Non-racemic amino acids in the Murray and Murchison meteorites

Two-dimensional LC-MS/MS and three-dimensional LC analysis of chiral amino acids and related compounds in real-world matrices

Amino acids normally have a chiral carbon and d/l-enantiomers are present. Due to the homochirality features on the present Earth, l-enantiomers are predominant in the living beings and the d-enantiomers are rare. Along with the progress and development of cutting edge analytical methods, several d-amino acids were found even in the higher animals including humans, and their biological functions and diagnostic values have also been reported. However, the amounts of these d-amino acids are much lower than the l-forms, and development/utilization of highly sensitive and selective methods are practically essential to avoid the disturbance from uncountable intrinsic substances. In the present review, multi-dimensional HPLC methods for the determination of chiral amino acids, especially two-dimensional LC-MS/MS and three-dimensional LC methods, and their applications to a variety of real-world matrices are summarized.

Solid-State Single-Molecule Sensing with the Electronic Life-Detection Instrument for Enceladus/Europa (ELIE)

Growing evidence of the potential habitability of Ocean Worlds across our solar system is motivating the advancement of technologies capable of detecting life as we know it-sharing a common ancestry or physicochemical origin with life on Earth-or don't know it, representing a distinct emergence of life different than our one known example. Here, we propose the Electronic Life-detection Instrument for Enceladus/Europa (ELIE), a solid-state single-molecule instrument payload that aims to search for life based on the detection of amino acids and informational polymers (IPs) at the parts per billion to trillion level. As a first proof-of-principle in a laboratory environment, we demonstrate the single-molecule detection of the amino acid L-proline at a 10 μM concentration in a compact system. Based on ELIE's solid-state quantum electronic tunneling sensing mechanism, we further propose the quantum property of the HOMO-LUMO gap (energy difference between a molecule's highest energy-occupied molecular orbital and lowest energy-unoccupied molecular orbital) as a novel metric to assess amino acid complexity. Finally, we assess the potential of ELIE to discriminate between abiotically and biotically derived α-amino acid abundance distributions to reduce the false positive risk for life detection. Nanogap technology can also be applied to the detection of nucleobases and short sequences of IPs such as, but not limited to, RNA and DNA. Future missions may utilize ELIE to target preserved biosignatures on the surface of Mars, extant life in its deep subsurface, or life or its biosignatures in a plume, surface, or subsurface of ice moons such as Enceladus or Europa. One-Sentence Summary: A solid-state nanogap can determine the abundance distribution of amino acids, detect nucleic acids, and shows potential for detecting life as we know it and life as we don't know it.

Uncovering the chiral bias of meteoritic isovaline through asymmetric photochemistry

Systematic enrichments of L-amino acids in meteorites is a strong indication that biological homochirality originated beyond Earth. Although still unresolved, stellar UV circularly polarized light (CPL) is the leading hypothesis to have caused the symmetry breaking in space. This involves the differential absorption of left- and right-CPL, a phenomenon called circular dichroism, which enables chiral discrimination. Here we unveil coherent chiroptical spectra of thin films of isovaline enantiomers, the first step towards asymmetric photolysis experiments using a tunable laser set-up. As analogues to amino acids adsorbed on interstellar dust grains, CPL-helicity dependent enantiomeric excesses of up to 2% were generated in isotropic racemic films of isovaline. The low efficiency of chirality transfer from broadband CPL to isovaline could explain why its enantiomeric excess is not detected in the most pristine chondrites. Notwithstanding, small, yet consistent L-biases induced by stellar CPL would have been crucial for its amplification during aqueous alteration of meteorite parent bodies.

Chiroptical activity of gas-phase propylene oxide predicting the handedness of interstellar circular polarization in the presolar nebula

Propylene oxide, the first chiral molecule recently detected in the interstellar medium, has once again raised the question whether biomolecular chirality might have cosmic origins. However, accurate chiroptical properties of propylene oxide in the ultraviolet spectral range necessary to suggest possible asymmetric synthetic routes in the gas phase are scarce. Here, we report on the first experimental measurements of the anisotropy spectra of gas-phase propylene oxide in the vacuum ultraviolet spectral range. Our experimental results provide novel insights into the handedness of interstellar circular polarization at the dawn of molecular evolution of our star- and planet-forming region. Besides the fundamental importance of this new investigation for understanding the origin and evolution of homochirality on Earth, our high-resolution experimental electronic circular dichroism data will inspire new efforts in quantum computational spectroscopy.

Gas-phase synthesis of racemic helicenes and their potential role in the enantiomeric enrichment of sugars and amino acids in meteorites

The molecular origins of homochirality on Earth is not understood well, particularly how enantiomerically enriched molecules of astrobiological significance like sugars and amino acids might have been synthesized on icy grains in space preceding their delivery to Earth. Polycyclic aromatic hydrocarbons (PAHs) identified in carbonaceous chondrites could have been processed in molecular clouds by circularly polarized light prior to the depletion of enantiomerically enriched helicenes onto carbonaceous grains resulting in chiral islands. However, the fundamental low temperature reaction mechanisms leading to racemic helicenes are still unknown. Here, by exploiting synchrotron based molecular beam photoionization mass spectrometry combined with electronic structure calculations, we provide compelling testimony on barrierless, low temperature pathways leading to racemates of [5] and [6]helicene. Astrochemical modeling advocates that gas-phase reactions in molecular clouds lead to racemates of helicenes suggesting a pathway for future astronomical observation and providing a fundamental understanding for the origin of homochirality on early Earth.

Asymmetric Autocatalysis as an Efficient Link Between the Origin of Homochirality and Highly Enantioenriched Compounds

Biological homochirality of essential components such as L-amino acids and D-sugars is prerequisite for the emergence, evolution and the maintenance of life. Implication of biological homochirality is described. Considerable interest has been focused on the origin and the process leading to the homochirality. Asymmetric autocatalysis with amplification of enantiomeric excess (ee), i.e., the Soai reaction, is capable to link the low ee induced by the proposed origins of chirality such as circularly polarized light and high ee of the organic compound. Absolute asymmetric synthesis without the intervention of any chiral factor was achieved in the Soai reaction.

The Coevolution of Biomolecules and Prebiotic Information Systems in the Origin of Life: A Visualization Model for Assembling the First Gene

Prebiotic information systems exist in three forms: analog, hybrid, and digital. The Analog Information System (AIS), manifested early in abiogenesis, was expressed in the chiral selection, nucleotide formation, self-assembly, polymerization, encapsulation of polymers, and division of protocells. It created noncoding RNAs by polymerizing nucleotides that gave rise to the Hybrid Information System (HIS). The HIS employed different species of noncoding RNAs, such as ribozymes, pre-tRNA and tRNA, ribosomes, and functional enzymes, including bridge peptides, pre-aaRS, and aaRS (aminoacyl-tRNA synthetase). Some of these hybrid components build the translation machinery step-by-step. The HIS ushered in the Digital Information System (DIS), where tRNA molecules become molecular architects for designing mRNAs step-by-step, employing their two distinct genetic codes. First, they created codons of mRNA by the base pair interaction (anticodon–codon mapping). Secondly, each charged tRNA transferred its amino acid information to the corresponding codon (codon–amino acid mapping), facilitated by an aaRS enzyme. With the advent of encoded mRNA molecules, the first genes emerged before DNA. With the genetic memory residing in the digital sequences of mRNA, a mapping mechanism was developed between each codon and its cognate amino acid. As more and more codons ‘remembered’ their respective amino acids, this mapping system developed the genetic code in their memory bank. We compared three kinds of biological information systems with similar types of human-made computer systems.

Possible chemical and physical scenarios towards biological homochirality

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Integrative analytical workflow to enhance comprehensive analysis of organic molecules in extraterrestrial objects

Molecular identification is a fundamental issue in astrobiology to investigate the routes of emergence of life on our planet involving in particular a potential seeding of extraterrestrial organic matter on the primitive Earth. However, this project encompasses major difficulties due to the low concentration of molecules present in bodies of the Solar System. This work proposes an integrative analytical workflow, no longer based on GC-MS instruments, to enhance comprehensive analysis of organic markers in these objects. Our strategy combines UPLC-HRMS and UPLC-MRM MS methods to bring both a broad molecular mapping and detailed data on indigenous compounds present in any extraterrestrial objects or laboratory analogs. Applied on water extracts from fresh meteorites, our workflow highlights a wide range of free molecules in the non-treated extracts and reveals the wide diversity of amino acid and nucleobase isomers that could lead to misinterpretation as far as the molecular composition of meteorite extracts cannot be anticipated. This strategy, never explored so far, would provide new clues for studying the organic matter in space and should offer new perspectives on its evolution and reactivity.

Perspectives on parity violation in chiral molecules: theory, spectroscopic experiment and biomolecular homochirality

The reflection (or ‘mirror’) symmetry of space is among the fundamental symmetries of physics. It is connected to the conservation law for the quantum number parity and a fundamental ‘non-observable’ property of space (as defined by an absolute ‘left-handed’ or ‘right-handed’ coordinate system). The discovery of the violation of this symmetry – the non-conservation of parity or ‘parity violation’ – in 1956/1957 had an important influence on the further development of physics. In chemistry the mirror symmetry of space is connected to the existence of enantiomers as isomers of chiral (‘handed’) molecules. These isomers would relate to each other as idealized left or right hand or as image and mirror image and would be energetically exactly equivalent with perfect space inversion symmetry. Parity violation results in an extremely small ‘parity violating’ energy difference between the ground states of the enantiomers which can be theoretically calculated to be about 100 aeV to 1 feV (equivalent to 10⁻¹¹ to 10⁻¹⁰ J mol⁻¹), depending on the molecule, but which has not yet been detected experimentally. Its detection remains one of the great challenges of current physical–chemical stereochemistry, with implications also for fundamental problems in physics. In biochemistry and molecular biology one finds a related fundamental question unanswered for more than 100 years: the evolution of ‘homochirality’, which is the practically exclusive preference of one chiral, enantiomeric form as building blocks in the biopolymers of all known forms of life (the l-amino acids in proteins and d-sugars in DNA, not the reverse d-amino acids or l-sugars). In astrobiology the spectroscopic detection of homochirality could be used as strong evidence for the existence of extraterrestrial life, if any. After a brief conceptual and historical introduction we review the development, current status, and progress along these three lines of research: theory, spectroscopic experiment and the outlook towards an understanding of the evolution of biomolecular homochirality.

The reflection (or ‘mirror’) symmetry of space is among the fundamental symmetries of physics. It is connected to the conservation law for the quantum number purity and its violation and has a fundamental relation to stereochemistry and molecular chirality.

Chirality and the Origin of Life

The origin of life, based on the homochirality of biomolecules, is a persistent mystery. Did life begin by using both forms of chirality, and then one of the forms disappeared? Or did the choice of homochirality precede the formation of biomolecules that could ensure replication and information transfer? Is the natural choice of L-amino acids and D-sugars on which life is based deterministic or random? Is the handedness present in/of the Universe from its beginning? The whole biosystem on the Earth, all living creatures are chiral. Many theories try to explain the origin of life and chirality on the Earth: e.g., the panspermia hypothesis, the primordial soup hypothesis, theory of parity violation in weak interactions. Additionally, heavy neutrinos and the impact of the fact that only left-handed particles decay, and even dark matter, all have to be considered.

Ronald C.D. Breslow (1931-2017): A career in review

Reviewed herein are key research accomplishments of Professor Ronald Charles D. Breslow (1931-2017) throughout his more than 60 year research career. These accomplishments span a wide range of topics, most notably physical organic chemistry, medicinal chemistry, and bioorganic chemistry. These topics are reviewed, as are topics of molecular electronics and origin of chirality, which combine to make up the bulk of this review. Also reviewed briefly are Breslow's contributions to the broader chemistry profession, including his work for the American Chemical Society and his work promoting gender equity. Throughout the article, efforts are made to put Breslow's accomplishments in the context of other work being done at the time, as well as to include subsequent iterations and elaborations of the research.

Constraints for Use of Ultraviolet Spectropolarimetry to Detect Chiral Amino Acids from Comets

Life is pervasive on planet Earth, but whether life is ubiquitous in the Galaxy and sustainable over timescales comparable to stellar evolution is unknown. Evidence suggests that life first appeared on Earth more than 3.77 Gyr ago, during a period of heavy meteoric bombardment. Amino acids, the building blocks of proteins, have been demonstrated to exist in interstellar ice. As such, the contribution of space-generated amino acids to those existing on Earth should be considered. However, detection of space amino acids is challenging. In this study, we used analytical data from several meteorites and in situ measurements of the comet 67P/Churyumov-Gerasimenko collected by the Rosetta probe to evaluate the detectability of alanine by ultraviolet spectropolarimetry. Alanine is the second-most abundant amino acid after glycine and is optically active. This chirality produces a unique signature that enables reliable identification of this amino acid using the imprint of optical rotatory dispersion (ORD) and circular dichroism (CD) in the ultraviolet spectrum (130-230 nm). Here, we show that the ORD signature could be detected in comets by using ultraviolet spectropolarimetric observations conducted at middle size space observatories. These observations can also provide crucial information for the study of sources of enantiomeric imbalance on Earth.

The Grayness of the Origin of Life

In the search for life beyond Earth, distinguishing the living from the non-living is paramount. However, this distinction is often elusive, as the origin of life is likely a stepwise evolutionary process, not a singular event. Regardless of the favored origin of life model, an inherent "grayness" blurs the theorized threshold defining life. Here, we explore the ambiguities between the biotic and the abiotic at the origin of life. The role of grayness extends into later transitions as well. By recognizing the limitations posed by grayness, life detection researchers will be better able to develop methods sensitive to prebiotic chemical systems and life with alternative biochemistries.

Integrated Molar Absorptivity of Mid- and Far-Infrared Spectra of Glycine and Other Selected Amino Acids

A selection of five proteinogenic amino acids-glycine, isoleucine, phenylalanine, tyrosine, and tryptophan-were studied in the mid-infrared and in the far-infrared with the purpose to facilitate the search and identification of these astrobiologically and astrochemically relevant molecules in space environments. The molar extinction coefficients (ɛ) of all mid- and far-infrared bands were determined as well as the integrated molar absorptivities (ψ). The mid-infrared spectra of the five selected amino acids were recorded also at three different temperatures from -180°C to ambient temperature to +200°C. We measured the wavelength shift of the infrared bands caused by temperature; and for the most relevant or temperature-sensitive infrared bands, a series of linear equations were determined relating wavelength position with temperature. Such equations may provide estimates of the temperature of these molecules once detected in astrophysical objects; and with the reported values of ɛ and ψ, it will be possible to estimate the relative abundance of these molecules in space environments.

Step by Step Strecker Amino Acid Synthesis from Ab Initio Prebiotic Chemistry

The amino acids synthesis from elementary precursors in abiotic conditions is traditionally described according to the Strecker reaction, thoroughly invoked to justify the observation of amino acids in extraterrestrial samples and their emergence in the primordial Earth. To this day, however, a quantitative microscopic description of the mechanism, thermodynamics, and kinetics of the multistep Strecker reaction is still lacking. In the present work we tackle this study by adopting a state-of-the-art ab initio computational approach, combining an efficient scheme of exploration of the relevant chemical networks with a rigorous determination of the underlying free energy and transition states. We determine the step-by-step chemical pathway from "Strecker precursors" to glycine in solution and calculate the corresponding full free energy landscape. Our results agree well with the scarce available experimental data and complete them, thus providing the first end-to-end study of this complex reaction, a crucial bottleneck for the emergence of life.

Insoluble organic matter in chondrites: Archetypal melanin-like PAH-based multifunctionality at the origin of life?

An interdisciplinary review of the chemical literature that points to a unifying scenario for the origin of life, referred to as the Primordial Multifunctional organic Entity (PriME) scenario, is provided herein. In the PriME scenario it is suggested that the Insoluble Organic Matter (IOM) in carbonaceous chondrites, as well as interplanetary dust particles from meteorites and comets may have played an important role in the three most critical processes involved in the origin of life, namely 1) metabolism, via a) the provision and accumulation of molecules that are the building blocks of life, b) catalysis (e.g., by templation), and c) protection of developing life molecules against radiation by excited state deactivation; 2) compartmentalization, via adsorption of compounds on the exposed organic surfaces in fractured meteorites, and 3) replication, via deaggregation, desorption and related physical phenomena. This scenario is based on the hitherto overlooked structural and physicochemical similarities between the IOM and the dark, insoluble, multifunctional melanin polymers found in bacteria and fungi and associated with the ability of these microorganisms to survive extreme conditions, including ionizing radiation. The underlying conceptual link between these two materials is strengthened by the fact that primary precursors of bacterial and fungal melanins (collectively referred to herein as allomelanins) are hydroxylated aromatic compounds like homogentisic acid and 1,8-dihydroxynaphthalene, and that similar hydroxylated aromatic compounds, including hydroxynaphthalenes, figure prominently among possible components of the organic materials on dust grains and ices in the interstellar matter, and may be involved in the formation of IOM in meteorites. Inspired by this rationale, a vis-à-vis review of the properties of IOM from various chondrites and non-nitrogenous allomelanin pigments from bacteria and fungi is provided herein. The unrecognized similarities between these materials may pave the way for a novel scenario at the origin of life, in which IOM-related complex organic polymers delivered to the early Earth are proposed to serve as PriME and were preserved and transformed in those primitive forms of life that shared the ability to synthesize melanin polymers playing an important role in the critical processes underlying the establishment of terrestrial eukaryotes.

Electron Spin-Polarization Dependent Damage to Chiral Amino Acid l-Histidine

The damage of approximately monolayer films of l-histidine by low-energy spin-polarized electrons (SPE) ejected from a magnetized cobalt substrate has been probed using X-ray photoelectron spectroscopy (XPS). Average damage cross sections for N-containing motifs of l-histidine are 25-30 and 2-5 Mb for zwitterions and neutral molecules, respectively. The magnetization direction of the substrate, which controls the ejected SPE helicity, was reversed in situ, and statistically significant differences in the damage cross sections of 10-30% were measured between positive and negative electron helicities. This is the first measurement of spin dichroism (SD) in an amino acid. The differential cross sections suggest that inelastic scattering of SPE with chiral molecules could contribute to the persistence of one enantiomer vs the other under certain irradiation conditions, particularly for the zwitterionic species.

A Novel Proteomics-Based Strategy for the Investigation of Peptide Sequences in Extraterrestrial Samples

Method development is one of the objectives of the astrophysical community for characterizing the organic matter in objects of the solar system. In this context, we report on the development of an enzyme-catalyzed stereoselective hydrolysis, inspired by the proteomics discipline, which has enabled the indirect detection of peptide sequences in extraterrestrial samples. A proof of concept has been performed on a Murchison extract. We show that our approach can successfully highlight l- and d-amino acids involved in peptide bonds. While we show that some d-amino acids must have been involved in peptide bonds, we cannot at this stage conclude on the indigenous or exogenous nature of these biopolymers. However, our strategy constitutes the first step toward direct UPLC-MS evidence of peptide sequences in extraterrestrial samples. It should thus contribute to deepening knowledge on the molecules available in the solar system, hence providing new clues about their chemical history, especially on Earth.

Three-dimensional high-performance liquid chromatographic analysis of chiral amino acids in carbonaceous chondrites

A three-dimensional (3D) HPLC system in combination with fluorescence derivatization has been developed for the highly sensitive and selective analysis of chiral amino acids in extraterrestrial samples. As the targets, alanine (Ala), 2-aminobutyric acid (2AB), valine (Val), norvaline (nVal) and isovaline (iVal), frequently found chiral amino acids in the carbonaceous chondrites, were selected. These amino acids were pre-column derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), and the target analytes were separated from other amino acids and organic compounds by a reversed-phase column in the first dimension. The targets were further separated from interferences by an anion-exchange column in the second dimension, and their enantiomers were separated and determined in the third dimension by a Pirkle-type enantioselective column. The present 3D-HPLC system was validated and applied to the Murchison meteorite and the Antarctic meteorites, and all of the target amino acid enantiomers were clearly observed (0.78-22.33 nmol/g in the Murchison meteorite and 1.79-78.84 nmol/g in the Antarctic meteorites) without severe interferences. The %L values of the non-proteinogenic amino acids were almost 50% in both meteorites, and even the proteinogenic amino acids were almost racemic in the Antarctic meteorites.

Connections Between Nuclear Physics and the Origin of Life - Examining the Origin of Biomolecular Chirality

The discovery of bio-molecules in meteorites with an excess of one chiral state has created one of the biggest questions in astrobiology today. That is, what is the origin of bio-molecular homochirality? Studies of this question are highly interdisciplinary, and while several phenomenological models exist, we examine the relationship between fundamental symmetries at the particle level and the macroscopic formation of bio-molecules. A model has been developed which couples fundamental interactions with the formation of molecular chirality. In this magneto-chiral model atomic nuclei bound in amino acids interact via the weak interaction in stellar environments. Nuclei are coupled to the molecular geometry (chirality) via the shielding tensor, the same interaction responsible for NMR identification. Associated with this is the fact that isotopic abundances vary from solar system values. Interactions with leptons can selectively destroy one chiral state over the other while changing isotopic values. Possible sites are proposed in which this model may exist.

Magnetars and Magnetic Separation of Chiral Radicals in Interstellar Space: Homochirality

Pasteur was the first to realize Earth's homochirality. Consequently, he attempted to design experiments revealing a mechanism that would expose life's chiral preference. Some of these experiments involved the application of magnetic fields to chemical reactions. His experiments failed, in part, because B-fields are pseudo-vectors and cannot couple preferentially to one handedness. However, extremely large magnetic fields cause the Maxwell equations to break down. This allows the motions of spin and charge densities in paramagnetic anion radicals to produce polarized axial B-fields that can undergo preferential coupling to one handedness. Hence, when a racemic mixture of paramagnetic organic molecules passes by an extremely large external gradated magnetic field, the enantiomers experience different torque forces and acquire different translational directions. B-fields of the required magnitude are unknown on this planet. In fact, they would be lethal, thereby eliminating any chance of Pasteur's success. On the other hand, Duncan and co-workers have recently discovered and garnered physical understanding of magnetars in interstellar space. Some of these neutron star systems produce B-fields greater than the quantum electrodynamic field strength, which is more than enough to generate the required torque for the interstellar enantiomeric separation. In space, chiralitically enriched materials can be deposited on planetesimals and result in homochiral "islands" on the planets. The formation of magnetars is a consequence of weak force events. We assert that, in interstellar space, a plethora of enantiomerically enriched dust clouds resulted from inter-magnetar-paramagnetic molecule force fields.

Methodologies for Analyzing Soluble Organic Compounds in Extraterrestrial Samples: Amino Acids, Amines, Monocarboxylic Acids, Aldehydes, and Ketones

Soluble organic compositions of extraterrestrial samples offer valuable insights into the prebiotic organic chemistry of the solar system. This review provides a summary of the techniques commonly used for analyzing amino acids, amines, monocarboxylic acids, aldehydes, and ketones in extraterrestrial samples. Here, we discuss possible effects of various experimental factors (e.g., extraction protocols, derivatization methods, and chromatographic techniques) in order to highlight potential influences on the results obtained from different methodologies. This detailed summary and assessment of current techniques is intended to serve as a basic guide for selecting methodologies for soluble organic analyses and to emphasize some key considerations for future method development.

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.

The Astrophysical Formation of Asymmetric Molecules and the Emergence of a Chiral Bias

The biomolecular homochirality in living organisms has been investigated for decades, but its origin remains poorly understood. It has been shown that circular polarized light (CPL) and other energy sources are capable of inducing small enantiomeric excesses (ees) in some primary biomolecules, such as amino acids or sugars. Since the first findings of amino acids in carbonaceous meteorites, a scenario in which essential chiral biomolecules originate in space and are delivered by celestial bodies has arisen. Numerous studies have thus focused on their detection, identification, and enantiomeric excess calculations in extraterrestrial matrices. In this review we summarize the discoveries in amino acids, sugars, and organophosphorus compounds in meteorites, comets, and laboratory-simulated interstellar ices. Based on available analytical data, we also discuss their interactions with CPL in the ultraviolet (UV) and vacuum ultraviolet (VUV) regions, their abiotic chiral or achiral synthesis, and their enantiomeric distribution. Without doubt, further laboratory investigations and upcoming space missions are required to shed more light on our potential extraterrestrial molecular origins.

The Origin of Prebiotic Information System in the Peptide/RNA World: A Simulation Model of the Evolution of Translation and the Genetic Code

Information is the currency of life, but the origin of prebiotic information remains a mystery. We propose transitional pathways from the cosmic building blocks of life to the complex prebiotic organic chemistry that led to the origin of information systems. The prebiotic information system, specifically the genetic code, is segregated, linear, and digital, and it appeared before the emergence of DNA. In the peptide/RNA world, lipid membranes randomly encapsulated amino acids, RNA, and peptide molecules, which are drawn from the prebiotic soup, to initiate a molecular symbiosis inside the protocells. This endosymbiosis led to the hierarchical emergence of several requisite components of the translation machine: transfer RNAs (tRNAs), aminoacyl-tRNA synthetase (aaRS), messenger RNAs (mRNAs), ribosomes, and various enzymes. When assembled in the right order, the translation machine created proteins, a process that transferred information from mRNAs to assemble amino acids into polypeptide chains. This was the beginning of the prebiotic information age. The origin of the genetic code is enigmatic; herein, we propose an evolutionary explanation: the demand for a wide range of protein enzymes over peptides in the prebiotic reactions was the main selective pressure for the origin of information-directed protein synthesis. The molecular basis of the genetic code manifests itself in the interaction of aaRS and their cognate tRNAs. In the beginning, aminoacylated ribozymes used amino acids as a cofactor with the help of bridge peptides as a process for selection between amino acids and their cognate codons/anticodons. This process selects amino acids and RNA species for the next steps. The ribozymes would give rise to pre-tRNA and the bridge peptides to pre-aaRS. Later, variants would appear and evolution would produce different but specific aaRS-tRNA-amino acid combinations. Pre-tRNA designed and built pre-mRNA for the storage of information regarding its cognate amino acid. Each pre-mRNA strand became the storage device for the genetic information that encoded the amino acid sequences in triplet nucleotides. As information appeared in the digital languages of the codon within pre-mRNA and mRNA, and the genetic code for protein synthesis evolved, the prebiotic chemistry then became more organized and directional with the emergence of the translation and genetic code. The genetic code developed in three stages that are coincident with the refinement of the translation machines: the GNC code that was developed by the pre-tRNA/pre-aaRS /pre-mRNA machine, SNS code by the tRNA/aaRS/mRNA machine, and finally the universal genetic code by the tRNA/aaRS/mRNA/ribosome machine. We suggest the coevolution of translation machines and the genetic code. The emergence of the translation machines was the beginning of the Darwinian evolution, an interplay between information and its supporting structure. Our hypothesis provides the logical and incremental steps for the origin of the programmed protein synthesis. In order to better understand the prebiotic information system, we converted letter codons into numerical codons in the Universal Genetic Code Table. We have developed a software, called CATI (Codon-Amino Acid-Translator-Imitator), to translate randomly chosen numerical codons into corresponding amino acids and vice versa. This conversion has granted us insight into how the genetic code might have evolved in the peptide/RNA world. There is great potential in the application of numerical codons to bioinformatics, such as barcoding, DNA mining, or DNA fingerprinting. We constructed the likely biochemical pathways for the origin of translation and the genetic code using the Model-View-Controller (MVC) software framework, and the translation machinery step-by-step. While using AnyLogic software, we were able to simulate and visualize the entire evolution of the translation machines, amino acids, and the genetic code.

Molecular Distribution, 13C-Isotope, and Enantiomeric Compositions of Carbonaceous Chondrite Monocarboxylic Acids

The water-soluble organic compounds in carbonaceous chondrite meteorites constitute a record of the synthetic reactions occurring at the birth of the solar system and those taking place during parent body alteration and may have been important for the later origins and development of life on Earth. In this present work, we have developed a novel methodology for the simultaneous analysis of the molecular distribution, compound-specific δ13C and enantiomeric compositions of aliphatic monocarboxylic acids (MCA) extracted from the hot-water extracts of sixteen carbonaceous chondrites from CM, CR, CO, CV and CK groups. We observed high concentrations of meteoritic MCAs, with total carbon weight percentages which in some cases approached those of carbonates and insoluble organic matter. Moreover, we found that the concentration of MCAs in CR chondrites is higher than in the other meteorite groups, with acetic acid exhibiting the highest concentration in all samples. The abundance of MCAs decreased with increasing molecular weight and with increasing aqueous and/or thermal alteration experienced by the meteorite sample. The δ13C isotopic values of MCAs ranged from -52 to +27‰, and aside from an inverse relationship between δ13C value and carbon straight-chain length for C3-C6 MCAs in Murchison, the 13C-isotopic values did not correlate with the number of carbon atoms per molecule. We also observed racemic compositions of 2-methylbutanoic acid in CM and CR chondrites. We used this novel analytical protocol and collective data to shed new light on the prebiotic origins of chondritic MCAs.

The Birthplace of Proto-Life: Role of Secondary Minerals in Forming Metallo-Proteins through Water-Rock Interaction of Hadean Rocks

The surface of Hadean Earth was mainly covered with three types of rocks-komatiite, KREEP basalt and anorthosite-which were remarkably different from those on the modern Earth. The water-rock interaction between these rocks and water provided a highly reducing environment and formed secondary minerals on the surface of the rocks that are important for producing metallo-enzymes for the emergence of primordial life. Previous studies suggested a correlation between the active site of metallo-enzymes and sulfide minerals based on the affinity of their structures, but they did not discuss the origin of metallic elements contained in these minerals which is critical to understanding where life began. We investigated secondary minerals formed through water-rock interactions of komatiite in a subaerial geyser system, then discussed the relationship between the active site of metallo-enzymes and secondary minerals. Instead of komatiite, we used serpentinite collected from the Hakuba Happo area, Nagano Prefecture in central-north Japan, which is thought to be a modern analog for the Hadean environment. We found several minor minerals, such as magnetite, chromite, pyrite and pentlandite in addition to serpentine minerals. Pentlandite has not been mentioned in previous studies as one of the candidates that could supply important metallic elements to build metallo-enzymes. It has been shown to be a catalyst for hydrogen generation possibly, because of structural similarity to the active site of hydrogenases. We consider the possibility that nickel-iron sulfide, pentlandite, could be important minerals for the origin of life. In addition, we estimated what kinds of minor minerals would be obtained from the water-rock interaction of these rocks using thermodynamic calculations. KREEP basalt contains a large amount of iron and it could be useful for producing metallo-enzymes, especially ferredoxins-electron transfer enzymes, which may have assisted in the emergence of life.

Monosaccharides and Their Derivatives in Carbonaceous Meteorites: A Scenario for Their Synthesis and Onset of Enantiomeric Excesses

Carbonaceous meteorites provide the best glimpse into the solar system’s earliest physical and chemical processes. These ancient objects, ~4.56 billion years old, contain evidence of phenomena ranging from solar system formation to the synthesis of organic compounds by aqueous and (likely) low-temperature photolytic reactions. Collectively, chemical reactions resulted in an insoluble kerogen-like carbon phase and a complex mixture of discrete soluble compounds including amino acids, nucleobases, and monosaccharide (or “sugar”) derivatives. This review presents the documented search for sugars and their derivatives in carbonaceous meteorites. We examine early papers, published in the early 1960s, and note the analytical methods used for meteorite analysis as well as conclusions on the results. We then present the recent finding of sugar derivatives including sugar alcohols and several sugar acids: The latter compounds were found to possess unusual “d” enantiomeric (mirror-image) excesses. After discussions on the possible roles of interstellar grain chemistry and meteorite parent body aqueous activity in the synthesis of sugar derivatives, we present a scenario that suggests that most of Earth’s extraterrestrial sugar alcohols (e.g., glycerol) were synthesized by interstellar irradiation and/or cold grain chemistry and that the early solar disk was the location of the initial enantiomeric excesses in meteoritic sugar derivatives.

Amino Acid Chiral Selection Via Weak Interactions in Stellar Environments: Implications for the Origin of Life

Magnetochiral phenomena may be responsible for the selection of chiral states of biomolecules in meteoric environments. For example, the Supernova Amino Acid Processing (SNAAP) Model was proposed previously as a possible mode of magnetochiral selection of amino acids by way of the weak interaction in strong magnetic fields. In earlier work, this model was shown to produce an enantiomeric excess (ee) as high as 0.014% for alanine. In this paper we present the results of molecular quantum chemistry calculations from which ees are determined for the α-amino acids plus isovaline and norvaline, which were found to have positive ees in meteorites. Calculations are performed for both isolated and aqueous states. In some cases, the aqueous state was found to produce larger ees reaching values as high as a few percent under plausible conditions.

Insights into Abiotically-Generated Amino Acid Enantiomeric Excesses Found in Meteorites

Biology exhibits homochirality, in that only one of two possible molecular configurations (called enantiomers) is used in both proteins and nucleic acids. The origin of this phenomenon is currently unknown, as nearly all known abiotic mechanisms for generating these compounds result in equal (racemic) mixtures of both enantiomers. However, analyses of primitive meteorites have revealed that a number of amino acids of extraterrestrial origin are present in enantiomeric excess, suggesting that there was an abiotic route to synthesize amino acids in a non-racemic manner. Here we review the amino acid contents of a range of meteorites, describe mechanisms for amino acid formation and their potential to produce amino acid enantiomeric excesses, and identify processes that could have amplified enantiomeric excesses.

A symbiotic view of the origin of life at hydrothermal impact crater-lakes

Submarine hydrothermal vents are generally considered as the likely habitats for the origin and evolution of early life on Earth. The theory suffers from the 'concentration problem' of cosmic and terrestrial biomolecules because of the vastness of the Eoarchean global ocean. An attractive alternative site would be highly sequestered, small, hydrothermal crater-lakes that might have cradled life on early Earth. A new symbiotic model for the origin of life at hydrothermal crater-lakes is proposed here. Meteoritic impacts on the Eoarchean crust at the tail end of the Heavy Bombardment period might have played important roles in the origin of life. Impacts and collisions that created hydrothermal crater lakes on the Eoarchean crust inadvertently became the perfect crucibles for prebiotic chemistry with building blocks of life, which ultimately led to the first organisms by prebiotic synthesis. In this scenario, life arose through four hierarchical stages of increasing molecular complexity in multiple niches of crater basins. In the cosmic stage (≥4.6 Ga), the building blocks of life had their beginnings in the interstellar space during the explosion of a nearby star. Both comets and carbonaceous chondrites delivered building blocks of life and ice to early Earth, which were accumulated in hydrothermal impact crater-lakes. In the geologic stage (∼4 Ga), crater basins contained an assortment of cosmic and terrestrial organic compounds, powered by hydrothermal, solar, tidal, and chemical energies, which drove the prebiotic synthesis. At the water surface, self-assembled primitive lipid membranes floated as a thick oil slick. Archean Greenstone belts in Greenland, Australia, and South Africa possibly represent the relics of these Archean craters, where the oldest fossils of thermophilic life (∼3.5 Ga) have been detected. In the chemical stage, monomers such as nucleotides and amino acids were selected from random assemblies of the prebiotic soup; they were polymerized at pores of mineral surfaces with the coevolution of RNA and protein molecules to form the 'RNA/protein world'. Lipid membranes randomly encapsulated these RNA and protein molecules to initiate a molecular symbiosis in a 'RNA/protein/lipid world' that led to hierarchical emergence of several cell components: plasma membranes, ribosomes, coding RNA and proteins, DNA, and finally protocells with a primitive genetic code. In the biological stage, the emergence of the first cells capable of reproduction, heredity, variation, and Darwinian evolution is the key breakthrough in the origin of life. RNA virus and prions may represent the evolutionary relics of the RNA/protein world that survived as parasites for billions of years. Although the proposed endosymbiotic model is speculative it has intrinsic heuristic value. Future experiments on encapsulated RNA virus and prions have the potential to create a synthetic cell that may confirm a coherent narrative of this hierarchical evolutionary sequence.

d-Amino acids in molecular evolution in space - Absolute asymmetric photolysis and synthesis of amino acids by circularly polarized light

Living organisms on the Earth almost exclusively use l-amino acids for the molecular architecture of proteins. The biological occurrence of d-amino acids is rare, although their functions in various organisms are being gradually understood. A possible explanation for the origin of biomolecular homochirality is the delivery of enantioenriched molecules via extraterrestrial bodies, such as asteroids and comets on early Earth. For the asymmetric formation of amino acids and their precursor molecules in interstellar environments, the interaction with circularly polarized photons is considered to have played a potential role in causing chiral asymmetry. In this review, we summarize recent progress in the investigation of chirality transfer from chiral photons to amino acids involving the two major processes of asymmetric photolysis and asymmetric synthesis. We will discuss analytical data on cometary and meteoritic amino acids and their potential impact delivery to the early Earth. The ongoing and future ambitious space missions, Hayabusa2, OSIRIS-REx, ExoMars 2020, and MMX, are scheduled to provide new insights into the chirality of extraterrestrial organic molecules and their potential relation to the terrestrial homochirality. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

Cell chirality: its origin and roles in left-right asymmetric development

An item is chiral if it cannot be superimposed on its mirror image. Most biological molecules are chiral. The homochirality of amino acids ensures that proteins are chiral, which is essential for their functions. Chirality also occurs at the whole-cell level, which was first studied mostly in ciliates, single-celled protozoans. Ciliates show chirality in their cortical structures, which is not determined by genetics, but by 'cortical inheritance'. These studies suggested that molecular chirality directs whole-cell chirality. Intriguingly, chirality in cellular structures and functions is also found in metazoans. In Drosophila, intrinsic cell chirality is observed in various left-right (LR) asymmetric tissues, and appears to be responsible for their LR asymmetric morphogenesis. In other invertebrates, such as snails and Caenorhabditis elegans, blastomere chirality is responsible for subsequent LR asymmetric development. Various cultured cells of vertebrates also show intrinsic chirality in their cellular behaviours and intracellular structural dynamics. Thus, cell chirality may be a general property of eukaryotic cells. In Drosophila, cell chirality drives the LR asymmetric development of individual organs, without establishing the LR axis of the whole embryo. Considering that organ-intrinsic LR asymmetry is also reported in vertebrates, this mechanism may contribute to LR asymmetric development across phyla.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Inhabited or Uninhabited? Pitfalls in the Interpretation of Possible Chemical Signatures of Extraterrestrial Life

The "Rare Earth" hypothesis-put forward by Ward and Brownlee in their 2000 book of the same title-states that prokaryote-type organisms may be common in the universe but animals and higher plants are exceedingly rare. If this idea is correct, the search for extraterrestrial life is essentially the search for microorganisms. Various indicators may be used to detect extant or extinct microbial life beyond Earth. Among them are chemical biosignatures, such as biomolecules and stable isotope ratios. The present minireview focuses on the major problems associated with the identification of chemical biosignatures. Two main types of misinterpretation are distinguished, namely false positive and false negative results. The former can be caused by terrestrial biogenic contaminants or by abiotic products. Terrestrial contamination is a common problem in space missions that search for biosignatures on other planets and moons. Abiotic organics can lead to false positive results if erroneously interpreted as biomolecules, but also to false negatives, for example when an abiotic source obscures a less productive biological one. In principle, all types of putative chemical biosignatures are prone to misinterpretation. Some, however, are more reliable ("stronger") than others. These include: (i) homochiral polymers of defined length and sequence, comparable to proteins and polynucleotides; (ii) enantiopure compounds; (iii) the existence of only a subset of molecules when abiotic syntheses would produce a continuous range of molecules; the proteinogenic amino acids constitute such a subset. These considerations are particularly important for life detection missions to solar system bodies such as Mars, Europa, and Enceladus.

The Asymmetry of Existence: Do We Owe Our Existence to Cold Dark Matter and the Weak Force?

A common theme throughout biology is homochirality, including its origin and especially implications. Homochirality has also intrigued scientists because of the hypothesis that life, as it currently exists, could not have occurred without it. In this review, we discuss several hypotheses regarding homochirality and their linkage to processes that range from subatomic in scale to processes that help define the structure of the universe. More importantly, this exploration begins with the knowledge that humans inhabit the universe in which there is an excess of normal matter over antimatter. It is a universe characterized by homochirality but is nonetheless contained in what is most easily described as a 3+1 dimensional spacetime wherein most laws of physics are invariant under spacetime transformations. This restriction on spacetime poses significant constraints on the processes that can be invoked to explain homochirality. However, in dealing with such restraints, including the total mass contained in the universe, the concepts of cold dark matter and dark energy can be incorporated into cosmological models with resultant behaviors and predictions very much in accord with the findings of the cosmic background surveys. Indeed, the introduction of cold dark matter and dark energy to solve problems relating to the mass found in the universe may provide a means for generating the needed asymmetry to allow homochirality to arise.

Meteoritic Amino Acids: Diversity in Compositions Reflects Parent Body Histories

The analysis of amino acids in meteorites dates back over 50 years; however, it is only in recent years that research has expanded beyond investigations of a narrow set of meteorite groups (exemplified by the Murchison meteorite) into meteorites of other types and classes. These new studies have shown a wide diversity in the abundance and distribution of amino acids across carbonaceous chondrite groups, highlighting the role of parent body processes and composition in the creation, preservation, or alteration of amino acids. Although most chiral amino acids are racemic in meteorites, the enantiomeric distribution of some amino acids, particularly of the nonprotein amino acid isovaline, has also been shown to vary both within certain meteorites and across carbonaceous meteorite groups. Large l-enantiomeric excesses of some extraterrestrial protein amino acids (up to ∼60%) have also been observed in rare cases and point to nonbiological enantiomeric enrichment processes prior to the emergence of life. In this Outlook, we review these recent meteoritic analyses, focusing on variations in abundance, structural distributions, and enantiomeric distributions of amino acids and discussing possible explanations for these observations and the potential for future work.