38 Acta Polytechnica CTU Proceedings 1(1): 38–41, 2014 38 doi: 10.14311/APP.2014.01.0038 Fullerenes, PAHs, Amino Acids and High Energy Astrophysics Susana Iglesias-Groth1,2 1Istituto de Astrofisica de Canarias, La Laguna, Via Láctea sn, 38201 La Laguna, Spain 2Departamento de Astrof́ısica de Universidad de La Laguna, La Laguna, Tenerife, Spain Corresponding author: sigroth@iac.es Abstract We present theoretical, observational and laboratory work on the spectral properties of fullerenes and hydrogenated fullerenes. Fullerenes in its various forms (individual, endohedral, hydrogenated, etc.) can contribute to the UV bump in the extinction curves measured in many lines of sight of the Galaxy. They can also produce a large number of absorption features in the optical and near infrared which could be associated with diffuse interstellar bands. We summarise recent laboratory work on the spectral characterisation of fullerenes and hydrogenated fullerenes (for a range of temperatures). The recent detection of mid-IR bands of fullerenes in various astrophysical environments (planetary nebulae, reflection nebulae) provide additional evidence for a link between fullerene families and diffuse interstellar bands. We describe recent observational work on near IR bands of C60 + in a protoplanetary nebula which support fullerene formation during the post-AGB phase. We also report on the survival of fullerenes to irradiation by high energy particles and gamma photons and laboratory work to explore the chemical reactions that take place when fullerenes are exposed to this radiations in the presence of water, ammonia and other molecules as a potential path to form amino acids. Keywords: ISM - molecules: fullerenes, PAHs - nebulae. 1 Introduction The discovery of fullerenes in 1985 when trying to repro- duce the chemistry of the atmospheres of red giant stars (Kroto et al., 1985) led to the identification of the third allotropic form of carbon. The most abundant fullerene molecule produced in those experiments was C60, a hollow molecule with 60 carbon atoms distributed in 12 pentagons and 20 hexagons (truncated icosahedron symmetry). Comparetively, we recall Polycyclic Aro- matic Hydrocarbons (PAHs) are planar molecules con- sisting of carbon rings and hydrogen, these rings are similar to benzene. The naphthalene and anthracene are the most simple PAHs with two and three benzene rings, respectively. These molecules have been postu- lated as potential carriers of the Unidentified Infrared Emission bands and of the diffuse interstellar bands which are ubiquitous in the interstellar medium (see e.g. Léger and Puget 1984). Similarly, fullerenes and their hydrogenated forms, are potential carriers of the interstellar bands. Several studies have shown that car- bon ring based molecular forms are very stable against UV radiation. This is remarkable because the basic structures of amino acids and in general of the molecu- lar basis of life are essentially conformed by such carbon rings making them rather stable against possible mu- tations. Laboratory work byChen et al. (2008) have aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa demonstrated that under strong UV radiation a mixture of naphthalene, ice and methane at low temperature, it is possible to form a large variety of amino acids. Fullerenes with carbon atom number of 20 (m2 + n2 + nm) where n and m are integers have icosahedral symmetry groups Ih and exhibit high stability. These molecules are very stable against UV/gamma radiation and collisions with other particles. Fullerenes have been detected in carbonaceou chondrites, a type of meteorite that originated in the early Solar System,with abun- dances of the order of 0.1 ppm (Pizzarello et al. 2000). On Earth, fullerenes have been detected in sedimentary layers of the Cretaceous-Tertiary boundary (KTB) in China and Bulgaria and in the mineral shunghite of the region of Karelia (Russia). In the various phases of the interstellar medium it is likely the presence of both, fullerenes and hydrogenated fullerenes, the so- called fulleranes (CnHm). A review of the properties of fullerenes and fulleranes can be found in Cataldo & Iglesias-Groth (2009, 2010). Here, recent developments on the astrophysical search for ionised fullerenes and on the properties of fullerenes when exposed to high energy radiation. 38 http://dx.doi.org/10.14311/APP.2014.01.0038 Fullerenes, PAHs, Amino Acids and High Energy Astrophysics 2 Astrophysical Searches for Fullerenes Theoretical work on the photoabsorption spectrum of icosahedral fullerenes suggests that these molecules can be responsible of the strongest feature in the interstel- lar extinction curve, the UV bump at 2175 Å. For large fullerenes and buckyonions the experimental photoab- sorption spectrum is very poorly known and the po- tential role of these molecules in interstellar absorption has been explored on a theoretical basis (see, for exam- ple, Iglesias Groth et al., 2002, 2003). In these works a theoretical approach to the photoabsorption specturm was based on a Hückel and Pariser-Parr-Pople (PPP) model. Fits of the computed spectra to measurements of the interstellar extinction in the UV/Optical for dif- ferent lines of sight in our Galaxy (Fitzpatrick, 1999) were obtained (see details of the model can be found in Iglesias-Groth, 2004). Very good fits to the extinction curves are provided by the photoabsorption cross sec- tions computed for both individual fullerenes and buck- yonions. The comparison of these models with the ob- served UV bump leads to an estimate of the number density of these molecules (in the range 0.2-0.08 ppm) and of the percentage of carbon locked in fullerenes and buckyonions in the ISM (Iglesias-Groth 2004, 2007) which depending the size distribution of these molecules could reach up to 25 % of all carbon in the ISM. Fullerenes C60 and C70 present active vibrational bands in the near infrared (3-20 µm) which have been key for their identification in planetary nebulae, reflec- tion nebulae and post-AGB stars (see e.g. Cami et al. 2010, Zhang and Kwok, 2011). Experimental measure- ment of the molar absorptivity for these infrared transi- tions is essential for the determination of fullerene abun- dances in such astrophysical contexts. Laboratory work is being conducted to measure the dependence of the molar absorptivity of fullerene infrared bands with tem- perature from approx. 100 K to 500 K to cover a variety of conditions in the interstellar medium (Iglesias-Groth et al. 2011). 2.1 New results on ionised fullerenes: C+60 Recently, it was reported the detection of mid-IR vi- brational transitions of the fullerene C60 in a carbon rich protoplanetry nebula, IRAS 01005+7910 (Zhang and Kwok 2010). We recorded the spectrum of this ob- ject between 5500 and 10000 Å with resolving power of 57,000 and detected two bands at 9577 and 9632 Å which are consistent with laboratory measurements of the C60 cation (See figure 1. Iglesias-Groth and Es- posito, 2013). If these two bands were produced by C+60 in the material surrounding the central post-AGB star we could infer that ∼ 1 % of carbon is trapped in this ionized form of fullerenes and that the cation abundance is much higher than that of the neutral fullerene species. It appears that ionized fullerene species in this proto- planetary nebula are significantly more abundant than in neutral form, which is consistent with the predictions of ionization models of PAHs (Bakes and Tielens 1995, Salama et al. 1996) for irradiated clouds near a hot star. Figure 1: The two near IR bands in the spectrum of IRAS01005+7910, attributed to the cation of C60 (Iglesias-Groth & Esposito 2013) These observations provide additional evidence for the presence of fullerenes in protoplanetary nebulae and suggest that a significant production takes place in this late stage of stellar evolution. Mid-IR bands of C+60 could be present in the 7-20 µm spectrum of IRAS 01005+7910 and are also likely to be detected in the spectra of planetary nebulae. Accounting for the ion- ized species of fullerenes appears necessary in nebulae with a nearby source of ionizing photons. A caveat: the derived abundances and excitation temperatures of planetary nebulae with claimed detections of fullerenes may have to be revised if indeed there is a significant contribution of cations to the formation of the bands used in these analyses. High resolution spectroscopy will be required for a reliable determination of the rela- tive abundance of neutral and ionized fullerenes in these objects. 2.2 The stability of C60 and C70 towards copuscular and γ radiation The stability of C60 and C70 fullerenes in the interstel- lar medium and embedded in meteorites and comets has been investigated with γ irradiation and with He+ ion bombardment. The radiation dose generated by ra- dionuclides decay expected to be delivered to fullerenes buried at a depth of ≥ 20 m in comets and meteorites is about 3 MGy per million years. We have exposed fullerenes to various radiation doses and found that these molecules are by far resistant to this type of ra- diation. Laboratory measurements on the concentra- 39 Susana Iglesias-Groth tion of samples of fullerenes exposed to strong radiation doses indicate that these molecules can survive millions of years inside comets and meteorites (Cataldo et al. 2009). This provides a natural explanation to their presence inside certain carbonaceous chondrites. In this laboratory experiments, fullerenes were also exposed to intense particle bombardment. Fullerens adsorbed or deposited on the surface of carbon are exposed to cos- mic ray bombardment with estimated doses of radiation comprised between 30 and 65 MGy per 106 years. We carried out experiments to test the survival of both C60 and C70 fullerenes and found that the complete amor- phization occurs at about a radiation dose of 250 MGy. Thus we infer that after 4-8 millions of years exposure to cosmic rays it is expected to producce a complete amorphization (Cataldo et al. 2009). 2.3 Gamma Radiolysis of C60 fullerene in water and water/ammonia mixtures; relevance of fullerenes fate in ices of interstellar medium In order to explore the potential astrochemical role of fullerenes,Iglesias-Groth et al. (2013) have explored the radiolysis of fullerene C60 dispersed in H2O, H2O/NH3, H2O/methanol and H2O/NH3/methanol under gamma radiation doses of 250 and 500 kGy. It was found that C60 originally insoluble in the above mentioned host- ing matrix became soluble as a consequence of multiple hydroxylation and oxidation reaction produced by the free radicals generated by the radiolysis of the hosting matrix. The changes undergone by C60 were studied by infrared spectroscopy (FT-IR) and by electronic ab- sorption spectroscopy. The astrochemical consequences of the present study are that C60 ejected in the interstel- lar medium for instance from protoplanetary and plan- etary nebulae can condense together with water and other ices in dense molecular clouds. Under the action of high energy radiation C60 reacts with the free radi- cals generated from the matrix where it is embedded it is solubilized and consequently its carbon content be- comes available for further abiotic processes of synthesis of molecules of astrobiological interest. The behavior of C60 appears comparable to that of common PAHs which are also hydroxylated and oxidized under similar conditions. 3 Stability Toward High Energy Radiation of Non-Proteinogenic Amino Acids: Implications for the Origins of Life In order to investigate the response of amino acids to high energy radiation, Cataldo et al. (2013) have taken a series of non-proteinogenic amino acids, most of them found frequently in carbonaceous chondrites, and ex- posed them to solid state radiolysis in vacuum to a total radiation dose of 3.2 MGy. This corresponds to 23% of the total dose expected to be taken by organic molecules buried in asteroids and meteorites since the beginning of the Solar System in 4.6 x 109 years. The radiolyzed amino acids where studied by FT-IR spec- troscopy, Differential Scanning Calorimetry (DSC) and by polarimety and Optical Rotatory Dispersion (ORD). It is found that an important fraction of each type of amino acid is able to ”survive” to the massive dose of radiation and also the enantiomeric excess is partially preserved. Based on these results it is concluded that it is not a surprise to find amino acids even in enan- tiomeric excess in carbonaceous chondrites. 4 Conclusions The astrochemical consequences of the recent discov- eries of fullerenes in various astrophysical contexts are not fully understood yet. Fullerenes are far resistant to high doses of gamma radiation and bombardment by high energy particles. It is likely that fullerenes ejected in the interstellar medium,for instance from protoplane- tary and planetary nebulae can condense together with water and other ices in dense molecular clouds. Un- der the action of high energy radiation fullerenes re- act with the free radicals generated from the matrix where they are embedded. If C60 is trapped in wa- ter ices it is hydroxylated and oxidized by the radiol- ysis products of water. The oxidation of C60 makes this molecule hydrophilic and hence soluble in water. The same phenomenon occurs in water/ammonia, in water/methanol and in water/ammonia/methanol mix- tures. Thus, C60 which in the solid state displays a con- siderable radiation resistance, when embedded in radi- olytic sensitive matrices like those just mentioned, it reacts swiftly, it is solubilized and consequently its car- bon content becomes available for abiotic processes of synthesis of other molecules of astrobiological interest. This compares well with the behavior of PAHs which are also hydroxylated and oxidized by the free radicals produced by the radiolysis of the hosting matrix (Gudi- pati & Allamandola 2006, Ricca et al. 2007, Ashbourn et al. 2007, Cuylle et al. 2012, Nuevo et al. 2012). It is known that PAHs and ices exposed to strong UV radiation can lead to the formation of amino acids. 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M. 1996, ApJ, 458, 621 [23] Zhang, Y., Kwok, S. 2011,ApJ , 730, 126 doi:10.1088/0004-637X/730/2/126 41 http://dx.doi.org/10.1111/j.1365-2966.2009.15457.x http://dx.doi.org/10.1111/j.1365-2966.2012.21006.x http://dx.doi.org/10.1086/422216 http://dx.doi.org/10.1086/518832 http://dx.doi.org/10.1111/j.1365-2966.2011.18124.x http://dx.doi.org/10.1088/2041-8205/776/1/L2 http://dx.doi.org/10.1016/S0016-7037(99)00280-X http://dx.doi.org/10.1086/512037 http://dx.doi.org/10.1088/0004-637X/730/2/126 Introduction Astrophysical Searches for Fullerenes New results on ionised fullerenes: C60+ The stability of C60 and C70 towards copuscular and radiation Gamma Radiolysis of C60 fullerene in water and water/ammonia mixtures; relevance of fullerenes fate in ices of interstellar medium Stability Toward High Energy Radiation of Non-Proteinogenic Amino Acids: Implications for the Origins of Life Conclusions