Max P. Bernstein

Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues

The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites—over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body.

The composition and distribution of dust along the line of sight toward the Galactic center

We discuss the composition of dust and ice along the line of sight to the Galactic center (GC) based on analysis of mid-infrared spectra (2.4-13 μm) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 μm H2O and the 4.27 μm CO2 ice features that there is more molecular cloud material along the line of sight toward Sgr A* than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH4, NH3, and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 μm. Ground-based studies have identified the 3.4 μm feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 μm in the GCS 3 spectrum to the C–H stretch in aromatic hydrocarbons. This feature is not detected, and its C–C stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons.

The First Cell Membranes

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

THE PHOTOSTABILITY OF AMINO ACIDS IN SPACE

Organic compounds observed in the interstellar medium and in solar system bodies are of particular importance for revealing the chemistry that may have led to life’s origin. Among these compounds, amino acids may have played a crucial role since they are basic components of proteins, which are the essential constituents of all organisms. We present laboratory studies testing the stability of amino acids against ultraviolet (UV) photolysis. Two biological and two nonbiological amino acids have been irradiated in frozen Ar, N2, and H2O to simulate conditions in the interstellar gas and on interstellar grains. The experimental results can be interpreted to indicate that amino acids in the gas phase will likely be destroyed during the lifetime of a typical interstellar cloud. In regions with relatively low UV radiation, amino acids might be present as transient gas-phase species. Their survival in interstellar icy grain mantles and the surface layers of comets and planets is strongly limited in the presence of UV irradiation. The rate of destruction is rather insensitive to the amino acid structure and to the ice matrix. We consider the implications of these results for the survival and transfer of amino acids in space environments, and thus their possible availability for prebiotic chemistry. Subject headings: infrared: ISM: lines and bands — ISM: abundances — ISM: molecules — methods: laboratory — molecular processes

Mechanisms of Amino Acid Formation in Interstellar Ice Analogs

Amino acids have been identified in carbonaceous chondrites, but their origin is yet unknown. Previous work has shown that a variety of amino acids can be formed via ultraviolet photolysis of interstellar ice analogs. Two possible mechanisms of formation of these amino acids have been proposed: a Strecker-type synthesis or a radical-radical mechanism. In this work, we have used isotopic labeling techniques to test the predictions made by each of these proposed mechanisms for the formation of the amino acids glycine and serine. We observe that amino acid formation occurs via multiple pathways, with potentially different mechanisms for glycine and serine. The major reaction paths do not match either of the two predicted mechanisms, although a modified radical-radical mechanism may account for our observations. The observation of multiple routes suggests that the formation of amino acids in interstellar ice analogs is not narrowly dependent on ice composition, but may occur under a variety of conditions that influence product distributions.

Hydrogenated polycyclic aromatic hydrocarbons and the 2940 and 2850 wavenumber (3.40 and 3.51 micron) infrared emission features.

The 3150-2700 cm-1 (3.17-3.70 microns) range of the spectra of a number of Ar-matrix-isolated PAHs containing excess H atoms (Hn-PAHs) are presented. This region covers features produced by aromatic and aliphatic C-H stretching vibrations as well as overtone and combination bands involving lower lying fundamentals. The aliphatic C-H stretches in molecules of this type having low to modest excess H coverage provide excellent fits to a number of the weak emission features superposed on the plateau between 3080 and 2700 cm-1 (3.25 and 3.7 microns) in the spectra of many planetary nebulae, reflection nebulae, and H II regions. Higher H coverage is implied for a few objects. We compare these results in context with the other suggested identifications of the emission features in the 2950-2700 cm-1 (3.39-3.70 microns) region and briefly discuss their astrophysical implications.

Assessment of the interstellar processes leading to deuterium enrichment in meteoritic organics

The presence of isotopic anomalies is the most unequivocal demonstration that meteoritic material contains circumstellar or interstellar components. In the case of organic compounds in meteorites and interplanetary dust particles (IDPs), the most useful isotopic tracer has been deuterium (D). We discuss four processes that are expected to lead to D enrichment in interstellar materials and describe how their unique characteristics can be used to assess their relative importance for the organics in meteorites. These enrichment processes are low-temperature gas phase ion-molecule reactions, low-temperature gas-grain reactions, gas phase unimolecular photodissociation, and ultraviolet photolysis in D-enriched ice mantles. Each of these processes is expected to be associated with distinct regiochemical signatures (D placement on the product molecules, correlation with specific chemical functionalities, etc.), especially in the molecular population of polycyclic aromatic hydrocarbons (PAHs). We describe these differences and discuss how they may be used to delineate the various interstellar processes that may have contributed to meteoritic D enrichments. We also briefly discuss how these processes may affect the isotopic distributions in C, O, and N in the same compounds.

Evolution of Interstellar Ices

Infrared observations, combined with realistic laboratory simulations, have revolutionized our understanding of interstellar ice and dust, the building blocks of comets. Ices in molecular clouds are dominated by the very simple molecules H2O, CH3OH, NH3, CO, CO2, and probably H2CO and H2. More complex species including nitriles, ketones, and esters are also present, but at lower concentrations. The evidence for these, as well as the abundant, carbon-rich, interstellar, polycyclic aromatic hydrocarbons (PAHs) is reviewed. Other possible contributors to the interstellar/pre-cometary ice composition include accretion of gas-phase molecules and in situ photochemical processing. By virtue of their low abundance, accretion of simple gas-phase species is shown to be the least important of the processes considered in determining ice composition. On the other hand, photochemical processing does play an important role in driving dust evolution and the composition of minor species. Ultraviolet photolysis of realistic laboratory analogs readily produces H2, H2CO, CO2, CO, CH4, HCO, and the moderately complex organic molecules: CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), R-CN (nitriles), and hexamethylenetetramine (HMT, C6H12N4), as well as more complex species including amides, ketones, and polyoxymethylenes (POMs). Inclusion of PAHs in the ices produces many species similar to those found in meteorites including aromatic alcohols, quinones and ethers. Photon assisted PAH-ice deuterium exchange also occurs. All of these species are readily formed and are therefore likely cometary constituents.

Side Group Addition to the Polycyclic Aromatic Hydrocarbon Coronene by Ultraviolet Photolysis in Cosmic Ice Analogs

Ultraviolet photolysis of various coronene-ice mixtures at low temperature and pressure caused the addition of amino (”NH2), methyl (”CH3), methoxy (”OCH3), cyano/isocyano (”CN, ”NC), and acid (”COOH) functional groups to the polycyclic aromatic hydrocarbon (PAH) coronene (C24H12), in addition to previously reported alcohol (”OH) and ketone (>C»O) formation. This work represents the first experimental evidence that ice photochemistry may have contributed to the aromatics bearing carbon and nitrogen containing side groups that are detected in primitive meteorites and interplanetary dust particles. Furthermore, these results suggest that a wide range of modified PAHs should be expected in interstellar ices and materials that predated solar system formation. The implications of these results for interstellar and meteoritic chemistry are discussed. Subject headings: astrobiology — astrochemistry — ISM: molecules — meteors, meteoroids — molecular processes — ultraviolet: ISM

THE LIFETIMES OF NITRILES (CN) AND ACIDS (COOH) DURING ULTRAVIOLET PHOTOLYSIS AND THEIR SURVIVAL IN SPACE

Nitriles are one of the most common classes of molecules observed in the gas phase in space, with over a dozen having been positively identified in interstellar and circumstellar environments through the detection of their rotational transitions. Acids, in contrast, are much less common. In this paper we present laboratory data comparing the stability of two structurally related acid-nitrile pairs to ultraviolet (UV) photolytic destruction: acetic acid (CH3COOH) versus acetonitrile (CH3CN) and glycine (H2NCH2COOH) versus aminoacetonitrile (H2NCH2CN). We find that the nitriles are destroyed 10 and 5 times more slowly (respectively) by UV photolysis than are the corresponding acids. This suggests that whatever their relative formation rates, acids may be less abundant than nitriles in interstellar environments in part because they are more rapidly destroyed by photolysis. The results of this infrared (IR) spectral matrix isolation study indicate that during the lifetime of a typical interstellar cloud, even in its darkest regions, a population of acids in the gas phase will likely be diminished by at least half. Since aminoacetonitrile is a precursor to the amino acid glycine, and far more stable, presolar aminoacetonitrile may be a contributor to the deuterium-enriched glycine detected in meteorites. It would clearly be informative to search for aminoacetonitrile (the nitrile corresponding to glycine) in the regions where the amino acid glycine has been reported. Subject headings: astrobiology — astrochemistry — ISM: molecules — molecular processes