Proton-Induced Charge Transfer on Imidazole and 2-Aminoimidazole. Role of the Substituent and Influence of Stepwise Hydration.

The behavior of potential prebiotic species in space is of main concern in the chemistry at the origin of life. Their reactivity or stability in spatial conditions, under strong UV radiations or ion bombardments, remains an open question and needs wide investigations. As protons are by far the most abundant ions in space, we focus presently on proton-induced collisions on imidazole and 2-aminoimidazole evidenced as important prebiotic RNA intermediates. Unconstrained full optimization of the structures was performed with B3LYP/cc-pVTZ model chemistry. The calculations were performed in a wide collision energy range in order to model various astrophysical environments, from eV in the interstellar medium, up to keV for solar winds or supernovae shock-wave protons. Such a study provides for the first time a theoretical insight on the influence of the amino substituent on the proton-induced charge transfer. We evaluated the role of icy grain environments through a cluster approach modeling the effect of a stepwise microhydration on the process. Comparisons with oxygenated and sulfurated analogues address further qualitative trends on the respective stability or reactivity of such heterocycles which may be of tremendous interest in prebiotic chemistry. Charge transfer appears to be quite efficient for imidazole compounds and their sulfurated analogue compared to the oxygenated heterocycle.

Electron Transfer Induced Decomposition in Potassium-Nitroimidazoles Collisions: An Experimental and Theoretical Work.

Electron transfer induced decomposition mechanism of nitroimidazole and a selection of analogue molecules in collisions with neutral potassium (K) atoms from 10 to 1000 eV have been thoroughly investigated. In this laboratory collision regime, the formation of negative ions was time-of-flight mass analyzed and the fragmentation patterns and branching ratios have been obtained. The most abundant anions have been assigned to the parent molecule and the nitrogen oxide anion (NO2-) and the electron transfer mechanisms are comprehensively discussed. This work focuses on the analysis of all fragment anions produced and it is complementary of our recent work on selective hydrogen loss from the transient negative ions produced in these collisions. Ab initio theoretical calculations were performed for 4-nitroimidazole (4NI), 2-nitroimidazole (2NI), 1-methyl-4- (Me4NI) and 1-methyl-5-nitroimidazole (Me5NI), and imidazole (IMI) in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process.

Understanding H2 Formation on Hydroxylated Pyroxene Nanoclusters: Ab Initio Study of the Reaction Energetics and Kinetics.

The rate constants of H2 formation on five models of silicate nanoclusters with varying degrees of hydroxylation, (Mg4Si4O12)(H2O)N, were computed over a wide temperature range [180-2000 K]. We tested nine combinations of density functional methods and basis sets for their suitability for calculating reaction energies and barrier heights, and we computed the minimum energy H + H → H2 reaction paths on each nanocluster. Subsequently, we computed the rate constants employing three semiclassical approaches that take into account tunneling and nonclassical reflection effects by means of the zero curvature tunneling (ZCT), the small curvature tunneling (SCT), and the one-dimensional semiclassical transition state theory (SCTST) methods, which all provided comparable results. Our investigations show that the H2 formation process following the Langmuir-Hinshelwood (LH) mechanism is more efficient on the hydroxylated (N = 1-4) nanoclusters than on the bare (N = 0) one due to relatively higher reaction barrier height on the latter. H2 formation is found to have the smallest barrier and the most exothermic reaction for the moderately hydroxylated (Mg4Si4O12)(H2O)2 nanocluster for all nine considered methods. Overall, we conclude that all the considered nanoclusters are very efficient catalyzing grains for H2 formation in the physical conditions of the interstellar medium (ISM) with pyroxene nanosilicates having moderate to high hydroxylation being more efficient than bare nanograins.

Ion-Pair Formation in Neutral Potassium-Neutral Pyrimidine Collisions: Electron Transfer Experiments.

We report novel data on ion-pair formation in hyperthermal (30-800 eV) neutral potassium collisions with neutral pyrimidine (Pyr, C4H4N2) molecules. In this collision regime, negative ions formed by electron transfer from the alkali atom to the target molecule were time-of-flight mass analyzed and the fragmentation patterns and branching ratios have been obtained. The most abundant product anions have been assigned to CN- and C2H- and the electron transfer mechanisms are comprehensively discussed. Particular importance is also given to the efficient loss of integrity of the pyrimidine ring in the presence of an extra electron, which is in contrast to dissociative electron attachment experiments yielding the dehydrogenated parent anion. Theoretical calculations were performed for pyrimidine in the presence of a potassium atom and provided a strong basis for the assignment of the lowest unoccupied molecular orbitals accessed in the collision process. In order to further our knowledge about the collision dynamics, potassium cation (K+) energy loss spectrum has been obtained and within this context, we also discuss the role of the accessible electronic states. A vertical electron affinity of (-5.69 ± 0.20) eV was obtained and may be assigned to a π 3 * (b 1) state that leads to CN- formation.

Role of sulfur in proton-induced collisions of RNA prebiotic precursors.

A main objective in the synthesis of prebiotic compounds is to remain consistent with reasonable geochemical scenarios, while avoiding concomitant formation of undesirable by-products. In this context, 2-aminothiazole has shown enhanced selectivity in the addition reaction with sugars promoting interest in this sulfur species compared to its oxygenated analogue, 2-aminooxazole. More generally, the role of sulfur in prebiotic chemistry needs to be widely investigated with regard to the numerous sulfur-containing molecules detected recently in different astrophysical environments. However, in parallel to the problematic formation of building blocks of life, how prebiotic molecules could survive under extreme astrophysical conditions remains an open question. Intense UV radiation or ion bombardment may indeed lead to fragmentation and the specific behaviour of sulfur compounds has to be addressed. Focusing on its potentiality in prebiotic multistep synthesis, a detailed analysis of the proton impact on 2-aminothiazole has been investigated theoretically in a wide collision energy range chosen to model various astrophysical environments. The comparison with its oxygenated analogue may suggest qualitative trends on their respective stability under such processes which could be of crucial interest for prebiotic synthesis.

Proton-induced collision dynamics on potential prebiotic sulfur species.

The formation of the building blocks of life and how they could survive under harsh astrophysical environments remains an open question of prebiotic chemistry. Effectively, ion bombardments or intense UV radiation may drive the destruction of prebiotic compounds. The possible role of sulfur in such processes is addressed here for the first time, by comparing directly proton-induced charge transfer for sulfur containing molecules and that for related COMs. The proton collision on mercaptoacetonitrile HSCH2C[triple bond, length as m-dash]N conformers has thus been investigated theoretically in a wide impact energy range modelling various astrophysical environments and compared to related COMs, namely HCN oligomers, in order to exhibit qualitative tendencies.

Size-Induced Segregation in the Stepwise Microhydration of Hydantoin and Its Role in Proton-Induced Charge Transfer.

Recent photochemistry experiments provided evidence for the formation of hydantoin by irradiation of interstellar ice analogues. The significance of these results and the importance of hydantoin in prebiotic chemistry and polypeptide synthesis motivate the present theoretical investigation, in which we analyzed the effects of stepwise hydration on the electronic and thermodynamical properties of the structure of microhydrated hydantoin using a variety of computational approaches. We generally find microhydration to proceed around the hydantoin heterocycle until 5 water molecules are reached, at which stage hydration becomes segregated with a water cluster forming aside the heterocycle. The reactivity of microhydrated hydantoin caused by an impinging proton was evaluated through charge transfer collision cross sections for microhydrated compounds but also for hydantoin on icy grains modeled using a cluster approach mimicking the true hexagonal ice surface. The effects of hydration on charge transfer efficiency are mostly significant when few water molecules are present, and they progressively weaken and stabilize in larger clusters. On the ice substrate, charge transfer essentially contributes to a global increase in the cross sections.

Modelling charge transfer processes in C2+-tetrahydrofuran collision for ion-induced radiation damage in DNA building blocks.

Investigations of collision-induced processes involving carbon ions and molecules of biological interest, in particular DNA building blocks, are crucial to model the effect of radiation on cells in order to improve medical treatments for cancer therapy. Using carbon ions appears to be one of the most efficient ways to increase biological effectiveness to damage cancerous cells by irradiating deep-seated tumors. Therefore, interest in accurate calculations to understand fundamental processes occurring in ion-molecule collision systems has been growing recently. In this context, the charge transfer process in the collision of C2+(1s22s2) ions with the heterocyclic sugar moiety building block tetrahydrofuran (THF) was studied in order to interpret the mechanisms occurring at the molecular level. The molecular structure properties of THF were obtained by means of ab initio quantum chemistry methods. The role of the conformational structure and the orientation of the THF molecule in collision with C2+ ions are particularly discussed. Anisotropic effects of the process dynamics in the collision energy ranging from eV to keV by means of semiclassical treatment are also presented and compared to previous experimental and theoretical investigations. A detailed analysis of the obtained cross sections points out an increase in these values by three orders of magnitude by a change of the THF symmetry from C2v to Cs in collision with C2+, which determines a more efficient charge transfer in this case.

Theoretical investigation of proton collisions on prebiotic candidates: hydrogen cyanide polymers.

One of the major concerns in prebiotic chemistry is the formation and destruction routes of prebiotic compounds in space. As detected for a long time, hydrogen cyanide (HCN) has been suggested to be a feedstock molecule at the origin of life driving easy oligomerization, in particular to form adenine. This may focus on its first oligomers because its dimer cyanomethanimine was recently observed in star-forming regions, or its trimer aminomalononitrile. With regard to the assumption of an extra-terrestrial origin of life, the stability of such species under ultraviolet radiation or in ion-collisions is an open question. Thus, we investigated theoretically the proton collisions with dimer and trimer isomers of HCN in a wide impact energy range to model various astrophysical environments and exhibit qualitative tendencies.

Proton-Induced Collisions on Potential Prebiotic Species.

With regard to the fascinating question of the origin of life, special interest has been devoted to potential prebiotic molecules which could drive the emergence of life. In the widely discussed hypothesis of a possible exogen apparition of life, the transport of those prebiotic species and their survival under spatial conditions is of strong interest. In particular their stability under solar radiation or in collisions with bare nucleus has to be considered. In that sense, taking account of the abundance of protons in ionized clouds of the interstellar medium, we have developed a detailed theoretical study of the charge transfer collision dynamics induced by impact of protons on a series of possible prebiotic compounds. Three main types of molecules have been considered: first of all the DNA and RNA building blocks with on a one hand the nucleobases uracil and thymine, and on the other hand the 2-deoxy-D-ribose sugar skeleton in its furanose and pyranose forms. The study has been extended to the 2-aminooxazole suggested to be a possible precursor of RNA nucleotides. The theoretical treatment involves ab-initio quantum chemistry molecular calculations followed by a semiclassical collision dynamics. Some qualitative trends may be suggested for the proton-induced damage of such prebiotic species.

Nanohydration of uracil: emergence of three-dimensional structures and proton-induced charge transfer.

Stepwise hydration of uracil has been theoretically revisited using different methods ranging from classical force fields to quantum chemical approaches. Hydration initially begins within the uracil plane but proceeds at four molecules into three-dimensional configurations or even water clusters next to the nucleobase. The relative stability between the various structures is significantly affected by zero-point energy and finite temperature (entropy) effects and also gives rise to markedly different responses to an excitation by an impinging high-energy proton. In particular, charge transfer to the molecular complex is dramatically altered in collisions toward the coating cluster but barely modified for peripheral hydration patterns.

Proton-induced damage on 2-aminooxazole, a potential prebiotic compound.

Among the complex organic molecules detected in space, in the interstellar medium, on meteorites or comets, special interest is devoted to the potentially exobiologic-relevant species. In the hypothesis, widely discussed, of a possible exogen origin of life, the transport of such compounds and their survival is indeed a fundamental question. Recently, suggestion has been made that 2-aminooxazole could be a possible precursor of RNA nucleotides on early earth and its stability to UV radiation or to collisions may be determinant. We have thus undertaken a detailed theoretical study of the charge transfer collision dynamics induced by the impact of 2-aminooxazole with protons, which could be an important process in particular in proton-rich environments. The theoretical treatment has been developed through ab initio quantum chemistry molecular calculations followed by semiclassical collision dynamics. The results are compared to previous investigations on DNA and RNA building blocks in order to extract some qualitative trends in the damage of prebiotic species under spatial radiation.

Ab initio treatment of ion-induced charge transfer dynamics of isolated 2-deoxy-D-ribose.

Modeling-induced radiation damage in biological systems, in particular, in DNA building blocks, is of major concern in cancer therapy studies. Ion-induced charge-transfer dynamics may indeed be involved in proton and hadrontherapy treatments. We have thus performed a theoretical approach of the charge-transfer dynamics in collision of C(4+) ions and protons with isolated 2-deoxy-D-ribose in a wide collision energy range by means of ab initio quantum chemistry molecular methods. The comparison of both projectile ions has been performed with regard to previous theoretical and experimental results. The charge transfer appears markedly less efficient with the 2-deoxy-D-ribose target than that with pyrimidine nucleobases, which would induce an enhancement of the fragmentation process in agreement with experimental measurements. The mechanism has been analyzed with regard to inner orbital excitations, and qualitative tendencies have been pointed out for studies on DNA buiding block damage.

Looking at radiation damage on prebiotic building blocks.

A number of complex organic molecules have been detected in the interstellar medium, as well as in meteorites or comets. Among them, some exobiologic-relevant molecules have attracted particular interest. In the hypothesis of an exogen transport of prebiotic building blocks at the origin of life, the survival of such species and particularly their resistance to the solar UV radiation or cosmic rays is a key issue. For that purpose, we have performed a theoretical approach of the charge transfer dynamics induced by collision of protons with nucleobases and the 2-deoxy-d-ribose sugar moiety in a wide collision energy range. Calculations have been carried out by means of ab initio quantum chemistry molecular methods and compared to previous theoretical results using carbon projectile ions. Qualitative trends can be exhibited on DNA or RNA building blocks damage, which may concern studies on prebiotic species under spatial radiation.

An ab initio study of ion induced charge transfer dynamics in collision of carbon ions with thymine.

Charge transfer in collisions of carbon ions on a thymine target has been studied theoretically in a wide collision range by means of ab initio quantum chemistry molecular methods. The process appears markedly anisotropic in the whole energy domain, significantly favoured in the perpendicular orientation. A specific decrease of the charge transfer cross sections at low collision energies may be pointed out and could induce an enhancement of the complementary fragmentation processes for collision energies down to about 10 eV, as observed for the low-electron fragmentation process. Such feature may be of important interest in ion-induced biomolecular radiation damage.