11:00 〜 13:00
[MIS06-P05] Formation of Organic Compounds in Slightly-Reducing Planetary Atomsphere
キーワード：アミノ酸、太陽エネルギー粒子、紫外線、鉄(II)イオン、有機物、弱還元型大気
Bioorganic compounds such as amino acids should have been formed prior to the generation of life on the Earth. Since Miller’s spark discharge experiments in 1953 [1], a large number of experiments have been conducted. In the earlier experiments in 1950-70s, strongly reducing gas mixtures (e.g. mixtures mainly containing CH4 and NH3) were often used as simulated primitive Earth atmosphere, and it was reported that amino acids were easily formed from such gas mixtures by various energies [2]. In these days, however, it is assumed that primitive Earth atmosphere was not so strongly-reducing but only slightly reducing [3]. If so, formation of amino acids was not easy by conventional energies [4,5]. If the primitive Earth atmosphere contained some amount of CO, it was possible to form amino acids and some nucleic acid bases by galactic cosmic rays and/or meteor impacts[6,7]. It has not been clear that the concentration of CO in the primitive Earth atmosphere, so that there was a possiblility that the primitive Earth atmosphere was almost neutral (mostly CO2, N2 and H2O).
It was suggested that that the young Sun had huge and frequent flares, which would have shot out high-flux of solar energetic particles (SEPs) [8, 9]. Such high-energy particles would have caused chemical reactions in planetary atmospheres. In the present study, we examined possible formation of organic compounds from slightly- reducing gas mixture, mainly composed of N2, CO2 and H2O. We also examined possible roles of Fe(II) ion in the prebiotic chemistry, since Fe(II) concentration in paleoocean could have been quite high and it could be used as reducing agent.
A gas mixture (700 Torr) and pure water (5 mL) was put in a 400 mL Pyrex tube with a Havar foil window, and the gas mixture was irradiated with 2.5 MeV protons (2.0 mC) from a Tandem accelerator (Tokyo Tech). The gas mixtures used were (i) Pure CO2 and (ii) CO2, CO and N2 (molar ratio 9:1:10). The same kind of mixture was put in a 400 mL Pyrex tube with a quartz window, and then irradiated with UV light from a 450 W Xe lamp (Hamamatsu Photonics) for 4 hours. In some experiments, FeCl2 solution was used in place of pure water (CO2: Fe(II) = 3: 1 in molar ratio).
The resulting gaseous products were analyzed by GC/MS (Shimadzu GCMS-QP2020: column: Poraplot Q). Total organic carbon was determined with a Shimadzu TOC-VWP total organic carbon analyzer. Amino acids were determined by ion-exchange HPLC (Shimadzu LC-10AT) after acid-hydrolysis, where post-column derivatization with o-phthalaldehyde and N-acetyl-L-cysteine was applied.
After proton irradiation of a mixture of CO2 and H2O, small amount of CO (0.14-0.24% of CO2) was found in gaseous phase, and 6-11 mg/L of TOC was detected in aqueous phase. When Fe(II) was added, both CO/CO2 ratio and TOC were decreased: We are surveying that reason. On the other hand, UV irradiation yielded much less amount of CO and TOC than proton irradiation: No CO was detected when CO2 + H2O was UV-irradiated without Fe(II). It was suggested that Fe(II) could have helped the formation of CO by UV irradiation of CO2. Amino acids were yielded after the proton irradiation of the mixture of CO2, CO, N2 and H2O.
It was suggested that SEPs are effective energy for formation of organic compounds including amino acids from slightly-reducing atmosphere. Fe(II) in hydrosphere might have subserve photo-reduction of CO2 to CO, which could used to form organics by SEPs. Further studies to examine possible roles of SEPs in prebiotic synthesis is in progress.
[1] Miller, S. L., Science 117, 528 (1953)
[2] Miller, S. L. and Orgel, L. E., The Origins of Life on the Earth, Prentice-Hall, Englewood Cliff (1974).
[3] Catring, D. C. and Kasting, J. F. Atmospheric Evolution on Inhabited and Lifeless Worlds,
Cambridge University Press (2017).
[4] Schlesinger, G and Miller, S. L., J. Mol. Evol. 19, 376 (1983).
[5] Kuwahara, H. et al., Orig. Life Evol. Biosph. 42, 533 (2012).
[6] Kobayashi, K. et al., Orig. Life Evol. Biosph. 28, 155 (1998).
[7] Miyakawa, S. et al., Proc. Natl. Acad. Sci. USA, 28, 14628 (2002).
[8] Maehara, H. et al. (2012) Nature 485, 478.
[9] Airapetian, V. S. et al. Nat. Geosci. 9, 452 (2016).
It was suggested that that the young Sun had huge and frequent flares, which would have shot out high-flux of solar energetic particles (SEPs) [8, 9]. Such high-energy particles would have caused chemical reactions in planetary atmospheres. In the present study, we examined possible formation of organic compounds from slightly- reducing gas mixture, mainly composed of N2, CO2 and H2O. We also examined possible roles of Fe(II) ion in the prebiotic chemistry, since Fe(II) concentration in paleoocean could have been quite high and it could be used as reducing agent.
A gas mixture (700 Torr) and pure water (5 mL) was put in a 400 mL Pyrex tube with a Havar foil window, and the gas mixture was irradiated with 2.5 MeV protons (2.0 mC) from a Tandem accelerator (Tokyo Tech). The gas mixtures used were (i) Pure CO2 and (ii) CO2, CO and N2 (molar ratio 9:1:10). The same kind of mixture was put in a 400 mL Pyrex tube with a quartz window, and then irradiated with UV light from a 450 W Xe lamp (Hamamatsu Photonics) for 4 hours. In some experiments, FeCl2 solution was used in place of pure water (CO2: Fe(II) = 3: 1 in molar ratio).
The resulting gaseous products were analyzed by GC/MS (Shimadzu GCMS-QP2020: column: Poraplot Q). Total organic carbon was determined with a Shimadzu TOC-VWP total organic carbon analyzer. Amino acids were determined by ion-exchange HPLC (Shimadzu LC-10AT) after acid-hydrolysis, where post-column derivatization with o-phthalaldehyde and N-acetyl-L-cysteine was applied.
After proton irradiation of a mixture of CO2 and H2O, small amount of CO (0.14-0.24% of CO2) was found in gaseous phase, and 6-11 mg/L of TOC was detected in aqueous phase. When Fe(II) was added, both CO/CO2 ratio and TOC were decreased: We are surveying that reason. On the other hand, UV irradiation yielded much less amount of CO and TOC than proton irradiation: No CO was detected when CO2 + H2O was UV-irradiated without Fe(II). It was suggested that Fe(II) could have helped the formation of CO by UV irradiation of CO2. Amino acids were yielded after the proton irradiation of the mixture of CO2, CO, N2 and H2O.
It was suggested that SEPs are effective energy for formation of organic compounds including amino acids from slightly-reducing atmosphere. Fe(II) in hydrosphere might have subserve photo-reduction of CO2 to CO, which could used to form organics by SEPs. Further studies to examine possible roles of SEPs in prebiotic synthesis is in progress.
[1] Miller, S. L., Science 117, 528 (1953)
[2] Miller, S. L. and Orgel, L. E., The Origins of Life on the Earth, Prentice-Hall, Englewood Cliff (1974).
[3] Catring, D. C. and Kasting, J. F. Atmospheric Evolution on Inhabited and Lifeless Worlds,
Cambridge University Press (2017).
[4] Schlesinger, G and Miller, S. L., J. Mol. Evol. 19, 376 (1983).
[5] Kuwahara, H. et al., Orig. Life Evol. Biosph. 42, 533 (2012).
[6] Kobayashi, K. et al., Orig. Life Evol. Biosph. 28, 155 (1998).
[7] Miyakawa, S. et al., Proc. Natl. Acad. Sci. USA, 28, 14628 (2002).
[8] Maehara, H. et al. (2012) Nature 485, 478.
[9] Airapetian, V. S. et al. Nat. Geosci. 9, 452 (2016).