Browsing by Subject "Prebiotic chemistry"

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Metalloporphyrine als potentiell präbiotische Moleküle und chemische Biosignaturen
(2022) Pleyer, Hannes Lukas; Strasdeit, Henry
This doctoral thesis addresses two important astrobiological aspects of metalloporphyrins. First, the possible abiotic origin of metalloporphyrins under prebiotically plausible conditions, and second, metalloporphyrins as possible molecular biosignatures in the context of the search for extraterrestrial life. This two-pronged approach allowed us to obtain a more comprehensive overview of metalloporphyrins in an astrobiological context. In presently known living organisms, metal complexes of porphyrins and porphyrinoids occur ubiquitously, whereas free base porphyrins scarcely matter. Among the best known examples are chlorophylls (magnesium complexes), which are crucially involved in photosynthesis, and the heme group, for example as a cofactor of cytochromes, which play an important role in cel¬lular respiration. In terms of evolution, porphyrinoid cofactors appear to be very old, and indeed 1.1 billion-year-old geoporphyrins have been found in sedimentary rocks. Ancestors of current porphyrinoid cofactors, simple metalloporphyrins, may have been present in the first organisms or even earlier in abiotic protometabolisms. Furthermore, the wide distribution of porphyrinoid cofactors among known organisms, as well as their participation in basic biological functions, suggests that metalloporphyrinoids may also be present in potential life forms beyond Earth. Thus, metalloporphyrinoids could be useful as chemical biosignatures in the search for life on Mars, Europa, Enceladus and beyond. The first objective of this work was to investigate whether complex formation with selected metals (Fe, Mg, Co, Ni, and Cu) is possible under prebiotically plausible conditions. Based on a prebiotic synthesis of octaalkylporphyrins at simulated primordial volcanic coasts, which has been previously described by our group, the influence of wet-dry cycles on octaethylporphyrin (OEP) and selected metal sources was to be studied. For this purpose, first a novel, automated apparatus had to be developed that allowed the simulation of conditions on the early Earth, especially at primordial volcanic coasts. In particular, this apparatus had to ensure strict exclu-sion of atmospheric oxygen and allow fluctuating changes between wet and dry phases (simu-lation of tides or rainfall). Initially, experiments were conducted to test the new apparatus. It was shown that the appa-ratus worked completely automatically and reliably over a longer period of time. In further experiments, it was tested whether oxidation-sensitive substances can be handled in the apparatus. Indeed, it was shown that it is possible to strictly exclude oxygen. Thus, the apparatus proved to be suitable for investigating the formation of metal complexes from OEP and various metal sources under the influence of alternating wet-dry cycles. Metal sources used included metal(II) chlorides, metal sulfides, basalt, and iron meteorites. The focus was on iron sources. Indeed, it was shown that iron, magnesium, cobalt, nickel and copper complexes formed with the corresponding metal sources in an unusual reaction (completely water-insoluble OEP reacted with partly also insoluble metal sources), whereby wet-dry cycles turned out to be essential. Yields ranging from 20 to 78% (relative to the porphyrin) were obtained in fresh water. In addition, the influence of artificial seawater and low pH values on complex formation was investigated. In the second part of the thesis, the stability of metalloporphyrins was investigated using the compound chlorido(octaethylporphyrinato)iron(III), [FeCl(oep)], as a model. Potentially de-structive conditions relevant to metalloporphyrins as possible chemical biosignatures were selected for the experiments performed, as well as conditions that presumably prevailed on primordial volcanic coasts. Extensive series of experiments demonstrated, among other things, that (a) the iron OEP core is stable over a pH range of 0.0 to 13.5, (b) [FeCl(oep)] is stable up to ca. 250 °C in an inert atmosphere, (c) a salt matrix protects [FeCl(oep)] from X-ray radiation but not from iron particle radiation, and (d) hypochlorite, hydrogen peroxide, chlorate, and nitric acid oxidatively decompose [FeCl(oep)] (oxidation power in descending order). On the other hand, perchlorate, which is often mentioned in connection with its occurrence in the Martian regolith, did not show any oxidation effect. Finally, the importance of metalloporphyrins as potential biosignatures is discussed in the presented work, particularly in the light of their stability and possible abiotic synthetic pathways for these compounds.