STUDIES ON IRON OXIDES AS PREBIOTIC CATALYST

Abstract

Several experiments have been conducted to trace out possible steps of chemical evolution. Experimental results suggest that origin of life processes began with the formation of important biomonomers, such as amino acids and nucleotides, from simple molecules present in the prebiotic environment and their subsequent condensation to biopolymers. Small reactive intermediates are the backbone of prebiotic organic synthesis. These include hydrogen cynanide, formaldehyde, ethylene, cyanoacetylene, acetylene, and such other molecules that combine to form large and more complex precursors with ultimate formation of stable biomolecules. Most of these reactive intermediates might have been produced at a relatively slow rate, resulting in their low concentration in the primitive ocean, where many of such interesting reactions have occurred. Subsequent reactions would have depended on the balance between atmospheric production rates and the degradation rates of small intermediates, dependent on the temperature and pH of the early ocean. The crucial step in chemical evolution must have involved polymerization of biomonomers in aqueous medium of the primeval seas. It is evident that catalysts might have played an important role in catalyzing reactions leading to the origin of life as they tend to direct the reaction along a few reaction pathways so that a limited array of products are obtained. It is assumed that solid surfaces of inorganic mineral and clays might have played pivotal role in concentrating the biomonomers and in catalyzing a class of reactions of prebiotic relevance during the course of chemical evolution. Metal oxides are important constituents of the Earth's crust and of other planets, and therefore, the catalytic role of metal oxides in the course of chemical evolution and origin of life cannot be ruled out. The iron oxide hydroxide minerals, goethite and akaganeite, were the likely constituents of the sediments present, for instance, in geothermal regions of the primitive (i) Earth. These might have adsorbed organic molecules and catalyzed the condensation processes, which may have led to the origin of life. It is assumed that they might have catalyzed the important reactions of prebiotic relevance. Iron oxides are insoluble in water, it is proposed that they might have locally settled at the bottom or at the sea shore and have concentrated the biomonomers from their aqueous solutions and subsequently catalyzed a class of reactions of prebiotic relevance. In the thesis, results of work on the role of iron oxides towards different aspects of chemical evolution and origin of life have been presented. It has been proposed that adsorption was the first step for the polymerization of biomonomers. That is why the interaction of ribose nucleotides with iron oxides (goethite, akaganeite and hematite) has been studied. During the study it has been found that iron oxides are good adsorbents towards ribose nucleotides. Further in order to investigate catalytic efficiency of iron oxides these have been used in the formation of nucleobases from formamide. Iron oxides were also found suitable for the oligomerization of amino acids under the simulated conditions. The First chapter of the thesis deals with the introduction of the topic "chemical evolution and origin of life" and literature survey on inorganic minerals, clays, metal cyanogen complexes and metal oxides, which are efficient in concentrating the organic molecules on their surfaces and subsequently catalyzed a class of prebiotic reactions during the course of chemical evolution has been discussed. The Second chapter describes experimental methodology and instrumentation. This chapter presents the method of synthesis of some metal oxides, their characterization and methods of chemical analyses involved. The metal oxides have been synthesized by precipitation method and characterized using X-ray diffraction, FE-SEM and TEM analysis. Experimental conditions and techniques used for the adsorption of ribose nucleotides on metal (ii) oxides have been given. Methods used for the synthesis of nucleobases from formamide and oligomerization of amino acids in the presence of iron and other metal oxides. Further the conditions for the interaction of aromatic amines with iron oxides and their oxidation to various compounds have also been discussed. The Third chapter comprises the result of studies on the interaction of ribose nucleotides (5'-AMP, 5'-GMP, 5'-CMP, and 5'-UMP) with iron oxides (goethite, akaganeite and hematite). Adsorption trend was found to follow Langmuir Adsorption Isotherm. Maximum adsorption was found to occur at neutral pH (-7.0), whereas among ribose nucleotides 5'-GMP was found to be adsorbed more on iron oxides used. Infrared spectral studies on the adsorption adducts showed that adsorption of ribose nucleotides takes place due to interaction of positively charged surface of metal oxides and negatively charged sites of ribose nucleotides. The Fourth chapter presents the results of studies on the formation of several nucleobases from formamide in presence of iron oxides (goethite, akaganeite and hematite). It was observed that goethite and hematite afforded the maximum yield of products. However, the number of products formed in each case was the same. The only difference was in the yields. Possible explanation has been given on the basis of their structural arrangements. The Fifth chapter comprises the result of studies on the oligomerization of amino acids (glycine and alanine) in the presence of iron oxides (goethite, akaganeite and hematite), zinc oxide and titanium dioxide at various experimental conditions such as temperature range 50-120 °C and for 35 days. It was observed that all the three iron oxides catalyzed oligomerization of amino acids with glycine up to trimer whereas alanine afforded only dimer. Yield of the products was in accordance with their surface area. (iii) The Sixth chapter illustrates the results of studies on interaction of aromatic amines (aniline, p-toluidine, p-chloroaniline and p-anisidine) with iron oxides (goethite, akaganeite and hematite). Adsorption trend was found to follow Langmuir Adsorption Isotherm. Maximum adsorption was found to occur at neutral pH (-7.0), whereas p-toluidine was found to be adsorbed more on iron oxides. Infrared spectral studies on the adsorption adducts showed that adsorption of ribose nucleotides occurred due to interaction of positively charged surface of metal oxide and the basicity of amines. During adsorption studies of the aromatic amines on iron oxides, it was found that some of the amines were oxidized in alkaline medium (pH~9) and afforded several other products. The results of the above studies clearly support the idea that a specific iron oxide might have played its important role as a prebiotic catalyst for the concentration and polymerization of biomolecules.