Amino Acids Indigenous to the Murray Meteorite

Mechanisms of glycine formation in cold interstellar media: a theoretical study

The possibility of the formation of glycine (Gly) from fundamental gas molecules in cold interstellar media was studied using quantum chemical methods, transition state theory and microcanonical molecular dynamics simulations with surface hopping dynamics (NVE-MDSH). This theoretical study emphasized five photochemical pathways in the lowest singlet-excited (S 1) state, thermochemical processes after non-radiative S 1→S 0 relaxations, and photo-to-thermal energy conversion in the NVE ensemble. The optimized reaction pathways suggested that to generate a reactive singlet dihydroxy carbene (HOCOH) intermediate, photochemical pathways involving the H2O…CO van der Waals and H2O-OC hydrogen bond precursors (Ch (1)_Step (1)) possess considerably lower energy barriers than the S 0 state pathways. The Gibbs free energy barriers (∆G ǂ ) calculated after the non-radiative S 1 →S 0 relaxations indicated higher spontaneous temperatures (T s) for the formation of the HOCOH intermediate (Ch (1)_Step (1)) than for Gly formation (Ch (1)_Step (2) and Ch (4)). Although the termolecular reaction in Ch (4) possesses a low energy barrier, and is thermodynamically favourable, the high exothermic S 1 →S 0 relaxation energy leads to the separation of the weakly associated H2O…CH2NH…CO complex into single molecules. The NVE-MDSH results also confirmed that the molecular processes after the S 1 →S 0 relaxations are thermally selective, and because the non-radiative S 1 →S 0 relaxation temperatures are exceedingly higher than T s, the formation of Gly on consecutive reaction pathways is non-synergistic with low yields and several side products. Based on the theoretical results, photo-to-thermal control strategies to promote desirable photochemical products are proposed. They could be used as guidelines for future theoretical and experimental research on photochemical reactions.

Glycine Imine-The Elusive α-Imino Acid Intermediate in the Reductive Amination of Glyoxylic Acid

Simple unhindered aldimines tend to hydrolyze or oligomerize and are therefore spectroscopically not well characterized. Herein we report the formation and spectroscopic characterization of the simplest imino acid, namely glycine imine, by cryogenic matrix isolation IR and UV/Vis spectroscopy. Glycine imine forms after UV irradiation of 2-azidoacetic acid by N₂ extrusion in anti-(E,E)- and anti-(Z,Z)-conformation that can be photochemically interconverted. In matrix isolation pyrolysis experiments with 2-azidoacetic acid, glycine imine cannot be trapped as it further decarboxylates to aminomethylene. In aqueous solution glycine imine is hydrolyzed to hydroxy glycine and hydrated glyoxylic acid. At higher concentrations or in the presence of Fe^II SO₄ as a reducing agent glycine imine undergoes self-reduction by oxidative decarboxylation chemistry. Glycine imine may be seen as one of the key reaction intermediates connecting prebiotic amino acid and sugar formation chemistry.

Insights into the molecular structure and infrared spectrum of the prebiotic species aminoacetonitrile

Aminoacetonitrile is an interstellar molecule with a prominent prebiotic role, already detected in the chemically-rich molecular cloud Sagittarius B2(N) and postulated to be present in the atmosphere of the largest Saturn's moon, Titan. To further support its observation in such remote environments and laboratory experiments aimed at improving our understanding of interstellar chemistry, we report a thorough spectroscopic and structural characterization of aminoacetonitrile. Equilibrium geometry, fundamental bands as well as spectroscopic and molecular parameters have been accurately computed by exploiting a composite scheme rooted in the coupled-cluster theory that accounts for the extrapolation to the complete basis set limit and core-correlation effects. In addition, a semi-experimental approach that combines ground-state rotational constants for different isotopic species and calculated vibrational corrections has been employed for the structure determination. From the experimental side, we report the analysis of the three strongest fundamental bands of aminoacetonitrile observed between 500 and 1000 cm^-1 in high-resolution infrared spectra. More generally, all computed band positions are in excellent agreement with the present and previous experiments. The only exception is the ν₁₅ band, for which we provide a revision of the experimental assignment, now in good agreement with theory.

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.

Hemiaminal route for the formation of interstellar glycine: a computational study

Calculations related to two simple two-step paths (path-I: [Formula: see text] path-II: [Formula: see text]) for the formation of glycine have been discussed. Calculations show that at interstellar conditions these two paths are feasible only in hot cores, not in the cold interstellar clouds (cold core formation is possible only if CH₂ = NH, H₂O (excess) and CO of path-II, react in a concerted manner). For the laboratory synthesis of glycine, the possibility suggested is via path-I and the reaction being carried out as controlled temperature one-pot synthesis. This study can also be extended to other α-amino acids and possibly enantiomeric excess can be expected. We think this work will not only be able to enrich our future understanding about the formation of amino acids in interstellar medium but also be able to suggest alternative paths for laboratory synthesis of amino acids using either Strecker's or Miller's ingredients. Graphical abstract Using computational calculations, two different reaction paths which go through a hemiaminal (α-hydroxyamine) intermediate have been proposed. It has been proposed that the reaction [Formula: see text] is a thermodynamically favorable reaction path in the laboratory conditions, if carried out as a controlled temperature one-pot synthesis. On the hand, it has been argued that the reaction[Formula: see text] is a feasible reaction path in the interstellar conditions, if it proceeds not via the hemiaminal route, rather in a concerted reaction path.

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.

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.

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.

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.

Possible interstellar formation of glycine through a concerted mechanism: a computational study on the reaction of CH2NH, CO2 and H2

Computational studies on the reaction of CH₂NH, CO₂ and H₂ show the possible interstellar formation of glycine in both hot-cores and cold interstellar clouds.

Possible interstellar formation of glycine from the reaction of CH2NH, CO and H2O: catalysis by extra water molecules through the hydrogen relay transport

Computational study shows the interstellar formation of glycine from CH₂NH, CO₂ and H₂, both in the hot-cores and cold interstellar clouds.

The first total synthesis of the peptaibol hypomurocin A1 and its conformation analysis: an application of the 'azirine/oxazolone method'

The first total synthesis of Hypomurocin A1 (HM A1) in solution phase is described. As members of the peptaibol family, hypomurocins are constituted by two groups of peptides: six undecapeptides (undecamers) in the HM A group and six octadecapeptides (18-mers) in the HM B group. The synthesis presented has been successfully achieved by the 'azirine/oxazolone method' to introduce the two Aib-Pro sequences included in this undecapeptaibol in one step with methyl 2,2-dimethyl-2H-azirine-3-prolinate as the building block. The coupling reactions of the Z-protected amino acids or peptide acids involved the use of N,N,N',N'-tetramethyluronium tetrafluoroborate (TBTU) and 1-hydroxybenzotriazole (HOBt), and led to the peptides in good-to-very-good yields. The peptides were purified by reverse-phase HPLC and characterized by NMR spectroscopy (1H, 13C, COSY, TOCSY, HSQC, HMBC, ROESY), ESI-MS, IR, elemental analysis, optical rotation, and X-ray crystallography. An NMR analysis of HM A1 was also carried out in deuterated micelles to perform a structural comparison of the helix in solution and in membranes.

Organic compounds in meteorites

Recent studies of carbonaceous chondrites provide evidence that certain organic compounds are indigenous and the result of an abiotic, chemical synthesis. The results of several investigators have established the presence of amino acids and precursors, mono- and dicarboxylic acids, N-heterocycles, and hydrocarbons as well as other compounds. For example, studies of the Murchison and Murray meteorites have revealed the presence of at least 40 amino acids with nearly equal abundances of D and L isomers. The population consists of both protein and non-protein amino acids including a wide variety of linear, cyclic, and polyfunctional types. Results show a trend of decreasing concentration with increasing carbon number, with the most abundant being glycine (41 n Moles/g). The monocarboxylic acids are more abundant (1.83 micro Moles propanoic acid/g Murchison), with hydrocarbons present in even greater abundance. In addition, quantification of the monocarboxylic acids shows a nearly equal distribution of straight and branched chain isomers. These and other results to be reviewed provide persuasive support for the theory of chemical evolution and provide the only natural evidence for the protobiological subset of molecules from which life on earth may have arisen.

Dipeptides and diketopiperazines in the Yamato-791198 and Murchison carbonaceous chondrites

The Yamato-791198 and Murchison carbonaceous chondrites were analyzed for dipeptides and diketopiperazines as well as amino acids and hydantoins by gas chromatography combined with mass spectrometry. Glycylglycine (gly-gly) and cyclo(gly-gly) were detected at the concentrations of 11 and 18 pmol g-1, respectively, in Yamato-791198, and 4 and 23 pmol g-1, respectively, in Murchison. No other dipeptide and diketopiperazine were detected. Five hydantoins were detected at 8 to 65 pmol g-1 in Yamato-791198 and seven in Murchison at 6 to 104 pmol g-1. Total concentration of the glycine (gly) dimers is approximately four orders of magnitude less than the concentration of free gly in Yamato-791198, and three orders of magnitude less than that in Murchison. The absence of L- and LL-stereoisomers of dipeptides consisting of protein amino acids indicates that gly-gly and cyclo(gly-gly) detected are native to the chondries and not from terrestrial contaminants. A possibility was discussed that the gly dimers might have been formed by condensation of gly monomers but not formed through N-carboxyanhydrides of gly.

The radiolysis and radioracemization of amino acids on clays

L-Leucine and its hydrochloride salt have been deposited on the clay minerals kaolin and bentonite, and the amino acid/clay preparations have been irradiated in a 3000 Ci60Co gamma-ray source for radiation dosages that achieved 2-89% radiolysis of the leucine. The undecomposed leucine was thereupon recovered and both percent radiolysis and percent radioracemization were determined. Similar studies were made using solid L-leucine and its hydrochloride, and L-leucine in 0.1 M aqueous solution. It has been found that radiolysis and radio-racemization in these and the previously studied leucine systems follow pseudo-first-order rate laws, and the corresponding specific rate constants are evaluated and compared. Leucine and its hydrochloride salt proved to be the most stable to both radiolysis and radioracemization, followed by leucine and its HCl salt on kaolin, followed by leucine and its HCl salt on bentonite, with leucine (and its HCl and Na salts) in aqueous solution being least stable to both radiolysis and (except for the HCl salt) radioracemization. Implications of these observations as regards the Vester-Ulbricht mechanism for the origin of optical activity are discussed.

Survivability of biomolecules during extraterrestrial delivery: new results on pyrolysis of amino acids and poly-amino acids

The hypothesis on exogenous origin of organic matter on the early Earth is strongly supported by the detection of a large variety of organic compounds (including amino acids and nucleobases) in carbonaceous chondrites. Whether such complex species can be successively delivered by other space bodies (comets, asteroids and interplanetary dust particles) is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. Recent simulation experiments on amino acid and nucleic acid base pyrolysis under oxygen-free atmosphere demonstrated that simple representatives of these (considered thermally unstable) compounds can survive at 1-10% level a rapid heating at 500-600 degrees C. In the present work, we report on new data on the pyrolysis of amino acids and their homopolymers and discuss implications of their thermal behavior for extraterrestrial delivery.

Non-racemic amino acids in the Murray and Murchison meteorites

Small (1.0-9.2%) L-enantiomer excesses were found in six alpha-methyl-alpha-amino alkanoic acids from the Murchison (2.8-9.2%) and Murray (1.0-6.0%) carbonaceous chondrites by gas chromatography-mass spectroscopy of their N-trifluoroacetyl or N-pentafluoropropyl isopropyl esters. These amino acids [2-amino-2,3-dimethylpentanoic acid (both diastereomers), isovaline, alpha-methyl norvaline, alpha-methyl valine, and alpha-methyl norleucine] are either unknown or rare in the terrestrial biosphere. Enantiomeric excesses were either not observed in the four alpha-H-alpha-amino alkanoic acids analyzed (alpha-amino-n-butyric acid, norvaline, alanine, and valine) or were attributed to terrestrial contamination. The substantial excess of L-alanine reported by others was not found in the alanine in fractionated extracts of either meteorite. The enantiomeric excesses reported for the alpha-methyl amino acids may be the result of partial photoresolution of racemic mixtures caused by ultraviolet circularly polarized light in the presolar cloud. The alpha-methyl-alpha-amino alkanoic acids could have been significant in the origin of terrestrial homochirality given their resistance to racemization and the possibility for amplification of their enantiomeric excesses suggested by the strong tendency of their polymers to form chiral secondary structure.

Transport of extraterrestrial biomolecules to the Earth: problem of thermal stability

The idea of extraterrestrial delivery of organic matter to the early Earth is especially attractive at present and is strongly supported by the detection of a large variety of organic compounds, including amino acids and nucleobases, in carbonaceous chondrites. Whether these compounds can be delivered by other space bodies is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. In the present study we estimated survivability of simple amino acids (alpha-aminoisobutyric acid, L-alanine, L-valine and L-leucine), purines (adenine and guanine) and pyrimidines (uracil and cytosine) under rapid heating to temperatures of 400 to 1000 degrees C under N2 or CO2 atmosphere. We have found that most of the compounds studied cannot survive the temperatures substantially higher than 700 degrees C; however at 500-600 degrees C, the recovery can be at a per cent level (or even 10%-level for adenine, uracil, alanine, and valine). Implications of the data for extraterrestrial delivery of the biomolecules are discussed.

Behavior of amino acids when volatilized in the presence of silica gel and pulverized basaltic lava

To evaluate the types of amino acid thermal transformations caused by silicate materials, we studied the volatilization products of Aib, L-Ala, L-Val and L-Leu under temperatures of up to 270 degrees C in the presence of silica gel as a model catalyst and pulverized basaltic lava samples. It was found that silica gel catalyzes nearly quantitative condensation of amino acids, where piperazinediones are the major products, whereas lava samples have much lower catalytic efficiency. In addition bicyclic and tricyclic amidines and several products of their subsequent thermal decomposition have been identified using the coupled technique of GC-FTIR-MS and HPLC-PB-MS, with auxiliary computer simulation of IR spectra and NMR spectroscopy. The decomposition is due to dehydrogenation, elimination of the alkyl substituents and dehydration as well as cleavage of the bicyclic ring system. The imidazole ring appears to be more resistant to thermal decomposition as compared to the pyperazine moiety, giving rise to the formation of different substituted imidazolones. The amidines were found to hydrolyze under treatment with concentrated HCl, releasing the starting amino acids and thus behaving as amino acid anhydrides. The thermal transformations cause significant racemization of amino acid residues. Based on our observations, the formation of amidine-type products is suggested to be rather common in the high-temperature experiments on amino acid condensation.

Chemical studies on the possible existence of life on Mars

Although there is no direct evidence yet for the existence of life on Mars, it is reasonable to conclude that the emergence of life on Earth, which appears to have been controlled by universal laws of physics and chemistry, may have been repeated elsewhere in the universe. The dual approach of synthesis and analysis in our experimental studies has provided ample evidence in support of this hypothesis.

Characteristics and formation of amino acids and hydroxy acids of the Murchison meteorite

Eight characteristics of the unique suite of amino acids and hydroxy acids found in the Murchison meteorite can be recognized on the basis of detailed molecular and isotopic analyses. The marked structural correspondence between the alpha-amino acids and alpha-hydroxy acids and the high deuterium/hydrogen ratio argue persuasively for their formation by aqueous phase Strecker reactions in the meteorite parent body from presolar, i.e., interstellar, aldehydes, ketones, ammonia, and hydrogen cyanide. The characteristics of the meteoritic suite of amino acids and hydroxy acids are briefly enumerated and discussed with regard to their consonance with this interstellar-parent body formation hypothesis. The hypothesis has interesting implications for the organic composition of both the primitive parent body and the presolar nebula.

Summary and implications of reported amino acid concentrations in the Murchison meteorite

A study of literature reports of the concentrations of amino acids in extracts from the Murchison meteorite shows that many of the concentration ratios are constant. There are two possible interpretations of these ratios. One is that they are controlled by the pathways through which the amino acids formed, from which it follows that the amino acids are distributed in the same proportions throughout the meteorite. The other interpretation is that the ratios result from the analytical procedures used to extract the amino acids from the meteorite. These methods rely heavily on high-temperature (100 degrees C) aqueous extraction and subsequent high-temperature acid hydrolysis. A correlation was observed in the present study between the relative concentrations of several amino acids in the meteorite extracts and their relative aqueous solubilities at 100 degrees C (alanine, valine, leucine, isoleucine, norleucine, aspartic acid, glutamic acid and glycine). The extract solutions are dilute, and far from the saturation limits, but these correlations suggest that the sampling procedure affects directly the reported concentrations for these amino acids. Ratios of the concentration of serine to those of glycine are also constant but cannot be accounted for solely by relative solubilities, and, as suggested elsewhere, serine as well as phenylalanine and methionine may be terrestrial contaminants. Data for beta-alanine, alpha-aminobutyric acid, proline, sarcosine, alloisoleucine, beta-aminoisobutyric acid, beta-aminobutyric acid, and threonine also show constant abundances relative to glycine, but lack of solubility data at extraction conditions prohibits evaluating the extent of possible sampling bias for these amino acids. If the extraction process does not bias the results, and all extractable amino acids are removed from meteorite samples, then the properties of amino acids which control both their solubilities and their concentrations in the meteorite need to be established. The possibility of sampling bias needs to be tested experimentally before concluding that extraction is complete, and that the constant relative abundances indicate that the relative concentrations of amino acids are homogeneous in the meteorite.

Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids from Murchison meteorite

The origin of the organic matter in carbonaceous meteorites remains controversial despite extensive study over the past 20 yr. Motivated by the expectation that the patterns of isotopic variation with molecular structure among the organic compounds would contain important clues to their origin, we have measured the carbon isotopic compositions for individual hydrocarbons and monocarboxylic acids from Murchison meteorite, a C2 carbonaceous chondrite which fell in Australia in 1969. With few exceptions, notably benzene, the volatile products are substantially isotopically heavier than their terrestrial counterparts, signifying their extraterrestrial origin. For both classes of compounds, the ratio of 13C to 12C decreases with increasing carbon number in a roughly parallel manner, and each carboxylic acid exhibits a higher isotopic ratio than the hydrocarbon containing the same number of carbon atoms. These trends are consistent with the kinetically controlled synthesis of higher homologues from lower ones. The results suggest the possibility that the production mechanisms for hydrocarbons and carboxylic acids may be similar; they also impose constraints on the identity of the reactant species.

Probing the presently tenuous link between comets and the origin of life

A unique set of millimeter-wave experiments for future cometary space missions is discussed. These experiments could yield answers to many basic questions about the presently undetermined nature of cometary nuclei and inner comae. This same set of experiments, designed to do fundamental cometary research, could simultaneously provide information on whether the accepted biological requirements necessary for the development of life are met in comets.

Adsorption of protein and non-protein amino acids on a clay mineral: a possible role of selection in chemical evolution

The adsorption of protein and non-protein amino acids by Na-montmorillonite was studied at pH 3, 7, and 10, in order to determine whether clays could have played a part in selection of protein over non-protein amino acids in prebiotic times. Five pairs of amino acids, containing two to six carbons, were used at a concentration equal to 100% cation exchange capacity of the clay in adsorption experiments. The following pairs of protein and non-protein amino acids were used: glycine and sarcosine, alpha-alanine and beta-alanine, alpha-aminobutyric acid and gamma-aminobutyric acid, valine and norvaline, L-isoleucine and D-alloisoleucine. No selective adsorption of protein amino acids occurred at varying hydrogen ion concentrations. The one difference observed in the adsorption of amino acids in the mixtures was a three- and four-fold greater adsorption of beta- and gamma-amino acids, respectively, than their alpha-amino acid counterparts under acidic and neutral conditions. Strong and weak adsorption of amino acids on the clay were correlated with mechanisms such as cation exchange and hydrogen bonding. The results of this research are significant to understanding the role of clay in chemical evolution because they do not support the role of preferential adsorption of protein over non-protein amino acids by clays.

The role of metal ions in chemical evolution: polymerization of alanine and glycine in a cation-exchanged clay environment

The effect of the exchangeable cation on the condensation of glycine and alanine was investigated using a series of homoinic bentonites. A cycling procedure of drying, warming and wetting was employed. Peptide bond formation was observed, and the effectiveness of metal ions to catalyze the condensation was Cu2+ greater than Ni2+ approximately Zn2+ greater than Na+. Glycine showed 6% of the monomer incorporated into oligomers with the largest detected being the pentamer. Alanine showed less peptide bond formation (a maximum of 2%) and only the dimer was observed.

Amino acids in a carbonaceous chondrite from Antarctica

A carbonaceous chondrite from the Antarctic, referred to as the Allan Hills meteorite 77306, appears to be free from terrestrial organic contamination. The presence of both protein and non-protein amino acids and an equal abundance of D- and L-enantiomers of amino acids, is testimony to the extraterrestrial nature of these compounds.

The radioracemization of amino acids by ionizing radiation: geochemical and cosmochemical implications

A number of optically active amino acids, both in the solid state and as sodium or hydrochloride salts in aqueous solution, have been exposed to ionizing radiation from a 3000 Ci60Co gamma-ray source to see if radioracemization might accompany their well-known radiolysis. gamma-Ray doses causing 55--68% radiolysis of solid amino acids typically engendered 2--5% racemization, while aqueous solutions of the sodium salts of amino acids which underwent 53--66% radiolysis showed 5--11% racemization. Amino acid hydrochloride salts in aqueous solution, on the other hand, showed little or no radioracemization accompanying their radiolysis. Both radiolysis and radioracemization were roughly proportional to gamma-ray dose in the range studied (1--36 x 10(6) rads). Mechanisms for the radioracemization of amino acids in the solid state and as aqueous sodium salts are discussed, and the absence of radioracemization for aqueous hydrochloride salts is rationalized. Isovaline, a non-protein amino acid which has been isolated from the Murchison meteorite, contains no alpha-hydrogen atom and is therefore incapable of racemization via the chemical mechanisms by which ordinary amino acids racemize. Nevertheless, isovaline suffers radioracemization in the solid state to an extent comparable to that shown by ordinary amino acids, as do its sodium and hydrochloride salts in the solid state. The sodium salt of isovaline in aqueous solution, however, fails to racemize during its radiolysis. Several implications of the newly described phenomenon of radiomization are pointed out for the fields of geochemistry and cosmochemistry.

Inadequacy of prebiotic synthesis as origin of proteinous amino acids

The production of some nonproteinous, and lack of production of other proteinous, amino acids in model prebiotic synthesis, along with the instability of glutamine and asparagine, suggest that not all of the 20 present day proteinous amino acids gained entry into proteins directly from the primordial soup. Instead, a process of active co-evolution of the genetic code and its constituent amino acids would have to precede the final selection of these proteinous amono acids.

Present status of enantiomeric analysis by gas chromatography

The development of selective chiral solute-solvent systems showing separation of enantiomers by gas chromatography is reviewed. The enantiomeric analysis of alpha-amino-acids and its application to various problems, including notably the analysis of meteorites, is discussed. Some recent results with the highly efficient diamide phases, which permit resolution on packed columns, are presented. Some new ideas on the mechanism of resolution are briefly mentioned.

Gas chromatographic confirmation of amino acid structure via diastereomer preparation

The partial gas chromatographic (GC) resolution of several amino acid N-trifluoroacetyl 2-butyl ester diastereomeric derivatives on a short packed column has been used to confirm GC peak assignments in analysis of a marine sediment sample.

Gas chromatography of amino acids

This review summarizes all papers that have appeared on the gas chromatography of amino acids (including the iodoamino acids) and their enantiomers in the period 1956-mid-1974. It has been found that the methods used for analysis of amino acids can be divided into three classes: (1) degradative procedures and techniques for converting the amino acid into another chemical compound; (2) procedures based on esterification of the carboxyl group and derivatization of the a-amino and other reactive groups in at least two steps; and (3) procedures based on a simultaneous derivatization of the carboxyl and a-amino groups in one reaction medium. For the treatment of the amino acid or its alkyl ester, three approaches can be distinguished for the two latter cases, i.e., acylation, alkylation (including silylation) and condensation. Of the procedures used for the resolution of optical antipodes, two methods are discussed, namely analysis of diastereoisomers on optically inactive stationary phases and separation of enantiomers on optically active stationary phases.

The origin of proteins: Heteropolypeptides from hydrogen cyanide and water

Evidence from laboratory and extraterrestrial chemistry is presented consistent with the hypothesis that the original heteropolypeptides on Earth were synthesized spontaneously from hydrogen cyanide and water without the intervening formation of chi-amino acids, a key step being the direct polymerization of atmospheric hydrogen cyanide to polyaminomalononitrile (IV) via dimeric HCN. Molecular orbital calculations (INDO) show that the most probable structure for (HCN)2 is azacyclopropenylidenimine. Successive reactions of hydrogen cyanide with the reactive nitrile side chains of IV then yield heteropolyamidines which are converted by water to heteropolypeptides. To study this postulated modification of a homopolymer to a heteropolymer, poly-chi-cyanoglycine (IX) was prepared from the N-carboxyanhydride of chi-cyanoglycine. Hydrolysis of IX, a polyamide analog of the polyamidine IV, yielded glycine. However, when IX was hydrolysed after being treated with hydrogen cyanide, other chi-amino acids were also obtained including alanine, serine, aspartic acid and glutamic acid, suggesting that the nitrile groups of IX (and therfore of IV) are indeed readily attacked by hydrogen cyanide as predicted. Further theoretical and experimental studies support the view that hydrogen cyanide polymerization along these lines is a universal process that accounts not only for the past formation of primitive proteins on Earth, but also for the yellow-brown-orange colors of Jupiter today and for the presence of water-soluble compounds hydrolyzable to chi-amino acids in materials obtained from environments as diverse as the moon, carbonaceous chondrites and the reaction chambers used to simulate organic synthesis in planetary atmospheres.

Amino acids in carbonaceous chondrites

For almost 20 years laboratory experiments have advanced the concepts of chemical evolution, particularly with regard to formation of the amino acids. What has been generally lacking is concrete natural evidence for this chemical evolution hypothesis. The recent development of sophisticated analytical techniques and availability of carbonaceous chondrites with a minimum of terrestrial contamination has resulted in the identification of amino acids which provide strong evidence for a natural extraterrestrial chemical synthesis. Since the initial find in the Murchison meteorite (a type II carbonaceous chondrite) of both protein and nonprotein amino acids with nearly equal abundances of D and L isomers, further studies have been carried out. These studies have revealed the presence of at least 35 amino acids; the population consists of a wide variety of linear, cyclic and polyfunctional amino acids which shows a trend of decreasing concentration with increasing carbon number. Investigations of the Murray meteorite (a type II carbonaceous chondrite) has produced similar results, but studies of the Orgueil meteorite (a type I carbonaceous chondrite) show only a limited suite of amino acids, some of which appear to be indigenous while others appear to be terrestrial contaminanats. A sample of the Murchison meteorite was extracted with D2O and in addition of 'free' amino acids, showing no deuterium incorporation, some amino acids showed the presence of deuterium suggesting either a 'precursor(s)' or hydrogen-deuterium exchange which require(s) formation of carbon-hydrogen bonds.

Organic Compounds in Meteorites: They may have formed in the solar nebula, by catalytic reactions of carbon monoxide, hydrogen, and ammonia

Organic compounds in meteorites seem to have formed by catalytic reactions of CO, H2, and NH3 in the solar nebula, at 360 degrees to 400 degrees K and (4 to 10) x 10-6 atm. The onset of these reactions was triggered by the formation of suitable catalysts (magnetite, hydrated silicates) at these temperatures. These reactions may be a source of prebiotic carbon compounds on the inner planets, and interstellar molecules.