH2CH2COOH, is the simplest amino acid. Although it has not been directly detected in the interstellar gas-phase medium, it has been identified in comets and meteorites, and its synthesis in these environments has been simulated in terrestrial laboratory experiments. Likewise, condensation of Gly to form peptides in scenarios resembling those present in a primordial Earth has been demonstrated experimentally. Thus, Gly is a paradigmatic system for biomolecular building blocks to investigate how they can be synthesized in astrophysical environments, transported and delivered by fragments of asteroids (meteorites, once they land on Earth) and comets (interplanetary dust particles that land on Earth) to the primitive Earth, and there react to form biopolymers as a step towards the emergence of life. Quantum chemical investigations addressing these Gly-related events have been performed, providing fundamental atomic-scale information and quantitative energetic data. However, they are spread in the literature and difficult to harmonize in a consistent way due to different computational chemistry methodologies and model systems. This review aims to collect the work done so far to characterize, at a quantum mechanical level, the chemical life of Gly, i.e., from its synthesis in the interstellar medium up to its polymerization on Earth.

Photochemical Synthesis of Ammonia and Amino Acids from Nitrous Oxide

Abiotic synthesis of ammonia (NH₃) and amino acids is important for the origin of life and early evolution. Ammonia and organic nitrogen species may be produced from nitrous oxide (N₂O), which is a second abundant nitrogen species in the atmosphere. Here, we report a new photochemical experiment and evaluate whether N₂O can be used as a nitrogen source for prebiotic synthesis in the atmosphere. We conducted a series of experiments by using a gas mixture of N₂O+CO, N₂O+CO₂, or N₂O + H₂ in the presence of liquid water. The results demonstrate that NH₃, methylamine (CH₃NH₂), and some amino acids such as glycine, alanine, and serine can be synthesized through photochemistry from N₂O even without metal catalysts. NH₃ can be produced not only from CO + N₂O, but also from H₂+N₂O. Glycine can be synthesized from CH₃NH₂ and CO₂, which can be produced from N₂O and CO under ultraviolet irradiation. Our work demonstrates, for the first time, that N₂O could be an important nitrogen source and provide a new process for synthesizing ammonia and organic nitrogen species, which has not been previously considered. The contribution of organic synthesis from N₂O should, therefore, be considered when discussing the prebiotic chemistry on primitive Earth.

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.

Important features of the potential energy surface of the methylamine plus O(1D) reaction

This research presents an ab initio characterization of the potential energy surface for the methylamine plus ¹D oxygen atom reaction, which may be relevant to interstellar chemistry. Geometries and harmonic vibrational frequencies were determined for all stationary points at the CCSD(T)/aug-cc-pVTZ level of theory. The focal point method along with several additive corrections was used to obtain reliable CCSDT(Q)/CBS potential energy surface features. Extensive conformational analysis and intrinsic reaction coordinate computations were performed to ensure accurate chemical connectivity of the stationary points. Five minima were determined to be possible products of this reaction and three novel transition states were found that were previously unreported or mislabeled in the literature. The pathways we present can be used to guide further searches for NH₂ containing species in the interstellar medium.

Microscopic origin of the chiroptical response of optical media

The potential for enhancing the optical activity of natural chiral media using engineered nanophotonic components has been central in the quest toward developing next-generation circular-dichroism spectroscopic techniques. Through confinement and manipulation of optical fields at the nanoscale, ultrathin optical elements have enabled a path toward achieving order-of-magnitude enhancements in the chiroptical response. Here, we develop a model framework to describe the underlying physics governing the origin of the chiroptical response in optical media. The model identifies optical activity to originate from electromagnetic coupling to the hybridized eigenstates of a coupled electron-oscillator system, whereas differential absorption of opposite handedness light, though resulting in a far-field chiroptical response, is shown to have incorrectly been identified as optical activity. We validate the model predictions using experimental measurements and show them to also be consistent with observations in the literature. The work provides a generalized framework for the design and study of chiroptical systems.

Analyses of Aliphatic Aldehydes and Ketones in Carbonaceous Chondrites

Aliphatic aldehydes and ketones are essential building blocks for the synthesis of more complex organic compounds. Despite their potentially key role as precursors of astrobiologically important molecules, such as amino acids and carboxylic acids, this family of compounds has scarcely been evaluated in carbonaceous chondrites. The paucity of such analyses likely derives from the low concentration of aldehydes and ketones in the meteorites and from the currently used chromatographic methodologies that have not been optimized for meteorite analysis. In this work, we report the development of a novel analytical method to quantify the molecular distribution and compound-specific isotopic analysis of 29 aliphatic aldehydes and ketones. Using this method, we have investigated the molecular distribution and 13C-isotopic composition of aldehydes and ketones in 10 carbonaceous chondrites from the CI, CM, CR, and CV groups. The total concentration of carbonyl compounds ranged from 130 to 1000 nmol g-1 of meteorite with formaldehyde, acetaldehyde, and acetone being the most abundant species in all investigated samples. The 13C-isotopic values ranged from -67 to +64‰ and we did not observe clear relationships between 13C-content and molecular weight. Accurately measuring the relative abundances, determining the molecular distribution, and isotopic composition of chondritic organic compounds is central in assessing both their formation chemistry and synthetic relationships.

Compound-Specific Carbon Isotope Compositions of Aldehydes and Ketones in the Murchison Meteorite

Compound-specific carbon isotope analysis (δ13C) of meteoritic organic compounds can be used to elucidate the abiotic chemical reactions involved in their synthesis. The soluble organic content of the Murchison carbonaceous chondrite has been extensively investigated over the years, with a focus on the origins of amino acids and the potential role of Strecker-cyanohydrin synthesis in the early solar system. Previous δ13C investigations have targeted α-amino acid and α-hydroxy acid Strecker products and reactant HCN; however, δ13C values for meteoritic aldehydes and ketones (Strecker precursors) have not yet been reported. As such, the distribution of aldehydes and ketones in the cosmos and their role in prebiotic reactions have not been fully investigated. Here, we have applied an optimized O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) derivatization procedure to the extraction, identification and δ13C analysis of carbonyl compounds in the Murchison meteorite. A suite of aldehydes and ketones, dominated by acetaldehyde, propionaldehyde and acetone, were detected in the sample. δ13C values, ranging from -10.0‰ to +66.4‰, were more 13C-depleted than would be expected for aldehydes and ketones derived from the interstellar medium, based on interstellar 12C/13C ratios. These relatively 13C-depleted values suggest that chemical processes taking place in asteroid parent bodies (e.g. oxidation of the IOM) may provide a secondary source of aldehydes and ketones in the solar system. Comparisons between δ13C compositions of meteoritic aldehydes and ketones and other organic compound classes were used to evaluate potential structural relationships and associated reactions, including Strecker synthesis and alteration-driven chemical pathways.

A Light-Releasable Potentially Prebiotic Nucleotide Activating Agent

Investigations into the chemical origin of life have recently benefitted from a holistic approach in which possible atmospheric, organic, and inorganic systems chemistries are taken into consideration. In this way, we now report that a selective phosphate activating agent, namely methyl isocyanide, could plausibly have been produced from simple prebiotic feedstocks. We show that methyl isocyanide drives the conversion of nucleoside monophosphates to phosphorimidazolides under potentially prebiotic conditions and in excellent yields for the first time. Importantly, this chemistry allows for repeated reactivation cycles, a property long sought in nonenzymatic oligomerization studies. Further, as the isocyanide is released upon irradiation, the possibility of spatially and temporally controlled activation chemistry is thus raised.

Hydrothermal Decomposition of Amino Acids and Origins of Prebiotic Meteoritic Organic Compounds

The organic compounds found in carbonaceous chondrite meteorites provide insight into primordial solar system chemistry. Evaluating the formation and decomposition mechanisms of meteoritic amino acids may aid our understanding of the origins of life and homochirality on Earth. The amino acid glycine is widespread in meteorites and other extraterrestrial environments; other amino acids, such as isovaline, are found with enantiomeric excesses in some meteorites. The relationship between meteoritic amino acids and other compounds with similar molecular structures, such as aliphatic monoamines and monocarboxylic acids is unclear; experimental results evaluating the decomposition of amino acids have produced inconclusive results about the preferred pathways, reaction intermediates, and if the conditions applied may be compatible with those occurring inside meteoritic parent bodies. In this work, we performed extensive tandem metadynamics, umbrella sampling, and committor analysis to simulate the neutral mild hydrothermal decomposition mechanisms of glycine and isovaline and put them into context for the origins of meteoritic organic compounds. Our ab initio simulations aimed to determine free energy profiles and decomposition pathways for glycine and isovaline. We found that under our modeled conditions, methylammonium, glycolic acid, and sec-butylamine are the most likely decomposition products. These results suggest that meteoritic aliphatic monocarboxylic acids are not produced from decomposition of meteoritic amino acids. Our results also indicate that the decomposition of L-isovaline prefers an enantioselective pathway resulting in the production of (S)-sec-butylamine.

Adsorption of Methylamine on Amorphous Ice under Interstellar Conditions. A Grand Canonical Monte Carlo Simulation Study

The adsorption of methylamine at the surface of amorphous ice is studied at various temperatures, ranging from 20 to 200 K, by grand canonical Monte Carlo simulations under conditions that are characteristic to the interstellar medium (ISM). The results are also compared with those obtained earlier on crystalline ( I_h) ice. We found that methylamine has a strong ability of being adsorbed on amorphous ice, involving also multilayer adsorption. The decrease of the temperature leads to a substantial increase of this adsorption ability; thus, considerable adsorption is seen at 20-50 K even at bulk gas phase concentrations that are comparable with that of the ISM. Further, methylamine molecules can also be dissolved in the bulk amorphous ice phase. Both the adsorption capacity of amorphous ice and the strength of the adsorption on it are found to be clearly larger than those corresponding to crystalline ( I_h) ice, due to the molecular scale roughness of the amorphous ice surface as well as to the lack of clear orientational preferences of the water molecules at this surface. Thus, the surface density of the saturated adsorption monolayer is estimated to be 12.6 ± 0.4 μmol/m², 20% larger than the value of 10.35 μmol/m², obtained earlier for I_h ice, and at low enough surface coverages the adsorbed methylamine molecules are found to easily form up to three hydrogen bonds with the surface water molecules. The estimated heat of adsorption at infinitely low surface coverage is calculated to be -69 ± 5 kJ/mol, being rather close to the estimated heat of solvation in the bulk amorphous ice phase of -74 ± 7 kJ/mol, indicating that there are at least a few positions at the surface where the adsorbed methylamine molecules experience a bulk-like local environment.

Distribution of Aliphatic Amines in CO, CV and CK Carbonaceous Chondrites and Relation to Mineralogy and Processing History

The analysis of water-soluble organic compounds in meteorites provides valuable insights into the prebiotic synthesis of organic matter and the processes that occurred during the formation of the solar system. We investigated the concentration of aliphatic monoamines present in the hot acid-water extracts of the unaltered Antarctic carbonaceous chondrites DOM 08006 (CO3) and MIL 05013 (CO3), and the thermally altered meteorites Allende (CV3), LAP 02206 (CV3), GRA 06101 (CV3), ALH 85002 (CK4), and EET 92002 (CK5). We have also reviewed and assessed the petrologic characteristics of the meteorites studied here, to evaluate the effects of asteroidal processing on the abundance and molecular distributions of monoamines. The CO3, CV3, CK4, and CK5 meteorites studied here contain total concentrations of amines ranging from 1.2 to 4.0 nmol/g of meteorite; these amounts are one to three orders of magnitude below those observed in carbonaceous chondrites from the CI, CM and CR groups. The low amine abundances for CV and CK chondrites may be related to their extensive degree of thermal metamorphism and/or to their low original amine content. Although the CO3 meteorites DOM 08006 and MIL 05013 do not show signs of thermal and aqueous alteration, their monoamine contents are comparable to those observed in moderately/extensively thermally altered CV3, CK4, and CK5 carbonaceous chondrites. The low content of monoamines in pristine CO carbonaceous chondrites suggests that the initial amounts, and not asteroidal processes, play a dominant role in the content of monoamines in carbonaceous chondrites. The primary monoamines, methylamine, ethylamine and n-propylamine constitute the most abundant amines in the CO3, CV3, CK4, and CK5 meteorites studied here. Contrary to the predominance of n-ω-amino acid isomers in CO3 and thermally altered meteorites, there appears to be no preference for the larger n-α-amines.