Grégoire Danger

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.

Hydrogenation of solid hydrogen cyanide HCN and methanimine CH2NH at low temperature

Context. Hydrogenation reactions dominate grain surface chemistry in dense molecular clouds and lead to the formation of complex saturated molecules in the interstellar medium. Aims. We investigate in the laboratory the hydrogenation reaction network of hydrogen cyanide HCN. Methods. Pure hydrogen cyanide HCN and methanimine CH2NH ices are bombarded at room temperature by H-atoms in an ultra-high vacuum experiment. Warm H-atoms are generated in an H2 plasma source. The ices are monitored with Fourier-transform infrared spectroscopy in reflection absorption mode. The hydrogenation products are detected in the gas phase by mass spectroscopy during temperature-programmed desorption experiments. Results. HCN hydrogenation leads to the formation of methylamine CH3NH2 ,a nd CH 2NH hydrogenation leads to the formation of methylamine CH3NH2, suggesting that CH2NH can be a hydrogenation-intermediate species between HCN and CH3NH2. Conclusions. In cold environments the HCN hydrogenation reaction can produce CH3NH2, which is known to be a glycine precursor, and to destroy solid-state HCN, preventing its observation in molecular clouds ices.

Experimental investigation of aminoacetonitrile formation through the Strecker synthesis in astrophysical-like conditions: reactivity of methanimine (CH2NH), ammonia (NH3), and hydrogen cyanide (HCN)

Astronomy & Astrophysics Experimental investigation of aminoacetonitrile formation through the Strecker synthesis in astrophysical-like conditions: reactivity of methanimine (CH 2 NH), ammonia (NH 3), and hydrogen cyanide (HCN) ABSTRACT Context. Studing chemical reactivity in astrophysical environments is an important means for improving our understanding of the origin of the organic matter in molecular clouds, in protoplanetary disks, and possibly, as a final destination, in our solar system. Laboratory simulations of the reactivity of ice analogs provide important insight into the reactivity in these environments. Here, we use these experimental simulations to investigate the Strecker synthesis leading to the formation of aminoacetonitrile in astrophysical-like conditions. The aminoacetonitrile is an interesting compound because it was detected in SgrB2, hence could be a precursor of the smallest amino acid molecule, glycine, in astrophysical environments. Aims. We present the first experimental investigation of the formation of aminoacetonitrile NH 2 CH 2 CN from the thermal processing of ices including methanimine (CH 2 NH), ammonia (NH 3), and hydrogen cyanide (HCN) in interstellar-like conditions without VUV photons or particules. Methods. We use Fourier Transform InfraRed (FTIR) spectroscopy to monitor the ice evolution during its warming. Infrared spec-troscopy and mass spectroscopy are then used to identify the aminoacetonitrile formation. Results. We demonstrate that methanimine can react with − CN during the warming of ice analogs containing at 20 K methanimine, ammonia, and [NH + 4 − CN] salt. During the ice warming, this reaction leads to the formation of poly(methylene-imine) polymers. The polymer length depend on the initial ratio of mass contained in methanimine to that in the [NH + 4 − CN] salt. In a methanimine excess, long polymers are formed. As the methanimine is progressively diluted in the [NH + 4 − CN] salt, the polymer length decreases until the aminoacetonitrile formation at 135 K. Therefore, these results demonstrate that aminoacetonitrile can be formed through the second step of the Strecker synthesis in astrophysical-like conditions.

The desorption of H2CO from interstellar grains analogues

Context. Much of the formaldehyde (H2CO) is formed from the hydrogenation of CO on interstellar dust grains, and is released in the gas phase in hot core regions. Radio-astronomical observations in these regions are directly related to its desorption from grains. Aims. We study experimentally the thermal desorption of H2CO from bare silicate surfaces, from water ice surfaces and from bulk water ice in order to model its desorption from interstellar grains. Methods. Temperature-programmed desorption experiments, monitored by mass spectrometry, and Fourier transform infrared spectroscopy are performed in the laboratory to determine the thermal desorption energies in: (i.) the multilayer regime where H2CO is bound to other H2CO molecules; (ii.) the submonolayer regime where H2CO is bound on top of a water ice surface; (iii.) the mixed submonolayer regime where H2CO is bound to a silicate surface; and (iv.) the multilayer regime in water ice, where H2CO is embedded within a H2O matrix. Results. In the submonolayer regime, we find the zeroth-order desorption kinetic parameters nu(0) = 10(28) mol cm(-2) s(-1) and E = 31.0 +/- 0.9 kJmol(-1) for desorption from an olivine surface. The zeroth-order desorption kinetic parameters are nu(0) = 10(28) mol cm(-2) s(-1) and E = 27.1 +/- 0.5 kJmol(-1) for desorption from a water ice surface in the submonolayer regime. In a H2CO:H2O mixture, the desorption is in competition with the H2CO + H2O reaction, which produces polyoxymethylene, the polymer of H2CO. This polymerization reaction prevents the volcano desorption and co-desorption from happening. Conclusions. H2CO is only desorbed from interstellar ices via a dominant sub-monolayer desorption process (E = 27.1 +/- 0.5 kJmol-1). The H2CO which has not desorbed during this sub-monolayer desorption polymerises upon reaction with H2O, and does not desorb as H2CO at higher temperature.

Diffusion measurements of CO, HNCO, H2CO, and NH3 in amorphous water ice

Context. Water is the major component of the interstellar ice mantle. In interstellar ice, chemical reactivity is limited by the diffusion of the reacting molecules, which are usually present at abundances of a few percent with respect to water. Aims. We want to study the thermal diffusion of H2CO, NH3, HNCO, and CO in amorphous water ice experimentally to account for the mobility of these molecules in the interstellar grain ice mantle. Methods. In laboratory experiments performed at fixed temperatures, the diffusion of molecules in ice analogues was monitored by Fourier transform infrared spectroscopy. Diffusion coefficients were extracted from isothermal experiments using Ficks second law of diffusion. Results. We measured the surface diffusion coefficients and their dependence with the temperature in porous amorphous ice for HNCO, H2CO, NH3, and CO. They range from 10(-15) to 10(-11) cm(2) s(-1) for HNCO, H2CO, and NH3 between 110 K and 140 K, and between 5-8 x 10(-13) cm(2) s(-1) for CO between 35 K and 40 K. The bulk diffusion coefficients in compact amorphous ice are too low to be measured by our technique and a 10(-15) cm(2) s(-1) upper limit can be estimated. The amorphous ice framework reorganization at low temperature is also put in evidence. Conclusions. Surface diffusion of molecular species in amorphous ice can be experimentally measured, while their bulk diffusion may be slower than the ice mantle desorption kinetics.

HYDROXYACETONITRILE (HOCH2CN) FORMATION IN ASTROPHYSICAL CONDITIONS. COMPETITION WITH THE AMINOMETHANOL, A GLYCINE PRECURSOR

This contribution is focused on the concurrent pathway to the Strecker synthesis of amino acids in an astrophysicallike environment. We indeed use experimental and modeling simulations to investigate the possibility to form the aminomethanol (HOCH2NH2) in concurrence with the hydroxyacetonitrile (HOCH2CN) from ices containing at 40 K formaldehyde (CH2O), ammonia (NH3), and cyanide ion (CN − ). We demonstrate using infrared spectroscopy and mass spectrometry that the formation of the aminomethanol (Ea = 4.5 kJ mol −1 ) is competing with the hydroxyacetonitrile formation (Ea = 3.9 kJ mol −1 ). The ratio between aminomethanol and hydroxyacetonitrile depends on the initial ratio in the ice between ammonia and cyanide. An increase of cyanide ion provides a decrease in aminomethanol formation. Since the aminomethanol is the first step through the formation of glycine in astrophysical environments, these data are important for understanding the possibility of forming glycine in such environments. Furthermore, using a reduced kinetic model, we evaluate the astrophysical environments in which the aminomethanol and hydroxyacetonitrile can be formed and evolved without degradation. The results suggest that these two molecules could be formed in molecular clouds or protostellar disks, and subsequently incorporated inside comets or asteroids. Therefore, hydroxyacetonitrile and aminomethanol could be formed before the formation of the solar system, which suggests that hydroxyacids and amino acids, such as those detected inside meteorites, have been formed in various astrophysical environments.

Experimental investigation of nitrile formation from VUV photochemistry of interstellar ices analogs: acetonitrile and amino acetonitrile

Context. The study of the chemical reactivity in interstellar ices in astrophysical environments is an important tool for understanding the origin of the organic matter in molecular clouds, in protoplanetary disks, and possibly, as a final destination, in our solar system. The laboratory simulations of the reactivity in ice analogs provide important information for understanding the reactivity in these environments. Here, we used these experimental simulations to trace some formation pathways of two nitriles, acetonitrile and amino acetonitrile, which are two potential precursors of amino acids in astrophysical environments. Aims. The purpose of this work is to present the first experimental approach for the formation of acetonitrile and amino acetonitrile in interstellar-like conditions. Methods. We use Fourier Transform InfraRed (FTIR) spectroscopy and mass spectrometry to study the formation at 20 K of ace-tonitrile CH 3 CN from VUV irradiation of ethylamine and of amino acetonitrile NH 2 CH 2 CN from VUV irradiation of ammonia: acetonitrile mixture. Isotopic substitutions are used to confirm identifications. Results. We demonstrate that acetonitrile can be formed at 20 K from the VUV irradiation of ethylamine with a yield of 4%. Furthermore, in presence of ammonia, at 20 K and under VUV irradiation, the acetonitrile can lead to the amino acetonitrile for-mation. These results suggest that acetonitrile and amino acetonitrile can be formed in astrophysical environments that are submitted to VUV irradiations.

Kinetics of the OCN− and HOCN formation from the HNCO + H2O thermal reaction in interstellar ice analogs

Aims. We study in the laboratory the kinetics of the low-temperature OCN − and HOCN formation from the purely thermal reaction of solid HNCO and H 2 O. The cyanate ion OCN − is an intermediate in the isomerization process of isocyanic acid HNCO into cyanic acid HOCN in water ice. Methods. We study the reaction, isomerization and desorption kinetics of the HNCO/OCN − /HOCN system using Fourier transform infrared spectroscopy. Results. Activation energies of 26 ± 2 kJ mol −1 (3127 K) and 36 ± 1 kJ mol −1 (4330 K) are found for the HNCO + H 2 O → OCN − + H 3 O + and OCN − + H 3 O + → HOCN + H 2 O reactions respectively. Desorption energies of 37 ± 3 kJ mol −1 (4450 K) and 40 ± 3 kJ mol −1 (4811 K) are measured for HNCO and OCN − , respectively. Conclusions. The present experiment has the important implication that the H 2 O + HNCO reaction alone cannot account for the observed abundances of solid OCN − in astronomical IR sources.

New insight into the formation of hexamethylenetetramine (HMT) in interstellar and cometary ice analogs

Aims. We investigate the purely thermal formation of hexamethylenetetramine (HMT, C6H12N4) in interstellar ice analogs from nonphotolysed ice and compare our results with those for the formation from photolysed ice. Methods. We use Fourier transform-infrared spectroscopy to follow residue formation from VUV irradiation of H2CO:NH3 ice mixture in different concentration ratios. We also report the warming of the H2CO:NH3:HCOOH ice mixture. Results. We present the characterization of organic residues obtained at 330 K from VUV irradiation of H2CO:NH3 ice mixtures. The organic residues contain compounds related to polyoxomethylene (POM, [-CH2-O-]n )a nd HMT (C 6H12N4). We report, for the first time, the formation of HMT from the warming of an interstellar ice analogs, H2CO:NH3:HCOOH, without any energetic processing (i.e. photons or particles). New insights into HMT formation mechanism are proposed. These results strengthen the hypothesis that HMT is present in interstellar grains or in comets, where it may be detected with the COSAC instrument of the Rosetta mission.

DYNAMIC CO-EVOLUTION OF PEPTIDES AND CHEMICAL ENERGETICS, A GATEWAY TO THE EMERGENCE OF HOMOCHIRALITY AND THE CATALYTIC ACTIVITY OF PEPTIDES

We propose a scenario for the dynamic co-evolution of peptides and energy on the primitive Earth. From a multi component system consisting of hydrogen cyanide, several carbonyl compounds, ammonia, alkyl amine, carbonic anhydride, borate and isocyanic acid, we show that the reversibility of this system leads to several intermediate nitriles, that irreversibly evolve to α-amino acids and N-carbamoyl amino acids via selective catalytic processes. On the primitive Earth these N-carbamoyl amino acids combined with energetic molecules (NOx) may have been the core of a molecular engine producing peptides permanently and assuring their recycling and evolution. We present this molecular engine, a production example, and its various selectivities. The perspectives for such a dynamic approach to the emergence of peptides are evoked in the conclusion.