Uwe J. Meierhenrich

Amino acids from ultraviolet irradiation of interstellar ice analogues

Amino acids are the essential molecular components of living organisms on Earth, but the proposed mechanisms for their spontaneous generation have been unable to account for their presence in Earths early history. The delivery of extraterrestrial organic compounds has been proposed as an alternative to generation on Earth, and some amino acids have been found in several meteorites. Here we report the detection of amino acids in the room-temperature residue of an interstellar ice analogue that was ultraviolet-irradiated in a high vacuum at 12 K. We identified 16 amino acids; the chiral ones showed enantiomeric separation. Some of the identified amino acids are also found in meteorites. Our results demonstrate that the spontaneous generation of amino acids in the interstellar medium is possible, supporting the suggestion that prebiotic molecules could have been delivered to the early Earth by cometary dust, meteorites or interplanetary dust particles.

Asymmetric photochemistry and photochirogenesis.

One of the most interesting phenomena on Earth is the chirality of biomolecules, the origin of which remains unknown. A challenge arising from this phenomenon is the selective, atom-economic synthesis of enantiomerically pure target molecules from nonchiral starting materials. Herein, new developments in the field of asymmetric photochemistry and photochirogenesis are described with special emphasis on absolute asymmetric synthesis. In this context, the elucidation of the ultimate cause of homochirality phenomena on earth and the possible correlation with physicochemical parameters are also presented.

Non-racemic Amino Acid Production by Ultraviolet Irradiation of Achiral Interstellar Ice Analogs with Circularly Polarized Light

The delivery of organic matter to the primitive Earth via comets and meteorites has long been hypothesized to be an important source for prebiotic compounds such as amino acids or their chemical precursors that contributed to the development of prebiotic chemistry leading, on Earth, to the emergence of life. Photochemistry of inter/circumstellar ices around protostellar objects is a potential process leading to complex organic species, although difficult to establish from limited infrared observations only. Here we report the first abiotic cosmic ice simulation experiments that produce species with enantiomeric excesses (e.e.s). Circularly polarized ultraviolet light (UV-CPL) from a synchrotron source induces asymmetric photochemistry on initially achiral inter/circumstellar ice analogs. Enantioselective multidimensional gas chromatography measurements show significant e.e.s of up to 1.34% for (13C)-alanine, for which the signs and absolute values are related to the helicity and number of CPL photons per deposited molecule. This result, directly comparable with some L excesses measured in meteorites, supports a scenario in which exogenous delivery of organics displaying a slight L excess, produced in an extraterrestrial environment by an asymmetric astrophysical process, is at the origin of biomolecular asymmetry on Earth. As a consequence, a fraction of the meteoritic organic material consisting of non-racemic compounds may well have been formed outside the solar system. Finally, following this hypothesis, we support the idea that the protosolar nebula has indeed been formed in a region of massive star formation, regions where UV-CPL of the same helicity is actually observed over large spatial areas.

Amino Acids and the Asymmetry of Life

‘How did life start on Earth?’ and ‘Why were left-handed amino acids selected for the architecture of proteins?’ A new attempt to answer these questions of high public and interdisciplinary scientific interest will be provided by this review. It will describe most recent experimental data on how the basic and molecular building blocks of life, amino acids, formed in a prebiotic setting. Most amino acids are chiral, that is that they cannot be superimposed with their mirror image molecules (enantiomers). In processes triggering the origin of life on Earth, the equal occurrence, i.e. the parity between left-handed amino acids and their right-handed mirror images, was violated. In the case of amino acids, the balance was tipped to the left – as a result of which lifes proteins today exclusively implement the left-handed form of amino acids, called l -amino acid enantiomers. Neither plants, nor animals, including humans, make use of d -amino acids for the molecular architecture of their proteins (enzymes). This review addresses the molecular asymmetry of amino acids in living organisms, namely the preference for left-handedness. What was the cause for the violation of molecular parity of amino acids in the emergence of life on Earth? All the fascinating models proposed by physicists, chemists, and biologists will be vividly presented including the scientific conflicts. Special emphasis will be given to amino acid enantiomers that were subjected to chiral photons. The interaction between racemic molecules and chiral photons was shown to produce an enantiomeric enrichment that will be discussed in the context of absolute asymmetric synthesis. The concluding paragraphs will describe the attempt to verify any of those models with the chirality-module of the Rosetta mission. This European space mission contains probe Philae that was launched on board the Rosetta spacecraft with the aim of landing on the icy surface of comet 67P/Churyumov-Gerasimenko and analysing whether chiral organic compounds are present that could have been brought to the Earth by comet impacts.

Ribose and related sugars from ultraviolet irradiation of interstellar ice analogs

Making ribose in interstellar ices Astrobiologists have long speculated on the origin of prebiotic molecules such as amino acids and sugars. Meinert et al. demonstrated that numerous prebiotic molecules can be formed in an interstellar-analog sample containing a mixture of simple ices of water, methanol, and ammonia. They irradiated the sample with ultraviolet light under conditions similar to those expected during the formation of the solar system. This yielded a wide variety of sugars, including ribose—a major constituent of ribonucleic acid (RNA). Science, this issue p. 208 Prebiotic sugars, including ribose, can be made by irradiating simple ices under interstellar conditions. Ribose is the central molecular subunit in RNA, but the prebiotic origin of ribose remains unknown. We observed the formation of substantial quantities of ribose and a diversity of structurally related sugar molecules such as arabinose, xylose, and lyxose in the room-temperature organic residues of photo-processed interstellar ice analogs initially composed of H2O, CH3OH, and NH3. Our results suggest that the generation of numerous sugar molecules, including the aldopentose ribose, may be possible from photochemical and thermal treatment of cosmic ices in the late stages of the solar nebula. Our detection of ribose provides plausible insights into the chemical processes that could lead to formation of biologically relevant molecules in suitable planetary environments.

Urea, Glycolic Acid, and Glycerol in an Organic Residue Produced by Ultraviolet Irradiation of Interstellar/Pre-Cometary Ice Analogs

More than 50 stable organic molecules have been detected in the interstellar medium (ISM), from ground-based and onboard-satellite astronomical observations, in the gas and solid phases. Some of these organics may be prebiotic compounds that were delivered to early Earth by comets and meteorites and may have triggered the first chemical reactions involved in the origin of life. Ultraviolet irradiation of ices simulating photoprocesses of cold solid matter in astrophysical environments have shown that photochemistry can lead to the formation of amino acids and related compounds. In this work, we experimentally searched for other organic molecules of prebiotic interest, namely, oxidized acid labile compounds. In a setup that simulates conditions relevant to the ISM and Solar System icy bodies such as comets, a condensed CH(3)OH:NH(3) = 1:1 ice mixture was UV irradiated at approximately 80 K. The molecular constituents of the nonvolatile organic residue that remained at room temperature were separated by capillary gas chromatography and identified by mass spectrometry. Urea, glycolic acid, and glycerol were detected in this residue, as well as hydroxyacetamide, glycerolic acid, and glycerol amide. These organics are interesting target molecules to be searched for in space. Finally, tentative mechanisms of formation for these compounds under interstellar/pre-cometary conditions are proposed.

Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry

Comets harbor the most pristine material in our solar system in the form of ice, dust, silicates, and refractory organic material with some interstellar heritage. The evolved gas analyzer Cometary Sampling and Composition (COSAC) experiment aboard Rosetta’s Philae lander was designed for in situ analysis of organic molecules on comet 67P/Churyumov-Gerasimenko. Twenty-five minutes after Philae’s initial comet touchdown, the COSAC mass spectrometer took a spectrum in sniffing mode, which displayed a suite of 16 organic compounds, including many nitrogen-bearing species but no sulfur-bearing species, and four compounds—methyl isocyanate, acetone, propionaldehyde, and acetamide—that had not previously been reported in comets.

Photochirogenesis: photochemical models on the absolute asymmetric formation of amino acids in interstellar space.

Proteins of all living organisms including plants, animals, and humans are made up of amino acid monomers that show identical stereochemical L-configuration. Hypotheses for the origin of this symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which interstellar ultraviolet (UV) circularly polarized light (CPL) induces an enantiomeric excess in chiral organic molecules in the interstellar/circumstellar media. This scenario is supported by a) the detection of amino acids in the organic residues of UV-photo-processed interstellar ice analogues, b) the occurrence of L-enantiomer-enriched amino acids in carbonaceous meteorites, and c) the observation of CPL of the same helicity over large distance scales in the massive star-forming region of Orion. These topics are of high importance in topical biophysical research and will be discussed in this review. Further evidence that amino acids and other molecules of prebiotic interest are asymmetrically formed in space comes from studies on the enantioselective photolysis of amino acids by UV-CPL. Also, experiments have been performed on the absolute asymmetric photochemical synthesis of enantiomer-enriched amino acids from mixtures of astrophysically relevant achiral precursor molecules using UV-circularly polarized photons. Both approaches are based on circular dichroic transitions of amino acids that will be highlighted here as well. These results have strong implications on our current understanding of how lifes precursor molecules were possibly built and how life selected the left-handed form of proteinogenic amino acids.

The effects of circularly polarized light on amino acid enantiomers produced by the UV irradiation of interstellar ice analogs

Two irradiation experiments on interstellar ice analogs at 80 K under interstellar-like conditions were performed with the LURE SU5 synchrotron beamline to assess, for the first time, the photochemical effect of circularly polarized ultraviolet light (UV CPL) at 167 nm (7.45 eV) with right and left polarizations on such ice mixtures. Methods. This effect was measured by determining the enantiomeric excesses (e.e.s) for two amino acids formed in the solid organic residues produced during the subsequent warm-up of the irradiated samples to room temperature: alanine, the most abundant chiral proteinaceous amino acid produced (both polarizations) and 2,3-diaminopropanoic acid (DAP), a non-proteinaceous amino acid (rightpolarization experiment). These excesses were compared to those measured for the same amino acids produced after unpolarized UV irradiation of the same ice mixtures (expected to be zero), in order to determine the contribution of CPL only. A careful estimate of all the associated uncertainties (statistical and systematic errors) was also developed. Results. It appears that the enantiomeric photochemical effect at this wavelength is weak, since both alanine and DAP e.e.s were found to be small, at most of the order of 1% in absolute values, and tends to be inconclusive since the effects obtained for both amino acids and both polarizations are not those expected. In light of these results, the hypothesis that CPL may be one source responsible for the e.e.s measured for such amino acids in some meteorites and, more generally, that CPL may be directly related to the origin of biomolecular homochirality on Earth is discussed.

A New Dimension in Separation Science: Comprehensive Two‐Dimensional Gas Chromatography

The introduction and development of comprehensive two-dimensional gas chromatography offers greatly enhanced resolution and identification of organic analytes in complex mixtures compared to any one-dimensional separation technique. Initially promoted by the need to resolve highly complex petroleum samples, the techniques enormous separation power and enhanced ability to gather information has rapidly attracted the attention of analysts from all scientific fields. In this Minireview, we highlight the fundamental theory, recent advances, and future trends in the instrumentation and application of comprehensive two-dimensional column separation.