"Is it possible to have life on other planets? When I say life, I am referring to any type of life-human life, bacteria, etc. "
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Quantum Chemistry Study of Prebiotic Geochemistry Mechanisms: Reduction Reactions on Mineral Surfaces
PI: Jaffe, Richard
Wachtershauser, Russell, Hall and others have hypothesized that many prebiotic chemical reactions were catalyzed by mineral surfaces at hydrothermal vents on the ocean floor during the Hadean and early Archean eras. Geochemical processes could have produced a variety of minerals and feedstock organic molecules from the chemicals outgassing from these hydrothermal vents. This early environment included minerals replete with transition metals and sulfur, including the iron sulfide group (pyrite, machinawite, troilite, greigite) and violarite (similar to greigite, but containing nickel and iron). The structure of some of these minerals can be considered as being built up from smaller units that have analogues in extant Fe-S protein cofactors. Most notable are the clusters Fe 2 S 2, Fe 4 S~4~ (a cubane structure) and Fe8S9(SR)2. Here R is an organic ligand, which can also attach to some or all of the iron atoms in the other clusters. In modern iron-sulfur proteins these clusters are responsible for catalyzing some important basic chemical processes such as dinitrogen fixation and hydrogenation. Also, there are hints from studies of anaerobic bacteria deprived of common nutrient sources, that these cofactors can catalyze a variety of other chemical processes.
In order to evaluate this hypothesis, we propose to carry out a series of electronic structure calculations to characterize the interactions between selected Fe-S clusters and simple adsorbates that would have been present in the prebiotic atmosphere or hydrosphere (e.g., N 2, H 2 , H 2 S, CO, CO 2 , H 2 O, CH 4 , and simple ions such as HS-, HCO 3 - , and CO 32 - ) in order to establish the most probable structures for these clusteradsorbate complexes and the mechanisms for their chemical transformations. In particular, we will study the coupling between oxidative growth of pyrite surfaces and the reduction of CO, CO 2 and N 2 . We propose to use conventional quantum chemistry methods like Density Functional Theory (DFT) to determine the geometries of the complexes coupled with the Quantum Monte Carlo (QMC) method to determine binding energies and activation energy barriers. DFT calculations are more readily applied these systems, but suffer from deficiencies due to truncated atomic orbital basis set expansions and an incomplete treatment of electron correlation effects. On the other hand, QMC calculations do not use basis set expansions and provide a complete accounting of electron correlation. However, they have much greater computer resource requirements. Recent QMC calculations have been used for obtaining accurate total electronic energies for transition-metal systems and for organic molecules containing more than 100 valence electrons. They will be used to calibrate the DFT calculations in the proposed study.
Specifically, we will focus on iron sulfide cluster-catalyzed chemical processes in which CO 2 and N 2 are reduced, leading to nitrogen fixation, carboxylic acid formation and hydrogenation. For these systems, initial complexes formed between the cluster and molecule result from weak chemical interactions that are difficult to describe using most modern theoretical chemistry tools with the exception of QMC. By focusing on small transition metal-sulfide clusters and simple molecular or ionic adsorbates, basic chemical interactions of the initial steps in these prebiotic cycles will be addressed. Thermodynamic and kinetic analysis of the chemical reaction pathways will enable us to directly assess the plausibility of the hypothesis that important prebiotic chemical processes could have been catalyzed by mineral surfaces at hydrothermal vents. This work will also be applicable to the biochemistry of extant organisms.
