RT Dissertation/Thesis
T1 Thermal behavior of amino acids in inorganic matrices : relevance for chemical evolution
A1 Dalai,Punam
WP 2014/02/24
AB The onset of life on Earth was preceded by an abiotic chemistry in which complex molecules were formed from simpler ones. In the presence of energy sources such as UV radiation, lightning and geothermal energy, a wide range of organic compounds probably formed on the young Earth. Stanley Miller was the first to study this scenario experimentally. He showed that amino acids were synthesized under simulated conditions of the primitive Earth?s atmosphere. Initially, it was believed that the Earth?s early atmosphere contained high concentrations of CH4, NH3, CO, and H2 and was thus strongly reducing. However, later it was assumed that the early atmosphere was redox neutral and was composed of N2, CO2, and H2O as main constituents. Isotopic data from zircons indicate that liquid water might have been present already around 4.2 billion years ago. So called banded iron formations confirm the presence of liquid water at least 3.8 billion years ago. The early geological histories of Earth and Mars were probably very similar. About 4 billion years ago, both planets had liquid water, volcanoes, and a dense atmosphere without free oxygen, and they experienced intense meteoritic and cometary impacts. Therefore, the simulation experiments described in the present thesis may also be relevant to the early Mars. Among the possible prebiotic molecules, amino acids are generally considered especially important for the origin of life. The main reason for this is that they serve as building blocks of proteins which are the pillars of metabolism in all organisms. There is practically no doubt that amino acids were present on the young Earth. They originated from endogenous and exogenous (i.e. extraterrestrial) sources. Glycine is the most abundant amino acid in carbonaceous meteorites and Miller-type experiments.
In the present work, the deep black residue was studied that forms when glycine is heated at 200 °C. Similar residues have been named ?thermo-melanoid? by others. The experiments were performed under a pure nitrogen atmosphere in order to simulate the oxygen-free early atmosphere of the Earth. It was found that the formation of the thermomelanoid from neat glycine started at 160 °C and was relatively fast and complete at 200 °C. However, the residues that formed at high temperatures (250?350 °C) were different from the thermo-melanoid. The thermo-melanoid was also present in the residues obtained by heating the glycine homopeptides 2,5-diketopiperazine (DKP), diglycine, triglycine, and tetraglycine at 200 °C. In contrast, penta- and hexaglycine remained almost unreacted at this temperature. Deuterolysis experiments revealed that C=C bonds are a characteristic structural feature of the thermo-melanoid. These bonds form by an unusual condensation reaction between C=O and CH2 groups. Glycine, DKP, and diglycine were released during hydrolysis of the thermomelanoid in water at 100 °C. In these experiments, the thermo-melanoid slowly dissolved. After 10 days, for example, a mass loss of ~75 % was observed. Therefore, the thermomelanoid can be regarded as a kind of storage form of glycine and glycine oligopeptides. The lower solubility of the thermo-melanoid as compared to glycine and its homopeptides may have influenced the distribution of glycine units on the early Earth. Moreover, additional experiments have shown that the thermo-melanoid mixed with soil continuously produced a higher amount of CO2 during a six-months period than samples without the thermo-melanoid. Obviously, the thermo-melanoid was decomposed in the soil. The decomposition was probably caused by microorganisms. Therefore, one can hypothesize that the thermomelanoid could have served as nutrient for early heterotrophic (pre-)organisms.
The salt concentration of the late Hadean/early Archean ocean was at least twice as high as the concentration in the present-day oceans. There are good reasons to assume that the ions were Na+, K+, Ca2+, Mg2+, and Cl?. SO4 2? and PO4 3? were possibly not present in significant concentrations as the early atmosphere of the Earth was anoxic. In relation to this, the thermal behavior of glycine was investigated in the presence of various salts. It was found that glycine changed from the initial α- to the γ-modification when it crystallized together with NaCl and NaCl?KCl mixtures. At 200 °C, the glycine that was embedded in the NaCl or NaCl?KCl salt crusts transformed into the thermo-melanoid and a small amount of DKP. Only ~5 % of unreacted glycine was left after seven days in the presence of NaCl. The results showed that the presence of these salts and the change in the modification were nearly ineffective in protecting glycine from transformation into the thermo-melanoid. In contrast to NaCl and KCl, CaCl2 formed a coordination compound with glycine, namely CaCl2(Hgly) ⋅ H2O, when solutions of CaCl2 ⋅ 2H2O and glycine were evaporated. It was found that more than 90 % of the glycine were still present in CaCl2(Hgly) ⋅ H2O after heating at 200 °C for seven days. The coordination of glycine to Ca2+ prevented the transformation of glycine into the thermo-melanoid up to 250 °C. Yusenko et al. reported that at 350 °C, small volatile N-heterocycles such as pyrroles formed from CaCl2(Hgly) ⋅ H2O. Pyrroles are the building blocks of porphyrin-type biomolecules such as cytochromes and chlorophylls. CaCl2(Hgly) ⋅ H2O was also identified in mixtures of glycine with artificial sea salt (AS) prepared from NaCl (705 mmol), KCl (15 mmol), MgCl2 ⋅ 6H2O (80 mmol), CaCl2 ⋅ 2H2O (15 mmol), and glycine (10 mmol). About 84 % of the initial glycine had survived after heating an AS?Hgly mixture for seven days at 200 °C. In contrast, neither complex formation nor change in the modification of glycine was observed in gypsum?Hgly and MgCO3?Hgly mixtures.
Clay minerals are mainly produced by the weathering of volcanic rock. They are not only found on Earth, but also on Mars. A possible role of clay minerals in chemical evolution was first suggested by Bernal more than half a century ago. In the present work, the thermal behavior of glycine embedded in smectites (Ca-montmorillonite, Na-montmorillonite, and nontronite) and kaolinite was investigated. The glycine-loaded clay minerals were heated at 200 and 250 °C for two days. HPLC and MALDI?TOF/TOF MS analyses of glycine-loaded Ca-montmorillonite that had been heated at 200 °C showed the presence of unreacted glycine, DKP, and linear peptides up to decaglycine. The comparison between the smectite clay minerals revealed that glycine was best protected by Ca-montmorillonite. ~63 % of the amino acid survived in its free form at 200 °C. This was followed by Na-montmorillonite (~53 %) and nontronite (~39 %) under similar experimental conditions. These results demonstrated that smectite clay minerals protect glycine from complete decomposition and sublimation, partly by promoting its polymerization. In contrast to smectites, kaolinite has no interlayer spaces available for intercalation. Therefore, glycine is only attached to the surface of the kaolinite particles. Sublimation of glycine and newly formed DKP was observed when kaolinite mixed with glycine was heated at 200 and 250 °C. All investigated clay minerals prevented the transformation of glycine into the thermo-melanoid during thermal treatments.
Various heating experiments were conducted with mixtures of glycine and volcanic rock (basaltic sand from the island of La Réunion, Indian Ocean) or Martian soil simulants (JSC Mars-1A, P-MRS, and S-MRS). Glycine and DKP were identified in the residues and sublimates after heating basaltic sand?Hgly and JSC Mars-1A?Hgly at 200 °C for two days. Additionally, the thermo-melanoid was found in the residue of basaltic sand?Hgly. JSC Mars-1A contains mainly volcanic glass. Forsterite (Mg2SiO4) was identified as the major crystalline mineral in the basaltic sand. Glycine cannot be intercalated in JSC Mars-1A and basaltic sand as they have no clay minerals. As a result, glycine in these two matrices undergoes thermal alterations similar to neat glycine. In contrast, glycine, DKP, and linear peptides from di- to hexaglycine were detected after heating a P-MRS?glycine mixture. This observation can be easily explained by the fact that P-MRS contains 70 % of clay minerals that protect the amino acid from complete decomposition and thus allow the formation of larger peptides. The S-MRS?glycine residue contained only DKP, glycine, and diglycine, obviously because the mineral matrix consisted only of rock, anhydrous iron oxides, and gypsum, but not clay minerals. These experiments again demonstrated the influence of clay minerals on the behavior of glycine when exposed to higher temperatures.
Another focus of the work was on the thermal behavior of chiral amino acids intercalated in Ca-montmorillonite. The heating experiments were conducted with different L-enantiomeric excesses (ee) of alanine [L-ee = 0 (i.e. racemic), 4, 20, 50, and 100 %] under a pure nitrogen atmosphere. The residues were analyzed by GC-MS/FID after derivatization. It was found that the racemization process was fast during the first 2?3 days and thereafter slowed down considerably. After eight weeks at 200 °C, the residues still contained 17.5?25.0 % of the respective starting L-ee. Complete racemization of L-alanine was not observed even after 24 weeks of heating. It was also found that, as expected, Camontmorillonite did not have any specific preference for the formation of either the D- or L-enantiomer. Interestingly, it could be observed that L-isovaline influenced the racemization of alanine. The presence of L-isovaline increased the rate of formation of D-alanine. In addition, higher temperatures greatly accelerated the racemization. For instance, after eight weeks at 220 °C, 85 % of the initial L-ee of alanine had been lost by racemization, whereas at 120 °C only 25 % racemization was observed. These experiments made use of the fact that Ca-montmorillonite largely protects amino acids from sublimation. In contrast, neat amino acids such as alanine undergo considerable sublimation in a few hours or less, depending on the temperature. Using a racemization kinetics model, it was estimated that L-alanine can survive in Ca-montmorillonite at elevated temperatures for years.
In the literature, there are several reports on enantiomeric excesses of certain amino acids in meteorites. In relation to this, experiments were performed to demonstrate the enantiomeric enrichment of amino acids by partial sublimation. After 3 and 24 hours at 200 °C, the L-ee of alanine and valine increased in the sublimation residue, whereas the L-ee of the sublimates was lower than the initial one. Thus, it seems that racemic alanine and racemic valine crystals are more volatile than enantiomerically pure crystals. It may be assumed that similar processes take place during the atmospheric entry of meteorites and in the aqueous alteration phase of asteroids.
The experimental results described in the present thesis suggest that various modes of interaction of amino acids with inorganic matrices such as salt mixtures and clay minerals existed on the young Earth. These results may help to better understand some of the processes of the prebiotic chemical evolution.
K1 Aminosäuren
PP Hohenheim
PB Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
UL http://opus.uni-hohenheim.de/volltexte/2014/960