Jason P. Dworkin

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.

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases

All terrestrial organisms depend on nucleic acids (RNA and DNA), which use pyrimidine and purine nucleobases to encode genetic information. Carbon-rich meteorites may have been important sources of organic compounds required for the emergence of life on the early Earth; however, the origin and formation of nucleobases in meteorites has been debated for over 50 y. So far, the few nucleobases reported in meteorites are biologically common and lacked the structural diversity typical of other indigenous meteoritic organics. Here, we investigated the abundance and distribution of nucleobases and nucleobase analogs in formic acid extracts of 12 different meteorites by liquid chromatography–mass spectrometry. The Murchison and Lonewolf Nunataks 94102 meteorites contained a diverse suite of nucleobases, which included three unusual and terrestrially rare nucleobase analogs: purine, 2,6-diaminopurine, and 6,8-diaminopurine. In a parallel experiment, we found an identical suite of nucleobases and nucleobase analogs generated in reactions of ammonium cyanide. Additionally, these nucleobase analogs were not detected above our parts-per-billion detection limits in any of the procedural blanks, control samples, a terrestrial soil sample, and an Antarctic ice sample. Our results demonstrate that the purines detected in meteorites are consistent with products of ammonium cyanide chemistry, which provides a plausible mechanism for their synthesis in the asteroid parent bodies, and strongly supports an extraterrestrial origin. The discovery of new nucleobase analogs in meteorites also expands the prebiotic molecular inventory available for constructing the first genetic molecules.

The Miller volcanic spark discharge experiment

Millers 1950s experiments used, besides the apparatus known in textbooks, one that generated a hot water mist in the spark flask, simulating a water vapor‐rich volcanic eruption. We found the original extracts of this experiment in Millers material and reanalyzed them. The volcanic apparatus produced a wider variety of amino acids than the classic one. Release of reduced gases in volcanic eruptions accompanied by lightning could have been common on the early Earth. Prebotic compounds synthesized in these environments could have locally accumulated, where they could have undergone further processing.

Enrichment of the amino acid l-isovaline by aqueous alteration on CI and CM meteorite parent bodies

The distribution and enantiomeric composition of the 5-carbon (C5) amino acids found in CI-, CM-, and CR-type carbonaceous meteorites were investigated by using liquid chromatography fluorescence detection/TOF-MS coupled with o-phthaldialdehyde/N-acetyl-l-cysteine derivatization. A large l-enantiomeric excess (ee) of the α-methyl amino acid isovaline was found in the CM meteorite Murchison (lee = 18.5 ± 2.6%) and the CI meteorite Orgueil (lee = 15.2 ± 4.0%). The measured value for Murchison is the largest enantiomeric excess in any meteorite reported to date, and the Orgueil measurement of an isovaline excess has not been reported previously for this or any CI meteorite. The l-isovaline enrichments in these two carbonaceous meteorites cannot be the result of interference from other C5 amino acid isomers present in the samples, analytical biases, or terrestrial amino acid contamination. We observed no l-isovaline enrichment for the most primitive unaltered Antarctic CR meteorites EET 92042 and QUE 99177. These results are inconsistent with UV circularly polarized light as the primary mechanism for l-isovaline enrichment and indicate that amplification of a small initial isovaline asymmetry in Murchison and Orgueil occurred during an extended aqueous alteration phase on the meteorite parent bodies. The large asymmetry in isovaline and other α-dialkyl amino acids found in altered CI and CM meteorites suggests that amino acids delivered by asteroids, comets, and their fragments would have biased the Earths prebiotic organic inventory with left-handed molecules before the origin of life.

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.

Origin and Evolution of Prebiotic Organic Matter As Inferred from the Tagish Lake Meteorite

The study of organic matter in a well-preserved meteorite provides insight into processes that affected its parent asteroids. The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites’ asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration. At least some molecules of prebiotic importance formed during the alteration.

The Effects of Parent Body Processes on Amino Acids in Carbonaceous Chondrites

To investigate the effect of parent body processes on the abundance, distribution, and enantiomeric composition of amino acids in carbonaceous chondrites, the water extracts from nine different powdered CI, CM, and CR carbonaceous chondrites were analyzed for amino acids by ultra performance liquid chromatography-fluorescence detection and time-of-flight mass spectrometry (UPLC-FD ⁄ ToF-MS). Four aqueously altered type 1 carbonaceous chondrites including Orgueil (CI1), Meteorite Hills (MET) 01070 (CM1), Scott Glacier (SCO) 06043 (CM1), and Grosvenor Mountains (GRO) 95577 (CR1) were analyzed using this technique for the first time. Analyses of these meteorites revealed low levels of two- to five-carbon acyclic amino alkanoic acids with concentrations ranging from approximately 1 to 2,700 parts-per-billion (ppb). The type 1 carbonaceous chondrites have a distinct distribution of the five-carbon (C5) amino acids with much higher relative abundances of the c- and d-amino acids compared to the type 2 and type 3 carbonaceous chondrites, which are dominated by a-amino acids. Much higher amino acid abundances were found in the CM2 chondrites Murchison, Lonewolf Nunataks (LON) 94102, and Lewis Cliffs (LEW) 90500, the CR2 Elephant Moraine (EET) 92042, and the CR3 Queen Alexandra Range (QUE) 99177. For example, a-aminoisobutyric acid (a-AIB) and isovaline were approximately 100 to 1000 times more abundant in the type 2 and 3 chondrites compared to the more aqueously altered type 1 chondrites. Most of the chiral amino acids identified in these meteorites were racemic, indicating an extraterrestrial abiotic origin. However, nonracemic isovaline was observed in the aqueously altered carbonaceous chondrites Murchison, Orgueil, SCO 06043, and GRO 95577 with l-isovaline excesses ranging from approximately 11 to 19%, whereas the most pristine, unaltered carbonaceous chondrites analyzed in this study had no detectable l-isovaline excesses. These results are consistent with the theory that aqueous alteration played an important role in amplification of small initial left handed isovaline excesses on the parent bodies.