2000 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 1999 – JUN 2000
Hydrothermal Organic Synthesis
Experiments designed to explore the potential hydrothermal roots of primitive biochemistry are being performed by researchers at both the Carnegie Institution of Washington and the U.S. Geological Survey under the auspices of the NASA Astrobiology Institute. The prinicipal task areas within this program include (1) the experimentally constrained analysis of the potential for the development of primitive metabolism under conditions that mimic that of deep sea hydrothermal vents (CIW), (2) the relationship between mineral surface topology and the development of homochirality (CIW), and (3) mechanistic studies on the formation of aminoacids, oligopeptides, and polypeptides under hydrothermal conditions (USGS). Integral to the successful completion of these task areas is the development of state-of-the-art experimental methods exploiting flow-through hydrothermal reactors, hydrothermal diamond anvil cell, as well as the development of micro-analytical methods enabling the interrogation of organic reaction progress in-situ, i.e., under conditions of high temperature and pressure. The first year of this project focused on the assembly, testing, and optimization of these instruments; the second year has focused on implementation of these methods to address astrobiological research (see individual task areas).
Task 1. Experimental Search for the Geochemical Roots of Biochemistry (Cody).
This task area focuses on establishing the roots of metabolic chemistry potentially intrinsic to the geochemical environments of deep sea hydrothermal vents. Over the past year we have been successful on a number of fronts. Cody et al. have established a potential entry point into the reductive TCA cycle utilizing transition metal sulfides and reduced carbon bearing fluids. This pathway, named the hydrothermal redox pathway, is not used in any extant organisms but may have been the ignition point for primary metabolism. Cody et al. have begun to explore the potential role of organometallic phases that may be intrinsic to hydrothermal systems as sources of primitive biological energy conversion functionality. The potentially critical catalytic role of such species may be a crucial link between geochemistry and biochemistry at the point of life’s emergence. Sharma et al. have developed new techniques for in-situ study of biochemically relevant reactions at elevated temperatures and pressure utilizing a hydrothermal diamond anvil cell (DAC). Loading nanoliter quantities of sample within the DAC, Sharma et al. have combined optical microscopy and Raman spectroscopy allowing for the analysis of complex reaction kinetics at high T and P. Additionally procedures have been developed to extract organic samples from the DAC following reaction for further analysis using GC/MS allowing us to compare the in-situ data with those previously derived using sealed reactor experiments. Filley et al. have begun studying carbonylation reactions using the high T and P flow reactor; preliminary results point to this method as being an effective tool for deconvolution of the reaction mechanisms. Integrating all of this, we find ourselves within one reaction of demonstrating a purely geochemical carbon fixation pathway that closely mimics the combined acetyl-CoA and reductive TCA pathways. Whether this pathway was the focal point for emergence of primary metabolism remains to be established, but the facility of the reactions makes a strong case for such a pathway lying at pre-enzymatic roots of biochemistry.
Task 2. The Possible Role of Mineral Surfaces in Prebiotic Chiral Selectivity (Hazen).
Life employs left-handed amino acids almost exclusively, with the notable exception of some bacterial cell wall components. The emergence of such chiral selectivity in life from a supposedly racemic prebiotic amino acid pool remains a central problem in origin of life research. In abiotic synthesis optically active products may be synthesized if either the substrate, reagent or catalyst are optically active. No plausible scenario has accounted for the production of a large chiral excess of substrate or reagent in prebiotic amino acid and peptide syntheses. Minerals can provide a simple solution to this problem, because most common minerals feature surfaces that are inherently chirally selective. Quartz, which occurs in both right-handed and left-handed structural variants, has long been recognized in this regard. However, little attention has been focused on the majority of centrosymmetric minerals, including rock-forming silicates, carbonates, sulphates and phosphates, which commonly possess pairs of crystal faces whose surface structures are mirror images of each other. Such surfaces are ideally suited to select and concentrate L- and R- centers in molecules, such as amino acids. Left- and right-handed mineral surfaces occur in approximately equal numbers, so they cannot produce chiral selection on a global scale. However, the origin of life was a local, not a global, event; the first life form arose at a specific place and time. The origin of life is in some respects analogous to the formation of a crystal. In each case nucleation and growth are essential and independent steps. It is reasonable to assume that nucleation of life – the self-organization of molecules (perhaps on a mineral surface) – is rare, perhaps even a singular event. But, once formed, this proto-life likely came to become dominant. Experiments are currently being conducted to test for chiral selectivity on carefully selected minerals.
Amino Acid Synthesis Under Hydrothermal Conditions (Bischoff).
An inherent mark of past studies of hydrothermal amino acid synthesis is the presence of formaldehyde in the short list of primitive starting materials. Formaldehyde is quite easily oxidized, however, and a fundamental notion can be developed questioning its presence in the prebiotic atmosphere at meaningful levels. With that view in mind, and in response to recent literature accounts describing the rapid conversion of CO2 to formic acid on common mineral surfaces in both hydrothermal and sunlight-promoted settings the USGS team has begun studies in hydrothermal media excluding formaldehyde. The experiments conducted at 210°C over 2-3 hr periods included cyanide (CN-), which is also considered a key primitive starting material, but which under the prevailing conditions is rapidly converted to a formic acid salt. Our study shows that amino acids are formed rapidly in the absence of formaldehyde with the product mixture containing glycine, racemic alanine, and racemic aspartic acid (note that the racemic quality of the acids assures that no contamination has taken place.) While further work is underway to confirm these results, this finding appears to put into question the long-held view that the formaldehyde based Strecker synthesis is the basis of prebiotic formation of amino acids. Another experimental series showed that amino acids were not produced when the feed contained formaldehyde but no cynanide. However, when the same system was spiked with glycine, significant quantities of racemic alanine, serine, and aspartic acid were formed. This work is continuing, but a preliminary proposition stemming from these results is that the higher acids are formed from the parent glycine. Such a scenario, if confirmed, would immediately explain why almost all of the essential amino acids are alpha-substituted.
PROJECT MEMBERS:James Bischoff
RELATED OBJECTIVES:Objective 1.0
Determine whether the atmosphere of the early Earth, hydrothermal systems or exogenous matter were significant sources of organic matter.
Develop and test plausible pathways by which ancient counterparts of membrane systems, proteins and nucleic acids were synthesized from simpler precursors and assembled into protocells.
Identify the environmental limits for life by examining biological adaptations to extremes in environmental conditions.
Search for evidence of ancient climates, extinct life and potential habitats for extant life on Mars.
Determine the presence of life's chemical precursors and potential habitats for life in the outer solar system.
Determine (theoretically and empirically) the ultimate outcome of the planet-forming process around other stars, especially the habitable ones.
Define an array of astronomically detectable spectroscopic features that indicate habitable conditions and/or the presence of life on an extrasolar planet.