Photostability of gas- and solid-phase biomolecules within dense molecular clouds due to soft X-rays

An experimental photochemistry study involving gas- and solid-phase amino acids (glycine, DL-valine, DL-proline) and nucleobases (adenine and uracil) under soft X-rays was performed. The aim was to test the molecular stabilities of essential biomolecules against ionizing photon fields inside dense molecular clouds and protostellar disks analogs. In these environments, the main energy sources are the cosmic rays and soft X-rays. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing 150 eV photons. In-situ sample analysis was performed by Time-of-flight mass spectrometer (TOF-MS) and Fourier transform infrared (FTIR) spectrometer, for gas- and solid- phase analysis, respectively. The half-life of solid phase amino acids, assumed to be present at grain mantles, is at least 3E5 years and 3E8 years inside dense molecular clouds and protoplanetary disks, respectively. We estimate that for gas-phase compounds these values increase one order of magnitude since the dissociation cross section of glycine is lower at gas-phase than at solid phase for the same photon energy. The half-life of solid phase nucleobases is about 2-3 orders of magnitude higher than found for amino acids. The results indicate that nucleobases are much more resistant to ionizing radiation than amino acids. We consider these implications for the survival and transfer of biomolecules in space environments.Comment: 10 pages, 5 figures, 2 tables. Accepted to be published in MNRA

Etudes théoriques à propos de l'origine exogène des molécules prébiotiques

The hunt for prebiotic species in the interstellar medium is a major concern for understanding the possible role of interstellar chemistry in the synthesis of the molecules at the origin of life. This study was motivated by the fact that some amino acids are currently identified in meteorites but none are yet observed in the interstellar medium. On the one hand, we use DFT and ab-initio methods of quantum chemical calculations to study the relative stability of the isomers detected in the interstellar medium. This study leads to a "Minimum Energy Principle" (MEP) specifying that the most thermodynamically stable isomer corresponds to the larger abundance observed. Applying this principle, we evaluate the possibility of detecting new prebiotic species, particularly amino acids, in the interstellar medium. Then, within the same approach, we verify that the link between abundance and thermodynamic order is valid for amino acids in the meteorites, which allows us to constrain the formation conditions in the parent bodies. On the other hand, we use the plane-wave periodic method in order to study the sticking of molecular species on carbonaceous and icy interstellar grains. We show that there is an efficient selective adsorption for the different isomers. This could lead to an observational bias for the relative abundances of these isomers, which could provide a possible explanation for the few exceptions to the MEP.La recherche de molécules prébiotiques dans le milieu interstellaire est importante pour comprendre le rôle potentiel de la chimie interstellaire dans la synthèse des molécules à l'origine de la vie. Aujourd'hui nous sommes confrontés à une apparente contradiction observationnelle : des acides aminés ont été identifiés dans les météorites mais aucun n'a été observé de façon certaine dans le milieu interstellaire. Dans un premier temps, nous utilisons les méthodes de calcul moléculaire DFT et ab-initio de la chimie quantique pour étudier la stabilité relative des isomères détectés dans le milieu interstellaire, ce qui nous conduit à établir un "Principe d'Energie Minimale" (PEM) stipulant que l'isomère le plus stable thermodynamiquement est le plus abondant. En s'appuyant sur ce principe, nous évaluons les possibilités de détection de nouvelles molécules prébiotiques dans le milieu interstellaire, en particulier les acides aminés. Puis, en suivant la même démarche, nous vérifions pour les météorites, que le lien entre abondance et ordre thermodynamique est respecté au niveau des acides aminés, ce qui nous permet de contraindre les conditions de formation dans les corps parents. Dans un second temps, nous utilisons les méthodes périodiques en ondes planes pour étudier les collages d'espèces moléculaires sur les grains carbonés et glacés interstellaires. Nous montrons l'existence d'une physisorption sélective pour certains types d'isomères. Elle entraînerait un biais observationnel sur les abondances relatives de ces isomères, ce qui nous donnerait une possibilité d'explication pour les quelques apparentes exceptions au PEM

Etudes théoriques à propos de l'origine exogène des molécules prébiotiques

L objectif de ce travail est de résoudre une apparente contradiction observationnelle : si des acides aminés ont été identifiés dans les météorites, aucun n a encore été observé dans le milieu interstellaire (MI). En étudiant la stabilité relative des isomères détectés dans le milieu interstellaire par les méthodes DFT et ab-initio de la chimie quantique, nous proposons un Principe d Energie Minimale (PEM) selon lequel l isomère de plus basse énergie est le plus abondant. Nous évaluons ainsi les possibilités de détection de nouvelles molécules prébiotiques dans le MI, dont les acides aminés. Puis nous vérifions que le lien entre abondance et ordre énergétique est respecté pour les acides aminés dans les météorites, ce qui contraint les conditions de formation dans les corps parents. En étudiant les collages de molécules sur les grains carbonés ou glacés interstellaires par les méthodes périodiques en ondes planes, nous montrons l existence d une physisorption sélective pour certains types d isomères. Ceci entraîne un éventuel biais observationnel sur leurs abondances relatives et une possibilité d explication des quelques exceptions au PEM.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Cycloaddition of Metal Porphines on Metal-Supported Graphene: A Computational Study

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About the detectability of glycine in the interstellar medium

Context. Glycine, the simplest of aminoacids, has been found in several carbonaceous meteorites. It remains unclear, however, wether glycine is formed in the interstellar medium (ISM) and therefore available everywhere in the Universe. For this reason, radioastronomers have searched for many years unsuccessfully to detect glycine in the ISM. Aims. We provide possible guidelines to optimize the return of these searches. Since, for most of the species observed so far in the ISM, the most abundant isomer of a given generic chemical formula is the most stable one (minimum energy principle (MEP)), we assess whether neutral glycine is the best molecule to search for or whether one of its isomers/conformers or ionic, protonated, or zwitterionic derivatives would have a higher probability of being detected. Methods. The question of the relative stability of these different species is addressed by means of quantum density functional theory (DFT) simulations within the hybrid B3LYP formalism. Each fully optimized structure is verified as a stationary point by means of a vibrational analysis. A comprehensive screening of 32 isomers/conformers of the C2H5O2N chemical formula (neutral, negative, and positive ions together with the corresponding protonated species and the possible zwitterionic structures) is carried out. In the sensitive case of the neutral compounds, more accurate relative energies were obtained by means of high level post Hartree-Fock coupled cluster calculations with large basis sets (CCSD(T)/cc-pVQZ). Results. We find that neutral glycine is not the most stable isomer and, therefore, probably not the most abundant one, which might explain why it has escaped detection so far. We find instead that N-methyl carbamic acid and methyl carbamate are the two most stable isomers and, therefore, probably the two most abundant ones. Among the non-neutral forms, we found that glycine is the most stable isomer only if protonated or zwitterionic if present in interstellar ices. Conclusions. Assuming that MEP can be applied to optimize our search for glycine, our conclusion is that this search will remain extremely difficult with the present instruments and we propose searching instead for other examples among the most stable isomers

About the presence of arsenic in prebiotic species

The recent publication that some bacteria could use arsenic instead of phosphorus for building their DNA triggered a large controversy in the astro/exobiology community. Most comments claim that such a substitution is not possible. Here, we address the same question of the presence of As in DNA from a pure theoretical point of view, beyond any biological consideration. By means of “First principle“ quantum calculations we found that there is no energetical or structural argument to reject the As to P substitution in the DNA helix. However, a topological analysis of the electron density shows that As-DNA is much more fragile and most probably will not survive because it lacks the covalent bonds that insure the stability of biological P-DNA