Prebiotic Fingerprints

Citation

Chimiak, Laura Marie (2021) Prebiotic Fingerprints. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0hvh-xz81. https://resolver.caltech.edu/CaltechTHESIS:01102021-045431757

Abstract

Meteorites contain organic compounds that occur in all known life. These compounds, commonly referred to as prebiotic compounds, include α-amino acids and are most prevalent on carbonaceous chondrites. As carbonaceous chondrites are pristine samples from early in the solar system that have not had living organisms on them, we can study the chemistry that produced α-amino acids on them to better understand the processes by which they might have formed on early Earth or on other bodies. Multiple syntheses have been put forth as routes to form amino acids on meteorites and include ice-grain chemistry on interstellar ices and Strecker synthesis in meteorite parent bodies. Prior measurements of molecular-average carbon isotope ratios (¹³C/¹²C) have found ¹³C enrichments of up to 53‰ in certain α-amino acids and molecular-average hydrogen isotope ratios (D/H) have found enrichments of 100s of ‰. With this data, it has been suggested that Strecker synthesis—a synthesis in which an aldehyde or ketone reacts with ammonia and cyanide to produce an α-aminonitrile that is hydrolyzed into an α-amino amide and then an α-amino acid—is the primary pathway to produce α-amino acids on aqueously altered meteorites.

Here, we develop an instrument that can measure site-specific isotope ratios (SSIR) for carbon — that is the ¹²C/¹³C at each site in a molecule — and use it to first constrain the site-specific isotope effects associated with Strecker synthesis and then the carbon SSIR of an alanine sample extracted from the Murchison meteorite. The instrument, the Q-Exactive Orbitrap, is a Fourier Transform Mass Spectrometer that has resolution of 240,000 full width-half maximum and can measure site-specific carbon isotope ratios on samples as small as 1 picomole. When we use it to measure the carbon SSIR in multiple samples of alanine produced by Strecker synthesis, we find a -20 ‰ equilibrium isotope effect between the product alanine's C-2 site (amine carbon, ¹³C-depleted) reactant acetaldehyde’s carbonyl carbon (¹³C-enriched), a potential -15 ‰ kinetic isotope effect on the C-1 site (eventual carboxyl carbon) for the first hydrolysis of α-aminopropanenitrile (¹³C-enriched) into alaninamide (¹³C-depleted), and a -15.4 ‰ kinetic isotope effect on the C-1 carbon for the second hydrolysis step in which α-alaninamide (¹³C-enriched) becomes alanine (¹³C-depleted). Through conventional isotope ratio mass spectrometry, we also measure a +56.4 ‰ equilibrium isotope effect between ammonia (¹⁵N-depleted) and the amine site on alanine (¹⁵N-enriched). When we measure the sample of alanine from the Murchison meteorite, we find site-specific carbon isotope ratios of -29 ± 10 ‰, 142 ± 20 ‰, and -36 ± 20 ‰ for the C-1, C-2, and C-3 (methyl) sites, respectively. This pattern agrees with the hypothesis that Strecker synthesis created alanine in Murchison. Combining these data with the isotope effects found for Strecker synthesis, we find initial site values of -7 ± 10 ‰, 162 ± 20 ‰, and 36 ± 20 ‰ for the C-1, C-2, and C-3 sites, respectively. With these values, we create a model of potential organic synthesis on the Murchison parent body that predicts the molecular-average δ¹³C values of 19 other prebiotic compounds.

Finally, we create a model that uses the previously measured molecular average carbon and deuterium isotope ratios for organics on Murchison to create models that predict site-specific and molecular average isotope ratios for organic compounds. This model finds that organic compounds with have methyl sites that are enriched in deuterium by up to 3000 ‰ relative to other sites in the compound and that the degree of enrichment scales both with a compound class’s solubility in water and with a sample’s degree of aqueous alteration and terrestrial weathering. These patterns suggest that a primordial ISM-derived deuterium signal exchanges with water and that the methyl site hosts the highest amount of this enrichment due to its low acidity. The carbon model demonstrates that using only the aldehyde and cyanide values measured on Murchison and isotope effects inferred from other studies, we can predict 59 of 82 organic compounds on it (72%) that have δ¹³C values spanning over 149 ‰ with an average residual of 6 ‰. To achieve this level of prediction, the model combines Strecker synthesis, reductive amination, and oxidation of aldehydes to create straight-chain α-H hydroxy and amino acids, amines, and monocarboxylic acids with subsequent formaldehyde addition to these compounds to create branches.

Thesis Files