uv photolysis of quinoline in interstellar ice analogs

Meteoritics amp Planetary Science 41 Nr 5 785ndash796 (2006)Abstract available online at httpmeteoriticsorg

785 copy The Meteoritical Society 2006 Printed in USA

UV photolysis of quinoline in interstellar ice analogs

Jamie E ELSILA1 Matthew R HAMMOND2 Max P BERNSTEIN1Scott A SANDFORD1 and Richard N ZARE2

1NASA Ames Research Center MS 245-6 Moffett Field California 94035ndash1000 USA2Department of Chemistry Stanford University Stanford California 94305ndash5080 USA

Corresponding author E-mail jelsilamailarcnasagov

(Received 16 November 2005 revision accepted 03 February 2006)

AbstractndashThe polycyclic aromatic nitrogen heterocycle (PANH) quinoline (C9H7N) was frozen at20 K in interstellar ice analogs containing either pure water or water mixed with methanol or methaneand exposed to ultraviolet (UV) radiation Upon warming the photolysis products were analyzed byhigh-performance liquid chromatography and nanoscale liquid chromatography-electrosprayionization mass spectrometry

A suite of hydroxyquinolines which were formed by the addition of oxygen atoms to quinolinewas observed as the primary product in all the ices Quinoline N oxide was not formed but fivehydroxyquinoline isomers were produced with no clear dominance of one isomer Reductionproducts formed by hydrogen atom addition were also created Ices created at 20 K with H2Oquinoline ratios of 101 to 1001 showed similar product distributions to those at 122 K with noapparent temperature or concentration dependence Increasing the UV dose led to a decrease inoverall yield indicating that quinoline and its products may be photo-destroyed

Methylquinolines were formed upon photolysis of the methanol- and methane-containing ices Inaddition possible methoxyquinolines or quinoline methylene alcohols were formed in the methanol-containing ice while methylhydroxyquinolines were created in the methane-containing ice

This work indicates that oxidation of PANHs could occur in icy extraterrestrial environments andsuggests that a search for such compounds in carbonaceous meteorites could illuminate the possiblelink between interstellar ice chemistry and meteoritic organics Given the importance of oxidized andalkylated PANHs to biochemistry the formation and delivery of such molecules to the early Earthmay have played a role in the origin and evolution of life

INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs) are believed tobe among the most widespread and abundant organiccompounds in the universe (Allamandola et al 1999 Cox andKessler 1999) These compounds are a ubiquitous componentof the organic carbon in meteorites and interplanetary dustparticles (IDPs) (see Ehrenfreund and Charnley [2000] for areview) In addition infrared features generally attributed toPAHs (the so-called ldquounidentified infrared bandsrdquo) have beenobserved in the gas phase in a wide range of extraterrestrialenvironments including protoplanetary and planetarynebulae reflection nebulae HII regions the diffuseinterstellar medium and in lines of sight through densemolecular clouds (Sellgren et al 1995 Boulanger et al 1998Allamandola et al 1999 Brooke et al 1999 Bregman et al2000 Chiar et al 2000 Bregman and Temi 2001)

One reason for the abundance and ubiquity of PAHs inextraterrestrial environments is the stability of their aromaticstructures Aromaticity is not limited to species containingonly carbon and hydrogen Nitrogen the fourth mostabundant reactive interstellar element can also beincorporated into aromatic compounds Interstellar PAHs arethought to form predominantly in the circumstellar shells andoutflows of carbon-rich late-type giants by repeatedhydrogen abstraction and addition of acetylene (C2H2)molecules to form rings (Allamandola et al 1989 Wang andFrenklach 1994) Computational studies have shown thatnitrogen can be incorporated into a PAH molecule by thereplacement of a C2H2 feedstock molecule with an HCNmolecule (Ricca et al 2001a Ricca et al 2001b) Thissubstitution may lead to the formation of a polycyclicaromatic nitrogen heterocycle (PANH) Although PANHs aresubject to photodestruction evidence suggests that they may

786 J E Elsila et al

be protected in dense cloud environments (Peeters et al2005) Recent studies suggest that larger PANHs with one ormore nitrogen atoms in the interior of the molecular skeletoncould account for the 62 microm interstellar emission band andthat such molecules may contain at least 1ndash2 of all cosmicnitrogen (Hudgins et al 2005) In addition PANHs such aspurines pyrimidines quinolines isoquinolines and pyridineshave been identified in carbonaceous chondrite meteorites atlevels greater than 1 ppm providing more evidence for theexistence of extraterrestrial PANHs (see Sephton 2002 for areview) It is thus a reasonable assumption that PANHscomprise some portion of interstellar organic compounds

In dense molecular clouds where temperatures are low(10ndash20 K) PAHs PANHs and other chemical species areexpected to form mixed molecular ice mantles that arecondensed on silicate dust grains (Sellgren et al 1995Sandford 1996 Chiar et al 2000) These icy mantles whichare usually dominated by water ice may then be subjected tochemical processing driven by ultraviolet radiation (eitherfrom the galactic background embedded protostars orcosmic-ray excitation of hydrogen) (Prasad and Tarafdar1983 Bernstein et al 1995 Cecchi-Pestellini et al 2005) andor by the impact of low-energy (sim1 MeV) cosmic rays(Strazzulla 1997) The chemical processing of certain PAHsin laboratory analogs of these ices has been studied and a richchemistry has been observed in which PAHs acquire a varietyof side groups including hydroxy carbonyl methyl andamino moieties (Bernstein et al 2002 Bernstein et al 2003)

Although the spectroscopic properties of PANHs havebeen studied (Mattioda et al 2003 Bernstein et al 2005Hudgins et al 2005) there has been no investigation of thecompounds that are produced when PANHs undergo chemicalprocessing in icy interstellar environments The possibilitythat various side groups may be added to PANHs throughinterstellar ice processing is of interest for several reasonsFirst as mentioned above PANHs have been detected incarbonaceous chondrites The detected compounds includePANHs with side groups such as alkylated quinolinesisoquinolines and pyridines (Hayatsu et al 1975 Stoks andSchwartz 1982) and oxidized PANHs such as carboxylatedpyridines and hydroxy- and keto-containing purines(xanthine hypoxanthine guanine adenine) (Van der Veldenand Schwartz 1977 Stoks and Schwartz 1981a 1981b)Quinoline-quinones another form of oxidized PANH havealso been tentatively identified in preliminary analysis ofinterstellar dust particles captured by the Stardust spacecraft(Krueger et al 2003) Previous studies have investigated theconnection between interstellar ice chemistry and meteoriticorganics pointing out that deuterium enrichments that areobserved in meteorites suggest an interstellar origin for somemeteoritic organics and that these deuterated excesses may becreated in interstellar ice processing (Kerridge et al 1987Messenger et al 1995 Sandford et al 2000 Messenger 2002Butterworth et al 2004 Huang et al 2005 Pizzarello andHuang 2005)

Another point of interest in the possible interstellarprocessing of PANHs is that substituted PANHs play animportant role in biochemistry on Earth The nucleobasesfound in DNA and RNA consist of substituted purines andpyrimidines while other substituted PANHs such as flavinsare involved in electron transfer processes The possibleinterstellar formation of such compounds therefore hasrelevance to astrobiology

In the work presented here we report on thephotochemical processing of the PANH quinoline in H2O-dominated interstellar ice analogs Quinoline was chosen as anitrogen-containing analog of the PAH naphthalene whichwas previously studied in similar investigations in thislaboratory (Bernstein et al 2001) A number of variablesincluding deposition temperature UV flux and icecomposition were explored in these experiments in order tosimulate a variety of extraterrestrial environments

EXPERIMENTAL TECHNIQUE

The quinoline-containing interstellar ice analogs wereprepared in a manner that has been described in detailelsewhere (Allamandola et al 1988 Bernstein et al 1995) Inbrief a gas mixture is deposited onto a rotatable cold fingercontained in an evacuated cryogenic chamber surrounded byan infrared spectrometer The gas mixtures were preparedwith quinoline (98 Sigma-Aldrich distilled prior to use)water (purified via a Millipore Milli-Q system to 182 M)methanol (HPLC grade Fisher) and methane (ultra-highpurity Matheson) The quinoline water and methanol weretriply freeze-pump-thawed to remove dissolved gases PureH2O deposited under these conditions is in its high-densityamorphous form when deposited at 15 K and goes throughseveral phase transitions during warming (Jenniskens andBlake 1994 Jenniskens et al 1995) Clearly the morphologyof ice mixtures can be more complex but previous IR spectraof H2Oquinoline mixtures are consistent with amorphousH2O (Bernstein et al 2005) This form is believed to berepresentative of H2O-rich ices in interstellar molecularclouds

For the work described here gas mixtures typicallycontained lt1 mbar of H2O and quinoline at a ratio of 501 andwere deposited onto a 20 K aluminum foil substrate for sim6 hrat an ice growth deposition rate of sim07 micromhr Residualpressure in the vacuum chamber was sim5 times 10minus8 bar Severalvariations on this typical preparation were also used to assessthe effects of different parameters on ice photochemistryThese variations included the following

1 Using a CsI substrate instead of aluminum foil in order totake infrared spectra

2 Slowing the rate of ice growth deposition to sim007 micromhrto increase UV dose while depositing for 48 hr

3 Changing the H2Oquinoline ratio to 101 and 10014 Including CH4 or CH3OH vapor at levels sim10 of the

H2O

UV photoanalysis of quinoline in interstellar ice analogs 787

5 Changing the deposition temperature to 122 K tosimulate outer solar system rather than interstellarconditions

6 Simultaneously depositing water and quinoline vaporsfrom separate borosilicate tubes for 48 hr in order toincrease the yield of material for analysisUltraviolet photolysis occurred simultaneously with

vapor deposition photolysis was provided by a microwave-powered flowing-hydrogen discharge lamp (Warneck 1962)that produces an output nearly evenly divided between theLyman α line and a 20 nm wide molecular transition centeredat 160 nm The lamp was operated at sim100 mbar of pressurefor the H2 gas and with microwave settings of 70 forwardpower and 6 reflected power The flux of such lamps hasbeen shown to vary with time and operating conditions but inthis work we have assumed a nominal flux of sim2 times 1015

photons cmminus2 sminus1 (Warneck 1962 Cottin et al 2003 Loeffleret al 2005) Based on the absorption cross-section of waterice it has been estimated that 90 of the flux from this typeof UV irradiation is absorbed within the first 01 microm of a purewater ice (Cottin et al 2003) Given this optical depth themolecules in the ices deposited at 07 micromhour would haveexperienced sim85 min of radiation (sim1 times 1018 photons cmminus2)The molecules in the ices deposited at 007 micromhr would haveexperienced more photolysis (sim85 min) prior to shielding byice layers and were used to assess the effects of UV flux onproduct distribution and yield

After deposition and photolysis the ices were warmed to220 K at 2 Kmin under dynamic vacuum The foil substratecontaining the sample was then removed from the system andquickly placed in a glass vial containing 500 microL of HPLC-grade methanol (Fisher) and sonicated to dissolve the iceresidue This methanol extract was then analyzed by themethods described below For the ices on CsI substratesinfrared spectra were measured both before and after warm-up High-performance liquid chromatography (HPLC) wasused to separate and analyze aliquots of the methanol extractsHPLC was performed on a 5 microL sample injection with aHewlett-Packard 1100 series HPLC equipped with both adiode array UVVIS detector and a fluorescence detector Theseparation utilized a Vydac 218TP54 column (46 times 250 mm5 microm C18) and a mobile phase gradient beginning with30 A (methanol) and 70 B (pH 55 50 mM sodium acetate+ 8 methanol) changing to 567 A and 433 B over40 min at 05 mLmin The absorbance at 230 nm wasmonitored to measure elution of peaks The HPLC grademethanol sodium hydroxide and acetic acid for the HPLCbuffer were all obtained from Fisher

Peaks were identified by comparison of retention timeand UV spectrum with those of a given standard Standardswere purchased from Sigma-Aldrich and included thefollowing compounds 2-hydroxyquinoline 4-hydroxyquinoline 5-hydroxyquinoline 6-hydroxyquinoline7-hydroxyquinoline 8-hydroxyquinoline quinoline N oxide24-dihydroxyquinoline 26-dihydroxyquinoline and 28-

dihydroxyquinoline Abundances were estimated bycomparing peak areas to a linear fit through zero of thestandard at five different concentrations Unfortunately 2-hydroxyquinoline and 5-hydroxyquinoline co-eluted underthe conditions used so it was impossible to determineabundances for these compounds The possible presence oftwo other hydroxyquinolines could not be assessed in thisstudy 8-hydroxyquinoline was not detectable by our methods(Nagiel-Ostaszewski et al 1991) and 3-hydroxyquinolinecould not be purchased for comparison

The methanol extracts of the ice residues were alsoseparated by nanoscale liquid chromatography (nanoLC)performed using a photopolymerized sol-gel (PSG)monolithic stationary phase and analyzed by electrosprayionization-mass spectrometry (ESI-MS) The ability to injectlong sample plugs on the PSG columns without losingchromatographic resolution (preconcentration) enhances thedetection sensitivity for this scheme (Quirino et al 2001)Preparation of 30 cm long pentafluorophenyl-derivatizedPSG monolith capillary columns is described elsewhere(Dulay et al 2001) with the modification that 75 microm idfused-silica capillary columns with UV-transparent coating(purchased from Polymicro Technologies Phoenix ArizonaUSA) were used Methanol extracts were pressure-injected at9 psi onto the column Injection time was optimized for eachsample and ranged from 60 s up to 12 hr NanoLC wasperformed on an Eksigent NanoLC-1D system at a flow rateof 300 microLmin The mobile phase was a binary mixtureconsisting of 40 acetonitrile in a 5 solution of acetic acidin water ESI-MS was performed on a Finnigan SSQ 700quadrupole mass spectrometer (Finnigan MAT San Jose CA)equipped with an electrospray ion source In this techniqueanalyte ions are ionized directly from the liquid phaseresulting in minimal fragmentation Sheathless electrospraywas performed by holding the electrospray tip voltage at 3 kVby an independent power supply The quadrupole wasscanned from mz 10 to 300 at 20 scans

RESULTS

H2OQuinoline Ices

Our analyses indicate that the UV photolysis of H2Oquinoline ices produces a mixture of unreacted quinoline anda variety of hydroxyquinoline compounds formed by thesubstitution of hydroxy (-OH) groups for hydrogen atoms onthe quinoline structure (Reaction 1)

The product composition was largely unaffected bychanges in concentration deposition temperature or UV fluxFigure 1 presents an HPLC trace from the separation of thematerial resulting from photolysis of a H2Oquinoline = 501mixture at 20 K under the typical UV flux conditionsdescribed above as well as traces from two controlexperiments The irradiated H2Oquinoline ice clearly showsa large peak caused by unreacted quinoline and smaller peaks


identified as 4-hydroxyquinoline 6-hydroxyquinoline theco-elution of 5-hydroxyquinoline and 2-hydroxyquinolineand 7-hydroxyquinoline No peaks matching the quinoline Noxide or the dihydroxyquinoline standards were observed(Reaction 2)

We were unable to identify the peak at 168 min since wecould not obtain standard samples of all the possible oxidizedand reduced quinoline derivatives It is reasonable tospeculate that this peak could result from 3-hydroxyquinoline a quinoline ketone or quinone or adihydroxyquinoline not searched for in this work Controlexperiments carried out in the absence of either quinoline orUV radiation showed no evidence of hydroxyquinolines(Fig 1) These experiments indicate that thehydroxyquinolines are not the result of contamination or non-photolytic reaction processes

NanoLC-ESI-MS analysis (Fig 2) of the methanol extractfrom the H2Oquinoline = 501 ice supported the HPLC dataThis analysis clearly showed the presence of unreacted

quinoline as well as multiple peaks with mass 145 amu whichcorrespond to the addition of an oxygen atom to the quinoline(most likely through the substitution of an alcohol group for ahydrogen atom forming the hydroxyquinoline isomers seen inthe HPLC data) In addition the nanoLC-ESI-MS analyses ofan ice created by a 48 hr long deposition (not shown) showedpeaks at 159 amu indicating the formation of a quinolinediketone possibly quinoline-quinone (Reaction 3) Thiswould be analogous to the photochemistry observed ofnaphthalene in H2O ices (Bernstein et al 2001) We alsoobserved small peaks at 161 amu (possibledihydroxyquinolines see trace (f) in Fig 2)

Species with mass 131 amu and 133 amu were alsoobserved (see traces (d) and (e) in Figure 2) which could beformed by partial reduction of the quinoline skeletonalthough these were minor species compared to the oxidationproducts (Reaction 4) Reduction of this sort was observed inthe photolysis of PAHs in H2O-rich ices (Bernstein et al1999 Bernstein et al 2002)

Fig 1 An HPLC trace of the methanol extracts of the residues from (a) a H2Oquinoline = 501 photolyzed ice (b) an unphotolyzed H2Oquinoline = 501 ice and (c) a photolyzed H2O ice (no quinoline) Hydroxyquinolines are produced on photolysis of the H2Oquinoline icebut not in the two controls Peaks were identified by their elution time and UV spectrum

(2)

(1)

(2)

(3)

(4)


A comparison of the infrared spectra taken of H2Oquinoline = 501 ices deposited with and without UVphotolysis (Fig 3) was consistent with the formation of anaromatic ketone but was not definitive This observationwould be consistent with the interpretation of the nanoLC-ESI-MS peak at 159 amu as a diketone or might suggest thatsome of the hydroxyquinolines reported above may be inequilibrium with the keto form with the exact ratio of alcoholto ketone unknown Again keto-enol tautomerism is thoughtto play a similar role in the photolysis of PAHs in H2O-richices (Bernstein et al 1999)

We calculated the yields for the 4-hydroxyquinoline 6-hydroxyquinoline and 7-hydroxyquinoline products In thetypical experiments described above (UV flux = sim1018

photons cmminus2) these yields were approximately 12 37and 48 respectively relative to unreacted quinoline Forcomparison we examined the HPLC traces of the methanolextracts from ices formed with the ldquoslowrdquo deposition rate (UVflux = sim1019 photons cmminus2) The increase in UV flux yieldedthe same qualitative mix of compounds as the lower fluxhigher deposition rate but the overall yield of productsrelative to unreacted quinoline increased to approximately

Fig 2 NanoLC-ESI-MS chromatograms of the methanol extracts of the residue from a H2Oquinoline = 501 photolyzed ice The top boxshows species with mass (a) 129 amu corresponding to unreacted quinoline and (b) 145 amu corresponding to hydroxyquinolines The bottombox shows a magnified portion of the chromatograms for the following masses c) 129 amu (unreacted quinoline) d) 131 amu (possiblypartially reduced quinoline) e) 133 amu (possibly partially reduced quinoline) and f) 161 amu (possibly dihydroxyquinolines)


45 72 and 115 respectively We found howeverthat although the relative yields increased the absolute yields(HPLC signal strength observed) of both thehydroxyquinolines and the unreacted quinoline decreased byapproximately an order of magnitude indicating a loss in theoverall amount of quinoline and photoproducts

The UV photolysis of H2Oquinoline = 101 and H2Oquinoline = 1001 mixtures showed qualitatively similarproduct distributions as the H2Oquinoline = 501 mixturewith a range of hydroxyquinolines identified Yields of 4-hydroxyquinoline and 7-hydroxyquinoline were slightlydecreased for the 101 H2Oquinoline ice but the yields forthe 501 and 1001 ices were comparable Overall theproducts observed are the same and their relative proportionsseem fairly insensitive to concentration

Ices Containing Methanol or Methane

Analysis of the methanol extracts from the icescontaining methane or methanol revealed the addition ofcarbon atoms to the quinoline skeleton as well as thepreviously observed oxygen atom addition The HPLC traceof the H2OCH3OHquinoline = 5051 ice) (Fig 4b) showedthe same qualitative mix of hydroxyquinolines identified inthe H2Oquinoline = 501 ice (see Reaction 1 for structuresand Fig 4c for comparison) However an additional peak at221 min was also observed This peak did not correspond toany of the methylquinoline standards or the 6-methoxyquinoline standard measured but probably resultsfrom a methylquinoline or methoxyquinoline for which wedid not have the standard Yields for the hydroxyquinolineswere much lower than those observed in the ices containingonly H2O and quinoline 4-hydroxyquinoline was

undetectable while 6-hydroxyquinoline and 7-hydroxyquinoline produced yields of approximately 05each relative to unreacted quinoline

The nanoLC-ESI-MS analysis of the H2OCH3OHquinoline ice revealed masses corresponding to unreactedquinoline (129 amu) methylquinolines (substituting a ndashCH3group for a hydrogen atom 143 amu) hydroxyquinolines(145 amu) and the possible simultaneous substitution of botha carbon-containing substituent (eg a methyl group) and anoxygen-containing substituent (eg a hydroxy group)(159 amu) Several methylquinoline isomers were thought tobe present but in the absence of relevant HPLC standards thespecific position of the methyl substituent could not beidentified (see Reaction 5 for general structure)

The methylquinoline peaks were observed as minorproducts only the preconcentration step in nanoLC-ESI-MSimproves sample detection limits which may explain thedetection of methylquinolines by this method but not by theHPLC analysis The peaks representing hydroxyquinolinesand possible simultaneous substitution were quite large incomparison The multiple species with mass 159 amu couldbe caused by substitution of a methyl group on to ahydroxyquinoline or by substitution of either a ndashCH2OH or andashOCH3 group as a single moiety (Reaction 6) The 159 amupeaks could also indicate the presence of a diketone assuggested in the H2Oquinoline results however this

Fig 3 Infrared (IR) spectra of (a) photolyzed H2Oquinoline = 501 ice and (b) unphotolyzed H2Oquinoline = 501 ice Ices were formed at20 K then warmed to 220 K for IR spectroscopy Several changes are noticeable the arrows in trace (a) point out the peaks at 1657 cmminus1 and1277 cmminus1 that are suggestive of an aromatic ketone Spectra were normalized to the height of the largest peak

(5)


assignment seems less likely because the potential diketonewas only weakly observed in the H2Oquinoline ice after amuch longer deposit

Photolysis of a H2OCH4quinoline = 5051 ice againproduced the hydroxyquinolines described above but HPLCanalysis of this mixture also indicated the presence ofmethylquinoline compounds (Fig 4a) The peak at 324 minis tentatively identified as 8-methylquinoline while the otherpeaks in the range of 33ndash37 min are likely to be othermethylquinolines It is difficult to assign exact identificationsboth because of the limited supply of methylquinolinestandards and because of the similarity in the retention timesand UV spectra of these compounds Yields for thehydroxyquinolines produced in this ice were sim 075 16and 18 for 4- 6- and 7-hydroxyquinoline relative tounreacted quinoline The peak identified as 8-methylquinoline was produced with a yield of approximately13 relative to unreacted quinoline

NanoLC-ESI-MS analysis of the extract from the H2OCH4quinoline = 5051 ice revealed species with massessimilar to those observed in the CH3OH-containing iceAgain methylquinolines (143 amu) and hydroxyquinolines(145 amu) were observed consistent with the HPLC analysisSeveral peaks at 159 amu corresponding to species arising

from possible simultaneous substitution of carbon- andoxygen-containing groups were also present

A comparison of the chromatograms for the 159 amumass between the CH3OH-containing ice and the CH4-containing ice reveals differences in the distribution ofspecies formed by possible simultaneous addition of carbon-and oxygen-containing moieties (Fig 5) In the CH3OH-containing ice there appears to be one major species and oneminor species while the CH4-containing ice contains severalspecies of similar prominence It is reasonable to suggestthat the products in the CH3OH-containing ice result fromthe addition of one single moiety either as the ndashCH2OHalcohol or the ndashOCH3 methyl ether (Reaction 6) This isconsistent with previous analyses of ices containing PAHsand CH3OH where formation of the methyl ether of thePAH was observed (Bernstein et al 2002) In the CH4-containing ice however it is possible that the 159 amuspecies are the result of the addition of separate ndashCH3groups and ndashOH groups producing several isomers ofmethylhydroxyquinoline (Reaction 7)

We examined the effects of deposition temperature onphotolysis in the H2O- and CH4-containing ices A change indeposition temperature from 20 K to 122 K did not affect the

Fig 4 HPLC traces of methanol extracts of residues from ices containing (a) H2OCH4quinoline = 5051 (b) H2OCH3OHquinoline = 5051 and (c) H2Oquinoline = 501 Regions with peaks corresponding to hydroxyquinolines unknown species unreacted quinoline andmethylquinolines are indicated

(6)

(7)


range of hydroxyquinolines produced in the H2Oquinoline =501 ice although the overall yield of the products relative tounreacted quinoline dropped When the H2OCH4quinoline =5051 ice was deposited and photolyzed at 122 K howevermethylquinolines were not observed in the HPLC analysis ofthe product mixture although hydroxyquinolines were stilldetected This lack of methylquinoline formation is mostlikely because the volatile methane did not freeze efficientlyinto the ices at this temperature

DISCUSSION

We have subjected quinoline-containing interstellar iceanalogs to UV photolysis and then analyzed the resultingphotoproducts We demonstrate that substituted quinolinesare readily produced under astrophysically relevantconditions Specifically hydroxyquinolines are formed inquinoline-containing H2O ices under a range of conditionswhich mirrors the results observed from photolysis of PAHsin H2O ice (Bernstein et al 2001) This result suggests thatoxidation of aromatics in water-rich icy environments is afacile and favorable reaction Our observations correlate wellwith the tentative preliminary identification of quinoline-quinones in apparently interstellar dust particles detected bythe Stardust spacecraft (Krueger et al 2003) Although wecould not confirm the presence of these quinones directly in

our laboratory analogs (and standards were not available forcomparison) we did see species with the appropriate mass(159 amu) produced in a long-term ice deposit Theirformation is also a logical extension of the formation of theoxidized quinolines that we could confirm

We also observed products resulting from the possibleaddition of hydrogen to the quinoline structure indicating thatPANHs might also be reduced in these environmentsNevertheless reduction products were of low abundancecompared to oxidation products Again these observationscorrelate well with previous studies of PAHs (Bernstein et al1999)

Our results also indicate that in environments with a highUV flux photodestruction of quinoline may compete with theoxidation reaction This observation is in agreement withrecent studies that indicate that although small PANHs are notexpected to survive the radiation environment in the diffuseinterstellar medium they could survive within a dense cloud(Peeters et al 2005) The UV flux within a dense molecularcloud has been estimated in various literature reports as 14 times103 photons cmminus2 sminus1 or 48 times 103 photons cmminus2 sminus1 (Prasad andTarafdar 1983 Menella et al 2003) At the edge of such acloud the flux can be approximated as sim108 photons cmminus2 sminus1

(Mathis et al 1983) Given this range our typical UV dosecorresponds to approximately 6 times 106ndash2 times 107 yr in the interiorof a dense cloud (a reasonable time scale representing the

Fig 5 NanoLC-MS-ESI chromatograms for species with a mass of 159 amu in the methanol extracts of the residue of (a) H2OCH3OHquinoline = 5051 ice and (b) H2OCH4quinoline = 5051 ice A single large peak dominates the former consistent with the substitution ofCH3OH as a single moiety (ndashOCH3 or ndashCH2OH) while the multiple peaks in trace (b) suggest substitution of two separate groups (minusCH3 plusndashOH)


lifetime of the cloud) or sim500 yr at the edge of the cloud Theexperiments done at slower deposition rate would receiveapproximately an order of magnitude more UV exposure theseare the experiments that showed evidence of photodestructionof quinoline Thus the production of significant amounts ofoxidized PANHs may be limited to the interior of dense cloudswhere shielding protects the compounds from completephotodestruction but sufficient amounts of UV are present todrive the photooxidation reactions

The observations related to the oxidation of PANHs areinsensitive to concentration or temperature Thus we expectthat if PANHS were delivered to planetary icy surfaces theywould undergo oxidation in those environments as well Weobserved formation of hydroxyquinolines at temperatures ofsim120 K an appropriate temperature for modeling outer solarsystem bodies such as Europa (Spencer et al 1999) If PANHsare present on such bodies it is likely that a significantfraction will be oxidized through photochemistry in H2O-dominated ices

Previous studies of the photochemistry of PAHs focusedon coronene (C24H12) a highly symmetric molecule that hasonly one possible mono-alcohol or mono-ketone andnaphthalene (C10H8) a structure with two possible mono-alcohols In contrast the regiochemistry of quinoline is farmore complex with eight possible distinct oxygensubstitution sites (including the nitrogen oxide) Our resultsdid not show a clear dominance of any single site a mixtureof at least five monohydroxyquinolines was observed withthe 7-hydroxyquinoline having a slightly higher yield than theother products Quinoline N oxide was not observedsuggesting that oxidation is much more likely to occur at thecarbon atoms than the lone nitrogen atom

We also observed the substitution of methyl and methoxymoieties on to the quinoline skeleton when CH4 or CH3OHwas present in the interstellar ice analogs The methane ormethanol was present at levels sim10 that of the H2O in theice The observed abundance ratio for CH3OH along mostlines of sight in interstellar dense clouds is sim10 althoughthis may be an average value caused by two distinct icy grainmantle populations (one methanol-rich and the othermethanol-poor) (Skinner et al 1992 Dartois et al 1999) Weview the levels used in our experiments as a representativeconcentration for methanol in interstellar dense clouds (Chiaret al 1996 Pontopiddan et al 2003) and a slightly high butexperimentally expedient approximation for methane (Lacyet al 1991 Boogert et al 1996) This formation ofmethylquinolines and possible methoxyquinolines ormethylhydroxyquinolines in our experiments parallels resultsobserved in photolysis of PAHs in interstellar ice analogs(Bernstein et al 2002) The formation of methylquinolines isof particular interest to meteoritics Substituted quinolineswith up to four methyl side groups have been identified in theMurchison meteorite (Stoks and Schwartz 1982) Previousstudies based on deuterium enrichments suggest a connection

between interstellar ice chemistry and meteoritic organics(Kerridge et al 1987 Messenger et al 1995 Sandford et al2000 Messenger 2002) The formation in our ice experimentsof methylquinolines similar to those observed in Murchisonlends support to this proposed connection

If these ice reactions of quinoline are linked to theproduction of meteoritic organic material then a portion of themeteoritic aromatic compounds would be expected to beoxidized Although PANHs have been identified in theMurchison meteorite there have been very few studiesfocusing on meteoritic PANHs and almost no attention hasbeen paid to the possibility of oxidized PANHs in meteorites Infact there has been relatively little study of any oxidizedaromatics in meteorites although a number of aromaticketones (eg fluorenone anthracenone anthracenedionebenzoanthracenone) have been identified (Basile et al 1984Krishnamurthy et al 1992) Targeted searches for PANHs andtheir oxidized and reduced derivatives are needed to determineif these compounds are present in meteorites There arecertainly several possible pathways to the formation ofmeteoritic organics and chemistry in icy environments is onlyone proposed method Because oxidized PANHs are the mostabundant products observed in our work information on therelative prevalence of oxidation in PANH meteoriticcompounds could provide insight into the possible significanceof ice chemistry in the formation of meteoritic organics

In addition PANHs are of significant astrobiologicalinterest Purine- and pyrimidine-based nucleobases whichmake up the information component of DNA and RNA areessentially PANHs with side groups including hydroxy andmethyl moieties Other important biomolecules includingtwo amino acids flavins and nicotinamides are built onPANH skeletons modified with side groups If the simplePANH skeletons such as purine and pyrimidine are present inthe interstellar medium or on icy planetary surfaces then theice chemistry described here could provide one facile methodfor attaching the side groups and making putatively pre-biologically relevant molecules (eg hypoxanthine orxanthine both formed by substitution of hydroxy groups forhydrogen atoms on purine and both playing roles in modernbiological processes) This possibility is potentially importantboth as a means of contributing these compounds to prebioticenvironments where they may play a role in the origination oflife and because these molecules could act as falsebiomarkers confusing the search for compounds that couldindicate life elsewhere in the galaxy

CONCLUSIONS

We have shown that the polycyclic aromatic nitrogenheterocycle quinoline is readily oxidized by UV photolysiswhen frozen in astrophysically relevant H2O-dominated icesA variety of hydroxyquinolines is produced with no singledominant product Quinoline N oxide was not formed


Oxidation occurs at temperatures relevant to dense molecularclouds and to icy planetary surfaces and is qualitativelyunaffected by quinoline concentration Reduction is alsoconsistent with these observations UV doses comparable tothose expected in the interior of a dense molecular cloud overits lifetime (sim1018 photons cmminus2) produce significant levels ofoxidation without large amounts of photodestruction of theresulting photoproducts although an increase in UV exposureby a factor of 10 appears to cause photodestruction ofquinoline

We have also shown that side groups such as methyl andmethoxy moieties may be added to the quinoline skeletonwhen methane or methanol is present in these ices atappropriate levels The methylquinolines formed are similarto compounds detected in the Murchison meteorite Theformation of these compounds is of interest because it mayprovide insight into the connection between interstellar icechemistry and meteoritic organics For example given thatmethylquinolines have previously been detected in theMurchison meteorite our current results lead us to suspect thepresence of hydroxyquinolines as well Future targetedsearches for oxidized aromatics in meteorites should provideinformation on this possibility In addition the astrobiologicalrelevance of PANHs and their substituted derivatives presentsintriguing reasons to study further the formation and reactionsof these compounds

AcknowledgmentsndashThis work was supported by NASArsquosExobiology (grant 34458-21-02) Astrobiology (grants 344-50-92-02 and NNA04CC05A) and Origins of Solar Systems(grant 344-37-44-01) programs This research was performedwhile J Elsila held a National Research Council ResearchAssociateship Award through the NASA AstrobiologyInstitute at NASA Ames Research Center We gratefullyacknowledge the technical expertise of Maria Dulay andEvan Pearce in assisting with the nanoLC-ESI-MSexperiments We also thank Lou Allamandola for helpfuldiscussions

Editorial HandlingmdashDr Christine Floss

REFERENCES

Allamandola L J Sandford S A and Valero G J 1988Photochemical and thermal evolution of interstellarprecometaryice analogs Icarus 76225ndash252

Allamandola L J Tielens A G G M and Barker J R 1989Interstellar polycyclic aromatic hydrocarbons The infraredemission bands the excitationemission mechanism and theastrophysical implications The Astrophysical Journal 71773ndash775

Allamandola L J Hudgins D J and Sandford S A 1999 Modelingthe unidentified infrared emission with combinations ofpolycyclic aromatic hydrocarbons The Astrophysical Journal511L115ndashL119

Basile B P Middleditch B S and Oro J 1984 Polycyclic aromatic

hydrocarbons in the Murchison meteorite OrganicGeochemistry 5211ndash216

Bernstein M P Sandford S A Allamandola L J Chang S andScharberg M A 1995 Organic compounds produced byphotolysis of realistic interstellar and cometary ice analogscontaining methanol The Astrophysical Journal 454327ndash344

Bernstein M P Sandford S A Allamandola L J Gillette J SClemett S J and Zare R N 1999 UV irradiation of polycyclicaromatic hydrocarbons in ices Production of alcohols quinonesand ethers Science 2831135ndash1138

Bernstein M P Dworkin J P Sandford S A and Allamandola L J2001 Ultraviolet irradiation of naphthalene in H2O iceImplications for meteorites and biogenesis Meteoritics ampPlanetary Science 36351ndash358

Bernstein M P Elsila J E Dworkin J P Sandford S AAllamandola L J and Zare R N J 2002 Side group addition tothe PAH coronene by UV photolysis in cosmic ice analogs TheAstrophysical Journal 5761115ndash1120

Bernstein M P Moore M H Elsila J E Sandford S AAllamandola L J and Zare R N 2003 Side group addition tothe polycyclic aromatic hydrocarbon coronene by protonirradiation in cosmic ice analogs The Astrophysical Journal 582L25ndashL29

Bernstein M P Mattioda A L Sandford S A and Hudgins D M2005 Laboratory infrared spectra of polycyclic aromaticnitrogen heterocycles Quinoline and phenanthridine in solidargon and H2O The Astrophysical Journal 626909ndash918

Boogert A C A Schutte W A Tielens A G G M WhittetD C B Helmich F P Ehrenfreund P Wesselius P R deGraauw T and Prusti T 1996 Solid methane toward deeplyembedded protostars Astronomy and Astrophysics 315L377ndashL380

Boulanger F Abergel A Bernard J P Cesarsky D Puget J LReach W T Ryter C Cesarsky C J Sauvage M Tran DVigroux L Falgarone E Lequeux J Perault M and Rouan D1998 The nature of small interstellar dust particles In Starformation with the Infrared Space Observatory edited by YunJ L and Liseau R San Francisco The Astronomical Society ofthe Pacific pp 15ndash23

Bregman J D Hayward T L and Sloan G C 2000 Discovery ofthe 112 micron polycyclic aromatic hydrocarbon band inabsorption toward Monoceros R2 IRS 3 The AstrophysicalJournal 544L75ndashL78

Bregman J D and Temi P 2001 Gas-phase polycyclic aromatichydrocarbons in absorption toward protostellar sources TheAstrophysical Journal 554126ndash131

Brooke T Y Sellgren K and Geballe T R 1999 New 3 micronspectra of young stellar objects with H2O ice bands TheAstrophysical Journal 517883ndash900

Butterworth A L Aballain O Chappellaz J and Sephton M A2005 Combined element (H and C) stable isotope ratios ofmethane in carbonaceous chondrites Monthly Notices of theRoyal Astronomical Society 347807ndash812

Cecchi-Pestellini C Saija R Iati M A Guisto A Borghese FDenti P and Aiello S 2005 Ultraviolet radiation insideinterstellar grain aggregates I The density of radiation TheAstrophysical Journal 624223ndash231

Chiar J E Adamson A J and Whittet D C B 1996 Three micronhydrocarbon and methanol absorption in Taurus TheAstrophysical Journal 472665ndash672

Chiar J E Tielens A G G M Whittet D C B Schutte W ABoogert A C A Lutz D van Dishoeck E F and BernsteinM P 2000 The composition and distribution of dust along theline of sight towards the Galactic Center The AstrophysicalJournal 537749ndash762


Cottin H Moore M H and Benilan Y 2003 Photodestruction ofrelevant interstellar molecules in ice mixtures The AstrophysicalJournal 590874ndash881

Cox P and Kessler M F 1999 The universe as seen by ISONoordwijk The Netherlands European Space AgencyPublications Division 2 vol 1090 p

Dartois E Schutte W Geballe T R Demyk K Ehrenfreund P andDrsquoHendecourt L 1999 Methanol The second most abundant icespecies towards the high-mass protostars RAFGL7009S andW33A Astronomy and Astrophysics 342L32ndashL35

Dulay M T Quirino J P Bennett B D Kato M and Zare R N2001 Photopolymerized sol-gel monoliths for capillaryelectrochromatography Analytical Chemistry 733921ndash3926

Ehrenfreund P and Charnley S B 2000 Organic molecules in theinterstellar medium comets and meteorites A voyage from darkclouds to the early Earth Annual Review of Astronomy andAstrophysics 38427ndash483

Hayatsu R Studier M H Moore L P and Anders E 1975 Purinesand triazines in the Murchison meteorite Geochimica etCosmochimica Acta 39471ndash488

Huang Y S Wang Y Alexandre M R Lee T Rose-Petruck CFuller M and Pizzarello S 2005 Molecular and compounds-specific isotopic characterization of monocarboxylic acids incarbonaceous meteorites Geochimica et Cosmochimica Acta 691073ndash1084

Hudgins D M Bauschlicher C W Jr and Allamandola L J 2005Variations in the peak position of the 62 um interstellar emissionfeature A tracer of N in the interstellar polycyclic aromatichydrocarbon population The Astrophysical Journal 632316ndash332

Jenniskens P and Blake D F 1994 Structural transitions inamorphous water ice and astrophysical implications Science265753ndash756

Jenniskens P Blake D F Wilson M A and Pohorille A 1995High-density amorphous ice the frost on interstellar grains(abstract) The Astrophysical Journal 455389

Kerridge J F Chang S and Shipp R 1987 Isotopic characterizationof kerogen-like material in the Murchison carbonaceouschondrite Geochimica et Cosmochimica Acta 512527ndash2540

Krishnamurthy R V Epstein S Cronin J R Pizzarello S and YuenG U 1992 Isotopic and molecular analyses of hydrocarbons andmonocarboxylic acids of the Murchison meteorite Geochimicaet Cosmochimica Acta 564045ndash4058

Krueger F R Werther W Kissel J and Schmid E R 2003Assignment of quinone derivatives as the main compound classcomposing interstellar grains based on both polarity ions detectedby the cometary and interstellar dust analyser (CIDA) onboardthe spacecraft Stardust Rapid Communications in MassSpectrometry 18103ndash111

Lacy J Carr J Evans N Baas F Achtermann J and Arens J 1991Discovery of interstellar methane observations of gaseous andsolid CH4 absorption toward young stars in molecular cloudsThe Astrophysical Journal 376556ndash560

Loeffler M J Baratta G A Palumbo M E Strazzulla G andBaragiola R A 2005 CO2 synthesis in solid CO by Lyman-αphotons and 200 keV protons Astronomy and Astrophysics 435587ndash594

Mathis J S Mezger P G and Panagia N 1983 Interstellar radiationfield and dust temperatures in the diffuse interstellar matter andin giant molecular clouds Astronomy and Astrophysics 128212ndash229

Mattioda A L Hudgins D M Bauschlicher C W Jr Rosi M andAllamandola L J 2003 Infrared spectroscopy of matrix-isolatedpolycyclic aromatic compounds and their ions 6 Polycyclic

aromatic nitrogen heterocycles Journal of Physical Chemistry A1071486ndash1498

Menella V Baratta G A Esposito A Ferini G and Pendleton Y J2003 The effects of ion irradiation on the evolution of the carrierof the 34 micron interstellar absorption band The AstrophysicalJournal 587727ndash738

Messenger S Clemett S J Keller L P Thomas K L ChillierX D F and Zare Richard N 1995 Chemical and mineralogicalstudies of an extremely deuterium-rich IDP Meteoritics 30546ndash547

Messenger S 2002 Deuterium enrichments in interplanetary dustPlanetary and Space Science 501221ndash1225

Nagiel-Ostaszewski I Vavrek M T and Weisburger J H 1991Separation of hydroxyquinolines by high-performance liquidchromatography Xenobiotica 21751ndash754

Peeters Z Botta O Charnley S B Kisiel Z Kuan Y-J andEhrenfreund P 2005 Formation and photostability of N-heterocycles in space I The effect of nitrogen on thephotostability of small aromatic molecules Astronomy andAstrophysics 433583ndash590

Pizzarello S and Huang Y S 2005 The deuterium enrichment ofindividual amino acids in carbonaceous meteorites A case forthe presolar distribution of biomolecule precursors Geochimicaet Cosmochimica Acta 69599ndash605

Pontopiddan K M Dartois E van Dishoeck E F Thi W-F andDrsquoHendecourt L 2003 Detection of abundant solid methanoltoward young low mass stars Astronomy and Astrophysics 404L17ndashL20

Prasad S S and Tarafdar S P 1983 UV radiation field inside denseclouds Its possible existence and chemical implications TheAstrophysical Journal 267603ndash609

Quirino J P Dulay M T and Zare R N 2001 On-linepreconcentration in capillary electrochromatography using aporous monolith together with solvent gradient and samplestacking Analytical Chemistry 735557ndash5563

Ricca A Bauschlicher C W and Bakes E L O 2001a Acomputational study of the mechanisms for the incorporation ofa nitrogen atom into polycyclic aromatic hydrocarbons in theTitan haze Icarus 145516ndash521

Ricca A Bauschlicher C W and Rosi M 2001b Mechanisms forthe incorporation of a nitrogen atom into polycyclic aromatichydrocarbon cations Chemical Physics Letters 347473ndash480

Sandford S A 1996 The inventory of interstellar materials availablefor the formation of the solar system Meteoritics amp PlanetaryScience 31449ndash476

Sandford S A Bernstein M P Allamandola L J Gillette J S andZare R N 2000 Mechanisms for the deuterium enrichment ofPAHs in interstellar ices by UV irradiation The AstrophysicalJournal 538691ndash697

Sellgren K Brooke T Y Smith R G and Geballe T R 1995 A new325 micron absorption feature toward Monoceros R2IRS-3 TheAstrophysical Journal 449L69ndashL72

Sephton M A 2002 Organic compounds in carbonaceousmeteorites Natural Product Reports 19292ndash311

Skinner C J Tielens A G G M Barlow M J and Justtanont K1992 Methanol ice in the photostar GL 2136 The AstrophysicalJournal 399874ndash881

Spencer J R Tamppari L K Martin T Z and Travis L D 1999Temperatures on Europa from Galileo photopolarimeter-radiometer Nighttime thermal anomalies Science 2841514ndash1516

Stoks P G and Schwartz A W 1981a Nitrogen-heterocycliccompounds in meteorites Significance and mechanisms offormation Geochimica et Cosmochimica Acta 45563ndash589


Stoks P G and Schwartz A W 1981b Nitrogen compounds incarbonaceous meteorites A reassessment Origins of Life 1159ndash64

Stoks P G and Schwartz A W 1982 Basic nitrogen-containingheterocyclic compounds in the Murchison meteoriteGeochimica et Cosmochimica Acta 46309ndash315

Strazzulla G 1997 Ion irradiation Its relevance to the evolution ofcomplex organics in the outer solar system Advances in SpaceResearch 191077ndash1084

van der Velden W and Schwartz A W 1977 Search for purines and

pyrimidines in the Murchison meteorite Geochimica etCosmochimica Acta 41961ndash968

Wang H and Frenklach M 1994 Calculations of rate coefficients forthe chemically activated reactions of acetylene with vinylic andaromatic radicals Journal of Physical Chemistry 9811465ndash11489

Warneck P 1962 A microwave-powered hydrogen lamp forvacuum ultraviolet photochemical research Applied Optics 1721ndash726


be protected in dense cloud environments (Peeters et al2005) Recent studies suggest that larger PANHs with one ormore nitrogen atoms in the interior of the molecular skeletoncould account for the 62 microm interstellar emission band andthat such molecules may contain at least 1ndash2 of all cosmicnitrogen (Hudgins et al 2005) In addition PANHs such aspurines pyrimidines quinolines isoquinolines and pyridineshave been identified in carbonaceous chondrite meteorites atlevels greater than 1 ppm providing more evidence for theexistence of extraterrestrial PANHs (see Sephton 2002 for areview) It is thus a reasonable assumption that PANHscomprise some portion of interstellar organic compounds

In dense molecular clouds where temperatures are low(10ndash20 K) PAHs PANHs and other chemical species areexpected to form mixed molecular ice mantles that arecondensed on silicate dust grains (Sellgren et al 1995Sandford 1996 Chiar et al 2000) These icy mantles whichare usually dominated by water ice may then be subjected tochemical processing driven by ultraviolet radiation (eitherfrom the galactic background embedded protostars orcosmic-ray excitation of hydrogen) (Prasad and Tarafdar1983 Bernstein et al 1995 Cecchi-Pestellini et al 2005) andor by the impact of low-energy (sim1 MeV) cosmic rays(Strazzulla 1997) The chemical processing of certain PAHsin laboratory analogs of these ices has been studied and a richchemistry has been observed in which PAHs acquire a varietyof side groups including hydroxy carbonyl methyl andamino moieties (Bernstein et al 2002 Bernstein et al 2003)

Although the spectroscopic properties of PANHs havebeen studied (Mattioda et al 2003 Bernstein et al 2005Hudgins et al 2005) there has been no investigation of thecompounds that are produced when PANHs undergo chemicalprocessing in icy interstellar environments The possibilitythat various side groups may be added to PANHs throughinterstellar ice processing is of interest for several reasonsFirst as mentioned above PANHs have been detected incarbonaceous chondrites The detected compounds includePANHs with side groups such as alkylated quinolinesisoquinolines and pyridines (Hayatsu et al 1975 Stoks andSchwartz 1982) and oxidized PANHs such as carboxylatedpyridines and hydroxy- and keto-containing purines(xanthine hypoxanthine guanine adenine) (Van der Veldenand Schwartz 1977 Stoks and Schwartz 1981a 1981b)Quinoline-quinones another form of oxidized PANH havealso been tentatively identified in preliminary analysis ofinterstellar dust particles captured by the Stardust spacecraft(Krueger et al 2003) Previous studies have investigated theconnection between interstellar ice chemistry and meteoriticorganics pointing out that deuterium enrichments that areobserved in meteorites suggest an interstellar origin for somemeteoritic organics and that these deuterated excesses may becreated in interstellar ice processing (Kerridge et al 1987Messenger et al 1995 Sandford et al 2000 Messenger 2002Butterworth et al 2004 Huang et al 2005 Pizzarello andHuang 2005)

Another point of interest in the possible interstellarprocessing of PANHs is that substituted PANHs play animportant role in biochemistry on Earth The nucleobasesfound in DNA and RNA consist of substituted purines andpyrimidines while other substituted PANHs such as flavinsare involved in electron transfer processes The possibleinterstellar formation of such compounds therefore hasrelevance to astrobiology

In the work presented here we report on thephotochemical processing of the PANH quinoline in H2O-dominated interstellar ice analogs Quinoline was chosen as anitrogen-containing analog of the PAH naphthalene whichwas previously studied in similar investigations in thislaboratory (Bernstein et al 2001) A number of variablesincluding deposition temperature UV flux and icecomposition were explored in these experiments in order tosimulate a variety of extraterrestrial environments

EXPERIMENTAL TECHNIQUE

The quinoline-containing interstellar ice analogs wereprepared in a manner that has been described in detailelsewhere (Allamandola et al 1988 Bernstein et al 1995) Inbrief a gas mixture is deposited onto a rotatable cold fingercontained in an evacuated cryogenic chamber surrounded byan infrared spectrometer The gas mixtures were preparedwith quinoline (98 Sigma-Aldrich distilled prior to use)water (purified via a Millipore Milli-Q system to 182 M)methanol (HPLC grade Fisher) and methane (ultra-highpurity Matheson) The quinoline water and methanol weretriply freeze-pump-thawed to remove dissolved gases PureH2O deposited under these conditions is in its high-densityamorphous form when deposited at 15 K and goes throughseveral phase transitions during warming (Jenniskens andBlake 1994 Jenniskens et al 1995) Clearly the morphologyof ice mixtures can be more complex but previous IR spectraof H2Oquinoline mixtures are consistent with amorphousH2O (Bernstein et al 2005) This form is believed to berepresentative of H2O-rich ices in interstellar molecularclouds

For the work described here gas mixtures typicallycontained lt1 mbar of H2O and quinoline at a ratio of 501 andwere deposited onto a 20 K aluminum foil substrate for sim6 hrat an ice growth deposition rate of sim07 micromhr Residualpressure in the vacuum chamber was sim5 times 10minus8 bar Severalvariations on this typical preparation were also used to assessthe effects of different parameters on ice photochemistryThese variations included the following

1 Using a CsI substrate instead of aluminum foil in order totake infrared spectra

2 Slowing the rate of ice growth deposition to sim007 micromhrto increase UV dose while depositing for 48 hr

3 Changing the H2Oquinoline ratio to 101 and 10014 Including CH4 or CH3OH vapor at levels sim10 of the

H2O










RESULTS

H2OQuinoline Ices










(2)

(1)

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RESULTS

H2OQuinoline Ices










(2)

(1)

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(6)

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(2)

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uv photolysis of quinoline in interstellar ice analogs

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