life

Review Molecular Asymmetry in Prebiotic Chemistry: An Account from

Sandra Pizzarello School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA; [email protected]; Tel.: +1-480-965-3370

Academic Editors: Pier Luigi Luisi and David Deamer Received: 8 February 2016; Accepted: 7 April 2016; Published: 13 April 2016

Abstract: (CC) meteorites are fragments of asteroids, solar planetesimals that never became large enough to separate matter by their density, like terrestrial planets. CC contains various amounts of organic carbon and carry a record of chemical evolution as it came to be in the Solar System, at the time the Earth was formed and before the origins of life. We review this record as it pertains to the chiral asymmetry determined for several organic compounds in CC, which reaches a broad molecular distribution and enantiomeric excesses of up to 50%–60%. Because is an indispensable attribute of extant polymers and these meteoritic enantiomeric excesses are still, to date, the only case of chiral asymmetry in organic measured outside the biosphere, the possibility of an exogenous delivery of primed prebiotic compounds to early Earth from meteorites is often proposed. Whether this exogenous delivery held a chiral advantage in molecular evolution remains an open question, as many others regarding the origins of life are.

Keywords: meteorites; asteroids; abiotic; prebiotic; chemical evolution; enantiomeric excess

1. Carbon-Containing Meteorites and Cosmochemical Evolution One of the paradigms of chemical evolution proposes that the abiotic formation of increasingly complex molecules throughout cosmochemical and Earthly processes could eventually lead to life’s precursor molecules. It finds its rationale in the observation of phenomena, both leading to, and proceeding from, the origin of life, such as the formation of complex organic molecules in interstellar media (e.g., [1]), and a terrestrial phylogeny that evolved from much simpler organisms (e.g., [2]). To date, such chemical evolution has found direct experimental scrutiny, solely through the analyses of Carbonaceous Chondrites (CC) meteorites, stony fragments of asteroids containing abundant organic carbon, ranging in complexity from kerogen-like macromolecular materials to simple soluble compounds, several of which are identical to terrestrial . Because CC Asteroidal parent bodies are planetesimals found between Mars and Jupiter that never accreted large enough to undergo the extensive differentiation involved in planet formation, their meteoritic fragments uniquely offer a pristine record of abiotic organic chemistry in the early Solar System, as found in a planetary setting, at a unique juncture in the long history of the biogenic element, and at its closest to the onset of life. Their studies, therefore, have offered an understanding, not only of how compounds identical to life’s molecules were formed abiotically, but also about the organic inventory accreted by early Earth, e.g., through its heavy bombardment from comets and meteorites [3,4]. Comprehensive molecular and isotopic analyses for over forty-five years, and continuing today, have revealed that CCs may contain a complex organic suite of compounds, which are seen as the products of the synthetic capabilities of both solar and pre-solar environments [5]. Their complex range varies from polar compounds, such as amino acids, to non-polar hydrocarbons (e.g., [5–7]) and, when detected simply by their elemental compositions, may include thousands of molecular species [8]. From the perspective of a possible continuity between cosmochemical and prebiotic

Life 2016, 6, 18; doi:10.3390/life6020018 www.mdpi.com/journal/life Life 2016, 6, 18 2 of 9 evolution, this diverse suite would not appear to indicate any selective synthetic trend and, in fact, indicate a fundamental distinction between abiotic and biological chemical processes in their gaining of molecular complexity. To say it with the words of Shakespeare, therefore, a meteoritic organic input of thousands H, C, N, O containing molecules would have offered “too much of a good thing” for an evolutionary transition towards the origins of life. Nevertheless, within this evident heterogeneity, several meteoritic compounds have been found to contain L-enantiomeric excesses (ee), i.e., to be more abundant in the L-chiral form like the amino acids of terrestrial proteins [9], offering the first unequivocal indication that a purely chemical evolutionary process, as the one believed to be responsible for the formation of organics, may also have been at work in chiral selection, a property known since the time of Pasteur to be intimately associated with life. The following review reports on ee distributions found within meteorite organics so far, the possible venues that might have lead to their formation, and the effect that the input of such selected molecules might have had in prebiotic chemistry.

2. Enantiomeric Excesses in Meteoritic Compounds The search for chiral asymmetry in meteorites was first undertaken by Engel and Nagy [10] and, although carried out carefully, was controversial [11] because of the pervasive distribution of homochirality in the biosphere and the possibility of terrestrial contamination in carbonaceous meteorites resulting from bacteria, molds, or their remnants. However, ee were ultimately detected in the for some amino acids not common in terrestrial proteins and abundant in meteorites, the 2-methyl, 2-aminoacids (2maa), and to have the same configuration (L) as protein amino acids [9]. To date, L-ee have been detected for a number of amino acids, as well as other compounds, and their inventory is summarized in Table1[ 12]. As shown, the larger number comprises the amino acids of several meteorites and involves molecular species that lack an α-H, with the exception of the diastereomers of isoleucine, as further detailed below, and the one-time finding for aspartic acid [13]. Lactic acid is the only hydroxy acid to display ee [14], while two recent communications have reported their detection in several sugar acids derivatives [15] and two amines, sec-butyl- and 1-methylbutyl amines [16]. These latter studies, appear to point at the possibility of diverse asymmetric effects in . These meteoritic enantiomeric excesses are still, to date, the only case of molecular asymmetry measured outside the biosphere. The overall extent of ee in 2maa varies significantly, between 0% and 18%, and even within short distances in the same meteorite stone [17,18]. For Isovaline (2-amino, 2-methylbutanoic acid), the most abundant of the acids, the absence of any contribution from terrestrial contaminants within this chiral heterogeneity was validated by Murchison, using 13C-, and D isotopic analyses [17–19], where the two enantiomers were found to have comparable enrichments. Isovaline δD values were particularly high, +3600‰ [19], and near to those measured spectroscopically for organic molecules in the interstellar medium. It should be noted that racemization, the conversion of one enantiomer into another, is possible for 2H-amino acids in water through a repeat statistical abstraction and reacquisition of the α-H and because of its vicinity to the electron-withdrawing carboxyl group. Racemization, with time and proper conditions, will lead to racemic solutions (equal amounts of the two enantiomers). The fact that all amino acids found non-racemic in meteorites have a 2-methyl in place of a 2-H and cannot racemize, easily raises the question of whether the twelve racemizable meteoritic amino acids detected having terrestrial protein counterparts might have initially carried ee but racemized during the water phases known to have affected CC parent bodies. The understanding of the processes involved in meteoritic amino acids’ syntheses and the apparently odd ee findings for diastereomer amino acids in several meteorites may be able to address the question. Life 2016, 6, 18 2 of 9 complex range varies from polar compounds, such as amino acids, to non-polar hydrocarbons (e.g., [5–7]) and, when detected simply by their elemental compositions, may include thousands of molecular species [8]. 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terrestrialL-enantiomeric might process, proteinshave as excesses thelead [9], one to offering( believedee ),their i.e., totheformation, to be be firstmore responsible unequivocal abundant and forthe in the indicationeffectthe formation L-chiral that that form theof meteoritea likeinputpurely the of such selectedchemicalaminoorganics, molecules acids evolutionarymay of also mightterrestrial have process, have been proteins had asat the workin [9],prebioticone in offeringbelieved chiral chemistry. selection,theto befirst responsible unequivocal a property, for theknownindication formation since that ofthe meteoritea timepurely of organics,chemicalPasteur, to evolutionarymay be intimately also have process, associated been asat thework with one life.in believed chiral selection, to be responsible a property, for theknown formation since ofthe meteorite time of Pasteur,organics,The to followingmay be intimately also reviewhave associated been reports at workwithon ee life. indistributions chiral selection, found a within property, meteorite known organics since the so timefar, the of 2. Enantiomeric Excesses in Meteoritic Compounds Pasteur,possibleThe tovenuesfollowing be intimately that review might associated reports have leadonwith ee to life.distributions their formation, found and within the meteoriteeffect that organics the input so offar, such the possibleselectedTheThe search molecules venuesfollowing for that chiral mightreview might asymmetryhave reports have had leadinon prebiotic ee into distributions meteoritestheir chemistry. formation, foundwas andfirst within theundertaken meteoriteeffect that organics bythe Engel input so andoffar, such theNagy [10] and,selectedpossible although moleculesvenues carried that might out might carefully,have have had leadin was prebiotic to controversia their chemistry. formation,l [11] and because the effect of thethat pervasive the input distributionof such of 2. Enantiomeric Excesses in Meteoritic Compounds homochiralityselected molecules in the might biosphere have had and in prebioticthe possibilit chemistry.y of terrestrial contamination in carbonaceous 2. Enantiomeric Excesses in Meteoritic Compounds The search for chiral asymmetry in meteorites was first undertaken by Engel and Nagy [10] meteorites2. Enantiomeric resulting Excesses from bacteria, in Meteoritic molds, Compounds or their remnants. 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Lactic acid is the only hydroxy acid to display ee [14], while two recentα amines,withcomprises secthe-butyl- exception the amino and of acidsthe1-methylbutyl diastereomers of several meteorites amines of isoleuci [16].andne, involvesasThes furthere molecularlatter detailed studies, speciesbelow, appear andthat thelack toone-time an point -H, at the findingwithcommunications the for exception aspartic have ofacid the reported [13]. diastereomers Lactic their acid detection isof the isoleuci only in hydroxy ne,several as further acidsugar todetailed acidsdisplay derivatives below, ee [14], and while [15] the two one-timeand recent two possibility of diverse asymmetric effects in cosmochemistry. These meteoritic enantiomeric excesses communicationsfindingamines,Life 2016 forsec ,aspartic-butyl-6, 18 have andacid reported 1-methylbutyl[13]. Lactic their acid detectionamines is the only [16]. in hydroxy severalThese acidlattersugar to studies, acidsdisplay derivatives eeappear [14], whileto [15]point two and atrecent twothe 3 of 9 are still,amines,communicationspossibility to date, sec of-butyl- diversethe haveonly and asymmetric reportedcase1-methylbutyl of molecu theireffects detectionamines larin cosmochemistry. asymmetry [16]. in severalThes measurede latterThesesugar studies,meteoriticacids outside derivatives appear enantiomericthe biosphere. to [15]point and excesses at twothe possibilityamines,are still, tosec ofdate,-butyl- diverse the and only asymmetric 1-methylbutyl case of molecu effects laramines in asymmetrycosmochemistry. [16]. Thes measurede latterThese outside studies,meteoritic the appear biosphere. enantiomeric to point excesses at the arepossibilityTable still, 1.toTable date,ofAmines diverse 1.the onlyAminesand asymmetric caseamino-, andof molecu effects amino-,hydroxy,lar in asymmetrycosmochemistry. hydroxy, and sugar-ac andmeasured sugar-acidsids These displaying outside meteoritic displayingthe enantibiosphere. enantiomericomeric enantiomeric excessesexcesses excessesin in arecarbonaceous still,Table tocarbonaceous date,1. Amines chondrites. the only and chondrites. case amino-, of molecu hydroxy,lar asymmetry and sugar-ac measuredids displaying outside enanti the biosphere.omeric excesses in Tablecarbonaceous 1. Amines chondrites. and amino-, hydroxy, and sugar-acids displaying enantiomeric excesses in 1 2 CompoundcarbonaceousTableCompound 1. Amines chondrites. Formula Formula and amino-, hydroxy,CompoundCompound and sugar-ac ids displaying eeenanti%ee %omeric1 Distribution excesses inDistribution 2 Compound Formula Compound ee% 1 Distribution 2 carbonaceous chondrites. GRA 95229 3,* Compound Formula Compound ee% 1 DistributionGRA 95229 3,* 2 GRA 95229 3,* LAP 02342 Compound Formula Compound ee% 1 DistributionGRALAP 95229 02342 3, * 2 LAP 02342 (S) (S)(S) SecSec-butylamine-butylamine MIL3, 07525MIL 07525 NH Sec-butylamine 0-18GRALAPMIL 95229 0752502342 * NH2 2 0-18(S) 0-18 Sec-butylamine PCALAPMIL 075250234291082PCA 91082 PCA 91082 NH2 0-18(S) EET 92049 Sec-butylamine PCAMILEET 920490752591082EET 92049 NH 0-18 O O 2 PCAEET 9204991082 2amino2methylbutanoic a. 4 4 L MN, MY *, Or 5 5 O 2amino2methylbutanoic a. L MN, MY *, Or OH 4 4 EET 920496, 5 5 OH 2amino2methylbutanoic2amino2methylbutanoic(isovaline) a. a. 2.5-19.6L LMN,LAP02342 MY *, Or * MN,6, MY *, Or NHO 2 (isovaline) 2.5-19.6 LAP02342 * NH2 OH 4 6, 5 6, NHO 2amino2methylbutanoic(isovaline)(isovaline) a. 2.5-19.6L 2.5-19.6 MN,LAP02342 MY *, Or * LAP02342 * O 2OH 2amino2methylpentanoic L O (isovaline) 2.5-19.6 LAP02342 6,* NH2 OH 2amino2methylpentanoic L MN, MY NHOH 2amino2methylpentanoic(2methylnorvaline) 1.4-2.8L MN, MY O 2OH (2methylnorvaline) 1.4-2.8 MN, MY NHNH2O 2amino2methylpentanoic2amino2methylpentanoic(2methylnorvaline) 1.4-2.8L L 2OH 2amino2methylhexanoic L MN, MY MN, MY O O (2methylnorvaline)(2methylnorvaline) 1.4-2.8 1.4-2.8 MN, MY NH2 OH NH 2amino2methylhexanoic(2methylnorleucine) 1.8L L OOH2OH MN, MYMN, MY NH 2amino2methylhexanoic(2methylnorleucine)(2methylnorleucine) 1.8L 1.8 NH2 2OH MN, MY 2amino2methylhexanoic(2methylnorleucine) 1.8 L Life 2016, 6, 18 NH2 3 ofMN, 9 MY Life 2016, 6, 18 (2methylnorleucine) 1.8 3 of 9

Life 2016, 6, 18 O 3 of 9 O

Life 2016, 6, 18 OH 2amino2methylheptanoic L MN 3 of 9 NHO OH 2amino2methylheptanoic L MN Life 2016, 6, 18 NH2 2amino2methylheptanoic L3 of 9 MN 2 Life 2016, 6, 18 O O OH 2amino2methylheptanoic L MN,MN MY 3 of 9 ONH 2amino3methylpentanoic L MN, MY Life 2016, 6, 18 OOH2OH 2amino3methylpentanoic2amino2methylheptanoic L GRAMN 95229 3 of 9 O OH (isoleucine) 4-50 GRA 95229 MN, MY NH2 NHO 2 2amino2methylheptanoic L MN,MN MY Life 2016, 6, 18 NH OH 2amino3methylpentanoic2amino3methylpentanoic(isoleucine) 4-50L L LAP 02342 3 of 9 2O OH LAP 02342 GRA 95229 ONHO 2OH 2amino2methylheptanoic(isoleucine) L 4-50GRAMN,MN 95229 MY Life 2016, 6, 18 NH O (isoleucine) 4-50 MN, MY 3 of 9 2ONHO 2amino3methylpentanoic DL MN, MY LAP 02342 OH2OH 2amino2methylheptanoic L GRALAPMN,MN 0234295229 MY 2amino3methylpentanoic(isoleucine) 4-50DL GRA 95229 NH2ONHOH (alloisoleucine) 2-60 GRA 95229 H2N O 2OH 2amino2methylheptanoic L GRALAPMN,MN 0234295229 MY H2N 2amino3methylpentanoic(allo(isoleucine)isoleucine) 4-502-60DL LAP 02342 NH2ONHOH2 LAP 02342 O OH 2amino2methylheptanoic L GRALAPMN,MN 0234295229 MY MN, MY OO 2amino3methylpentanoic2amino3methylpentanoic(allo(isoleucine)isoleucine) 4-502-60L D H2NNH2ONHOH 2amino2,3methylbutanoic2amino3methylpentanoic DL GRAMN, 95229 MY GRA 95229 OHOH2 2amino2,3methylbutanoic L GRALAPMN, 0234295229 MY NH 2amino3methylpentanoic(allo((isoleucine)alloisoleucine)isoleucine) 4-502-60DL 2-60 H2N O2OOHOH (2methylvaline) 1.0 GRAMN, 95229 MY LAP 02342 NH2 (2methylvaline) 1.0 GRALAPMN, 0234295229 MY NHNH2 2amino2,3methylbutanoic2amino3methylpentanoic(allo(isoleucine)isoleucine) 2-604-50DL H2N O2OOHOH GRALAPMN, 0234295229 MY O (2methylvaline) 1.0L H NHN NHO2 2amino2,3methylbutanoic2amino3methylpentanoic(allo(isoleucine)isoleucine) 4-502-60DL 2 O2 OHOHOH 2amino2,3methylpentanoic L GRALAPMN, 0234295229 MY H N NHO OH 2amino2,3methylpentanoic2amino2,3methylbutanoic2amino3methylpentanoic2amino2,3methylbutanoic(2methylvaline)(alloisoleucine) 1.4-5.22-601.0DL L MN, MY 2 NHOO2OH2OH 1.4-5.2L GRALAPMN, 0234295229 MY MN, MY NH2 2amino2,3methylbutanoic(alloisoleucine) 2-60L H2N NHO OH 2amino2,3methylpentanoic2amino3methylpentanoic(2methylvaline)(2methylvaline) 1.0D 1.0 MN, MY O 2OHOH LAPMN, 02342 MY NHO allo 1.4-5.2L GRA 95229 NHOO 2 2amino2,3methylbutanoic(2methylvaline)(alloisoleucine) 2-601.0L H2N 2OHOH 2amino2,3methylpentanoicallo L MN, MY O OH 2amino2,3methylpentanoic 2.2-10.41.4-5.2L LAPMN, 02342 MY NHNHO 2amino2,3methylbutanoic(2methylvaline) 1.0L ONH2OH2OH 2amino2,3methylpentanoicallo 2.2-10.4L MN, MY O NHO 2O (2methylvaline) 1.4-5.21.0L L MN, MY NHO2 2OH 2amino2,3methylpentanoic2amino2,3methylbutanoic2amino2,3methylpentanoic L MN, MY MN, MY O OHO 2amino2,3methylpentanoicallo 2.2-10.4L MN, MY NHO 2 1.4-5.2L 1.4-5.2 HO NHO 2OHOH 2amino2,3methylpentanoic2methylglutamic(2methylvaline) 1.0L MN,MN MY NH2 allo L HO O NHONHO OH 2amino2,3methylpentanoic2methylglutamic 2.2-10.41.4-5.23-2L MN NHO 2OH2 2amino2,3methylpentanoic 3-2L MN, MY NH2 allo L O O O 2amino2,3methylpentanoic2methylglutamic 2.2-10.41.4-5.2 MN7, HO NHO 2OHOH L MN,MN MY*, O OH 2amino2,3methylpentanoic 3-2L MN, MY7, O NHO 2 2amino2,3methylpentanoicalloallo 2.2-10.4L L MN *, HO NHOOH2OHOH 2methylglutamicLactic 1.4-5.2L GRA95229MN,MN MY * MN, MY NH2 L O O OHNHO 2amino2,3methylpentanoic2amino2,3methylpentanoicLacticallo 2.2-10.43.0-12-33-2L 2.2-10.4 GRA95229MN 7, *, * HO OH NHO 2OH 2OH 2methylglutamic 3.0-12-3 LAPMN,MN 02342 MY * OH 3-2L O NH 2amino2,3methylpentanoicallo 2.2-10.4L LAP 023427, * HO OOH NHOHO 2O 2OH 2methylglutamicLactic GRA95229MNMN *, * OH OH 3.0-12-3L MN, MY O OHO ONHO 3-2L 7, HO NHOH 2OH 2amino2,3methylpentanoic2methylglutamicThreonicLactic 2.2-10.4D LAPGRA95229MNMNMN 02342 8*, * HO 2OH 3.0-12-33-2L L 8 OHOHO ONHOH Threonic D MN7, HOHOO OHOHO 2OH 2methylglutamic2methylglutamicLactic GRA95229LAPMNMN 02342 *, * MN OH 3.0-12-33-2L 3-2 8 1 OHOHO NH2 Threonic2 DL MN7, ValuesHO HOfrom [12]OHO OHunless OH otherwise indicated;2methylglutamicLactic Meteorites where ee of the givenLAPGRA95229 MNcompoundsMN 02342 *, * are 1 Values from O[12] unless otherwise indicated; 2 Meteorites where3.0-12-3 ee Lof the given compounds are OHOHOHOHONH 3-2 MN 7,8*, found. MN,HO Murchison;OH 2 MY, Murray; ThreonicOr,Lactic Orgueil; * isotopic dataD acquiredGRA95229LAP forMN 02342 individual * found.1 MN, Murchison; MY, Murray; Or, Orgueil;2 * isotopic3.0-12-3 dataL acquired for individual 7, Values fromOHOH3 O[12]OHO unless4 5otherwise6 indicated;7 8 Meteorites where ee of the given compounds7,8 are enantiomers;HO [16];OH OH acid; [18]; [39]; [14];ThreonicLactic [15], this last study detectedD other eeLAPGRA95229-containingMNMN 02342 *, * sugar MN *, enantiomers; 3 [16]; 4 acid; 5 [18]; 6 [39]; 7 [14]; 8 [15], this last study detected3.0-12-3L other L ee-containing sugar 1found. MN,OHOH Murchison;OHO MY, Murray; Or,Lactic Orgueil;2 * isotopic data acquired for individual8 GRA95229 * acids. Values HOfrom [12]OH OHunless otherwise indicated;ThreonicLactic Meteorites where ee Dof the3.0-12-3 givenGRA95229LAP compoundsMN 02342 * are 1acids.enantiomers; OH 3 [16];O 4 acid; 5 [18]; 6 [39]; 7 [14]; 8 [15],2 this last study detected3.0-12-3 other ee-containing8 sugar LAP 02342 * found. Values MN, HOfromOH Murchison;[12]OH OHunless otherwiseMY, Murray; indicated; ThreonicOr, Orgueil; Meteorites * isotopic where eedata Dof theacquired givenLAP compoundsMNfor 02342 individual * are found.1enantiomers;acids.The Values overall MN, fromOH 3extent Murchison; [16];OH[12]O OHunless4 acid;of ee 5 otherwise MY,in[18]; 2maa 6Murray; [39]; indicated;varies 7 [14]; ThreonicOr, 8significantly, [15], Orgueil;2 Meteorites this last * studyisotopic betweenwhere detected eedata D0%of the acquiredotherand given 18%,ee-containing compoundsforMN and individual 8 even sugar are within The overallHO extent of ee in 2maa varies significantly, between 0% and 18%, and even within 1enantiomers;found.acids. Values MN, from 3 Murchison;[16];[12]OH unless4 acid; 5otherwise MY,[18]; 6Murray; [39]; indicated; 7 [14]; Or, 8 [15], Orgueil;2 Meteorites this last * studyisotopic where detected eedata of the otheracquired given ee-containing compoundsfor individual8 sugar are short distancesHO in the sameOH meteorite stoneThreonic [17,18]. For Isovaline (2-amino,D 2-methylbutanoicMN acid), short1The distances overall in 3extent the same4 of ee meteorite5 in 2maa6 stonevaries7 [17,18]. 8significantly,2 For Isovaline between (2-amino, 0% and 2-methylbutanoic 18%, and even within acid), the mostfound.acids.enantiomers; Values abundant MN, from Murchison; [16];OH[12] of unless acid;the acids,otherwiseMY, [18]; Murray; the[39]; indicated; absence [14]; Or,Threonic [15], Orgueil; of Meteorites thisany last contribution* isotopicstudy where detected eedata offrom the acquiredother given Dterrestrial ee-containing compoundsfor individualcontaminants sugar are MN 8 theshort most distances abundant in3 the of same4 the meteoriteacids,5 6 the stoneabsence7 [17,18].8 of any For contributionIsovaline (2-amino, from terrestrial2-methylbutanoic contaminants acid), withinenantiomers;1found.acids.The Values this overall chiralMN, from extent Murchison;[16];[12]heterogeneity unless acid;of ee otherwise MY,[18];in 2maawas Murray; [39]; validated indicated;varies [14]; Or, significantly, [15], Orgueil;by2 Meteorites thisMurchison, last * studyisotopic betweenwhere usingdetected eedata of0% 13 theC-, otheracquiredand givenand 18%,ee-containing D compoundsfor isotopicand individual even sugar analysesare within 1 Values from [12] unless otherwise indicated; 2 Meteorites where13ee of the given compounds are found. MN, withinthe mostacids.Theenantiomers; this overall abundant chiral 3extent [16];heterogeneity of 4 acid;theof ee acids,5 in[18]; 2maawas 6 the[39]; validated varies absence7 [14]; 8significantly, [15], byof thisanyMurchison, last contribution study between usingdetected 0% fromC-, otherand andterrestrial 18%,ee -containingD isotopicand contaminants even sugar analyses within short[17–19],found. distances where Murchison;MN, thein Murchison; the two MY,same enantiomers Murray; meteoriteMY, Or,Murray; were Orgueil; stone found Or, [17,18]. * to isotopicOrgueil; have For co data mparableIsovaline* isotopic acquired enrichments. (2-amino,data for individual acquired 2-methylbutanoic Isovaline enantiomers;for individualδD values3 [acid),16 were]; 4 acid; 5 [18]; [17–19], where the two enantiomers were found to have comparable enrichments.13 Isovaline δD values were shortthewithinparticularly mostenantiomers;acids.The distances this overall abundant 6high,chiral[39 ]; in+3600‰ 73extent [16];[theheterogeneity14 ];of same48 acid;[theof 15[19], ],ee meteoriteacids, this5 [18];andin last2maawas 6near the[39]; study validated stone variesto absence7 [14]; detectedthose [17,18]. 8significantly, [15], measuredbyof other thisany Murchison,For last ee contributionIsovaline-containing spectroscopicallystudy between usingdetected (2-amino, sugar 0% fromC-, otherand acids. andterrestrialfor2-methylbutanoic 18%,ee -containing organicD isotopicand contaminants moleculeseven sugar analyses within acid), in particularly high, +3600‰enantiomers [19], and near were to found those to measured have comparable spectroscopically enrichments.13 for Isovaline organic δ Dmolecules values were in theshortwithin[17–19], interstellarmostTheacids. distances this where overall abundant chiral medium.thein extent the heterogeneitytwo of same theof ee meteoriteacids, in 2maawas the validated stonevariesabsence [17,18]. significantly, ofby any Murchison,For contributionIsovaline between using (2-amino, 0% fromC-, and and terrestrial2-methylbutanoic 18%, D isotopicand contaminants even analyses within acid), theparticularly interstellarThe overall high, medium. +3600‰ extent of ee in 2maa varies significantly, between 0%13 and 18%, and even within shortwithinthe[17–19], mostIt distances shouldthis where abundant chiral be thein noted the heterogeneitytwo of same enantiomersthatthe[19], meteoriteracemization,acids, and was near thewere validatedstone toabsence found thosethe [17,18]. conversito measured byofhave anyMurchison,For coon mparableIsovalinecontribution of spectroscopically one using enantiomerenrichments. (2-amino, fromC-, andterrestrial for2-methylbutanoic intoIsovaline organicD another, isotopic δcontaminants Dmolecules valuesis analysespossible acid), were in shortIt distances should be in noted the same that meteoriteracemization, stone the [17,18]. conversi Foron Isovaline of one enantiomer (2-amino,13 2-methylbutanoicinto another, is possible acid), the[17–19],withinparticularlyfor 2H-aminointerstellarmostThe this where overallabundant high, chiral acids medium.the +3600‰ extent heterogeneitytwo inof water enantiomersthe of[19], ee acids,through inand 2maawas near thewere a validated variestorepeatabsence found those significantly,statisticalto measured ofbyhave anyMurchison, co mparableabstractioncontribution spectroscopically between using enrichments. and 0% from C-, reacquisitionand andterrestrialfor Isovaline18%, organicD isotopicand ofδcontaminants Dmolecules eventhe values analysesα -Hwithin were and in forthe 2H-aminomost abundant acids in of water the acids,through the a repeatabsence statistical of any abstractioncontribution and from13 reacquisition terrestrial ofcontaminants the α-H and withinshortparticularly[17–19],thebecause interstellarIt distances shouldthis whereof high,itschiral be vicinity medium.the in+3600‰ noted theheterogeneity two same toenantiomersthat [19], the meteoriteracemization, andelectron-withdrawi was near were validatedstone to found those the [17,18]. conversito measured byhaveng Murchison,For carboxylcoon mparableIsovaline ofspectroscopically one group. using enantiomerenrichments. (2-amino, Racemization,C-, and for2-methylbutanoic intoIsovaline organicD another, isotopic withδ Dmolecules valuesis time analysespossible acid), wereand in withinbecause this of itschiral vicinity heterogeneity toenantiomers the electron-withdrawi was were validated found to byhaveng Murchison, carboxylcomparable group. usingenrichments. Racemization,13C-, and Isovaline D isotopic withδD values timeanalysesα wereand [17–19],theparticularlyforproper 2H-aminointerstellarmostIt shouldconditions, where abundant high, acidsbe medium.the +3600‰ noted two will inof water thatleadthe [19], racemization, acids,throughto and racemic nearthe a torepeatabsence solutions thosethe conversistatistical measuredof (equal anyon abstractioncontribution amounts of spectroscopically one enantiomer of and thefrom reacquisition two terrestrial forinto enantiomers). organic another, ofcontaminants molecules the is possibleThe-H andfact in particularly[17–19],proper conditions, where high, the +3600‰ twowill enantiomerslead to racemic were solutionsfound to have (equal comparable amounts enrichments. of the13 two Isovalineenantiomers). δD values The were fact withintheforbecausethat 2H-aminointerstellar allIt should thisamino of itschiral acidsbe acidsvicinitymedium. noted heterogeneity in found water tothat [19], the non-racemic racemization, through andelectron-withdrawi was near avalidated torepeat in those the meteorites conversistatistical measuredbyng Murchison, carboxyl haveon abstraction ofspectroscopically aone 2-methyl group. usingenantiomer and Racemization, in C-,reacquisition place and forinto organicofD another, aisotopic 2-H withof molecules the and is time analysespossibleα -Hcannot and in particularlythat all amino high, acids +3600‰ found [19], non-racemic and near to in those meteorites measured have spectroscopically a 2-methyl in place for organicof a 2-H molecules and cannot in the[17–19],forbecauseproperracemize, 2H-aminointerstellarIt shouldconditions, whereof easily its acidsbe vicinitymedium.the notedraises two willin water toenantiomersthatlead the the racemization,questionthroughto electron-withdrawi racemic were aof repeat solutionsfound whether the statisticalconversito have ng (equalthe carboxylco ontwelve mparableabstraction amounts of one group.racemizable enantiomerenrichments. of and the Racemization, reacquisition two meteoriticinto Isovalineenantiomers). another, withofδ Damino the valuesis time αpossibleThe-H acids were andfact theracemize, interstellar easily medium. raises the question of whether the twelve racemizable meteoritic amino acids particularlybecauseforproperthatdetected 2H-amino allIt should conditions,amino ofhaving high,its acidsbeacidsvicinity +3600‰ terrestrialnoted willin found water tothatlead [19], the proteinnon-racemic racemization, throughto andelectron-withdrawi racemic near counterparts a torepeat solutionsin thosethe meteorites conversistatistical measuredmight ng(equal carboxyl onhavehave abstraction amountsof spectroscopically aoneinitially 2-methyl group. enantiomer of and carriedthe Racemization, in reacquisition two place foreeinto enantiomers). but organicof another, racemizeda 2-H withof molecules the and is time possibleαThe -Hduringcannot andfact in detectedIt should having be terrestrial noted that protein racemization, counterparts the conversi might onhave of initiallyone enantiomer carried eeinto but another, racemized is αpossible during fortheproperbecausethatracemize, 2H-aminointerstellar waterall conditions,amino of phaseseasily its acids acidsvicinitymedium. raisesknown willin found water tolead the theto non-racemic throughquestiontohave electron-withdrawi racemic affected aof repeat solutions inwhether meteoritesCC statistical pare ng(equal the ntcarboxyl havebodies.twelve abstraction amounts a 2-methyl group.racemizableThe of andunderstanding the Racemization, in reacquisition two place meteoritic enantiomers). of a of 2-H withof the aminothe and processestime The -Hcannot acids andfact forthe 2H-aminowater phases acids known in water to throughhave affected a repeat CC statistical parent bodies. abstraction The andunderstanding reacquisition of of the the processes α-H and becausethatproperracemize,detectedinvolved allIt should conditions,amino of having in easilyits meteoritic beacidsvicinity terrestrialnotedraises will found amino tothatleadthe the proteinnon-racemic racemization, questionto acids’electron-withdrawi racemic counterparts syntheses of solutions inwhether the meteorites conversi and might ng (equal thethe carboxyl haveonhave twelveapparently amountsof aoneinitially 2-methyl group.racemizable enantiomer of odd carriedthe Racemization, inee two placefindings eeintometeoritic enantiomers). but of another, racemizeda for 2-Hwith diastereomeramino and is time possibleThe duringcannot acids andfact becauseinvolved of in itsmeteoritic vicinity aminoto the acids’electron-withdrawi syntheses andng the carboxyl apparently group. odd Racemization, ee findings for with diastereomer time and properforracemize,thatdetectedtheamino 2H-amino waterall acids conditions,amino having easilyphases in acids acidsseveral terrestrialraises known willin found watermeteorites leadthe to proteinnon-racemic questionthroughtohave racemic may counterpartsaffected aof berepeat solutions in whetherable meteoritesCC statisticalto paremight address (equal thent have havetwelvebodies. abstraction amounts the ainitially question.2-methyl racemizableThe of and understanding carriedthe in reacquisition two place eemeteoritic enantiomers). but of racemizeda of 2-H of theamino the and processes αThe -Hduringcannot acids andfact aminoproper acidsconditions, in several will meteorites lead to racemic may be solutions able to address (equal amountsthe question. of the two enantiomers). The fact thatbecausedetectedracemize,theinvolved waterallA possibleamino of having in phaseseasily its meteoritic acidsvicinity pathway terrestrial raisesknown found amino tothe proposedtheto proteinnon-racemic questionhave acids’electron-withdrawi counterpartsaffected for syntheses ofthe in whether formation meteoritesCC and paremightng thethe nt carboxylof have have bodies.twelveapparently2-amino-, ainitially 2-methyl group.racemizableThe and odd understandingcarried Racemization,2-hydroxy inee placefindings eemeteoritic but of acids racemizeda for of2-H with thediastereomerinamino and meteoritesprocessestime duringcannot acids and that allA possibleamino acids pathway found proposed non-racemic for the in formationmeteorites of have 2-amino-, a 2-methyl and 2-hydroxy in place of acids a 2-H in and meteorites cannot racemize,properthedetectedinvolvedaminois the water addition acidsconditions, having in easilyphases meteoriticin severalof terrestrial raises ammoniaknown will meteorites aminoleadthe to protein and questiontohave acids’ racemic HCN may affectedcounterparts syntheses of tobe solutions ketones whetherable CC to and paremight address and (equalthe thent aldehydes have twelvebodies.apparently amountsthe initially question. racemizableThe in of oddthe understanding carriedthe presence ee two findings eemeteoritic enantiomers). but of racemized waterforof thediastereomeramino (Figure processes The during acids fact 1a, racemize,is the addition easily of raisesammonia the andquestion HCN toof ketoneswhether and the aldehydes twelve racemizable in the presence meteoritic of water amino (Figure acids 1a, detectedthatinvolvedtheaminoReference waterallA acids possibleamino havingin [12]phases meteoriticin acidsandseveral pathwayterrestrial known the found meteorites aminoreferences proposed to proteinnon-racemic have acids’ may therein).counterpartsaffected for syntheses bethe in able formation meteoritesThisCC to and pare might isaddress athe nt reasonableof have have bodies.apparently2-amino-, the ainitially question.2-methyl The hypothesis, and odd understandingcarried 2-hydroxy inee placefindings ee for but ofat acids racemized a leastfor of2-H thediastereomerin someand meteoritesprocesses duringcannot of the detectedReference having [12] and terrestrial the references protein therein).counterparts This mightis a reasonable have initially hypothesis, carried eefor but at racemizedleast some during of the theracemize,aminoinvolvedis the waterA addition acidspossible in phaseseasily meteoriticinbecause severalof pathway raisesammoniaknown all meteoritesamino the proposedto andneeded questionhave acids’ HCN may affected forreagents syntheses oftobethe ketones whetherable formationCC are to andpare abunaddressand the thent ofaldehydesdant bodies.twelve apparently2-amino-, the components question. racemizableThe in and oddtheunderstanding 2-hydroxy presence ofee findingsinterstellar meteoritic of acids waterforof ices the diastereomerinamino (Figure meteoritesandprocesses likelyacids 1a, theamino water acids phases because known all the to needed have affected reagents CC are pare abunntdant bodies. components The understanding of interstellar of icesthe andprocesses likely involveddetectedaminoisReferenceconstituents theA addition acidspossible havingin [12] meteoriticinof andseveralof pathway terrestrialprimitive ammonia the meteorites aminoreferences proposed protein andasteroids acids’ HCN may therein).counterparts for syntheses to[20,21].bethe ketones able formation This toandAl might isaddressandthough athe reasonable ofaldehydes have apparently2-amino-, theproducing initially question. hypothesis,in and oddthe carried 2-hydroxyan presence ee findingsasymmetric ee for but atof acids racemized leastwaterfor diastereomerincarbon some (Figuremeteorites during of and, the1a, aminotheisinvolvedconstituentsReference the waterA addition acidspossible in [12]phases meteoriticinbecauseof severalofand pathwayprimitive ammoniaknown the all meteorites aminoreferences the proposedto andasteroidsneeded have acids’ HCN may therein).affected forreagents syntheses to[20,21].bethe ketones able formation CCThis are to andAlpare abunisaddressandthough athent reasonable ofaldehydesdant bodies. apparently2-amino-, theproducing components question. The hypothesis,in and oddtheunderstanding an 2-hydroxypresence ofee asymmetricfindingsinterstellar for atof acids leastwaterforof ices the diastereomerincarbon some (Figure meteoritesandprocesses oflikelyand, the1a,

involvedReferenceamino isconstituents theA addition acidspossible in [12] meteoriticinbecauseof andseveralof pathwayprimitive ammonia the all meteoritesaminoreferences the proposed andasteroidsneeded acids’ HCN maytherein). for reagentssyntheses to[20,21].thebe ketones able formation This are toandAl isabunaddressandthough athe reasonable ofaldehydesdant apparently2-amino-, theproducing components question. hypothesis,in and oddthe 2-hydroxyan presence eeof asymmetricfindingsinterstellar for atof acids leastwaterfor ices diastereomerin carbonsome (Figure meteoritesand of likelyand, the1a, A possible pathway proposed for the formation of 2-amino-, and 2-hydroxy acids in meteorites isaminoReferenceconstituents the addition acids [12] inbecauseof ofseveraland primitiveammonia the all meteorites referencesthe andasteroidsneeded HCN maytherein). reagents to[20,21].be ketones able This are toAl abunisaddressandthough a reasonablealdehydesdant the producingcomponents question. hypothesis,in the anpresence of asymmetricinterstellar for atof leastwater ices carbon some (Figure and of likelyand, the1a, Referenceconstituentsisamino the addition acids [12] becauseof andof primitive ammonia the all references the asteroidsandneeded HCN therein). reagents to[20,21]. ketones This are Al is abunandthough a reasonable aldehydesdant producing components hypothesis,in the an presence of asymmetricinterstellar for atof leastwater ices carbonsome (Figure and of likelyand, the1a, A possible pathway proposed for the formation of 2-amino-, and 2-hydroxy acids in meteorites aminoReference acids [12] because and the all referencesthe needed therein). reagents This are abunis a reasonabledant components hypothesis, of interstellar for at least ices some and oflikely the is constituents the addition of of primitiveammonia andasteroids HCN to[20,21]. ketones Al andthough aldehydes producing in the an presence asymmetric of water carbon (Figure and, 1a, amino acids because all the needed reagents are abundant components of interstellar ices and likely constituentsReference [12] of and primitive the references asteroids therein). [20,21]. This Al isthough a reasonable producing hypothesis, an asymmetric for at least carbon some ofand, the constituents of primitive asteroids [20,21]. Although producing an asymmetric carbon and, amino acids because all the needed reagents are abundant components of interstellar ices and likely constituents of primitive asteroids [20,21]. Although producing an asymmetric carbon and,

Life 2016, 6, 18 4 of 9

A possible pathway proposed for the formation of 2-amino-, and 2-hydroxy acids in meteorites is the addition of ammonia and HCN to ketones and aldehydes in the presence of water (Figure1a, Reference [12] and the references therein). This is a reasonable hypothesis, for at least some of the amino acids because all the needed reagents are abundant components of interstellar ices and likely constituentsLife 2016, 6 of, 18primitive asteroids [20,21]. Although producing an asymmetric carbon and, therefore,4 of 9 a chiral in most cases, this type of synthesis is non-stereospecific because the HCN addition therefore, a chiral molecule in most cases, this type of synthesis is non-stereospecific because the wouldHCN be randomaddition andwould give be equalrandom amounts and give ofequal D-, amounts and L-enantiomers. of D-, and L-enantiomers.

Figure 1. (a) The Strecker synthesis for the possible formation of amino acids in meteorites ([12] and Figure 1. (a) The Strecker synthesis for the possible formation of amino acids in meteorites ([12] references therein); (b) Structural relation of the alloisoleucine and isoleucine diastereomers; (c) and references therein); (b) Structural relation of the alloisoleucine and isoleucine diastereomers; Enantiomeric excesses detected for the isoleucine diastereomers in the GRA 95229 meteorites using (c) EnantiomericGas Chromatography-Mass excesses detected Spectrom for theetry, isoleucine single ion diastereomers traces [22]. in the GRA 95229 meteorites using Gas Chromatography-Mass Spectrometry, single ion traces [22]. However, reaction products become more complex for longer aldehydes that already contain an asymmetric carbon, such as in the synthesis of the 6-C isoleucine (ile) and alloisolucine (allo) diastereomers (Figure 1b) from DL 2-methylbutanal [12]. In this case, were an ee already present in

Life 2016, 6, 18 5 of 9

Life 2016, 6, 18 5 of 9 However, reaction products become more complex for longer aldehydes that already contain anthe asymmetricprecursor aldehyde, carbon, suche.g., of as the in (S) the configuratio synthesis ofn, thethose 6-C diastereomeric isoleucine (ile) product and allo aminoisolucine acids ( allothat) diastereomerscarried the (S) (Figure portion1b) of from the DLprecursor 2-methylbutanal through th [ 12eir]. synthesis In this case, will were be anmoreee already abundant present than in their the precursorrespective aldehyde, enantiomers e.g., and of the any (S) original configuration, ee would those be revealed diastereomeric in different product compounds. amino acids that carried the (S)In portion the above of theexample, precursor this throughwould be their the synthesis(RS) allo and will (SS) be more ile compounds abundantthan or, in their the respectiveformalism enantiomersused for amino and acids, any original D-allo andee would L-ile. As be revealedcan be seen, in different these two compounds. sets of diastereomers are also each other’sIn theracemization above example, product this would(called be epimerization the (RS) allo and in (SS)this ilecase,) compounds and, because or, in the the formalism 3C is too used far forremoved amino acids,from the D-allo carboxyland L-ile. and As will can racemize be seen, these much two more sets ofslowly, diastereomers if at all, areoriginal also each ee may other’s be racemizationpreserved much product longer. (called epimerization in this case,) and, because the 3C is too far removed from the carboxylSuch was and the will distribution racemize much found more in the slowly, extracts if at of all, a originalgroup ofee pristinemay be preservedmeteorites much collected longer. in AntarcticaSuch was (Renazzo-type the distribution or CR) found (Figure in the1c, [22,23]) extracts and, of a based group on of the pristine above meteorites formative collectedpremise, inas Antarcticawell as upon (Renazzo-type confirming orthe CR) indigeneity (Figure1c, of [ 22 all,23 four]) and, individual based on enantiomers the above formative by 13C isotopic premise, analyses as well as[22], upon the confirming findings were the indigeneity interpreted of to all signify four individual that their enantiomers precursor aldehyde by 13C isotopic carried analyses an original [22], theS-asymmetry findings were to the interpreted meteorite’s to signifyparent thatbody. their CR precursormeteorites’ aldehyde organic carriedcomposition, an original therefore, S-asymmetry offers a tonew the exciting meteorite’s account parent of body.cosmochemical CR meteorites’ evolution’s organic capabilities composition, toward therefore, the selective offers a production new exciting of accountvery large of cosmochemicalabiotic ee, of up evolution’s to 60% in capabilities the case of toward the isoleu the selectivecine diastereomers, production ofin verysome large meteorites abiotic ee(Table, of up 1). to 60% in the case of the isoleucine diastereomers, in some meteorites (Table1).

3.3. The The Likely Likely Locals Locals for for Asymmetric Asymmetric Syntheses Syntheses TheThe originorigin of ofee eein thein the amino amino acids acids of meteorites of meteorites has been has long been debated long debated since their since initial their detection, initial anddetection, is still and not is known still not (e.g., known [24]). (e.g., The [24]). first hypothesesThe first hypotheses to be put to forward be put proposedforward proposed their possible their asymmetricpossible asymmetric decomposition decomposition upon UV upon photolysis UV photolysis [25] and, following[25] and, thefollowing hint of the amino hint acids’ of amino high isotopicacids’ high enrichments isotopic inenrichments deuterium, thein likelihooddeuterium, of the their likelihood syntheses inof cold,their deuterium-enrichingsyntheses in cold, cosmicdeuterium-enriching environments [7cosmic]. The twoenvironments enantiomers [7]. of aTh chirale two molecule enantiomers have identical of a chiral chemical molecule properties, have butidentical differ inchemical some physico-chemical properties, but differ features, in e.g.,some polarizability physico-chemical [26], and, features, as a result, e.g., maypolarizability react/interact [26], differentlyand, as a result, with other may chiralreact/interact molecules differently or entities wi (ath common other chiral illustration molecules could or be entities that of (a the common distinct waysillustration two people could canbe that join of hands the distinct for different ways aims).two people can join hands for different aims). TheThe effectseffects ofof UVUV circularlycircularly polarizedpolarized lightlight (CPL)(CPL) irradiation,irradiation, whichwhich cancan havehave right-handedright-handed oror left-handedleft-handed helicalhelical pathspaths of of propagation propagation (Figure (Figure2), 2), were were discussed discussed and and studied studied in detail in detail [ 27 ][27] and and for aminofor amino acids acids in particular. in particular. Several Several experiments experiments did show did UVCPL show irradiation UVCPL generatingirradiation adsorptiongenerating bandsadsorption by the bands compounds by the compounds with the same with the sign same as the sign irradiating as the irradiating CPL [28 CPL], and [28], even and over even a over large a wavelengthlarge wavelength range range [29], however,[29], however, the maximum the maximumee achieved ee achieved were were found found to be, to atbe, most, at most, 9% 9% (at an(at unlikelyan unlikely pH pH 1 [30 1 ]),[30]),i.e., i.e. far, lowerfar lower than than found found in meteorites. in meteorites.

Figure 2.2. Graphic representation ofof LeftLeft andand RightRight CircularlyCircularly PolarizedPolarized Light.Light.

These experimental data leave open the possibility that ee were first formed in precursor molecules, such as the aldehydes mentioned above, which would also justify the findings for ile and allo. Aldehydes have not been studied in this regard, e.g., their circular dichroism is not known,

Life 2016, 6, 18 6 of 9

These experimental data leave open the possibility that ee were first formed in precursor molecules, such as the aldehydes mentioned above, which would also justify the findings for ile and allo. Aldehydes have not been studied in this regard, e.g., their circular dichroism is not known, nor is their behavior upon CPL irradiation, and the field is, in fact, yet open to inquiry. However, the proposal of their participation as precursors to the syntheses of amino acids in meteorites would draw the important inference that, because chiral aldehydes racemize quickly in water, in fact, much more rapidly than amino acids, the synthetic window for amino acids in meteorites from non-racemic aldehydes during the asteroidal water processes should have been short and icy [22]. Early parent body aggregates or protoplanetary disc environments would fit the scenario, with ammonia abundance possibly providing a lower freezing point of water. Although the scenario also implies that meteoritic ee would be rare and could be detected for 2-amino acids only in the special cases, such as for the diastereomer compounds described above, the hypothesis does not explain the ee for 2maa, of which precursors would be ketones that cannot racemize. Additionally, the large heterogeneity of ee values demonstrated throughout these studies of meteoritic chiral compounds is very puzzling and has brought other proposals that the formation of ee could be related to asteroidal parent body effects during aqueous stages [12,31] and, in particular, catalysis from their mineral phases. This was first addressed to justify the large variability of isovaline ee between contiguous fragments of the Murchison meteorite and, by their X-ray diffraction analyses, showed a possible correlation between isovaline ee and hydrous silicates abundances [16]. In the case of CR meteorites mentioned above, that their aqueous processes are found to be variable within the meteorite matrix [31] would also satisfy the observation of varying ee detected between meteorites’ samples. The recent findings of two non-racemic amines in several CR2 meteorites with ee as high as 60% [15] were able to further this discussion. Because, as for amino acids, amines’ circular dichroism [15] and anisotropy spectra [32] do not encourage suggestions of possible asymmetric effects by UV CPL, it has been proposed that the ketones precursor to these amines, while not chiral, could have benefited by adsorption upon mineral phases possessing asymmetry. Such minerals, e.g., magnetite seen to have in meteorites helical structures [33], could then provide chiral induction during syntheses. For 2-methylamines, it could have been a reductive amination process made possible by the large abundance of ammonia known to be present in this type of meteorites, e.g., [34]. If further confirmed experimentally, processes like these would point to possible important roles for Asteroidal bodies, their aqueous history and the minerals ensuing from warm hydrous stages in the development of chiral asymmetry in prebiotic molecules.

4. Prebiotic Chiral Asymmetry and the Origins of Life The origins of life are utterly unknown, have been debated for close to a century, and the only achieved consensus is one of life likely emerging from inanimate molecules, about 3.5 billion years ago and by diverse possible scenarios, such as syntheses in the early atmosphere, the deep Earth or by input of organics materials from comets and meteorites (e.g., [5,35,36]). As mentioned already, and as Cronin and Chang [37] remarked pointedly, the known meteoritic inventory discussed above “ ... is clearly not the recipe for constructing the progenote ... and a basic challenge resides in finding ... where diversity that characterizes chemical evolution ... gives way to the selectivity of life”. The question therefore is: Has the molecular asymmetry of meteoritic compounds changed the odds of an exobiology? One distinguishing characteristic of Earth life is that the structure and functions of biopolymers rely on the exclusive one-handedness of their monomeric components, i.e., most protein amino acids have an L-configuration while sugars in RNA and DNA have a D-configuration and substitutions along the polymers with enantiomers of opposite handedness usually result in loss of function. That the trait of molecular asymmetry could be, or become by evolution, so pervasive as to define life Life 2016, 6, 18 7 of 9 processes and that several meteoritic compounds may have ee of the same L-configuration as terrestrial proteins do seem to provide a new dimension to this debate. However, the overall data also pose a caveat. As we have seen, the asteroidal aqueous environments, as recorded in the mineralogy of all meteorites containing organic compounds of prebiotic interest, tend to racemize either the α-H amino acids or their aldehyde precursors, making ee for these amino acids scarce in the mature parent bodies. The α-substituted species that do not racemize would be more abundant but possibly less reactive. On the other hand, their ketone precursors do not racemize at all and could, in fact, have been very desirable prebiotic reactants. Moreover, some of the CC meteorites, such as the Renazzo-type, contain, not only abundant amino acids, but also large abundance of ammonia [38–40], an indispensable reactant for their syntheses. The combination of mineral catalysts with exogenously delivered key ingredients and chiral compounds, therefore, may still represent a very good evolutionary chance in prebiotic chemistry.

Acknowledgments: The author is grateful for the near forty years of funding from the NASA Exobiology and Astrobiology Division and the current support from the National Aeronautics and Space Administration under Agreement No. NNX15AD94G for the program “Earths in Other Solar Systems”. Conflicts of Interest: The authors declare no conflict of interest.

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