NASA Technical Reports Server (NTRS) 19980162983: Detecting Amino Acids on Mars

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by  NASA Technical Reports Server (NTRS)

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DETECTINGAMINO
ACIDSONMARS
Jeffrey L. Bada
Scripps Institution of Oceanography
Gene D, M©Donald
Come//University
Reprinted with permission from Analytical Chemistry 1996, 68, 668 A--673 A
Copyright © 1996 American Chemical Society
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DETECTINAGMINO
ACIDSONMARS
The unambiguous detection of amino acids
on Mars could bepivotal in understanding
the origin of life
on Mars (3). Thus, traces of prebiotic The subsequent transition from the
led to the origin of life on Earth chemistry, or organic compounds de- abiotic chemistry of primitive Earth to the
nderstanding the events that
is complicated bythe lack of rived from an extinct Martian biota (40, first serf-replicating molecular systems
geological evidence from the period could be present on Mars. Although capable of Darwinian evolution marked
around 4 billion years (4 Gyr) ago when deemed unlikely, life may still exist to- the point of the origin oflife. On Earth,
the transition from prebiotic chemistry to day on Mars in some protected subsur- subsequent evolution ofthe first self-repli-
biochemistry is believed to have occurred. face environments (5). cating molecules gave rise tothe RNA
Although erosion and plate tectonics have The processes thought to be involved world and finally the DNA-pmtein world
since erased the terrestrial geological in the origin of life on Earth are summa- characteristic of all modem life.
record from the time of the origin of life, rized in Figure 1.The first requirement is Amajor goal of the NASA Space Explo-
there is apossibility that information the presence ofa prebiotic soup consist- ration Program isto search for evidence
about this period of Earth history may still ing of arich variety oforganic compounds, ofabiotic chemistry and extinct orextant
be preserved on Mars. although the exact composition of asoup life on Mars. During the next decade,
Compared with Earth, Mars is amore necessary for the origin oflife isnot known. spacecraft will orbit Mars, land on the sur-
placid planet. Surface alteration rates are The components of the soup may have face, and return with surface samples for
minimal, and there is no known plate been made directly on Earth or supplied analysis. The question is what compounds
tectonic activity. Extensive areas ofthe from space bycomets, asteroids, microme- should we search for, either directly on
Martian surface may date to >4Gyr ago teorites, orinterplanetary dust (6").Alarge the planet orin samples returned to
(i). Geomorphoiogic evidence suggests variety oforganic compounds--including Earth, that will answer unambiguously
that liquid water existed on the Martian those that play amajor role inbiochemis- whether abiotic and/or biotic organic mol-
surface at some point in the past and that try, such as amino acids, purines, and pyd- ecules are present.
early Mars may have had an atmosphere midines--have been identified in one class
similar to that of early Earth (2). Ifthis of meteorites, the carbonaceous chon- Previous organic analyses
is the case, then atleast some of the drites (Figure 2). In addition to demon- The detection of organic material on Mars
steps leading to the origin of terrestrial strafing that important biomolecules can was attempted in 1976 by the Viking 1and
biochemistry may have also taken place be synthesized by abiotic reactions in ex- 2landers, which carried GC/MS systems
traterrestrial environments, the presence (8). No organic compounds were detected
of these organic compounds in meteorites above the part-per-billion level inthe up-
also suggests that exogenous compounds per few centimeters of the Martian sur-
Jeffrey L. Bada
were periodically delivered to the surface face, but the results of other experiments
Scripps Institution of Oceanography
Gene D. McDonald of the Earth orother planetary bodies by aboard the ]anders indicated that the Mar-
Cornel/University various processes (6). tian surface is saturated with an unidentl-
0003- 2700/96/0368 -668A1512.00/0
© 1996 American Chemical Society
fled oxidant thatprobably destroyed any Although trace quantities ofabiotic origin inEETA79001 (13) suggests that
organics within ashorttimeframe (9). amino acids were notdetected inEETA- organic compounds associated with Earth-
Because the oxidizinglayermay onlyex- 79001,part-per-miUionquantities of the based contamination could generate mis-
tendafew meters below the surface (/0), L-enantiomersofthe amino acids com- leading conclusions aboutwhether or-
the preservation oforganics inthesubsur- monly found inthe proteinsofliving or- ganic molecules derived fromextinctlife
faceis stillapossibility. ganisms were found (!3). The L-amino areindeed present inMartianmeteorites.
Aside fromtheViking missions, the acidsinthis Martian meteorite appear to
onlyother opportunities todirectlyana- beterrestrial contaminates probably de- What should we look for in
ly_ Martiansamplescame from theSNC rivedfromAntarctic icemeltwater that future missions?
(shergottites, nakhlites, andChassigny) had percolated through the meteorite. Anystrategyforinvestigating the pres-
meteorites. Because theycontain trapped Failure todetect endogenous aminoacids ence oforganic molecules onMarsshould
noble gases thataresimilartothose mea- inthisonemeteorite does notnecessarily focus oncompounds thatarereadilysyn-
sured inthe Martianatmosphere bythe ruleoutthepresence ofaminoacids on thesized underplausible prebiotic condi-
Viking spacecraft (/1), the SNCmeteor- Mars,because the extreme temperatures tions, areabundantincarbonaceous mete-
ites are thought tobefragments ofthe duringtheimpactejection should have orites,andplayanessential role inbio-
Martiancrust ejected byimpactevents. destroyed anyamino acidsoriginally chemistry. Amino acids areoneofthe few
The Antarcticshergottite EETA79001 present (14). However, these results do compoundclasses that fulfillallthese re-
isofconsiderable interestbecause itcon- suggest thatthetransfer oforganic com- quirements. They are synthesized inhigh
rainsacarbonatecomponent with600-700 pounds such asaminoacids fromMarsto yields inprebiotic simulation experiments
ppmcombustible carbon,which wassug- Earth,orviceversa, byimpactejecta ap- (15), areone ofthe more abundant types
gested tobe endogenous Martianorganic pears unlikely. oforganic compounds incarbonaceous
material (12). However, analyses ofa The carbonate component ofanother meteorites (Figure 2), are thebuilding
smallfragment ofthe EETA79001 Martianmeteorite collected inthe Antarc. blocks ofproteins andenzymes, andare
carbonate (13)found<1ppbofct-amin- tic,ALH84001,has recently been reported ubiquitous on Earth(14).
oisobutyric acid (bib), anaminoacidcon- tocontainendogenous polycyclic aro- Infutureorganic analyses ofMartian
sidered tobe primarilyofabioticorigin matichydrocarbons (PAHs), which are samples, amajorchallenge isnotjust
(14).Aibisoneofthe mostabundant suggested to beofbioticorigin (4).The identifying andquantifying the organic
aminoacids incarbonaceous meteorites part-per-millionlevels ofPAl-Isreported in compounds thatmay be present, but also
andisreadilysynthesized inlaboratory- thismeteorite greatly exceed the part-per- determining whether the molecules were
based prebiotic experiments, but itisnot billionupperlimitfororganic compounds produced byabioticreactions orarethe
oneofthemajoraminoacidsfoundinthe determinedbyViking. The presence of productofextinctorextantlife.Amino
proteins ofterrestrialorganisms. onlyL-aminoacidsofapparentterrestrial acidhomochirality provides anunambigu-
Martian lifethat could nothave been de- the entire Martian surface isnot known.
rived from seeding the planetwithterres- Terrestrial aminoacidcontamination could
trial life.In contrast, the presence ofrace- greatly compromise assessments of
mieaminoacids--along withamino acids whether anyamino acids are endogenous
such asAiband racemic isovaline (14)- toMars. Because ofthe distinctive L-amino
would beindicative ofanabiofic origin, acidsignature associated withlifeonEarth,
although wehave toconsider the possibil- enantiomeric analyses ofprotein amino
itythat the racemic amino acids were gen- acidscouldbeused tomonitor the levelof
erated fromthe racemization ofbiotic, contamination onMars duringthe course
homochiral amino acids (18). ofplanetary exploration. This requires that
Whenan organismdies, itsaminoacids databe acquired asearly as possible inthe
begin toracemize at arate dependent on Mars exploration program toprovideause-
the particularamino acid,the temperature, fulbaseline dataset forcomparison with
Figure 1, General scheme depicting and the chemical environment (14).Race- future analyses.
the key steps Inthe origin and early mizationreactions are rapid ontheterres-
evolution oflife on Earth. trial geological timescale, and evenat deep Detecting amino acids on Mars
Lifeisdefinedasanautonomourseplicating ocean temperatures of2°C, aminoacids InTable iwehave evaluated the space-
systemthatevolvesbynaturaslelection.
are totallyracemized inabout 5millionto craft worthiness ofaminoacidanalytical
10millionyears. Aminoacidsfroman ex- methods routinely used inthe laboratory
tinctMartian biotamaintained inadry,cold that might be used toperform insitu anal-
pusway ofdistinguishing between abiotic (<250K)environment wouldnothaverace- yses onMars.GC/MS and HPLCare
andbioticorigins. Allknownlaboratory mized significantlyoverthe entire4.5-Gyr aboutequally suitedfor spacecraR instru-
abioticsynthetic processes yield racemic lifetimeoftheplanet.However,complete mentation;however, because ofthe pros-
mixturesof aminoacids, andtheamino racemization wouldhave takenplacein pects forminiaturization, CEappears to
acidsincarbonaceous chondfites arera- environments where liquidwaterwas be thebest choice ofthe three methods.
cemic when terrestrial contamination is presentforperiods ofonlyafewmillion GC/MS isanobviouschoice because of
absent (16). Incontrast, terrestrial organ- yearsfollowingbioticextinction. the success withsimilarinstrumentation
ismsuse L-aminoacids almost exclusively The best preservationof amino acid duringtheV'ddngmissions and because of
inprotein biosynthesis. homochirality associated with extinct the longhistory ofaminoacid analysis by
Amino acid homochirality isan im- Martianlifewould be inthe polarregions. GCinEarth-basedlaboratories. However,
portant aspect ofbiology, because pro- When biogenic aminoacids arecom- anyGC/MS system onfuturemissions
teins cannot foldinto bioactive configu- pletelyracemized, theyare indistinguish- mustbeabletodistinguish between abiotic
rationssuch as the c_-helixor _sheets if ablefromachiralitypointofviewfi'omthe andbioticorigins throughenantiomeric
the amino acids are racemic. However, racemicaminoacidsproduced byabiotic resolution. Eitherchemical derivatizatlon
enzymes made up entirely of D-amino organicsyntheses or those derived from proceduresthatproducediastereomeric
acids function just aswell asthose made exogenous sources. Although wdialkyl derivativesorachiralstationary phase that
upofonly L-aminoacids, but the twoen- amino acids withachiralcenter,which canseparatederivatizedenantiomers would
zymes use the opposite stereoisomeric are common incarbonaceous meteorites be required.
substrates (17). (7), are veryresistant toracemization
There are nobiochemical reasons that (14), these aminoacids arenotusually
L-aminoacids would befavoredover foundinthe proteins ofterrestrialorgan-
D-aminoacids. OnEarth,the use ofonly isms However, wecannotexclude the
L-aminoacids bylifewas probablysimply possibility thatwdialkyl aminoacids
amatter ofchance (16).We assume that if mightbeused bylifeelsewhere. Finding
proteins andenzymes wereacomponent that the amino acidswith an--hydrogen
ofextinctor extantlifeonMars,amino were racemic,whereas forthe wdialkyl
acidhomochirality would have been are- aminoacidswith achiralcarbonthere
quirement. However, the possibility that wasasignificant excess ofone enanti-
Martian lifewas (oris) based onD-amino omer, could suggest thatlife did,orstill
acidswould beequal tothe possibility that does, exist onMars.
itisbased onL-aminoacids. Preventingcross-contaminationbe-
The detection ofanonracemic mixture tween Earthandothersolarsystembodies
ofaminoacidsinaMartiansample would isacentralconcern oftheNASASpaceEx-
Figure 2.Approximate amounts of
be strong evidence forthe presence ofan plorationProgram (19).Althoughthe sur- several classes of organic
extinct orextant biota on Mars.The find- faceoxidantlayercouldretard theaccumu- compounds detected in
ing ofanexcess ofD-aminoacidswould lationofEarth-derivedorganiccontami- carbonaceous chondrltes.
provideirrefutable evidence of unique nates,howwidespread the oxidantisover DatafromReference7.
These derivatization procedures re- plinggeologically recent impactcraters or
quire additional hardware such as reac- Table t. Evaluation of amino exposing fresh materialbyusing explosive
tion chambers, valves, and pumps, which acid analytical techniques for projectilesoffersalternative subsurface
cangreatly increase the size, weight, and spacecraft operation. sampling strategies.
mechanical complexity ofthe GC/MS Once asample has been taken,the
system, although there has been some GC/MSHPLC CE amino acidsmust be separated fromthe
progress inminiaturizing GCcomponents Sensitivity 1 3 3 rock ormineralmatrix. The extraction
(20). MSand thermal conductivity detec- Analysistime 1 2 3 procedure could parallelthatused inlabo-
tors lack the sensitivity needed todetect Weight 2 1 3 ratories onEarthforthe analyses of
amino acids at the sub-part-per-billion Mechanical 1 1 3 amino acidsincarbonaceous meteorites
complexity
level, and although flame ionization detec- Reagentvolume 2 1 3 (27). Apulverizedsample isextracted
tors havegreater sensitivity, they are andstorage with hot water, andaportionofthe super-
probably too unstable and dangerous for Eadseeriovfatization 1 3 3 natantisdesaltedby ion-exchange chro-
use ona spacecraft. Confirmationof 3 1 1 matography, derivatized,andanalyzed.
HPLCismore suited tochiral amino peakidentity Another portionofthe water extract can
acidanalysis than GC.Simple chiral deri- Provenspace 3 1 1 be hydrolyzed inHC1toconvert amino
worthiness
vatization procedures and fluorescence Enantiomeric 2 3 3 acidprecursors toamino acids,which are
detection canbe used toachieve sensitivi- resolution
subsequently analyzed.Although this pro-
ties well below the part-per-billionlevel. Miniaturization 2 1 3 cedure issuitable forlaboratory-based
potential
Reversed-phase IX;with o-phthaldialde- Totalscore 18 17 26 analyses, inaspacecraftinstrument pack-
hyde/N-acetyl-L-cysteine (0PA/NAC) age, acomplex series ofvalves, reagent
derivatization and fluorescence detection Scoring:1=leastsuitable, reservoirs, and reaction chambers would
2=suitable,3=mostsuitable
has been used to search forextraterres- berequired.
trial amino acids inmeteorites (13),lunar Another possible extraction procedure
samples (21), sediments fromthe Creta- wouldbe to sublime theamino acidsdi-
ceous--Tertiary boundary (22), andpolar recdyfrom the sample.Aminoacids have
icecore samples (23). However, the hard- methodology offersthe best potential fora appreciablevaporpressures attempera-
ware isheavy and mechanically complex compact,rugged, low-mass instrument turesinthe range 150-250 °C (28). Thus,
and requires large volumes ofsolvents, packageforinsituamino acidanalyses on amino acidscould beisolatedbyheating
which are alldisadvantages when design- Mars. the sample underpartialvacuum (easily
ing instrumentation fora spacecraft. obtained on Marswhere thesurface pres-
Arelatively newtechnology thatshows Sample acquisition and sure isonly4--6ton')inaclosed chamber
promiseforspacecraft-basedaminoadd preparation interfaced withthe sample acquisition
analysis ismicrochip-based CE (20,24,25), Beforetheaminoacidcontentcanbedeter- component oftheanalytical system main-
which canbeused withthe same chiral mIned, asample ofMars must beacquired, rainedatthe Martiansurface temperature.
derivatization reagents and detection tech- prepared, and delivered tothe insitu analyt- This would requirenoextensive sample
niques used inHPLC,such as laser-in- icalinstrumenL Simplescooping upof manipulation procedures orreagents.
duced fluorescence orelectrochemical de- loose soiland droppingitintoareceptacle However, the possible decomposition of
tection, toachieve the same levelofsensi- onthe analytical Instrument wasused by aminoacids duringsublimation needs to
tivity.The separation hardware, including the Viking landers. However, because of becarefullyinvestigated. (We recently
buffer reservoirs and derivatizatlon reaction the highly oxidizingconditions onthe Mar- conducted anexperiment withalanine at
chambers, canbeetched ontomicrochips tiansurface (9),this method would not 500°Cand abouti tort and foundthat it
withdimensions onthe order ofcentime- likelyyieldsamples containing amino adds, readilysublimed.Although there was
ters (26). Inthese systems,multiple reac- and other samplingmethods are needed. some decomposition, itcouldbemini-
tions canbeperformed under computer One possibility isthe removal ofintact mized byusing lowertemperatures.)
control inafew minutes, consuming only fragments from the interior ofrocks, al-
100nLofreagents. thougharemotely operated technology to Samples returned to Earth
The reagents, sample,and solventscan dothis has notbeen developed. To obtain Acompleteevaluationoftheinventoryof
alsobe manipulated usingtheelectro- samples notaffected bythe oxidant layer, it organiccompoundsthatmaybepresent
osmotic forcesthateffectthe separation, willprobablybe necessary topenetrateto onMars requiresthatsamplesbere-
with noneed formechanical pumps or the deepsubsurface.Although samplecol- turned,especiallyifpriorinsituanalyses
valves Such asystem has great advantages lectionbydrillingtodepthsofameter or are positive. Theoretically, samples re-
over GCor LCsystems inweight, size,and morewas achievedbyRussian unmanned turned toEarth could heanalyzed byany
power requirements. Although theenantio- lunarvehicles, drillingequipmentcapable suitable technique; however, there are
meric resolution ofamino acidsusing mi- ofreachingdepths of10mormore maybe limitations inreturned-sample analyses.
crochip-based CEhas notyet beenexten- necessary for Mars;thispresents major The cost ofasample return mission
sivelyinvestigated, itappears thatthis logistical andtechnological problems.Sam- could limit sampling toonly afewgeologi-
callydistinct sites. The size ofsample that nants thatwillaffectdetection limits of Terrestrialcontamination has limited
can bereturned usingpresently available extraterrestrial organic compounds. Al- the detection ofAlbinlunar soils to ~ 0.1
space transportation technology may limit though this could also be apotential prob- ppb (21)and to ~ 1pptr inpolar ices
the number oflaboratory-based analyses lemforinsitu Martian analyses, lhere are (23).For example, results our ofanalyses
that can be performed and may eliminate waystominimize it.Anyspacecraft land- ofsmall samples ofthe organic-rich
techniques withlarge sample require- ingonthe Martian surface would bere- Murchison meteorite are shown inFigure
ments. Compound-specific organic analy- quired toundergo rigorous decontamina- 3.With a10-ragsample, the extraterres-
ses ofareturned sample might be limited tiontoensure that the planet isnot inocu- trial amino acids Aib and racemic isova-
totechniques that are compatible with latedwithterres_ial organisms andorganic line areclearly preseni, whereas inthe
other areas ofinvestigation, such as min- compounds (19).Reagents used ininsitu 100-pgsample, Albis onlybarely detected
eralogical and stable isotope analyses. analyticalsystems wouldbe extensively compared withthe blank.
The main limitation oforganic analy- purifiedand sterilized before the mission Contamination problems are even
ses ofreturned samples willbe the omni- andprobably transported dry.Water re- more crucialinthe detection ofthe pro-
present problem ofterrestrial contamina- quired foraqueous buffers andsample pro- teinamino acids inextraterrestrial sam-
tion. Eventhe best and most sensitive cessing could be made using anHJO 2fuel ples (16). To detect part-per-billion levels
analytical methods are limited bycontami- celldirectly onthe Martian surface. ofendogenous amino acids with aS/N
Zare's Martian technique that results inlittle fragmenta- life."We've got organics,"said Zare.
measurements tionofthe analytes. "But you canfindorganics inmeteor-
The announcementinAugustofpossible Whatisthe technique? Apulse from itesthatyou don't necessarily think
fossilsofearlylifeonMarsfarsmaller anIRlaser isused todesorb molecules are alive.We've got PAHs. Does this
than eventhesmallestbacteriaonEarth from the samplesurface.The IRpulse itselfprove life?Itdoes not."
willundoubtedly accelerate the search heats the surfacewithout ionization,and The PAHevidence was used in
formore signs oflifeonthe surface of the lowpowerdensity minimizes decom- combination withelectron microscopy
the RedPlanetitself.Whether ornotthe position. Asecond laser pulse, this one of"carbonate globules" found inthe
claimofancient lifestands upunder UV,uses 1+1resonance-enhanced mul- meteorite, which are similar inshape
further scrutiny,RichardN.Zare,a tiphoton ionization, inwhich afirstphoton and textureto carbonate precipitates
chemist atStanfordUniversity,isconfi- promotes the molecule toanexcited state induced bybacteria onEarth. The
dent thatthePAilsinthemeteorite, andasecond photonionizes it,toionize carbonate globules are associated with
which hisgroupfoundusing their micro- preferentiallygas-phase organic mole- magnetite and iron sulfides. The coex-
probetwo-st_laseMrS method cules. Analytes canbeselectively ionized istence ofnoncorroded iron sulfides
(id..,_VIS)r,epresentthe firstmeasured byadjustingthe I.IVwavelength used, and magnetite with partiatly dissolved
organiccompounds ofMartianorigin(40. 266nminthis case. The ions aredetected carbonate isnot easily explained with
Although theoverallprojectwas with atime-of-flight mass spectrometer. inorganic models but can be explained
headed byDavidS.McKayofNASA Zare and his co-workers identified two with abiogenic model. The concentra-
Johnson Space Center,Zareled the collections ofPAHs inthe mass spectra of tion ofPAHsalsopeaked atthe loca-
team responsible forthe mass spectral the meteorite samples. One mass enve- tion ofthe carbonate globules.
analysisofALH84001,ameteorite lope covered the mass range of178-276 Zareand his colleagues are quick
foundinthe AllanHillsofAntarcticain andwas assigned tophenanthrene (178), toacknowledge that noone lineofevi-
1984thathas been determined tobe of pyrene (202),chrysene (2283,perylene or dence points tobiological origins, but
Martianorigin.The MStechnique benzopyrene (252),and anthanthracene they believe that the combination is
they used wasdeveloped byZarefor (278).The second mass envelope ranges difficultto explain inany other way.
the studyofinterplanetary dust patti- from 300to>450with periodicities of "Each thing has a'maybe'associated
des. "It's onlybecause ofthe reputa- both 14and 2,indicating the presence of withit,"saidZare."When you put
tion wegotfor ourwork [with inter- alkylated sidechains. The concentration 'maybes'together, you stillhave 'may-
planetarydust] that wewere ap- profile ofthe PAHs increases withsam- be,'butatsome point, your'maybe'
proached bythepeople fromNASA," piingdepth, ruling outterrestrial orlabo- gets yourconfidence upto the point
saidZare."The ideatostudy this Mar- ratory contamination. where yousayit'slikely. Finally,you
tianmeteorite didnotoriginate with Despite being the first organic mole- get toastage where you say'Idon't
me."The pL2MStechnique canmea- cules associated withMars anddespite knowanother way.There may be an-
sure the PAils inthe samplewith the factthat PAHs are well-known biomar- otherway,but Idon'tknow what itis.'
40-prospatialresolution andsubatto- kers forprehistoric lifeonEarth,the dis- Still,ahealthy skepticism isan impor-
mole sensitivity without requiring a covery ofPAHsinthe Martian meteorite tantpart ofthescientific process."
separation step.It isa"soft" ionization does notinitself suggest the existence of CeliaHenry
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