Monosaccharides and Their Derivatives in Carbonaceous Meteorites

29 Pages·2017·4.13 MB·English

life
Review
Monosaccharides and Their Derivatives in
Carbonaceous Meteorites: A Scenario for Their
Synthesis and Onset of Enantiomeric Excesses
GeorgeCooper1,*,AndroC.Rios1,2,*andMichelNuevo1,3 ID
1 NASA-AmesResearchCenter,MoffettField,CA94035,USA;[email protected]
2 BlueMarbleSpace,10014thAve,Ste3201,Seattle,WA98154,USA
3 BayAreaEnvironmentalResearchInstitute,NASAResearchPark,MoffettField,CA94035,USA
* Correspondence:[email protected](G.C.);[email protected](A.C.R.)
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:5June2018;Accepted:22August2018;Published:27August2018
Abstract: Carbonaceous meteorites provide the best glimpse into the solar system’s earliest
physicalandchemicalprocesses. Theseancientobjects,~4.56billionyearsold,containevidenceof
phenomenarangingfromsolarsystemformationtothesynthesisoforganiccompoundsbyaqueous
and (likely) low-temperature photolytic reactions. Collectively, chemical reactions resulted in an
insolublekerogen-likecarbonphaseandacomplexmixtureofdiscretesolublecompoundsincluding
aminoacids,nucleobases,andmonosaccharide(or“sugar”)derivatives. Thisreviewpresentsthe
documentedsearchforsugarsandtheirderivativesincarbonaceousmeteorites. Weexamineearly
papers,publishedintheearly1960s,andnotetheanalyticalmethodsusedformeteoriteanalysisas
wellasconclusionsontheresults. Wethenpresenttherecentfindingofsugarderivativesincluding
sugar alcohols and several sugar acids: The latter compounds were found to possess unusual
“D” enantiomeric (mirror-image) excesses. After discussions on the possible roles of interstellar
grainchemistryandmeteoriteparentbodyaqueousactivityinthesynthesisofsugarderivatives,
wepresentascenariothatsuggeststhatmostofEarth’sextraterrestrialsugaralcohols(e.g.,glycerol)
weresynthesizedbyinterstellarirradiationand/orcoldgrainchemistryandthattheearlysolardisk
wasthelocationoftheinitialenantiomericexcessesinmeteoriticsugarderivatives.
Keywords: carbonaceousmeteorites;interstellarphotolysis;enantiomericexcess;monosaccharide;
sugars;sugaracid;aldoses;aldonicacid;sugaralcohol;glycerol
1. Introduction
Asubsetofmeteoritesclassifiedas“carbonaceous”preservesavaluableglimpseofearlysolar
systemchemicalandphysicalprocesses. ThisisparticularlytrueoftheCI,CM,andCRclassesof
carbonaceousmeteorites,whichareviewedasthemostunaltered(pristine)sincetheirformation[1].
The majority of carbonaceous meteorites are assigned to the “carbonaceous chondrite” class of
meteorites, and we will therefore refer to them as CCs. Their constituent pre-solar grains [2] are
an intriguing illustration of their antiquity, as they include the oldest known condensates in the
solarsystem,~4.56-billion-year-oldcalcium-aluminuminclusionsintheAllendeandotherCCs[3].
Presolar grains also contain evidence of even earlier events such as the origin of the solar system.
Isotopes(60Fe,etc.) showthatsupernovaelikelyhadaroleintriggeringsolarsystemformation[3–6].
SiC grains, direct condensates from supernova, are found in several CCs such as the Murchison
meteorite[5,7]. TheanalysisofCrisotopesinseveraloxidegrainsfromtheOrgueilCCalsoimplicates
supernovaeinsolarsystemformation[3,8].
Inadditiontosupernovae,thepresenceofmultipleothertypesofstarsat(orbefore)thebirthofthe
solarsystem[2–7]indicatesthatsufficientambientradiationwasavailableforthechemicalsynthesis
Life2018,8,36;doi:10.3390/life8030036 www.mdpi.com/journal/life
Life 2018, 8, x 2 of 29
In addition to supernovae, the presence of multiple other types of stars at (or before) the birth of
Lthifee2 0s1o8l,a8r, 3s6ystem [2–7] indicates that sufficient ambient radiation was available for the chem2oifc2a9l
synthesis of organic compounds. For example, analysis of CCs yielded strong evidence of the role of
gas-phase photolysis in shaping the properties of certain meteoritic organic sulfur compounds,
oforganiccompounds. Forexample,analysisofCCsyieldedstrongevidenceoftheroleofgas-phase
sulfonates acids (R-SO3−) (see Section 3.3.3). Characteristics of the organic phase of CCs provide
photolysisinshapingthepropertiesofcertainmeteoriticorganicsulfurcompounds,sulfonatesacids
evidence that organic synthetic processes were taking place throughout solar system formation and
(R-SO −)(seeSection3.3.3). CharacteristicsoftheorganicphaseofCCsprovideevidencethatorganic
evolut3ion. This is significant for origin of life research as CCs provide the only window to this
synthetic processes were taking place throughout solar system formation and evolution. This is
prebiotic organic chemistry and can be studied in great detail. The most studied CC, to date, is the
significantfororiginofliferesearchasCCsprovidetheonlywindowtothisprebioticorganicchemistry
Murchison meteorite. The fall of this meteorite (in Murchison, Australia, 1969) was directly observed
andcanbestudiedingreatdetail. ThemoststudiedCC,todate,istheMurchisonmeteorite. Thefallof
and it was collected very soon after. This minimized contamination at a time when the lunar
thismeteorite(inMurchison,Australia,1969)wasdirectlyobservedanditwascollectedverysoonafter.
program of NASA had developed protocols for the sampling and collection of very clean
ThisminimizedcontaminationatatimewhenthelunarprogramofNASAhaddevelopedprotocolsfor
extraterrestrial samples. In the last few decades, examination of several CCs has revealed that the
thesamplingandcollectionofverycleanextraterrestrialsamples. Inthelastfewdecades,examination
organic carbon consists of an insoluble “macromolecular” carbon phase, ~50–99% of the total organic
ofseveralCCshasrevealedthattheorganiccarbonconsistsofaninsoluble“macromolecular”carbon
carbon [9–12], as well as multitude of discreet and relatively labile soluble organic compounds
phase,~50–99%ofthetotalorganiccarbon[9–12],aswellasmultitudeofdiscreetandrelativelylabile
[9,10,13,14] that demonstrated an active organic chemistry network in the pre-biological solar
solubleorganiccompounds[9,10,13,14]thatdemonstratedanactiveorganicchemistrynetworkinthe
system. Identified meteoritic organic compounds are numerous [9] and many are biologically
pre-biologicalsolarsystem. Identifiedmeteoriticorganiccompoundsarenumerous[9]andmanyare
relevant. They include amino acids [9,13,14], polyaromatic hydrocarbons nucleobases [15,16], and
biologicallyrelevant.Theyincludeaminoacids[9,13,14],polyaromatichydrocarbonsnucleobases[15,16],
early reports of monosaccharides (Section 2.1) and monosaccharide derivatives [17]. Hereafter, we
andearlyreportsofmonosaccharides(Section2.1)andmonosaccharidederivatives[17]. Hereafter,
will refer to monosaccharides as simply “sugars”, or refer collectively to monosaccharides and their
wewillrefertomonosaccharidesassimply“sugars”,orrefercollectivelytomonosaccharidesand
derivatives as “polyols” (Figure 1). In this article we focus on aldehyde sugars or “aldoses” and their
their derivatives as “polyols” (Figure 1). In this article we focus on aldehyde sugars or “aldoses”
acid and alcohol derivatives. Collectively, these compounds are essential components of DNA,
andtheiracidandalcoholderivatives. Collectively,thesecompoundsareessentialcomponentsof
RNA, polysaccharides, and metabolic processes, and are thought to have been necessary for the
DNA,RNA,polysaccharides,andmetabolicprocesses,andarethoughttohavebeennecessaryforthe
formation of Earth’s first life forms. Glucose and mannose are familiar examples of polyols in
formationofEarth’sfirstlifeforms. Glucoseandmannosearefamiliarexamplesofpolyolsinmodern
modern biological processes, but glyceric acid (and other sugar acids) also has a critical and
biologicalprocesses,butglycericacid(andothersugaracids)alsohasacriticalandubiquitousrolein
ubiquitous role in biology [18]. In this article, we will intermittently use the term “aldonic acid” to
biology[18]. Inthisarticle,wewillintermittentlyusetheterm“aldonicacid”torefertotheacidform
refer to the acid form of a sugar in which all carbons are bonded to an OH group. For example,
ofasugarinwhichallcarbonsarebondedtoanOHgroup. Forexample,glycericacidisthealdonic
glyceric acid is the aldonic acid form of glyceraldehyde, while 2-methylglyceric acid is a “deoxy”
acidformofglyceraldehyde,while2-methylglycericacidisa“deoxy”sugaracid(Figure1).
sugar acid (Figure 1).

Figure 1. Example structures of polyol classes: (1) glyceraldehyde; (2) glyceric acid (an aldonic acid);
Figure1.Examplestructuresofpolyolclasses:(1)glyceraldehyde;(2)glycericacid(analdonicacid);
((33)) ggllyycceerrooll;; ((44)) 22--mmeetthhyyllggllyycceerriicc aacciidd..
A note on pristine meteorites: although meteorites generally categorized as “pristine” do show
Anoteonpristinemeteorites: althoughmeteoritesgenerallycategorizedas“pristine”doshow
relatively less aqueous and thermal alteration, recent observations have reemphasized the fact that
relativelylessaqueousandthermalalteration,recentobservationshavereemphasizedthefactthat
that some do contain evidence of (very localized) aqueous activity. For example, from a study of
that some do contain evidence of (very localized) aqueous activity. For example, from a study of
components of CR2‒3 and CO3.0 carbonaceous meteorites [19], the authors concluded that
componentsofCR2-3andCO3.0carbonaceousmeteorites[19],theauthorsconcludedthatfine-grained
fine-grained material likely underwent small amounts of pre-accretionary aqueous alteration by
material likely underwent small amounts of pre-accretionary aqueous alteration by melted water
melted water ice. The melting could have taken place by multiple mechanisms including shock
ice. The melting could have taken place by multiple mechanisms including shock waves in the
waves in the solar nebula and heating from 26Al inside precursor planetesimals [19]. Another study
solarnebulaandheatingfrom26Alinsideprecursorplanetesimals[19]. Anotherstudyofthevery
of the very pristine CR3.0 chondrite MET 00426 [20] illustrated a close spatial association between
pristineCR3.0chondriteMET00426[20]illustratedaclosespatialassociationbetweenorganicmatter
organic matter and water, and suggested physical and chemical interactions between the two. After
andwater,andsuggestedphysicalandchemicalinteractionsbetweenthetwo. Afteranice-melting
an ice-melting event(s), apparently some fraction of the water-soluble component of organic matter
event(s),apparentlysomefractionofthewater-solublecomponentoforganicmatterwasdistributed
was distributed into surrounding cracks and grain boundaries. It was further suggested that this
intosurroundingcracksandgrainboundaries. Itwasfurthersuggestedthatthisaqueousalteration
aqueous alteration was of sufficient strength to have transformed soluble compounds into insoluble
was of sufficient strength to have transformed soluble compounds into insoluble material. In this
material. In this article, we will point out how very early aqueous-phase reactions, i.e., before those
article, we will point out how very early aqueous-phase reactions, i.e., before those in meteorite
parentbodies(asteroids),mayhaveplayedaroleinshapingtheenantiomericpropertiesofmeteoritic

organiccompounds.
Life2018,8,36 3of29
EnantiomerPropertiesofPolyolsandTheirRelevancetotheOriginofLife
The majority of biological polyols are chiral, i.e., their two mirror images cannot be perfectly
superimposeduponeachother(whereeveryatomcoincides),andeachindividualmemberofsucha
pairiscalledanenantiomer(usuallythe“D”or“L”). Inthisreview,compoundsthatarenotchiralwill
bereferredtoas“achiral”or“non-chiral”. Figure2isanexamplethathighlightstheenantiomersofall
3C–6Caldosesugarsanddenoteswhethertheyarerelativelycommonorrareinbiology. Aproperty
critical for life is the selective use of just one enantiomer of a pair. The chiral monomers (sugars,
aminoacids,etc.) ofbiologicalpolymerssuchasnucleicacidsandproteinsareexclusivelyoneofthe
twopossibleenantiomers, makingtheentirepolymer“homochiral”. InthecaseofRNA,onlythe
“righthanded”or“D”enantiomerofthesugarriboseispresent;inproteins,only“L”enantiomersof
aminoacidsareused. Itisstillamysteryastohowenantiomerpreferencesoriginatedinbiology[21],
however,thephenomenonmayhaveoriginsintheapparentdifficultyofbuildinganabioticreplicating
polymer from racemic monomers, i.e., from compounds composed of equal amounts of their two
enantiomers(“racemization”isdefinedastheprocessofconvertingoneenantiomerintotheother;
ifracemizationgoestocompletion, a1:1“racemic”mixtureisobtained. Ithasbeenshowninone
laboratorysettingthatabioticsynthesesofsignificantlysizedRNApolymersdonotproceedinthe
presenceofracemicribose,astheysufferfrom“enantiomericcross-inhibition”[22]. Thiswouldimply,
ifextrapolatedbacktotheoriginoflife,thataveryearlyandsignificantenantiomerpreferencewas
alreadyinplacebeforethefirstlivingcellsemerged. Ingeneral, thereisascarcityofmechanisms
knowntobecapableofinducingenantiomerasymmetry[23,24](ourtermforunequalenantiomer
abundances),thereforeanimportantendeavoristouncoverplausibleabioticmechanismsorsourcesof
earlyenantiomericenrichments.AswillbediscussedinSections2.2,5and6,allmeteoriticaldonicacids
andthevastmajorityofaminoacidsthatcontainenantiomericexcessesexhibittheexcessesinthesame
enantiomerasthecorrespondingbiologicalclassesofcompounds,theDandLenantiomer,respectively,
whichisanintriguingsimilaritytolife’senantiomerselection. Here,enantiomericexcess(es)(ee)are
determinedusingtheabundancesoftheindividualenantiomers: %ee=[(L−D)/(L+D)]×100.
Life 2018, 8, x 4 of 29

Figure 2. 3C–6C Straight-chain sugar (aldose) enantiomers. The biologically more abundant
Figure 2. 3C–6C Straight-chain sugar (aldose) enantiomers. The biologically more abundant
enantiomers are shown in black, and the relatively rare enantiomers are in red. In some cases, the less
enantiomersareshowninblack,andtherelativelyrareenantiomersareinred.Insomecases,theless
common enantiomer of a pair is still found in biology, e.g., both enantiomers of arabinose are shown
commoneinna bnltaicokm. Herowoefvaerp, athire iLs esntialnltfioomunerd isi nmbuicohl omgoyr,ee c.ogm.,mboonth. Tehnea Dn teinoamnteiormseorfs aorfa tbhirneoossee aanrde shownin
black.Howgaelvacetro,seth aereL meonrea cnotmiommoenr thisanm thuec Lh. morecommon.TheDenantiomersofthreoseandgalactose
aremorecommonthantheL.
2. The Reported History of Meteoritic Polyol Research
2.1. Early Reports of Meteoritic Sugars
Despite the scarcity of meteoritic polyol research, the essential roles of these compounds in
current life and their likely importance at the origin of life warrant a compilation of relevant
published literature. Pre-Murchison meteorite studies on the possible presence of polyols in
meteorites focused almost exclusively on sugars and no other polyol derivatives. Degens and Bajor
[27] presented a short report on the analysis of two meteorites: Bruderheim (an “achondrite”; year of
fall, 1960) and Murray (a CC; year of fall, 1950). Sugars were measured at 20 micrograms/gram (i.e.,
parts per million, or ppm) in Bruderheim and 70 ppm in Murray. However, no analytical details
were given, and chirality was not discussed. A follow-up paper that included the original author
stated that the original analyses were done with 80% ethanol extraction and acid hydrolysis of the
two meteorites [28]. The analytical techniques of the expanded work included thin-layer
chromatography, ultraviolet/visible spectroscopy, as well as infrared and nuclear magnetic
resonance spectroscopy. Meteorite samples were eight CCs, five non-CC chondrites, and one
achondrite. Of the CCs, Murray was the youngest specimen; all others had fallen from 54 to 124
years earlier. Ultraviolet/visible spectroscopy of extracts revealed weak maxima in the region of 225–
275 nm, suggesting tentative evidence of sugars, however, peaks were generally very broad or only
shoulders and simply suggested a complexity of compounds. The final determination of sugars
(qualitative and quantitative) was done by paper chromatography. Detected were the six-carbon
sugars glucose and/or mannose, and much smaller amounts of the five-carbon sugars arabinose and
xylose. For sugars above trace levels, concentrations ranged from 5 to 26 ppm, with mannose and
glucose being the most abundant: The general decrease in order of abundance was mannose,
glucose, xylose, and arabinose. No mention was made of enantiomer ratios of individual
Life2018,8,36 4of29
One of the unusual and relevant properties of CCs is that chiral members of one class of
compoundswillcontaineewhilethoseinotherclassescanbefoundasracemicinthesamemeteorite,
forexample,ee-containingaminoacids[9]andracemiccarboxylicacidsintheMurchisonmeteorite[25].
Wewilladdressthisgeneralquestioninthediscussionofpossiblelocalesandoriginsofpolyolee
(Section5). Whileaminoacidshavebeenthesubjectofalonghistoryofanalysis[9,26],therearefew
reportsofpolyolanalysisinmeteorites. Inthefollowingsectionwewillfocusonthesepublications,
whichincludemolecularidentificationsandtherecentanalysisofenantiomerratiosofindividual
polyols. We will then examine polyol synthesis in laboratory simulations under interstellar-type
conditionsandproposeascenarioforproducingearlysolarsystemenantiomericexcesses.
2. TheReportedHistoryofMeteoriticPolyolResearch
2.1. EarlyReportsofMeteoriticSugars
Despite the scarcity of meteoritic polyol research, the essential roles of these compounds in
currentlifeandtheirlikelyimportanceattheoriginoflifewarrantacompilationofrelevantpublished
literature. Pre-Murchisonmeteoritestudiesonthepossiblepresenceofpolyolsinmeteoritesfocused
almostexclusivelyonsugarsandnootherpolyolderivatives. DegensandBajor[27]presentedashort
reportontheanalysisoftwometeorites: Bruderheim(an“achondrite”;yearoffall,1960)andMurray
(aCC;yearoffall,1950).Sugarsweremeasuredat20micrograms/gram(i.e.,partspermillion,orppm)
inBruderheimand70ppminMurray. However,noanalyticaldetailsweregiven,andchiralitywas
notdiscussed. Afollow-uppaperthatincludedtheoriginalauthorstatedthattheoriginalanalyses
weredonewith80%ethanolextractionandacidhydrolysisofthetwometeorites[28]. The analytical
techniquesoftheexpandedworkincludedthin-layerchromatography,ultraviolet/visiblespectroscopy,
as well as infrared and nuclear magnetic resonance spectroscopy. Meteorite samples were eight
CCs,fivenon-CCchondrites,andoneachondrite. OftheCCs,Murraywastheyoungestspecimen;
allothershadfallenfrom54to124yearsearlier. Ultraviolet/visiblespectroscopyofextractsrevealed
weak maxima in the region of 225–275 nm, suggesting tentative evidence of sugars, however,
peaksweregenerallyverybroadoronlyshouldersandsimplysuggestedacomplexityofcompounds.
Thefinaldeterminationofsugars(qualitativeandquantitative)wasdonebypaperchromatography.
Detected were the six-carbon sugars glucose and/or mannose, and much smaller amounts of the
five-carbonsugarsarabinoseandxylose. Forsugarsabovetracelevels,concentrationsrangedfrom
5 to 26 ppm, with mannose and glucose being the most abundant: The general decrease in order
ofabundancewasmannose,glucose,xylose,andarabinose. Nomentionwasmadeofenantiomer
ratios of individual compounds. In controls and solvents, no sugars or amino acids were found.
However,serinewasfoundinrelativelylargeabundances,whilenon-biologicalcompoundswere
notreported. Asmorerecentanalyseshaveshown,serineisnotamajormeteoriticaminoacid[29].
Murchison meteorite analyses have nearly all shown that in a series of homologous compounds,
rare non-biological members of the series are also prominent and fit within the general pattern of
decreasingabundancewithincreasingcarbonnumber, i.e.,suchaseriesrepresentsanabioticand
thermodynamicprocess[9,10]. Thediscussionofcontaminationintheearlywork(describedabove)
wasexpandeduponinacontemporaneousreview[30]withageneralconclusionthatcontamination
wasalikelyprobleminmultipleaspectsoftheanalyses. Inaddition,themajoranalyticaltechnique,
paperchromatography,wasdescribedas“animpreciseanalyticalmethod”[30]. Itisquitepossible
thatthedifficultyofmeteoriteanalysisinlieuofcontaminationfromubiquitousbiologicalsugars
(includingallofthesugarsreportedabove)discouragedsubsequentresearchinthisareafordecades.
2.2. RecentReportsofMeteoriticPolyolsandTheirEnantiomerProperties
The most recent works on the search for meteoritic polyols reported the finding of one sugar
(dihydroxyacetone),severalsugaralcohols,sugarmono-anddi-acidsanddeoxysugaracids[17,31]
(Figure3).
Life 2018, 8, x 5 of 29
compounds. In controls and solvents, no sugars or amino acids were found. However, serine was
found in relatively large abundances, while non-biological compounds were not reported. As more
recent analyses have shown, serine is not a major meteoritic amino acid [29]. Murchison meteorite
analyses have nearly all shown that in a series of homologous compounds, rare non-biological
members of the series are also prominent and fit within the general pattern of decreasing abundance
with increasing carbon number, i.e., such a series represents an abiotic and thermodynamic process
[9,10]. The discussion of contamination in the early work (described above) was expanded upon in a
contemporaneous review [30] with a general conclusion that contamination was a likely problem in
multiple aspects of the analyses. In addition, the major analytical technique, paper chromatography,
was described as “an imprecise analytical method” [30]. It is quite possible that the difficulty of
meteorite analysis in lieu of contamination from ubiquitous biological sugars (including all of the
sugars reported above) discouraged subsequent research in this area for decades.
2.2. Recent Reports of Meteoritic Polyols and Their Enantiomer Properties
The most recent works on the search for meteoritic polyols reported the finding of one sugar
(dihydroxyacetone), several sugar alcohols, sugar mono- and di-acids and deoxy sugar acids [17,31]
Life2018,8,36 5of29
(Figure 3).

FFiigguurree 33.. SSttrruuccttuurreess aanndd nnaammeess ooff aallll kknnoowwnn ppoollyyoollss iiddeennttiififieedd iinn ccaarrbboonnaacceeoouuss mmeetteeoorriitteess [[1177,,3311]]..
DDeeooxxyy ssuuggaarr aacciiddss aarree mmaarrkkeedd wwiitthh ((**)),, ssuuggaarr aallccoohhoollss aarree uunnddeerrlliinneedd,, aanndd ddiiccaarrbbooxxyy ssuuggaarr aacciiddss aarree
iittaalliicciizzeedd.. TThheeo onnlylys suuggaarr( b(byyd defiefninitiitoino)ni)s isth tehe3 C3Cco cmompopuonudnddi hdyihdyrdoxroyxayceatcoentoe,nae,k ae tkoesteotshea ttheaxti setxsisints
eiqnu eiqliubirliiubmriuwmi twhiitths aitlsd aolsdeoisseo misoerm,gelry, cgelryacledreahldyedhey.de.
The identifications ranged from the two-carbon (2C) ethylene glycol to several six-carbon (6C)
Theidentificationsrangedfromthetwo-carbon(2C)ethyleneglycoltoseveralsix-carbon(6C)
compounds, including a multitude of both biological and non-biological polyols shown in Figure 3.
compounds,includingamultitudeofbothbiologicalandnon-biologicalpolyolsshowninFigure3.
Furthermore, each series of compounds featured a general decrease in abundance with increasing
Furthermore, eachseriesofcompoundsfeaturedageneraldecreaseinabundancewithincreasing
carbon number, as expected for an abiotic synthesis. However, the decrease in abundances from the
carbonnumber,asexpectedforanabioticsynthesis. However,thedecreaseinabundancesfromthe
3C sugar alcohol (glycerol) and 3C sugar acid (glyceric acid) to their respective 4C homologs was
3C sugar alcohol (glycerol) and 3C sugar acid (glyceric acid) to their respective 4C homologs was
unexpectedly large relative to previously observed homologous series of meteoritic compounds
unexpectedlylargerelativetopreviouslyobservedhomologousseriesofmeteoriticcompounds[9,10].
[9,10]. This may have implications for the initial mode of formation of polyols (Section 3).
Thismayhaveimplicationsfortheinitialmodeofformationofpolyols(Section3).
The origins of the meteoritic polyols were attributed to possible aqueous reactions and/or
The origins of the meteoritic polyols were attributed to possible aqueous reactions and/or
photolysis of precursors on interstellar grains. Among the rare, non-biological polyols, the detection
photolysisofprecursorsoninterstellargrains. Amongtherare,non-biologicalpolyols,thedetectionof

branched compounds such as the 4C sugar alcohol 2-hydroxymethylglycerol (HMGly), and the
sugar acid 2-hydroxymethylglyceric acid (HMG) (Figure 3) was supporting evidence of abiotic
chemistry. Inparticular,HMGwasfoundtobemoreabundantthanitstworelativelycommonisomers
(erythronic acid and threonic acid) in at least one meteorite (GRA 95229) and likely in others [31].
Ifaportionofthesetwobranchedcompoundswasformedinaqueousreactions[32–35]aplausible
syntheticmechanismistheadditionofformaldehydetoglyceraldehydetogivethebranchedsugar
2-hydroxymethylglyceraldehydewhichcouldthenundergoacross-Cannizzaroreactiontogiveits
correspondingalcohol(HMGly)andacid(HMG)[36]. Ofcourse,anysugarsoriginallypresenton
the meteorite parent body could simply have been oxidized to their corresponding aldonic acids
in solution, e.g., ribose → ribonic acid. HMGLy is also a product of low-temperature (~15–80 K)
photolysisofinterstellariceanalogs(Section3.2)[37].
The enantiomer analysis of polyols in multiple meteorites revealed a surprising pattern of
increasingD/Lratioswithincreasingcarbonnumber(Figure4). Thepercentexcessofanenantiomer
iscalculatedasshowninSection“EnantiomerPropertiesofPolyolsandTheirRelevancetotheOrigin
Life 2018, 8, x 6 of 29
sugar acid 2-hydroxymethylglyceric acid (HMG) (Figure 3) was supporting evidence of abiotic
chemistry. In particular, HMG was found to be more abundant than its two relatively common
isomers (erythronic acid and threonic acid) in at least one meteorite (GRA 95229) and likely in others
[31]. If a portion of these two branched compounds was formed in aqueous reactions [32–35] a
plausible synthetic mechanism is the addition of formaldehyde to glyceraldehyde to give the
branched sugar 2-hydroxymethylglyceraldehyde which could then undergo a cross-Cannizzaro
reaction to give its corresponding alcohol (HMGly) and acid (HMG) [36]. Of course, any sugars
originally present on the meteorite parent body could simply have been oxidized to their
corresponding aldonic acids in solution, e.g., ribose → ribonic acid. HMGLy is also a product of
low-temperature (~15–80 K) photolysis of interstellar ice analogs (Section 3.2) [37].
The enantiomer analysis of polyols in multiple meteorites revealed a surprising pattern of
Life2018,8,36 6of29
increasing D/L ratios with increasing carbon number (Figure 4). The percent excess of an enantiomer
is calculated as shown in Section “Enantiomer Properties of Polyols and Their Relevance to the
oOfrLigifien” o.fO Lnilfye”D. Oexncleys Dse esxwceesrseeos bwseerrve eodbsineravleddo ninic aaldciodnsico fa≥cid4Cs ,owf ≥h4iCle, dwehoixley dsueogxayr ascuigdasra ancdidcsh airnadl
scuhgiraarl aslucgoahro alslcwoheroelsr awceerme irca[c3e1m].ic [31].

FFiigguurree 44.. TThhee DD//LL eennaannttiioommeerricic rraattiiooss ooff mmeetteeoorriittiicc aallddoonniicc aacciiddss ((FFiigguurree 33)) iinnccrreeaassee wwiitthh ccaarrbboonn
nnuummbbeerr [[3311]].. Theasteriskshowninthe6CvalueisextrapolatedbasedontheD/Ltrendof3C→4C
sugaracidsasreportedin[31].
The smallest (3C) sugar acid, glyceric acid, was found to be approximately racemic in multiple
meteorites. This is significant given the ubiquitous nature and high abundance of D-glyceric acid in
Thesmallest(3C)sugaracid,glycericacid,wasfoundtobeapproximatelyracemicinmultiple
biology. Large D/L ratios are nearly always observed in soils and even the crust of contaminated
meteorites. ThisissignificantgiventheubiquitousnatureandhighabundanceofD-glycericacidin
meteorites (see Supporting Information (SI) in [31]). Relevant to ideas presented in this article, we
biology. Large D/L ratios are nearly always observed in soils and even the crust of contaminated
note here that since glyceric acid contains only one chiral carbon it would appear to be relatively
meteorites (see Supporting Information (SI) in [31]). Relevant to ideas presented in this article,
susceptible to racemization. However, in laboratory experiments simulating plausible meteorite
wenoteherethatsinceglycericacidcontainsonlyonechiralcarbonitwouldappeartoberelatively
parent body aqueous solutions, no racemization was seen in glyceric acid in the timescale (months)
susceptibletoracemization. However,inlaboratoryexperimentssimulatingplausiblemeteoriteparent
of the experiments (SI in [31]). In addition, the high range of temperatures (~70 °C) in the
bodyaqueoussolutions,noracemizationwasseeninglycericacidinthetimescale(months)ofthe
experiments was sometimes held for extended periods (several weeks), and the pH was often
experiments (SI in [31]). In addition, the high range of temperatures (~70 ◦C) in the experiments
brought to 11 (with Na2CO3) to hasten any possible racemization. In the astrophysical setting to be
was sometimes held for extended periods (several weeks), and the pH was often brought to 11
presented below, this might imply that, once formed with a particular D/L enantiomer ratio, glyceric
(with Na CO ) to hasten any possible racemization. In the astrophysical setting to be presented
2 3
acid’s final (observed) enantiomer ratio might reflect the original. But of course, even CC parent
below,thismightimplythat,onceformedwithaparticularD/Lenantiomerratio,glycericacid’sfinal
bodies may vary from relatively gentle to harsh conditions, and harsh conditions may have
(observed)enantiomerratiomightreflecttheoriginal. Butofcourse,evenCCparentbodiesmayvary
racemized any glyceric acid that was initially formed with enantiomeric excess. It is still worth
fromrelativelygentletoharshconditions,andharshconditionsmayhaveracemizedanyglycericacid
noting that in the recent study of meteoritic polyols, glyceric acid was racemic in a variety of CC [31],
thatwasinitiallyformedwithenantiomericexcess. Itisstillworthnotingthatintherecentstudyof
at least suggesting the possibility that its enantiomeric ratio reflects early formation processes.
meteoriticpolyols,glycericacidwasracemicinavarietyofCC[31],atleastsuggestingthepossibility
The chiral 4C aldonic acids erythronic and threonic were analyzed in multiple specimens of
thatitsenantiomericratioreflectsearlyformationprocesses.
Murchison and a sample each of GRA 95229 and ALH 85013. These two acids were found to possess
The chiral 4C aldonic acids erythronic and threonic were analyzed in multiple specimens of
MurchisonandasampleeachofGRA95229andALH85013. Thesetwoacidswerefoundtopossess

an average of 44% D excess in the range of 33–55%. The end-member values came from separate
Murchison specimens, and only one measurement of erythronic acid was reported. The authors
madenoteofthefactthatonemeasurementincludeda43%DexcessofthreonicacidinGRA95229,
ameteoritethatwasdemonstratedtobenearlyfreeofobservablecontamination. However,thisraises
aquestionofwhycertainsugaracids(thealdonicacids)possessenantiomericexcesseswhileothers
do not, i.e., the deoxy sugar acids are racemic [31]. This will be addressed below (Section 8) after
discussionsofpossiblemechanismstogeneratepre-solarenantiomericexcesses.
The5Caldonicacidsribonicarabinonic,xylonic,andlyxonicrevealedevenlarger D excesses,
bothindividuallyandasagroup(Figure5c),thanthe4Caldonicacids. InthemostcleanMurchison
Life 2018, 8, x 7 of 29
an average of 44% D excess in the range of 33–55%. The end-member values came from separate
Murchison specimens, and only one measurement of erythronic acid was reported. The authors
made note of the fact that one measurement included a 43% D excess of threonic acid in GRA 95229, a
meteorite that was demonstrated to be nearly free of observable contamination. However, this raises
a question of why certain sugar acids (the aldonic acids) possess enantiomeric excesses while others
do not, i.e., the deoxy sugar acids are racemic [31]. This will be addressed below (Section 8) after
discussions of possible mechanisms to generate pre-solar enantiomeric excesses.
Life2018,8,36 7of29
The 5C aldonic acids ribonic arabinonic, xylonic, and lyxonic revealed even larger D excesses,
both individually and as a group (Figure 5c), than the 4C aldonic acids. In the most clean Murchison
sample(M39)eerangedfrom57%(ribonicacid)to82%(xylonicacid). Excludingxylonicacid,theee
sample (M39) ee ranged from 57% (ribonic acid) to 82% (xylonic acid). Excluding xylonic acid, the ee
rangenarrowsconsiderably: ribonic,57%;arabinonic,60%;andlyxonic,61%.
range narrows considerably: ribonic, 57%; arabinonic, 60%; and lyxonic, 61%.

Figure 5. Chromatograms of aldonic acid enantiomer analyses of carbonaceous meteorites [31].
(AFi)g3uCre( g5l.y Ccehrriocmacaidto)gernaamnsti oomf aelrdsofnroicm acthide Menuarncthioismoner( Manuarlcyhs)esm oeft ecoarribteo.n(aBc)e4oCuse nmaentteioormiteerss .[3(C1]). 5(AC)
e3nCa n(tgiolymceerrisc. a(Dcid))6 Ceneannatinotmioemrse rfsr,oGmR tAhe0 6M1u00rc.hNisoon6C (MLuerncahn)t imometeeorsriwtee. r(eBf)o 4uCn de.naTnhteioemnaenrst.i o(mC)e r5sC
deenpainctteiodmasergsl.y (cDer)i c6Cac iedniann(tAio)m[3e1rs],m GaRyAa c0tu61a0ll0y. bNeof r6oCm La ecnhairnatlioalmkaelrisn ewoerrep ofolyumnder. rTehaec teionnanptrioodmuecrts
odfegplyiccteerdic aasc igdly(acsersihco awcnidb iyng (lAyc)e r[3ic1]a cmidasyt aancdtuaardllsy inbec afrrboomn aat ecshoirlualt ioanlksa).liHneo wore vpeorl,yomtheerr rreuancstioofn
GpRroAd9u5c2t 2o9f sghlyocwertihc aatcbido t(hasg slyhcoewrinc bayci dglayncedripco ascsiidb lsetarenadcatriodns ipnr ocdarubcotsnaatree sroalcuetmioinc.s). However, other
runs of GRA 95229 show that both glyceric acid and possible reaction products are racemic.
6CSugaracidenantiomeranalyseswerereportedfortwometeoritesMurchisonandGRA06100.
6C Sugar acid enantiomer analyses were reported for two meteorites Murchison and GRA
InMurchison, sixoftheeightpossiblealdonicacidswereidentified: Allonic, gluconic, mannonic,
06100. In Murchison, six of the eight possible aldonic acids were identified: Allonic, gluconic,
gulonic,galactonic,andtalonic. Mannonicacidwasthemostabundant(743nmol/g)followedby
mannonic, gulonic, galactonic, and talonic. Mannonic acid was the most abundant (743 nmol/g)
gluconicacid(243nmol/g). OnlyDenantiomerswerefound,whichwasattributedtothepossibility
thfoaltlotwoteadl abbyu gnludcaonnciecs aicnidth (i2s43g rnomupol/wg)e.r Oensilmy Dp leyntaonotiolomwertso woebrsee rfvouenthde, wLheinchan wtiaosm aettrrsi.buHteodw teov tehre,
dpeosspsiitbeiltihtye tohvaetr atolltalol wabuabnudnadncaensc eins, tthwiso gorfotuhpe w6Cerseu sgimarpalyci dtoso, mloawn ntoon oicbsaecrivde atnhde Lg leuncaonntiicomaceidrs,.
However, despite the overall low abundances, two of the 6C sugar acids, mannonic acid and
werecomparableinabundancestothe5Caldonicacids. Therefore,thequestion: Whywerethereno
gluconic acid, were comparable in abundances to the 5C aldonic acids. Therefore, the question: Why
observableLenantiomersofthe6Cacids,atleastformannonicacidandgluconicacid? Onepossibility
iswtehraet ,tbheecraeu nseo liofbe’ssecravrabboleh yLd erantaenptioolymmeresr soafr ethceo m6Cp oasceiddsp,r eadt olemaisnt aftoerly moafnthneoDniecn aacnitdio amnedr ,galnudcothneic
acid? One possibility is that, because life’s carbohydrate polymers are composed predominately of
lattertwo(D)acidsarecommoninbiology,theywerecontaminantsandthusthelackofobservable
mtheete Do reitnicanLteionmanetri,o amnedr sthsiem laptltyerr etflweoct s(Dt)h iasccidosn taarme icnoamtiomno.nO innt bhieoolothgeyr, hthaenyd ,witeirsep coosnstiabmleitnhaantttsh aesned
6 Cacidswereindeedindigenousandthelackof L enantiomerssimplyfollowsfromtheobserved
(indigenous)progressionofincreasingDexcesses,from4Cto6C(Figure4)[31]. Theauthorspresented
variousliteratureandlaboratoryexperimentsinaddressingthequestionofpotentialcontamination.
Asmentioned,resultsonsixMurchison6Caldonicacidswereshown(excludedwereidonicacidand
talonicacid). ShowninFigure5disatraceofGRA06100datafromthesameworkthatincludesidonic
acid,galactonicacid,andgluconicacid,andagain,noLenantiomerswereobserved. Unexpectedly,
Life2018,8,36 8of29
andincontrasttoMurchison, mannonicacid(andtheremaining6Cacids)inGRA06100iseither
absent or present in only a trace amount. Due to signs of contamination in GRA 06100, only the
6C sugar acids data was regarded as reliable because six of the eight members (including idonic
acid) are biologically rare on Earth. In addition to the above ee-containing discrete compounds,
themoreabundantorganicphaseofCC,themacromolecularcarbon,alsocontainsa(unknown)source
of asymmetry [34]. It was suggested that the increasing ee (and size) of discrete polyols and the
mysteriouseeofthemacromolecularcarbonarelinkedbyacommonsyntheticmechanism[31].
Itshouldbestressedthat,withineachgroup(4C,5C,6C),allofthesugar(aldonic)acidswitheeare
diastereomers(i.e.,stereoisomersthatarenotenantiomers). Thesecompoundsarerelativelydifficult
(butnotimpossible)toracemizeduetotheirpossessionofmorethanonechiralcarbon(SIin[31])and
thereforetheirD/Lratiosarevaluablerecordsofanearlyasymmetricalsynthesisinthesolarsystem.
However,the4Cdeoxysugaracids2,3-erythro-and2,3-threo-dihydroxybutyricacids(Figure3)are
alsodiastereomersbutracemic: Theirchiralpropertiesalsorecordearlysynthesisbutapparentlyone
thattookplaceunderdifferentconditionsfromthoseofthelargeraldonicacids.
3. TheSynthesisofMeteoriticPolyols
In the sections below, we will discuss possible formation pathways (aqueous, photolytic,
hydrogen addition) of polyols, without the addition of detailed chemical mechanisms. However,
wekeepinmindthatotherfactorslikelycontributedtogeneralsyntheticprocesses,e.g.,thereactionsof
radicals,assupportedbytheexaminationoftheCCmacromolecularcarbonphasewhichrevealedthat
radicalsarestilltrappedconstituents,~4.56billionyearsaftersolarsystemformation[38]. Inaddition
tomorestandardreactions,radicalscouldhaveplayedsignificantrolesinpotentialmagnetochiral
processes(Section7).
3.1. MeteoriteParentBody(Aqueous)SynthesisofPolyols
Aqueousreactionsofformaldehydeand/orglycolaldehydemayberesponsibleforthepresence
ofseveralmeteoriticpolyolssuchassugarsand/ortheirderivatives. Alikelyprebioticmechanism
fortheirformation,whetheronEarthorinextraterrestrialsettings,isthe“formosereaction”[39–41].
Thisreaction,whichisactuallyasetofreactions,wastraditionallydescribedasformaldehyde(CH O)
2
undergoingself-additioninstronglyalkaline(highpH)aqueoussolutiontograduallybuildavariety
of hydroxylated compounds of increasing carbon number. Among the products were said to be
glycolaldehyde, ethylene glycol, glyceraldehyde, glycerol, ribose, and other five and six carbon
sugars, including glucose, and ketone sugars. However, the formose reaction was also reported
nottooccurinpuresolutionsofformaldehydeduetotheabsenceofanenolizablecarbonneeded
to initiate aldol chemistry [42]. Instead, small amounts of organic catalysts, e.g., glycolaldehyde
or carbohydrates, were needed [33,42], which created doubt as to the prebiotic plausibility of the
formosereactionintheabsenceofsuchinitiators. Yet,ininterstellarenvironments,formaldehyde
is often found with glycolaldehyde [43,44] and many other molecules (for a list of interstellar
molecules,includingformaldehydeandglycolaldehyde,see[45]). Inaddition,formaldehyde[46],
and glycolaldehyde [47] are constituents of CC and thus a formose-type reaction during aqueous
alterationonameteoriteparentbodywouldbechemicallyplausible.
Historically,thestrongbasecalciumhydroxide(Ca(OH) )isthemostcommonalkalinecatalyst
2
inlaboratoryformosereactions. However,carbonates,i.e.,muchweakerbases,willalsocatalyzethe
reactionbutproductyieldscanbesubstantiallylowerforanygiventemperature[41]. Still,asjudged
fromCCs,theasteroidparentbodieslikelyprovided(atleastperiodically)adequateconditionsfor
theseformose-typereactionstooccur.Inadditiontoaqueousactivityandthepresenceofformaldehyde
andglycolaldehyde,carbonatesarerelativelyabundantinCCsanditisnotuncommontoobserve
up to several percent carbonate in these meteorites [48–50]. In addition, pH measurements of CC
extractsareofteninthe~7–10range,partlyduetocarbonates. However,whilelaboratoryformose
reactionsaregenerallyperformedintemperaturesrangesof~50–100◦C,estimatesofaqueous(water)
Life 2018, 8, x 9 of 29
Life2018,8,36 9of29
from CCs, the asteroid parent bodies likely provided (at least periodically) adequate conditions for
these formose-type reactions to occur. In addition to aqueous activity and the presence of
alteration temperatures in CC include ~0–50 ◦C [51] and ~20–71 ◦C [52]. Measurement of oxygen
formaldehyde and glycolaldehyde, carbonates are relatively abundant in CCs and it is not
isotoperautniocosmomfwona ttoe roibnsecrevret auipn tCo Csesveinradl ipcaertceecnat rcbaorbnoantaetef oinrm thaetsieo nmeatteoar“itleosw [48te–m50p]. eIrna atuddreit”iorna, npgHe of
<150 ◦C [m53e]a.suHreomweenvtse or,f mCCo setxterastcitms aartee osftoefn ainq utheeo ~u7s–1a0l trearnagtei,o pnarttelmy dpueer atot ucarerbionnathtees. mHoowstevperri,s wtinheileC C
favorthellaobworeartoterym pfoerrmatousree rreaancgtieo,n<s 5a0re◦ Cg.eMneertaelloyr itpiecrffoorrmmeods ei-nty tpeemrpeearcattiuorness arta<ng5e0s◦ Cof c~o5m0–b1i0n0e d°Cw, ith
weakercaersbtiomnaatetse ocfa atqaulyesotussm (wigathetr)e axlptelraaitniotnh teemlopwereartuorvees rianl ClrCe liantcilvuedea ~b0u‒n50d a°Cn c[5e1s] oafneda ~c2h0–h7o1m °Co [l5o2g]o. us
Measurement of oxygen isotope ratios of water in certain CCs indicate carbonate formation at a “low
seriesofpolyols,i.e.,sugars,sugaralcohols,andsugaracids[17].
temperature” range of <150 °C [53]. However, most estimates of aqueous alteration temperature in
Surprisingly, the meteoritic lower mass (2C) alcohol (ethylene glycol), the 3C sugar alcohol
the most pristine CC favor the lower temperature range, <50 °C. Meteoritic formose-type reactions at
(glycerol), and the 3C sugar acid (glyceric acid) were notable for their “over abundance” relative
<50 °C combined with weaker carbonate catalysts might explain the lower overall relative
tohighermembersoftheirrespectivehomologousseries[17]. Forexample,the4Csugaralcohols
abundances of each homologous series of polyols, i.e., sugars, sugar alcohols, and sugar acids [17].
(erythritol anSdurtphrriesiintogll)y,w theer emeestteiomriatitce ldowtoerb meaosns l(y2C~)1 a%lcoohfotl h(eethaybluenned galnycceol)o, fthgel y3cCe rsoulg.aAr aslceophaorla te
studyinv(ogllyvcienrgolo), ragnadn tihcec 3oCm spuoguarn adcsidf r(oglmycearirca ancgide) owfeCreR nmotaebteleo froitre tsheoifr d“oifvfeerr eanbutnddeagnrceee”s roelfataiqveu etoo us
alterationhi(gdhieffre mreenmtbpeertsr oogf rtahpeihr ircetsyppecetsiv)ea lhsoomfooluongodutsh siesrsietsr ik[1i7n]g. lFyorc oenxasmistpelne,t tahbeu 4nCd asungcaer paalctotehronls for
(erythritol and threitol) were estimated to be only ~1% of the abundance of glycerol. A separate
sugaralcohols[47],asthemeteoritesrangedfromthoseexhibitingahighdegreeof(past)aqueous
study involving organic compounds from a range of CR meteorites of different degrees of aqueous
alterationtothosethatexperiencedextremelylittle. Fromthelatterstudyitcanbeseenthatthereis
alteration (different petrographic types) also found this strikingly consistent abundance pattern for
nocleartrendacrosstypes,withanaverageabundanceoferythritolrelativetoglycerolandethylene
sugar alcohols [47], as the meteorites ranged from those exhibiting a high degree of (past) aqueous
glycolof~1%(Table1;TableS7in[47]). Theratiosandabundancessuggestthat: (A)morethanone
alteration to those that experienced extremely little. From the latter study it can be seen that there is
mechanisnmo ccleoaur ltdrehnda vaecrocossn ttyrpibesu, tweditht oant haveefroagrem aabtuiondnaonfcet hofe elroywtherirtoml raeslasticvoem top golyucnerdosl ainnde eatchhylsenerei es,
perhaps tghlyrocoul gohf ~a1%co (mTabbilen a1;t iToanbloe fS7a qinu [e4o7u])s. ,Thpeh roattoiolys taincd, aanbudndnaonnc-eps hsuogtogleystt itchacto: l(dA-)g mraoirne tchhaenm oniset ry;
and (B) amquecehoaunsismre accotuilodn hsa(vaet cloenatsrtibaustewd etok tnhoe wfortmheatmio)n wofe trhee alpowpaerr emnatlsys cnoomtpnoeucnedsss ainr yeatcoh fsoerrmies,t he
perhaps through a combination of aqueous, photolytic, and non-photolytic cold-grain chemistry;
above-mentioned smaller polyols, i.e., some of the parent bodies of the meteorites had very little
and (B) aqueous reactions (at least as we know them) were apparently not necessary to form the
evidence of aqueous activity. The studies [47] also reported only trace levels of 5C sugar alcohols
above-mentioned smaller polyols, i.e., some of the parent bodies of the meteorites had very little
(arabinitol,etc.),whentheywerefoundatall(Table1). Theconstantabundanceratioscouldalsoresult
evidence of aqueous activity. The studies [47] also reported only trace levels of 5C sugar alcohols
fromtherelativelystable(unreactive)natureofthesugaralcohols,i.e.,onceinitiallyformedatanearly
(arabinitol, etc.), when they were found at all (Table 1). The constant abundance ratios could also
pre-aqueoreussulstt fargoemt thheey resliamtivpelyly pstearbslies t(uednrtehacrotiuveg)h navtaurrieo oufs thseta sguegsaro aflacoqhuoelos,u i.se.a, lotnecrea tiinoitnia.lly formed at
an early pre-aqueous stage they simply persisted through various stages of aqueous alteration.
Table1.Glycerol/erythritolratiosandabundancesof5Csugaralcoholsversusthedegreeofaqueous
alterationTianbclea r1b. oGnlyacceerooul/esrmythetreitoorl irtaetsio*s. aNndo acbleuanrdtarnecneds oifn 5rCa stiuogsara naldcoahboulsn vdearsnucse tshae rdeesgereeen o.f aqueous
alteration in carbonaceous meteorites *. No clear trend in ratios and abundances are seen.
DeDgreegereoef oAf AququeoeouussA Alltteerraattiioonn

MeteoritMeeteoriteG RO95G5R77O 955P7C7 A9P1C0A82 91082Q UEQ9U9E1 9797177 MurMchuirschoinson EETEE92T0 9422042 MMETET0 000442266 MMIILL 0077552255
(Petrolog(iPceTtryoploeg)ic Type)( 1.3) (1.3) (2.3) (2.3) (2.4)(2.4) (2.5()2.5) (2.5()2.5) (2(.26.)6) ((22..88))
Glycerol/erythritol 89 24 74 100 171 104 70
Glycerol/erythritol 89 24 74 100 171 104 70
5C Alcohols
tr tr/nf nf tr tr tr/nf tr/nf
5CAlcohaoblsundance
tr tr/nf nf tr tr tr/nf tr/nf
abundance* All data from [47] except for Murchison [17]. Meteorites are ordered by petrographic subtype
*Alldata(fproamren[4th7e]seexsc),e apst tfaokreMn ufrrocmhi s[o1]n e[x1c7e]p.tM foert eMoIrLit e0s75a2re5 [o5r4d]e. rOedthebry mpeetteroogrirtaeps hwicithsu qbutaylpiteat(ipvaerlyen ltahrgesee s),
astakenfrgolmyc[e1r]oel/xecreypthtrfiotorlM raILtio0s7 5a2re5 G[5R4]A. O95th2e2r9 manedte oArLitHes 8w5i0t1h3q (ucaallcitualtaivteedly dlaatrag efrgolmyc ererocel/net rpyuthbrliitcoaltiroant ios
areGRA95[3212]9). a5nCd SAugLaHr a8l5c0o1h3ol(sc arlecfuerla ttoe dribdiatotal, farroambinreitcoeln, tanpdu bxlyicliatotilo. ntr [=3 1T]r)a.c5eC leSvueglsa;r nafl =co nhoot lfsoruenfedr; ttor/nrifb =it ol,
arabinitol,andxylitol.tr=Tracelevels;nf=notfound;tr/nf=oneofthethreealcoholswasnotfound.
one of the three alcohols was not found.
3.2. Inters3t.e2l.l aInrtIerrrstaedlliaart iIornradSiyatniothn eSsyisntohfePsios loyfo Plsolyols
The foreTghoei nfgoredgioscinugs sdiiosncusssriaoinsse rtahisee pthoes spibosisliitbyilittyh atthasto smomeef rfarcactitoionn ooff mmeetteeoorritiitci cpoployloylso lwsewree re
synthesizseydntwheeslizlebde fwoerlel bthefeoroen tsheet oonfsmet eotfe moreitteeorpitaer epnartebnot dbyodayq auqeuoeuoussr ereaaccttiioonnss.. MMeetetoeroirtiictiisctiss thsavhea ve
previously concluded that pre-solar material (grains and organic matter) survived the collapse of the
previouslyconcludedthatpre-solarmaterial(grainsandorganicmatter)survivedthecollapseofthe
solar system’s precursor molecular cloud as well as parent body (asteroid) processes [55].
solarsystem’sprecursormolecularcloudaswellasparentbody(asteroid)processes[55].
Although “organic matter” in the above context generally refers to the relatively refractory
Although “organic matter” in the above context generally refers to the relatively refractory
insoluble carbon phase of meteorites [11,12], low-temperature photolytic reactions in interstellar
insoluble carbon phase of meteorites [11,12], low-temperature photolytic reactions in interstellar

environmentscouldprovideasourcefordiscretepolyolproductionandthereforeanexplanationfor
theanomalous(high)relativeabundancesoflowermassmeteoriticpolyolssuchasethyleneglycol,
glycerol, and glyceric acid. In this article, when we refer to laboratory simulations of interstellar
(molecularcloud)photolysiswearereferringtoexperimentsthatgenerallyincludeacombinationof
knownvolatileinterstellarmolecules(e.g.,H O,CO,CO ,CH ,CH OH,etc.) thatcondenseonthe
2 2 4 3
Life 2018, 8, x 10 of 29
environments could provide a source for discrete polyol production and therefore an explanation for
the anomalous (high) relative abundances of lower mass meteoritic polyols such as ethylene glycol,
glycerol, and glyceric acid. In this article, when we refer to laboratory simulations of interstellar
(molecular cloud) photolysis we are referring to experiments that generally include a combination of
Lkinfeo2w01n8, 8v,o3l6atile interstellar molecules (e.g., H2O, CO, CO2, CH4, CH3OH, etc.) that condense o10n otfh2e9
surface of a cold substrate to form “ices” at very low temperatures (~10‒80 K). These ices are then
subject to irradiation (typically, UV photons) for various periods of times before they are warmed to
surfaceofacoldsubstratetoform“ices”atverylowtemperatures(~10–80K).Theseicesarethen
room temperature under static vacuum, at which time they are recovered for analysis.
subjecttoirradiation(typically,UVphotons)forvariousperiodsoftimesbeforetheyarewarmedto
Such simulations have added considerable credibility to the hypothesis that irradiation of
roomtemperatureunderstaticvacuum,atwhichtimetheyarerecoveredforanalysis.
interstellar grains coated with ices of small precursor molecules was capable of synthesizing
Such simulations have added considerable credibility to the hypothesis that irradiation of
prebiotically important organic compounds [56–63]. In the case of polyols, included in the
interstellar grains coated with ices of small precursor molecules was capable of synthesizing
photoproducts are significant amounts of lower mass sugar alcohols and sugar acids [56,64–66],
prebiotically important organic compounds [56–63]. In the case of polyols, included in the
smaller amounts of sugars, and the larger molecular weight sugar acids. Initially, questions were
photoproducts are significant amounts of lower mass sugar alcohols and sugar acids [56,64–66],
raised as to whether the general formation of compounds actually occurred during the warm-up
smaller amounts of sugars, and the larger molecular weight sugar acids. Initially, questions were
procedure (in preparation for analyses) rather than at the temperature of irradiation. Subsequent
raised as to whether the general formation of compounds actually occurred during the warm-up
experiments have been conducted using laser ablation/ionization mass spectrometry and infrared
procedure (in preparation for analyses) rather than at the temperature of irradiation. Subsequent
spectroscopy as in situ analytical techniques that observed the production of compounds in frozen
experimentshavebeenconductedusinglaserablation/ionizationmassspectrometryandinfrared
ice mixtures starting at ~5 K [67,68], demonstrating that organic synthesis can occur at extremely low
spectroscopyasinsituanalyticaltechniquesthatobservedtheproductionofcompoundsinfrozen
interstellar temperatures.
icemixturesstartingat~5K[67,68],demonstratingthatorganicsynthesiscanoccuratextremelylow
interstellartemperatures.
3.3. Formation of Meteoritic Sugar Alcohols: Parent Body (Aqueous) versus Cold Interstellar Synthesis: Are
Meteoritic Sugar Alcohols Primarily Products of Cold Interstellar Synthesis?
3.3. FormationofMeteoriticSugarAlcohols: ParentBody(Aqueous)versusColdInterstellarSynthesis: Are
MeteoriticSugarAlcoholsPrimarilyProductsofColdInterstellarSynthesis?
3.3.1. Sugar Alcohol Production in Standard (Aqueous) Formose Reactions
3.3.1. SugarAlcoholProductioninStandard(Aqueous)FormoseReactions
An attempt to understand, at least qualitatively, the relative significance of the two
abovAe-nmeantttieomnepdt styontuhentdice rmsteacnhda,nisamt sl e(aaqstueqouuasl viteartsivuesl py,hotthoelytriecl)a atnivde tos idginscifiercna nthcee loocfattihoen(st)w oof
asybnotvhee-smise noft imoneetdeosryitnicth peotilcyomlse,c mhainghistm hsav(aeq iumepouliscavteiornsus sfoprh tohteoolyriteics) oann dthteo odriisgcienr onft (hoer lloaccakt ioofn) (tsh)eoirf
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wgloyuceldrabldeevhiyadthe e(Sccrhosesm-Ce a1n):n izzaroreactionofglyceraldehyde(Scheme1):

SScchheemmee 11.. TThhee pprroodduuccttiioonn ooff ggllyycceerrooll iinn tthhee ccrroosssseedd--CCaannnniizzzzaarroo rreeaaccttiioonn [[1144]]..
However, a significant limitation is that glyceraldehyde undergoes this reaction “to only a
However, a significant limitation is that glyceraldehyde undergoes this reaction “to only a
small extent” [32]. The relative rate of the reaction was thought to be a function of pH and base
small extent” [32]. The relative rate of the reaction was thought to be a function of pH and base
(catalyst) strength [32,41], as it was found that rates increased with increasing strength of the
(catalyst)strength[32,41],asitwasfoundthatratesincreasedwithincreasingstrengthofthecatalyst.
Fcaotraleyxsatm. Fpolre ,ewxaitmhpClea,( OwHith) Cthae(ObHra)n2 cthheed bsruangcahrehdy dsurogxayr mhyetdhryolxgylymceetrhayldlgelhyycedreailsdefohrymdee dis, yfoetrmweitdh,
2
yet with NaOH (a stronger base), the corresponding sugar alcohol, hydroxymethylglycerol (HMGly,
NaOH(astrongerbase),thecorrespondingsugaralcohol,hydroxymethylglycerol(HMGly,Figure3)
Figure 3) is formed. These observations underscore the point that even with a relatively strong base
isformed. Theseobservationsunderscorethepointthatevenwitharelativelystrongbasesuchas
Csuac(hO Has) ,Cita(isOoHft)e2n, idt ififis couflttetno odbitfafiicnuslitg ntoifi ocabntatiyni esldigsnoiffiscuagnat ryaileclodhso losfi nsutrgaadri taiolcnoahlofolsrm ino sterraedaictitoionnasl.
2
formose reactions. In fact, the general low abundance of glycerol and higher (≥4C) sugar alcohols in
In fact, the general low abundance of glycerol and higher (≥4C) sugar alcohols in formose-type

reactionsroutinelyallowmanyresearcherstoconverttheircomplexmixturesofsugarsintosugar
alcohols(i.e.,tocreatea“reducedformose”)inordertosimplifyanalysis(e.g.,[33,36]).
GiventhelowyieldsofsugaralcoholsusingastrongbasesuchasCa(OH) inrelativelypure
2
laboratoryformosesolutions,whatyields(ifany)aretobeexpectedonCCparentbodies?Asdescribed