2:45 PM - 3:00 PM
[MIS17-05] Chemical networks and organic syntheses in geochemical cycles on prebiotic Earth
Earth’s life is believed to emerge through chemical evolution from simple molecules to complex organic molecules with functions. Previous work showed, however, all the complex organic molecules, such as proteins, phospholipids, and nucleotides, cannot be produced in one environment under a single physical-chemical condition (e.g., temperature, pH, redox, and availability of light). This strongly suggests that the transport of materials through geochemical cycles among different environments (e.g., hydrothermal systems and shallow water) would have been important for formation of complex organic molecules and their assembling. However, there are no studies that investigated the chemical networks and prebiotic chemistry during transportation among different environments.
Here, we report our results of chemical network modeling of organic syntheses from simple molecules in the atmosphere (such as, CO2, CH4, and N2) and their photochemical products (such as, HCHO and HCN) to complex molecules during transportation in four distinct environments with different temperature and pH (i.e., low temperature and low pH, high temperature and low pH, high temperature and high pH, and low temperature and high pH). To this end, we constructed reaction database, including total 20879 reactions and 23776 compounds, for different environments depending on the reaction conditions. In the model, we first performed network expansion in one environment (low temperature and low pH environment: surface water) starting with the atmospheric molecules until reaching a steady state. Then, chemical compounds were filtered and transported to the high temperature and low pH environment (acidic hydrothermal vents). Filtering occurred depending on adsorption characteristics of chemical compounds onto particular secondary minerals that typically found in the environment (e.g., montmorillonite for the low temperature and low pH environment). In the high temperature and low pH environment, network expansion was again carried out until reaching a steady state. Then, chemical compounds were transported to the high temperature and high pH environment (alkaline hydrothermal vents) after filtering. This continues until chemical compounds returned to the low temperature and low pH environment.
In the chemical network calculations, we find that 1) compared with a single environment, transportation in four environments increases the number of total chemical compounds appeared in the network, 2) the alkaline hydrothermal environment is important to produce various organic compounds when the atmospheric composition is CO2 and N2, 3) formaldehyde is a key (hub) molecule that dramatically increases a diversity of organic compounds, 4) if HCN is present, reactions of HCN with aldehydes produce glycolonitriles, which in turn form various precursors of amino acids. We find the presence of a carbon redox cycle in our chemical network, in which methanol generated from CO2 and H2 at the alkaline hydrothermal environment is recycled back to CO2 and H2 in low temperature surface water, while generating formaldehyde. This redox cycle might be important for emergence of proto-metabolism. In addition, we find that no nucleotide is produced in our chemical network because of absence of cyanamide in any environments, suggesting that cyanamide would have been hard to form in geochemical cycles on early Earth. This implies the importance of exogenic inputs of cyanamide, such as carbonaceous chondrites and comets, for formation of nucleotide on early Earth.
Here, we report our results of chemical network modeling of organic syntheses from simple molecules in the atmosphere (such as, CO2, CH4, and N2) and their photochemical products (such as, HCHO and HCN) to complex molecules during transportation in four distinct environments with different temperature and pH (i.e., low temperature and low pH, high temperature and low pH, high temperature and high pH, and low temperature and high pH). To this end, we constructed reaction database, including total 20879 reactions and 23776 compounds, for different environments depending on the reaction conditions. In the model, we first performed network expansion in one environment (low temperature and low pH environment: surface water) starting with the atmospheric molecules until reaching a steady state. Then, chemical compounds were filtered and transported to the high temperature and low pH environment (acidic hydrothermal vents). Filtering occurred depending on adsorption characteristics of chemical compounds onto particular secondary minerals that typically found in the environment (e.g., montmorillonite for the low temperature and low pH environment). In the high temperature and low pH environment, network expansion was again carried out until reaching a steady state. Then, chemical compounds were transported to the high temperature and high pH environment (alkaline hydrothermal vents) after filtering. This continues until chemical compounds returned to the low temperature and low pH environment.
In the chemical network calculations, we find that 1) compared with a single environment, transportation in four environments increases the number of total chemical compounds appeared in the network, 2) the alkaline hydrothermal environment is important to produce various organic compounds when the atmospheric composition is CO2 and N2, 3) formaldehyde is a key (hub) molecule that dramatically increases a diversity of organic compounds, 4) if HCN is present, reactions of HCN with aldehydes produce glycolonitriles, which in turn form various precursors of amino acids. We find the presence of a carbon redox cycle in our chemical network, in which methanol generated from CO2 and H2 at the alkaline hydrothermal environment is recycled back to CO2 and H2 in low temperature surface water, while generating formaldehyde. This redox cycle might be important for emergence of proto-metabolism. In addition, we find that no nucleotide is produced in our chemical network because of absence of cyanamide in any environments, suggesting that cyanamide would have been hard to form in geochemical cycles on early Earth. This implies the importance of exogenic inputs of cyanamide, such as carbonaceous chondrites and comets, for formation of nucleotide on early Earth.