The possibility of parent body aqueous syntheses seems confirmed by the likelihood that at least some of Murchison amino acids were formed via a Strecker-like reaction of precursor aldehydes and ketones, ammonia and HCN Fig. These are compounds in which two carboxyl-containing alkyl chains are bonded at the same amino group and would likely result from a Strecker synthesis, e. However, there are isotopic as well as molecular trends within the Murchison organic suite that reveal significant formative distinctions between individual compounds and cannot be accounted for by any simplified model.
Because a similar branched versus linear difference in D-enrichment was also observed between 3- and 4-amino isomers, it seems reasonable to assume that branched molecular species were processed in cold environments to a different degree than the linear ones. On the other hand, 2-, 3- and 4-amino acids also show different trends of 13 C abundance with increasing chain lengths, which decreases in the case of the 2-amino acids while remains level, or even slightly increases, in the case of the others.
That is, within each level of D-enrichment, various processes of chain elongation seem to have been possible. The obvious conclusion from these Murchison detailed analyses is that diverse cosmic regimes and synthetic processes might have participated in producing the organic composition of this type of meteorites. The isotopic analyses of CR meteorites added to the above scenario. Because of the near absence of molecular 15 N values for cosmic environments 3 , only theoretical considerations can be offered for the CR2 findings.
The ones offered by Charnley and Rodgers , , describe a mechanism for higher nitrogen fractionations in regions of the ISM, where the enhanced density and pressure that precede star formation would cause the freeze-out of most carbon- and oxygen containing molecules; with their disappearance, the disruption of N 2 formation pathways in clouds of lesser density would result in a prevalence of gas-phase atomic nitrogen. These predictions are interesting in that they appear to match, albeit in broad terms, the findings in meteorites and the current interpretation of meteoritic amino acid formation.
Very little is known of the molecular sequence of events that would have taken place in a prestellar core; however, we can expect that several stages of temperature, pressure, and ensuing chemical regimes followed the initial collapse of the presolar portion of the ISM e.
We could hypothesize, therefore, that some of the warmer stages of star formation might have allowed selected environments, where the desorption, mixing, and reactions of radical, precursor molecules, water, and ammonia led to the syntheses of higher 15 N amino acids and favored shorter molecular species formation. It also appears that such locals and the kinetic processes they allow to envision could, rather than parent body reactions, explain some of the molecular distributions seen in the CR meteorites, such as: the far from unity diastereomer ratios seen for the thermodynamically similar amino acids allo ile and ile Chaban and Pizzarello , their erratic levels of enantiomeric excesses as well as the preponderance of lower chain length species and the abundance of unreacted carbonyl containing molecules Pizzarello and Holmes Meteorites probably present just a minuscule sample of the prebiotic potential of cosmic synthetic processes but, through their studies, we may be able to infer how common or widespread they may be.
Transformations of organic compounds, or their synthesis from inorganic compounds, occurs in response to thermodynamic drives, modulated by the kinetic properties of individual reactions. Setting aside the mechanistic details for a moment, it is useful to examine how reactions may or may not be favored by the thermodynamic properties of the system. Reactions involving organic compounds and occurring in aqueous solution may have occurred on meteorite parent bodies, smaller icy aggregates on their way to form asteroids or comets, and in selected prestellar environments; therefore, investigating relative stabilities of aqueous organic compounds may yield clues to these processes.
This approach can help to answer specific questions about relative abundances of organic compounds found in carbonaceous meteorites. The following discussion illustrates this approach, with the specific goal of understanding the relative abundances of ammonia, amino acids, and aldehydes. Stabilities of amino acids relative to other organic compounds during aqueous alteration can be assessed by considering a set of hypothetical overall reactions involving amino acids and other aqueous organic compounds.
As an example, the stability of alanine relative to the aldehyde propanal can be assessed by considering a reaction in which carbon is conserved in the two aqueous organic compounds, given by 1 where the aq indicates that the compounds of interest are all dissolved in H 2 O. This reaction is not meant to depict a specific synthetic process, but instead delineates relative stabilities.
The Organic Composition of Carbonaceous Meteorites: The Evolutionary Story Ahead of Biochemistry
It is evident from reaction 1 that there could be abundances of NH 3 aq that would favor the stability of alanine relative to propanal. Likewise, at strongly reduced conditions, where there may be considerable H 2 aq present, alanine would become unstable relative to propanal and NH 3 aq. Quantifying the activities and concentrations of NH 3 aq and H 2 aq , where such transformations become possible, can be accomplished by considering the equilibrium constant for reaction 1 , and manipulating its law of mass action expression.
With this assumption, Equation 2 can be rearranged to give 3 which represents the equation of a line on a plot of log aNH 3 aq vs log aH 2 aq with a slope of 2 and an intercept equal to At constant temperature and pressure, log K is a constant, which means that various lines can be determined based on the activity ratio of alanine to propanal. The bold contour labeled 1 in each plot shows the position of equal activities of the two organic solutes at equilibrium.
Ranges of relative predominance of propanal and alanine are indicated, with that of alanine in each plot falling at higher activities of NH 3 aq and lower activities of H 2 aq , consistent with Le Chatlier's principle applied to reaction 1.
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Also shown in these diagrams are the values of log a H 2 aq , at which hematite Fe 2 O 3 would be reduced to magnetite Fe 3 O 4 , consistent with 4 and where magnetite would be reduced to the ferrous silicate fayalite Fe 2 SiO 4 in the presence of quartz SiO 2 according to 5 Magnetite, which is one of the aqueous alteration products identified in CI, CM, CO, CR, CV meteorites and some LL3 chondrites Zolensky et al.
Selected contours of the equilibrium ratio of activities of alanine to propanal from to 0.
Equilibrium constants for reaction 1 were calculated with the revised Helgeson-Kirkham-Flowers equation of state Shock et al. Also shown are activities of H 2 aq corresponding to equilibrium between hematite and magnetite HM, reaction 4 , as well as magnetite, quartz, and fayalite FMQ, reaction 5.
Thermodynamic data for minerals come from Helgeson et al. The presence of magnetite brackets the equilibrium activities of H 2 aq that could have attained during at least a portion of the aqueous alteration processes occurring on the Murchison parent body. If, on the other hand, temperatures were warmer, these activities would change.
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In dilute solutions, activities of neutral solutes correspond closely to concentrations Amend and Shock These plots reveal the ranges of H 2 aq concentrations that are consistent with the occurrence of magnetite, and the NH 3 aq concentrations that would provide a thermodynamic drive for the formation of an amino acid rather than an aldehyde, and vice versa. Whether or not equilibrium is actually attained among organic compounds during aqueous alteration events on meteorite parent bodies, the persistent thermodynamic drive to form amino acids or aldehydes depends on the chemical composition of the system.
The plots in Figure 7 reveal the quantitative nature of those thermodynamic drives. They also make it possible to begin to understand the amounts of NH 3 aq that would be required if amino acid concentrations were similar to aldehyde concentrations or vastly different. These data can be combined with the thermodynamic analysis depicted in Figure 7 in an attempt to evaluate what conditions were like during aqueous alteration, if the relative abundances of aldehydes and amino acids were influenced by that stage of meteorite history.
It should be kept in mind that the data that exist are for what is present in the meteorites and not what may have been present in aqueous solution during the alteration process. Adsorption equilibria among solutions and various mineral phases may differ for these two classes of organic compounds, and much could have happened to alter ratios inherited from such an early stage in the history of the solar system.
Let us assume that the relative abundances of organic compounds in meteorite extracts reflects conditions on the parent bodies at the time the compounds formed and that they were not radically reset by subsequent history. If the amino acid to aldehyde ratio is a result of aqueous alteration, then it provides us with this locus of possibilities in log a NH 3 aq versus log a H 2 aq space.
Likewise, ratios from GRA and Murchison indicate that conditions during aqueous alteration may have generated conditions near or slightly above the equal activity contour. If there were estimates of the activity of either H 2 aq or NH 3 aq that prevailed during aqueous alteration, then the equilibrium value of the other would be uniquely defined by the ratio of organic compounds.
Assuming that the relative abundances of ammonia in the extracts are analogous to the relative abundances during aqueous alteration leads to the following assessment of relative oxidation-reduction redox states during aqueous alteration events.notdiwapudend.ml
The Organic Composition of Carbonaceous Meteorites: The Evolutionary Story Ahead of Biochemistry
The amino acid to aldehyde ratios in GRA and Murchison are about equal, but the abundance of ammonia that can be extracted from GRA is much greater. Therefore, it seems likely that conditions during aqueous alteration of the Murchison parent body would plot at a lower activity of NH 3 aq than those that attained during alteration of the GRA parent body. If so, then the fact that both meteorites fall on about the same contour means that the activity of H 2 aq was much greater during alteration of GRA than during alteration of Murchison if alteration processes happened at similar temperatures on both parent bodies.
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The abundance of ammonia in the LAP extracts is nearly as great as the GRA extracts, but the amino acid to aldehyde ratio is also greater. If the temperature of alteration of LAP was similar to that of GRA, then conditions during the alteration of LAP would fall somewhat lower in log a NH 3 aq , but also considerably lower in the activity of H 2 aq to maintain the higher amino acid to aldehyde ratio.
All else being equal, indications are that conditions were most oxidized during alteration of the Murchison parent body, most reduced during alteration of GRA, and intermediate during the alteration of LAP. Corroborating evidence may be found in the relative abundances of carboxylic acids, which are more oxidized than either amino acids or aldehydes.
Redox conditions during alteration directly affect the potential for abiotic organic synthesis Shock ; a ; Shock and Schulte ; ; Amend and Shock ; Shock and Canovas If the analysis outlined above survives deeper scrutiny, the overall potential for abiotic organic synthesis from inorganic starting compounds may have been greatest on the GRA parent body, despite its lower abundances of amino acids.
There are several ways that these predictions of relative redox states can be tested. One would be to examine the mineralogy of the alteration products in all three meteorites for evidence of mineral assemblages that could indicate redox conditions that prevailed during alteration. Another would be to seek evidence from mineral assemblages, organic compound associations, and isotopes oxygen in pairs or suites of minerals that formed together, for example that could bracket the temperatures of the alteration events on each parent body so that quantitatively appropriate versions of the plots in Figure 7 could be built.
Also, experimental studies of the adsorption of ammonia, amino acids, aldehydes, and other organic compounds commonly extracted from meteorites on minerals found in meteorite alteration assemblages would enable estimation of aqueous concentrations or activities from the abundances of these compounds in the meteorites. If we trust the record of impact craters observed in most of solar planets and satellites, meteorites have showered the Earth throughout geological ages and certainly did so soon after its accretion e. Abundant organic materials were just as certainly delivered to the early Earth and, it is reasonable to assume, a good portion of them survived the process.
We have learned from the analyses of two largely different types of meteorites that this exogenous input delivered both complex macromolecules of uncertain composition and free soluble compounds. Various molecular species must have interacted in the meteorites already prior to their fall, to a certain degree, because some derivative compounds such as the carboxamides Cooper and Cronin are released from their extracts upon hydrolysis; however, peptides have been carefully searched for in the Murchison meteorite and not found, with the exception of diglycine Shimoyama and Ogasawara If we are trying to estimate the potential of this delivery for prebiotic evolution and we believe that such evolution had to gain some polymeric complexity for life to ensue, then, we have to conclude that the bulk of meteoritic compounds could have provided, at best, monomeric constituents.
In general, however, any evolutionary path has to rely on monomeric material as well and, just comparing with other early planetary processes that could have led to organic compounds such as atmosphere-mediated Miller-Urey-type syntheses or the environment of hydrothermal vents, the molecular species ready-made in meteorites would not appear as too bad of a start.
Of these, meteoritic amino acids appear as likely candidates for further molecular evolution, particularly considering their selective and abundant suites found in CR2 chondrites. Amino acids, the components of extant proteins, are able to polymerize under a variety of laboratory conditions and could have done so in early Earth environments. Also Leman, Orgel, and Ghadiri showed that the presence of carbonyl sulfide, such as it is found around volcanoes, could lead to easy formation of peptides. When of the type found nonracemic in Murchison, amino acids readily form an activated carboxyl, e.
These findings suggest that it is plausible that exogenous amino acids acquired at least some polymeric complexity during early terrestrial evolution; it is as likely that their overall molecular properties might have been evolutionary factors as well. For example, all ee found so far for meteoritic amino acids have just one configuration, l -, whereas those obtained for chiral molecules in natural processes, designed experiments, or via theoretical schemes are all subjected to chance outcome in the absence of asymmetric influences.
Similarly, several terrestrial crystals such as quartz are chiral but their world-wide production is about equal in d - and l -forms.