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.

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 (13 C)-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.

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International audienc

Circularly polarized light (CPL) is known to be a true chiral entity capable of generating absolute molecular asymmetry. However, the degree of inducible optical activity depends on the λ of the incident CPL. Exposure of amorphous films of rac-alanine to tunable CPL led to enantiomeric excesses (ee) which not only follow the helicity but also the energy of driving electromagnetic radiation. Postirradiation analyses using enantioselective multidimensional GC revealed energy-controlled ee values of up to 4.2 %, which correlate with theoretical predictions based on newly recorded anisotropy spectra g(λ). The tunability of asymmetric photochemical induction implies that both magnitude and sign can be fully controlled by CPL. Such stereocontrol provides novel insights into the wavelength and polarization dependence of asymmetric photochemical reactions and are highly relevant for absolute asymmetric molecular synthesis and for understanding the origins of homochirality in living matter.

Evolved interstellar ices observed in dense protostellar molecular clouds may arguably be considered as part of precometary materials that will later fall on primitive telluric planets, bringing a wealth of complex organic compounds. In our laboratory, experiments reproducing the photo/thermochemical evolution of these ices are routinely performed. Following previous amino acid identifications in the resulting room temperature organic residues, we have searched for a different family of molecules of potential prebiotic interest. Using multidimensional gas chromatography coupled to time-of-flight mass spectrometry, we have detected 10 aldehydes, including the sugar-related glycolaldehyde and glyceraldehyde-two species considered as key prebiotic intermediates in the first steps toward the synthesis of ribonucleotides in a planetary environment. The presence of ammonia in water and methanol ice mixtures appears essential for the recovery of these aldehydes in the refractory organic residue at room temperature, although these products are free of nitrogen. We finally point out the importance of detecting aldehydes and sugars in extraterrestrial environments, in the gas phase of hot molecular clouds, and, more importantly, in comets and in primitive meteorites that have most probably seeded the Earth with organic material as early as 4.2 billion years ago.glyceraldehyde | glycolaldehyde | astrochemistry | precometary ices | prebiotic evolution

Dedicated to Professor Henri B. Kagan on the occasion of his 80th birthday Biopolymers such as nucleic acids and proteins are composed of chiral monomers that show identical stereochemical configuration. Naturally occurring proteins are made up of l-amino acids.[1] Hypotheses for the origin of symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which circularly polarized (CP) light induces an enantiomeric excess (ee) in chiral organic molecules. [2][3][4] This model is supported by both the observation of CP light in the star-forming region of Orion [3,5] and the occurrence of l-enantiomer-enriched amino acids in carbonaceous meteorites. [6][7][8] However, the differential absorption of CP light by amino acid enantiomers, which determines the speed and intensity of enantioselective photolysis, is unknown over a large spectral range. Here we show that significant circular dichroic transitions in amino acids can be observed by extending circular dichroism (CD) spectroscopy to the vacuum-ultraviolet (UV) spectral range. a-H amino acids show the same CD magnitude and sign over a large wavelength range. In a given spectral window [9] CP light is therefore capable of inducing enantiomeric excesses of the same handedness into the proteinogenic amino acids we have studied. Absolute asymmetric photochemistry might thus well have triggered the appearance of l-amino acid based life on Earth. Our results demonstrate that enantiomers of "meteoritic" a-methyl amino acids show dichroic absorption with equal magnitude, yet opposite sign to a-H amino acids. Therefore CP light cannot induce l enantiomeric excesses into a-methyl and a-H amino acids as found in meteorites.To explain the cause of symmetry breaking in biomolecules a well-known theory [2-4, 10, 11] proposes that CP interstellar UV radiation-similar to that identified in the starforming region of Orion in the infrared [3,5] -induced enantiomeric excesses into interstellar and circumstellar organic compounds by asymmetric photochemical reactions prior to their deposition on the early Earth. [12] In support of this theory chiral amino acid structures were identified in interstellar ice analogues [13] and a large number of l-enantiomer-enriched amino acids have been identified in the interior of the Murchison [6] and Murray [7] carbonaceous meteorites.[8] To verify the absolute asymmetric photochemistry model the differential CP-light absorption of proteinogenic and meteoritic amino acid enantiomers requires systematic examination.Until now, the popular and extensively used technique of CD spectroscopy has been used to record electronic CD for chiral molecules in aqueous solution above 190 nm.[14] Water absorbs photons of l < 190 nm, making the vacuum-UV region inaccessible for CD spectroscopy in aqueous solution. By using a synchrotron radiation source for CP light and preparing isotropic amorphous solid-state samples immobilized on MgF 2 windows, we have extended electronic CD measurements to the vacuum-UV spectral range.We observed...

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