ROLE OF METAL OXIDES AS PREBIOTIC CATALYST

Abstract

Several simulated experiments and theoretical evidences have led us to believe that origin of life processes began with the formation of important biomonomers such as amino acids, nucleotides and sugars from simple molecules under prebiotic environment. These biomonomers have undergone numerous complicated reactions producing more and more complex material leading to the formation of first living cell on the Earth. Early oceans seem as the probable birth place of life on our planet. The crucial step in chemical evolution must have involved to concentrate the biomonomers from dilute aqueous solutions and then to polymerize them into biopolymers. It is proposed that catalysis had a central role in the formation of biopolymers. Catalysts may have been important for the origins of life because they tend to direct the reaction along a few reaction 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. It is assumed that solid surfaces of inorganic mineral and clays could 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 constitute an important component of Earth crust and other planets. Aluminium oxide, iron oxide, zinc oxide, silica, titanium oxide is the important constituent of Earth crust and also of Mars. It is assumed thatthey might have catalyzed the important reactions of prebiotic relevance. Metal oxides are (ii) normally insoluble in water. It is proposed that they might have locally settled at the bottom or at the sea shores and could 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 metal oxides towards different aspects of chemical evolution and origin of life have been presented. We have studied the interaction of ribose nucleotides with aluminium oxide, zinc oxide and hematite. During these studies it is found that metal oxides are good adsorbent towards ribose nucleotides as compared to other inorganic minerals. Zinc oxide has also been found to catalyze the formation of purine and adenine from formamide and also plays a vital role in the dimerization of L - alanine to dialanine. Role of metal oxides in the hydrolysis of pnitrophenylphosphate (4-NPP) and p-nitrophenylacetate (4-NPA) has also been studied. Results of the above studies has been reported in the form of five chapters as follows. 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, metalcyanogencomplexes 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 is discussed. The second chapter describes experimental methodology and instrumentation. This chapter presents the method of the synthesis of some metal oxides, their characterization and methods of chemical analyses involved. The (Hi) f metal oxides have been synthesized by precipitation method and characterized using magnetic susceptibility measurements, TGA/DTA and X - ray diffraction patterns. Experimental conditions and techniques used for the adsorption of ribose nucleotides on metal oxides have been given. Methods used for the hydrolysis of 4-NPP and 4-NPA with metal oxides, procedure adopted for the synthesis of adenine and purine from formamide and dimerisation of L-alanine 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 zinc oxide. 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 zinc oxide. Infrared spectral studies of the adsorption adducts showed that adsorption of ribose nucleotides takes place due to interaction of positively charged surface of metal oxide and negatively charged groups of ribose nucleotides. Hydrolysis reaction of 4-NPA with zinc oxide has also been studied. Kinetic measurements have been done. This chapter comprises the result of studies on the catalytic role of zinc oxide in the formation of purine and adenine from formamide. Zinc oxide has been found as a good catalyst for the synthesis of adenine. In this chapter we also report the results on the catalytic role of zinc oxide in the polymerization of Lalanine to dialanine. Zinc oxide has been found to be good catalyst for the polymerization of amino acid.

The fourth chapter presents the results of studies on interaction of ribose nucleotides (5'-AMP, 5'-GMP, 5'-CMP, and 5'-UMP) with three forms of aluminium oxide namely, acidic, neutral and basic alumina. Adsorption trend was found to follow Langmuir Adsorption Isotherm. Maximum adsorption was found to occur on acidic alumina, whereas among ribose nucleotides 5'-GMP was found to be adsorbed more on alumina. Infrared spectral studies of the adsorption adducts showed that adsorption of ribose nucleotides takes place due to interaction of positively charged surface of metal oxide and negatively charged groups of ribose nucleotides. Further, hydrolysis of 4-nitro phenyl phosphate with aluminium oxide has also been studied. Kinetic studies for the hydrolysis reaction has been performed. Reaction has been found to be first order in nature. The fifth chapter comprises the result of studies on the synthesis of iron oxide with calcinations at different temperatures and interaction of ribose nucleotides (5'-AMP, 5'-GMP, 5'-CMP, and 5'-UMP) with iron oxide. Adsorption trend was found to be maximum on iron oxide hydroxide synthesized at 70°C. Adsorption trend 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 oxide. Infrared spectral studies of the adsorption adducts showed that adsorption of ribose nucleotides takes place due to interaction of positively charged surface of metal oxide and negatively charged groups of ribose nucleotides. Hydrolysis of 4-NPP has also been studied with iron oxide. Kinetic studies have also been performed.