Research Article

Ultraviolet Photostability of Adenineon Gold and Silicon Surfaces

Eva Mateo-Martı,1 Claire-Marie Pradier,2 and Jose-Angel Martın-Gago1,3

Abstract

The adenine molecule is a DNA nucleobase, an essential component of genetic material. Because of theimportant role of DNA nucleobases in terrestrial biochemistry, we have studied the molecular adsorption,orientation, and chemical binding of adenine on metallic and semiconducting surfaces, such as gold and silicon,respectively, and their stability toward ultraviolet radiation by X-ray photoelectron spectroscopy (XPS) andreflection absorption infrared spectroscopy (RAIRS) techniques. We have exposed the adenine surface system toUV radiation (200–400 nm) under a high-vacuum environment (10�7 mbar) to study the photostability andphotochemistry of adenine on different surfaces. After 10 or 24 hours of exposure under interplanetary spaceconditions, UV radiation induces desorption and partial dissociation of the molecule, which is dependant onthe nature of the surface. The electronic excitations, induced in the material by UV absorption, play a major rolein the photodestruction of the absorbed molecules on the solid surfaces. Key Words: Adenine—Ultravioletradiation—X-ray photoemission spectroscopy—Infrared spectroscopy—Gold and silicon surfaces. Astrobiology9, 573–579.

1. Introduction

Organic compounds observed in the interstellar mediumand in solar system bodies are of particular interest for

revealing the chemistry that may lead to the origin of life. Asan example, complex organic molecules such as polycyclicaromatic hydrocarbons (PAHs) are widespread throughoutspace. PAHs are of importance because of their ubiquity andhigh abundance, as inferred from infrared data (Bernstein etal., 1996; Hudgins and Allamandola 1999a, 1999b) and thepotential that they may link the molecular and dust phases ofthe interstellar medium (Clayton et al., 2003). PAHs containnitrogen, a key biochemical element found in many biologi-cally important processes. For instance, nucleobases, suchas purines and pyrimidines, are small N-containing aro-matic ring structures that play a major role in terrestrialbiochemistry.

It has been demonstrated that adenine can be formedunder certain conditions by hydrogen cyanide (HCN) pen-tamerization in gas, liquid, or condensed phases (Miller andUrey, 1959; Glaser et al., 2007). The presence of large amountsof HCN and HNC in interstellar space is well established(Ishii et al., 2006). The distribution of these isomers in pro-tostellar dust cores has been measured (Tennekes et al., 2006),

and the formation of small HCN oligomers in interstellarclouds has been discussed (Smith et al., 2001). Recently,studies using a theoretical model have predicted the exis-tence of adenine in interstellar dust clouds, in which highlyconcentrated HCN exists. Many models have been devel-oped since the 1940s to describe the chemistry in interstellarclouds, which range from pure ion-molecule gas-phasenetworks driven by cosmic-ray ionization (Herbst andKlemperer, 1973) to pure grain-surface chemistry models(Allen and Robinson, 1977; Tielens and Hagen, 1982). Mod-ern model networks contain up to 4000 different reactionsamong several hundred species (Lee et al., 1996; Millar et al.,1997).

Even if the molecule can be synthesized under spaceconditions, an important question remains open as to whe-ther this molecule is stable on surfaces upon irradiation.The photostability of adenine toward destructive photo-chemical reactions has therefore attracted much interest andhas been discussed as a possible evolutionary factor in theemergence of life on the Sun-irradiated surface of the prim-itive Earth.

To assess the availability of these organic compoundsfor prebiotic chemistry, it is imperative to investigate theirstability in environments similar to primitive Earth, for

1Centro de Astrobiologıa (CSIC-INTA), Torrejon de Ardoz, Madrid, Spain.2Laboratoire de Reactivite de Surface, Universite Pierre et Marie Curie, Paris, France.3Instituto de Ciencia de Materiales de Madrid (CSIC), Madrid, Spain.

ASTROBIOLOGYVolume 9, Number 6, 2009ª Mary Ann Liebert, Inc.DOI: 10.1089=ast.2008.0317

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instance, when they are adsorbed on solid surfaces. In thiscontext, we investigated the photostability of nucleobases inan effort to look for possible molecular damage upon UVirradiation.

N-heterocycle photostability depends on molecule envi-ronment. It has been deeply studied in the gas phase (dustclouds) and in a solid matrix. Previous studies (Peeters et al.,2005) have reported that, in an argon matrix (frozen sub-strate), the energy absorbed by the molecule can be easilydissipated, which thereby lowers the photodestruction effi-ciency compared to the same molecule in the gas phase.However, until now very little attention has been paid to therole of the surface in this process. The objective of this workis to mimic a system where, rather than occurring in a gasphase or within a matrix, molecules are in a more realisticprebiotic environment, that is, they are adsorbed on a sur-face. To know whether the irradiation of adsorbed moleculesis closer to the gas-phase or to the matrix inclusion models,we used two different surfaces. One of the two investigatedwas a semiconducting surface, the native oxide of Si wafers,which mimics sand and clay compounds. The other was ametallic surface, gold, to model interactions with metallicmineral surfaces.

A second motivation for the present work was to shedsome light on the possible mechanisms of nucleobase sur-vival on small interplanetary particles. We will show, here-after, that molecules adsorbed on mineral oxide surfaces,such as silicon oxide, will have more chances of enduringthan those adsorbed on metallic dust particles. This differ-ence is due to the several excitation and de-excitation pro-cesses induced by UV radiation on the metallic surfaces,which returns energy to the adsorbed nucleobase.

Finally, our initial motivation to attempt this work was thepotential yield of fundamental information about chemicalstructure, conformation, and bonding of biomolecules ad-sorbed on surfaces. The orientation as well as the nature ofthe anchoring chemical groups (functionalities) of the mole-cule on the surface can play a crucial role in the molecularstability due to the interaction with the surface. We firstchose a gold well-defined surface to study the interaction ofadenine with a solid surface, paying special attention to thestructure and orientation of the adsorbed molecule. In-vestigating the adsorption process of adenine on a goldplanar surface allows for the use of well-adapted and pow-erful surface science spectroscopies, which have been suc-cessfully applied when studying the adsorption of simplebiomolecules such as amino acids on reflecting surfaces(Mateo-Martı et al., 2002, 2004).

Thus, we report an experimental characterization of theinteraction and UV photostability of adenine adsorbed onsilicon oxide and gold surfaces by means of X-ray photo-electron spectroscopy (XPS) and reflection absorption infra-red spectroscopy (RAIRS). These two techniques can be usedto monitor chemical processes induced by surface irradia-tion. The UV irradiation experiments have been performed inthe planetary atmospheres simulation chamber under high-vacuum environment (P* 10�7 mbar) (Mateo-Martı et al.,2006), at exposure to photons in the wavelength range of200–400 nm, where the UV photobiology effects are relevant.For instance, it is well known that nucleic acid bases absorbUV radiation strongly, with a maximum of absorbance at260 nm contributing to the destruction of the nucleic acids.

2. Materials and Methods

Adenine (99%) was obtained from Aldrich and usedwithout further purification. The immobilization of adenineon gold and on silicon surfaces for UV radiation studies wasperformed for 17 hours, at concentration of 2 mM in Milli-Qgrade H2O. The adsorption was carried out on polycrystal-line Au layers evaporated on glass (Arrandee, Werther,Germany) and on silicon oxide wafers (SiO2). The gold sur-face was flame-annealed to produce a predominant (111)faceting of the surface. The Si surface was rinsed with ace-tone and water and dried with helium gas. The Au and Sisubstrates were immersed in adenine solution 2 mM for17 hours. After the immobilization step, the crystals wererinsed in H2O, dried by blowing argon, and placed insidethe simulation chamber, where they were exposed toUV radiation (200–400nm) under high-vacuum conditions(10�7 mbar) for 10 and 24 hours. Adenine=gold samples wereanalyzed by XPS, after transfer to a dedicated chamber, andin the air by polarization modulation RAIRS (PM-RAIRS),before and after UV exposure. Adenine=silicon samples wereanalyzed ex situ by XPS before and after UV exposure asreference surfaces to test the efficiency of the experimentalsetup.

Ultraviolet radiation experiments were performed inside aplanetary simulation chamber (see technique details Mateo-Martı et al., 2006). A 150 W water-cooled deuterium UV lamp(Hamamatsu C3150), placed perpendicular to the goldsample, was used to irradiate the sample. The lamp is apolychromatic source with a continuous spectrum in awavelength range from 200 to 400 nm. The UV radiationfrom the lamp enters the system through a quartz window.The UV light hits a beam splitter, placed very close to thelamp, which allows 88% of the radiation to pass through. Theother 12% of the beam is reflected onto another quartzwindow, where a UV detector is placed that permits thecontinuous monitoring of the incoming UV flux via a spec-troradiometer (Bentham DMc150FC). After the beam splitter,we set a focusing lens to focus the beam on the surface po-sition. The beam splitter, lens, and quartz windows can beisolated from the main chamber by a valve to avoid damageto the optical system when using corrosive gases or watervapor.

The irradiance spectrum of the deuterium lamp is a con-tinuum that decreases for increasing photon wavelength; theUV flux measured at the sample position, obtained by inte-gration of the irradiance over the 200–400 nm wavelengthrange, is 2.3�1014 photons=cm2 s in the current system.

The PM-RAIRS spectra were recorded on a commercialNICOLET Nexus spectrometer. The external beam was fo-cused on the sample, with a mirror, at an optimal incidentangle (see below). The incident beam was modulated be-tween p and s polarizations with a ZnSe grid polarizer and aZnSe photoelastic modulator (HINDS Instruments, PEM 90,modulation frequency¼ 37 kHz). The light reflected at thesample was then focused on a nitrogen-cooled mercury-cadmium-telluride detector. An important advantage of thePM-RAIRS technique, over the classical RAIRS mode ofanalysis, is that the signal is directly extracted from the DR=Rdata, which avoids the recording of a reference spectrum ona bare sample. Also, this technique provides an enhancedsensitivity to the vibration modes normal to the surface, at a

574 MATEO-MARTI ET AL.

short distance from the surface (<100 nm). All spectra re-ported below were recorded at 8 cm�1 resolution by co-adding 64 scans.

X-ray photoelectron spectroscopy analysis of the sampleswas carried out in an ultrahigh-vacuum chamber equippedwith a hemispherical electron analyzer and with the useof a Mg Ka X-ray source (1253.6 eV) with an aperture of4 mm�7 mm. The base pressure in the chamber was1�10�9 mbar, and the experiments were performed at roomtemperature. The core-level binding energies were calibratedagainst the binding energy of the Au(4f7=2) peak set to84.0 eV; with this calibration, the carbon peak attributed tohydrocarbon contamination was measured at 285.0 eV. Thepeak deconvolution in different components was shaped,after background subtraction, as a convolution of Lorenztianand Gaussian curves. Lorenztian and Gaussian widths of 0.1and 1 eV, respectively, common for all the components, wereused.

3. Results

Reflection absorption infrared spectroscopy and XPS datawere carefully analyzed after immersion of the surface in theadenine solution, rinsing and drying, and then after 10 or 24hours of UV irradiation. Special attention was paid to theinsight brought about by these techniques into the chemi-sorbed and orientation of the adsorbed molecule as well ason its stability. Figure 1 shows the chemical structure of theadenine molecule.

3.1. Adsorption and orientationof the adenine on surfaces

We recorded XPS spectra to prove the molecule was suc-cessfully adsorbed on gold and silicon surfaces. An unam-biguous signature of the presence of the molecule on thesurfaces is the detection of a N core-level peak, because ni-trogen is not adsorbed on these surfaces from air. Upperspectra of Fig. 2 show the N(1s) core-level peaks, on Au andon SiO2, after adenine adsorption. The total amount of ni-trogen on the SiO2 surface is larger than on gold (see N=Auand N=Si ratios in Table 1), which suggests a higher affinityof the semiconducting surface to the molecules. The N(1s)and C(1s) peaks, shown in Figs. 2 and 3, were decomposedby applying the criterion of using the lowest number ofcomponents.

The N(1s) peak, slightly asymmetric on either surface (seeFig. 2), was decomposed into two components—a main peak

centered at 399.1 eV and a shoulder at 400.6 eV—which areassigned to nitrogen in unsaturated chemical bonds (-N¼)and to (NHþNH2), respectively (Mateo-Martı et al., 2005;Furukawa et al., 2007; Magulick et al., 2008). Intensities of allN(1s) contributions are reported in Table 1. The ratio of thetwo XPS N(1s) contributions, N399.1=N400.6, is equal to 1.9 inthe case of gold and 1.5 in the case of SiO2, in close agree-ment with the (-N¼)=(NHþNH2) 1.5 ratio derived from thechemical formula of the molecule (see Fig. 1). In the case ofgold, deviation from the expected 1.5 ratio could originatefrom photoelectron diffraction effects that enhance the in-tensity of some of the components with respect to the other(Hufner, 2003), also from the interaction of some moleculeswith the Au surface, via the N atom of the NH2 group; thiswould induce some downshift of the binding energy, in away similar to the case of metal nitride formation (Haladaand Clayton, 1993). As will be seen below, some kind ofmolecular ordering can account for this process. In any case,these values confirm that the molecule mainly adsorbs as anentire entity.

After adsorption of adenine, the C(1s) core-level peak wasbest-fitted with the same two contributions on both surfaces(Fig. 3a), at 284.9 and 286.7� 0.5 eV, which have been as-signed to carbon in CC=CH groups and carbon in the C-Nbonds of the adenine rings, respectively (Magulick et al.,2008). The low energy contribution at 284.9 eV, not expectedfrom the chemical formula of the molecule, indicates asignificant hydrocarbon contamination of the surface, whichis not surprising given that samples were prepared in solu-tion and then rinsed and dried under ambient atmospherebefore introduction into the XPS analysis chamber. However,it is possible to identify and distinguish the carbon fromcontamination and the carbon from the adenine moleculeindependently, as they appear at different binding energies,which have been tabulated for different carbon functionalgroups: CC=CH groups assigned to a contamination andFIG. 1. Chemical structure of the adenine molecule.

FIG. 2. XPS core-level peak of N(1s) and its decompositionin curve components for adenine adsorbed on a gold (left)and on a SiO2 (right) surface before (a) and after UV expo-sure times of 10 hours (b) and 24 hours (c).

UV PHOTOSTABILITY OF ADENINE 575

carbon in the C-N bonds assigned to the adenine rings, re-spectively.

To obtain further information about the adsorption andorientation of the molecule on a gold surface, we consideredthe chemical functionalities of adenine for the infraredanalysis. Figure 1 shows the chemical structure of the ade-nine molecule.

The reflection absorption spectrum of adenine depositedon gold from a 2 mM aqueous solution is shown in Fig. 4a.The assignments of the most important infrared bands in thespectrum are given in Table 2. The spectrum exhibits a seriesof intense absorption bands that can be assigned to severalchemical groups of the adenine molecule.

In our study, the infrared spectrum shows a characteristicband at ca. 1672 cm�1 from a free adenine, which has beenassigned to the stretching vibration of the nitrogen atom ofthe ring and b(NH2)sciss. The extraordinary upwards shift (of20 cm�1) in this frequency [b(NH2)sciss] has been previouslyattributed to intermolecular hydrogen bonding interactionsin adenine adlayers mediated via the amino hydrogens(McNutt et al., 2003). Other relevant infrared bands for theadenine molecule are found at 1420 and 934 cm�1 and can beattributed to the in-plane cycle deformation, while those at1265 and 1458 cm�1 are clearly due to the C¼N and C-Nstretch modes, respectively; the latter is broad likely due to acontribution of the NH2 deformation mode. The signal at1111 cm�1 may be ascribed to the in-plane C-N stretch(Nowak et al., 1996; McNutt et al., 2003). To summarize theinfrared results, the presence of all the infrared featurescharacteristic of the chemical groups of adenine stronglysuggests that molecules adsorb on the gold surface as entireentities.

Next, we discuss the orientation adopted by the moleculeon the gold surface, based on the metal surface selection rule:only those vibrations producing a dynamic dipole perpen-dicular to the surface will be observed. Therefore, taking intoaccount the molecular vibrations in terms of in- and out-of-plane modes can provide significant information aboutorientation. Considering the rather strong intensity of the in-plane ring deformation bands (1420 and 934 cm�1), one maythink that the adenine molecule, an essentially planar mol-ecule, lies such that its rings are inclined from the surface.Moreover, the appearance of an intense NH2 scissoring bandat 1458 cm�1 rules out a flat orientation of this group. Strictlyspeaking, the amino hydrogen atoms lie just outside theplane of the rings, forming a dihedral angle between 108 and258 (Sponer and Hobza, 1994). The adenine molecule iscomplex in that it has a number of distinct vibrations that

reflect complex and coupled molecular motions, most ofwhich are in-plane modes. Thus, seeing rather intense ab-sorption signals is clear evidence that the rings of adenine arenot flat on the surface (Santamaria et al., 1999).

3.2. Chemical stability of adenine to the UV irradiation

With a better understanding of the adsorption of adenineon gold and silicon surfaces, we studied the chemical sta-bility of the molecule under irradiation conditions.

An XPS analysis of adenine adsorbed on gold or siliconsurfaces after 10 and 24 hours of UV irradiation was per-formed. The N(1s) and C(1s) peaks, together with their de-composition, are shown in Figs. 2 and 3 (curves b and c).

After UV irradiation of 2.3�1014 photons=cm2s on thesamples, the carbon peak intensity slightly increases, and itsdecomposition shows that it is due to the enhancement of thecomponent related to hydrocarbons. The carbon compo-nent, however, associated with the adenine molecule (C-N)

Table 1. XPS Binding Energies (eV), Assignment and Ratios

(-N¼)=(NHþNH2)

UV dose C(1s) N(1s) N(1s)=Au(4f ) N(1s)=Si(2p) Au Si

As deposited molecule 284.9 (CC=CH) 399.1 (-N¼) 0.019 0.067 1.9 1.5286.7 (C-N) 400.6 (NHþNH2) 0.010 0.045

10 h 284.9 (CC=CH) 399.1 (-N¼) 0.011 0.063 1.4 1.5286.7 (C-N) 400.6 (NHþNH2) 0.007 0.042

24 h 284.9 (CC=CH) 399.1 (-N¼) 0.006 0.058 1.1 1.5286.7 (C-N) 400.6 (NHþNH2) 0.004 0.039

Atomic ratios are calculated by dividing the peak areas by the Scoffield factors of the corresponding elements.

FIG. 3. XPS core-level peak of C(1s) and its decompositionin curve components for adenine adsorbed on a gold surfacebefore (a) and after UV exposure times of 10 hours (b) and 24hours (c).

576 MATEO-MARTI ET AL.

slightly decreases (see Fig. 3). This increase of the contami-nation may be reasonably attributed to outgassing inside theanalysis chamber due to the deuterium lamp.

The amount of nitrogen on the surface decreases 44% after10 hours and 67% after 24 hours of UV irradiation of theadenine=gold samples, and 14% and 35% for the same UVirradiation conditions on the adenine=silicon samples, re-gardless of whether carbon contamination increases or re-mains unchanged. The origin of the decrease of the nitrogenpeaks may be twofold: attenuation by the hydrocarbonsadsorbed on the surface and some molecular desorption.

The ratio of the two XPS N(1s) contributions in the case ofsilicon surface, N399.1=N400.6, is equal to 1.5, which is similarto what it was before the UV treatment and in agreementwith the (-N¼)=(NHþNH2) ratio derived from the chemicalformula of the molecule (see Fig. 1). The fact that this ratiodoes not change upon irradiation indicates that the UV ir-radiation induces desorption rather than fragmentation ofthe molecule. However, on the gold surface, the same ratio,N399.1=N400.6, decreases from 1.9 to 1.4 or 1.1 after 10 or 24hours of UV irradiation; the fact that this ratio changes uponirradiation indicates that the UV irradiation likely inducessome fragmentation and change in the adsorption mode, inaddition to desorption of the adenine molecule.

To reinforce the previous statement, we performed infra-red analysis on the adenine=gold samples after 10 and 24hours UV irradiation (Fig. 4, spectrum b and c). A compar-ison study of the adenine=gold samples before and after UVtreatment demonstrated that the RAIRS spectra show smallchanges in the intensity and shape of the bands, as opposedto the appearance or disappearance of new infrared features.The only significant changes are a net decrease of the band at1111–1122 cm�1 and a concomitant increase of that at 934–953 cm�1. Though both vibrations are ascribed to the in-plane cycle deformation, the former one is also due to acontribution of the C-N stretch, a vibration of the 5C cycle;this is likely the contribution that decreases upon irradiation.This may indicate that UV irradiation induces a slight changeof the orientation of the molecules and likely promotes littlecracking and desorption of a fraction of them from the sur-face, since the intensity of the infrared bands assigned to theC-N and N-H vibrations (1415–1458 and 1122 cm�1) de-creases as well. Infrared spectra after UV irradiation (b and c)exhibit a series of intense absorption bands that can be againassigned to several chemical groups of the adenine molecule.

4. Discussion and Conclusions

Previous study results on adenine in a solid matrix indi-cate that, in this environment, the molecule has several waysby which to release energy, which results in an enhancedphotostability with respect to the gas phase. A comparisonstudy between diminution of the N(1s)=Au(4f ) signal andthe N(1s)=Si(2p) signal before and after 24 hours of UV ir-radiation shows that the adenine molecule desorbs 67% fromgold and 35% from a silicon surface after 24 hours of UVirradiation. Therefore, adenine molecules present a lowerdesorption rate on silicon than on gold surfaces. Further-more, in the case of adenine on gold, we have shown thatthere is both desorption and dissociation of the adsorbedspecies. However, this is not the case for silicon surfaces,where the molecule desorption is smaller than that on a goldsurface and no fragmentation could be in evidence, at leastfrom the constant N399.1=N400.6 ratio. Therefore, it can beconcluded that a pure metallic surface, such as gold, plays amore significant role than that of a simple support for themolecule. Ultraviolet irradiation on gold surfaces not onlycauses a molecule to enter into an excited state but alsopromotes the emergence of surface-localized collective exci-tations. Plasmons and phonons are two of the possible

FIG. 4. PM-RAIRS of adenine on gold before (a) and afterUV exposure times of 10 hours (b), and 24 hours (c).

Table 2. Infrared Band Assignment from the Adsorption of Adenine on a Gold Surface

Exposed to Different UV Irradiation Times of 0, 10, and 24 Hours

Adenine=gold (as deposited) Adenine=gold UV (10 hours) Adenine=gold UV (24 hours) Assignment

1727 1730 17251672 1673 1677 n(N of the ring), b(NH2)sym

1627 1630 1630 n(CN) n(CC)1458 1458 1450 b(NH2)sym and n(CN) (Nutt)1420 1415 1411 n(CN) and n(CC) (Nutt)1315 1311 1315 n(CN) b(CH) (Nutt)1265 1261 1265 n(CN) b(CH) b(NH) (Nutt)1225 1225 1230 b(NH2) rock n(CN) (Nutt)1111 1122 1118 n(CN) and ring deformation (Nutt)934 949 953 in plane ring deformations (Nutt)

UV PHOTOSTABILITY OF ADENINE 577

quasiparticle excitations that these materials can produce.Gold has a greater efficiency for surface plasmon generationon UV absorption (Kittel 1996). This energy, absorbed by thesurface, can be released to the molecule and induce desorp-tion and some dissociation.

This is not the case, however, for silicon. On a siliconsurface, the electronic excitations are strongly reduced due tothe semiconducting nature of the materials; therefore, theenergy transfer to the molecule is significantly reduced. Thiscase resembles a matrix configuration, in which the vibra-tional energy gained by the molecules can dissipate.

Physicochemical surface spectroscopic techniques, such asXPS and RAIRS, were applied to characterize the adsorptionof adenine on a gold and on a silicon surface, as well as itsphotostability after exposure to UV irradiation. Our resultsconfirmed that adenine molecules were adsorbed on poly-crystalline gold and polycrystalline silicon surfaces.

Reflection absorption infrared spectroscopy data showthat characteristic features of the molecular infrared spectraare essentially retained, which indicates that the moleculeadsorbs intact on the gold surface. Adenine mainly adsorbs,on the gold surface, with its ring upright or strongly tiltedfrom the surface plane and with the NH2 group almostperpendicular to the surface. After UV irradiation, a smalldecrease in the intensity of the infrared bands assigned to theC-N and N-H vibrations takes place. XPS results show asmall diminution of the signal of the nitrogen peak and theC-N component from the carbon peak, which are related todesorption and partial dissociation of a small fraction ofadenine molecules on gold.

Therefore, XPS and RAIRS show that UV irradiationpromotes a small reorientation of the molecules, as well asdesorption and partial dissociation, depending on the natureof the surface. The nature of the surface plays an importantrole on the photostability of the molecules; photodecompo-sition on the solid phase is around 5–10 times lower than it isin the gas phase; this is particularly true for silicon, on whichthe energy absorbed by the molecule can be dissipated intothe matrix. The solid matrix can scatter radiation and dissi-pate energy and thereby lower the chance of photodestruc-tion that might otherwise result in molecular bond rupture(Andrews, 1973; Peeters et al., 2005). Furthermore, results oftheoretical studies suggest that adenine has a high level ofintrinsic photostability over a broad range of UV irradiation(Perun et al., 2005).

Those results reinforce the idea that, when adsorbed onisolating or semiconducting surfaces, nucleobases (precur-sors of terrestrial biomolecules) have an increased resis-tance against UV radiation (do not fragment), the underlyingmaterial playing an important role in dissipating energy andavoiding molecular fragmentation. Photostability studies ofnucleic bases are especially relevant to an understanding ofthe lifetime and abundance of these molecules in space; and,in that sense, we have proved that complementary surfacescience spectroscopies (XPS and RAIRS) provide useful in-formation about molecular photochemistry on surfaces.Surface analysis of adsorbed biomolecules, combined withthe experimental setup (UV irradiation–ultrahigh-vacuumconditions) of the planetary atmospheres simulation cham-ber, offer a novel approach to understanding chemical pro-cesses with regard to the origin of life and precursormolecules.

To summarize, the fragmentation of nucleobases (andlikely similar molecules as well) upon UV radiation is en-hanced on metallic surfaces due to the excitation of differentelectronic processes from the substrate. These electronicprocesses (plasmon, phonon, or electronic interband transi-tion excitations) provide an added source of energy to themolecule that could lead to desorption and fragmentationprocesses. These apparently basic results present severalimplications for astrobiology.

Two of the more relevant implications are related to in-terplanetary dust particles and prebiotic chemistry in that thecomposition and origin of interplanetary dust particles isdiverse (Grun et al., 2001), and it has been reported thatnebular dust analogues present impressive catalytic proper-ties for synthesizing prebiotic molecules (Hill and Nuth,2003). Our work supports the idea that condritic particles(mainly oxides) are more suitable than metallic particles incatalyzing reactions and carrying nucleobases throughoutspace. Thus, nucleobases and other similar chemical formsfragment easily when adsorbed on metallic grains.

Our results also present implications for studies in prebi-otic chemistry. Chemical reactions that allow for the assem-bly of superior molecular structures could be easily catalyzedon mineral oxide surfaces (as sand or clays) rather than onmetallic rocks.

Acknowledgments

Work carried out at Centro de Astrobiologıa was sup-ported by the European Union, Instituto Nacional de TecnicaAeroespacial, Ministerio de Ciencia e Innovacion and Co-munidad de Madrid.

Abbreviations

PAHs, polycyclic aromatic hydrocarbons; PM-RAIRS,polarization modulation RAIRS; RAIRS, reflection absorp-tion infrared spectroscopy; XPS, X-ray photoelectron spec-troscopy.

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Address correspondence to:Eva Mateo-Martı

Centro de Astrobiologıa (CSIC-INTA)Ctra. Ajalvir, Km. 4

28850-Torrejon de ArdozMadrid

Spain

E-mail: mateome@inta.es

UV PHOTOSTABILITY OF ADENINE 579