{"title":"Magnetic Circular Dichroism in Archean Stratospheric Oxygen: Enantiomeric Excess of Amino Acids Produced in Volcanic Plumes.","authors":"A Sharma","doi":"10.1007/s11084-023-09637-y","DOIUrl":null,"url":null,"abstract":"<p><p>While there is consensus that Archean atmosphere was anoxic with O<sub>2</sub> pressure, p(O<sub>2</sub>) <10<sup>-6</sup> PAL (present atmospheric level) at sea-level, evidence suggests that p(O<sub>2</sub>) at stratospheric altitudes of 10-50 km was orders of magnitude higher, a result of photodissociation of CO<sub>2</sub> by UVC sunlight and incomplete mixing of O<sub>2</sub> with other gases. Molecular O<sub>2</sub> is paramagnetic due to triplet ground state. Magnetic circular dichroism (MCD) by stratospheric O<sub>2</sub> is examined in earth's magnetic field and shows maximum circular polarization │(I<sub>+</sub> - I<sub>-</sub>)│ at altitude of 15-30 km (I<sub>+</sub>/I<sub>-</sub> is intensity of left/right circularly polarized light). While (I<sub>+</sub> - I<sub>-</sub>)/(I<sub>+</sub> + I<sub>-</sub>) is small (~10<sup>-10</sup>), it is an unexplored source of enantiomeric excess (EE) by asymmetric photolysis of amino acid precursors produced in volcanic eruptions. The precursors reside in stratosphere for periods of over a year due to relative absence of vertical transport. Due to negligible thermal gradient across equator, they are trapped in the hemisphere where they are produced, with interhemispheric exchange time of over a year. The precursors diffuse through altitudes of maximum circular polarization before getting hydrolyzed on ground to amino acids. Enantiomeric excess of ~10<sup>-12</sup> is calculated for precursors and amino acids. While small, this EE is orders of magnitude higher than predicted (~10<sup>-18</sup>) by parity violating energy differences (PVED) and could be the seed for growth of biological homochirality. Preferential crystallization (PC) is described as a plausible mechanism for amplification of solution EE of some amino acids from 10<sup>-12</sup> to 10<sup>-2</sup>, for period of several days.</p>","PeriodicalId":19614,"journal":{"name":"Origins of Life and Evolution of Biospheres","volume":"53 1-2","pages":"71-86"},"PeriodicalIF":1.9000,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"1","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Origins of Life and Evolution of Biospheres","FirstCategoryId":"101","ListUrlMain":"https://doi.org/10.1007/s11084-023-09637-y","RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"2023/6/6 0:00:00","PubModel":"Epub","JCR":"Q2","JCRName":"BIOLOGY","Score":null,"Total":0}
引用次数: 1
Abstract
While there is consensus that Archean atmosphere was anoxic with O2 pressure, p(O2) <10-6 PAL (present atmospheric level) at sea-level, evidence suggests that p(O2) at stratospheric altitudes of 10-50 km was orders of magnitude higher, a result of photodissociation of CO2 by UVC sunlight and incomplete mixing of O2 with other gases. Molecular O2 is paramagnetic due to triplet ground state. Magnetic circular dichroism (MCD) by stratospheric O2 is examined in earth's magnetic field and shows maximum circular polarization │(I+ - I-)│ at altitude of 15-30 km (I+/I- is intensity of left/right circularly polarized light). While (I+ - I-)/(I+ + I-) is small (~10-10), it is an unexplored source of enantiomeric excess (EE) by asymmetric photolysis of amino acid precursors produced in volcanic eruptions. The precursors reside in stratosphere for periods of over a year due to relative absence of vertical transport. Due to negligible thermal gradient across equator, they are trapped in the hemisphere where they are produced, with interhemispheric exchange time of over a year. The precursors diffuse through altitudes of maximum circular polarization before getting hydrolyzed on ground to amino acids. Enantiomeric excess of ~10-12 is calculated for precursors and amino acids. While small, this EE is orders of magnitude higher than predicted (~10-18) by parity violating energy differences (PVED) and could be the seed for growth of biological homochirality. Preferential crystallization (PC) is described as a plausible mechanism for amplification of solution EE of some amino acids from 10-12 to 10-2, for period of several days.
期刊介绍：
The subject of the origin and early evolution of life is an inseparable part of the general discipline of Astrobiology. The journal Origins of Life and Evolution of Biospheres places special importance on the interconnection as well as the interdisciplinary nature of these fields, as is reflected in its subject coverage. While any scientific study which contributes to our understanding of the origins, evolution and distribution of life in the Universe is suitable for inclusion in the journal, some examples of important areas of interest are: prebiotic chemistry and the nature of Earth''s early environment, self-replicating and self-organizing systems, the theory of the RNA world and of other possible precursor systems, and the problem of the origin of the genetic code. Early evolution of life - as revealed by such techniques as the elucidation of biochemical pathways, molecular phylogeny, the study of Precambrian sediments and fossils and of major innovations in microbial evolution - forms a second focus. As a larger and more general context for these areas, Astrobiology refers to the origin and evolution of life in a cosmic setting, and includes interstellar chemistry, planetary atmospheres and habitable zones, the organic chemistry of comets, meteorites, asteroids and other small bodies, biological adaptation to extreme environments, life detection and related areas. Experimental papers, theoretical articles and authorative literature reviews are all appropriate forms for submission to the journal. In the coming years, Astrobiology will play an even greater role in defining the journal''s coverage and keeping Origins of Life and Evolution of Biospheres well-placed in this growing interdisciplinary field.
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