ROLE OF MANGANESE OXIDES IN CHEMICAL EVOLUTION

Abstract

Through historical contributions by Oparin, Haldane, Miller, Urey, Cairns-Smith etc., the intellectual challenge of the origin of life enigma has unfolded that life originated on Earth through a series of physicochemical processes known as Chemical Evolution. Investigations revealed a predominantly reducing prebiotic atmosphere and the main compounds of the primordial atmosphere were CO2, N2, and water vapor. The early ocean where such physicochemical processes have been assumed to have occurred was a prebiotic soup, a rich diversity of organic and inorganic compounds suitable for assembling into more complex precursors of life forms. Several experiments have been conducted to trace out the possible steps of chemical evolution. Experimental results suggest that such chemical evolution 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 cyanide, formaldehyde, ethylene, cyanoacetylene, acetylene, and such other molecules that combine to form large and more complex precursors with ultimate formation of stable biomolecules. 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 polymerization of the biomonomers relies on the mechanism of concentrating the basic ingredients from vastly diluted early oceans and it was believed that natural minerals like clays would have provided a surface for the adsorption of organic molecules. Like natural minerals, transition metals may have been important as catalysts for the formation of biopolymers during chemical evolution and the origin of life. Catalysts may have been important for the origins of life because they tend to direct the reaction along a few reaction (i) pathways so that a limited array of products is obtained. Catalysts bind specific types of compounds to their surfaces and then convert them to a limited number of products. Manganese is the 10th most abundant element in the biosphere (,1014 kg of suspended and dissolved manganese found in oceans) and is second only to iron in relative terrestrial abundance of the transition metals. On an average, crustal rocks contain about 0.1% by weight of Mn, coordinated with oxygen, and may also exist in the bottom of seas as nodules. The existence of manganese on Mars has also been reported. In' the early stages of the Earth's evolution, volcanoes were a major source of such elements which in turn may have been involved in adsorption and catalytic reactions of biomolecules in molecular evolution. The catalytic activity of manganese for many reactions in the presence of nucleotides, mRNA to give oligoribonucleotides is already reported. Manganese exists in various oxidation states on Earth. The microbial oxidation of soluble Mn (II) is an important process for the formation of soluble Mn (III, IV) oxides in natural environments. We proposed that since the redox potential of the primitive Earth's atmosphere was low and the atmosphere was less oxidized, manganese oxides of lower oxidation states were more important for selectively adsorbing and concentrating bio-molecules during chemical evolution. In the thesis, results of work on the role of manganese 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 manganese oxides (manganosite (MnO), bixbyite (Mn203), hausmannite (Mn304) and pyrolusite (Mn02)) has been studied. During the study it has been found that manganese oxides are good adsorbents towards ribose nucleotides. Further in order to investigate catalytic efficiency of manganese oxides these have been used in the formation of nucleobases from formamide. Manganese 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 and literature review of the topic "chemical evolution and origin of life". Various aspects on prebiotic scenario, amino acid synthesis, synthesis of ribose, nucleobases, phosphates and their assembly studies, peptide synthesis, role of various inorganic minerals, clays, metal cyanogen complexes and metal oxides, which have been efficient in concentrating the organic molecules on their surfaces and subsequently catalyzed a class of prebiotic reactions during the course of chemical evolution have 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 oxides have been given. Discussions have also been made on methods used for the synthesis of nucleobases from formamide and oligomerization of amino acids in the presence of manganese oxides. Further the conditions for the interaction of aromatic amines with manganese 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 various manganese oxides (manganosite (MnO), bixbyite (Mn203), hausmannite (Mn304) and pyrolusite (Mn02)). Adsorption trend was found to follow the Langmuir Adsorption Isotherm. Maximum adsorption was found to occur at neutral pH (-'7.0), whereas among the ribose nucleotides, 5'- GMP was found to be adsorbed more on Manganosite (MnO) 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 the presence of manganese oxides (manganosite (MnO), bixbyite (Mn203), hausmannite (Mn304) and pyrolusite (Mn02)). It was observed that manganosite (MnO) 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 manganese oxides (manganosite (MnO), bixbyite (Mn203), hausmannite (Mn304) and pyrolusite (Mn02)) at various experimental conditions such as temperature range 50-120 °C and for a time span of 35 days. It was observed that all the four manganese oxides catalyzed the 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. The Sixth chapter illustrates the results of studies on interaction of aromatic amines (aniline, p-toluidine, p-chloroaniline and p-anisidine) with manganese oxides (manganosite (MnO), bixbyite. (Mn203), hausmannite (Mn304) and pyrolusite (Mn02)). 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 manganosite (MnO). Infrared spectral studies on the adsorption adducts showed that adsorption of ribose nucleotides occurred due to interaction of positively charged surface of manganese oxide and the basicity of amines. During adsorption studies of the aromatic amines on manganese (iv) 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 manganese oxide might have played its important role in chemical evolution for the concentration and polymerization of biomolecules.