ROLE OF METAL CYANOGEN COMPLEXES 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 pentose 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 steps in chemical evolution must have involved to concentrate the biomonomers from dilute aqueous solutions and then to polymerize them into biopolymers. 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. Cyanide ions were readily available in prebiotic environment. Since cyanide ion acts as strong field ligand and shows greater ease of coordination, it is assumed that cyanide ions might have complexed with transition metal ions abundantly present in primeval seas. Thus, a series of metal cyanogen complexes could have been formed in primeval seas. Arrhenius proposed the existence of ferri-ferrocyanide in Anoxic Archean Hydrosphere. As most of the metal cyanogen complexes of transition metal ions are insoluble in water, it is proposed that it might have locally settled at the bottom or at the sea shores. (ii) Ferrocyanide ions form a green rust interlayer complexe of higher stability. In view of the perceived potential of inorganic minerals, metal cyanogen complexes of general formula M'x [M(CN)6]y.nH20, where Mand M' represent transition metal ions, have been proposed as an important prebiotic catalyst. Metal cyanogen complexes could have concentrated the biomonomers from their dilute aqueous solutions and subsequently catalyzed a class of reactions of prebiotic relevance. Interaction of some aliphatic amino acids, nucleotides and aromatic amines with metal ferrocyanides has previously been studied in our laboratory. Metal ferrocyanides have also been found to catalyze some important reactions of prebiotic relevance. Present study deals with the interaction of ribose nucleotides, aromatic amino acids with metal ferrocyanides, metal cobalticyanides and metal chromicyanides. During our studies it has been observed that all the metal cyanogen complexes are efficient adsorbent towards ribonucleotides (5'-AMP, 5'- GMP, 5'-CMP and 5'-UMP) and aromatic amino acids (tryptophan and phenylalanine). Copper chromicyanide has been found to catalyze the dimerisation of cysteine to cystine and conversion of fructose to pyruvaldehyde etc. The results of our studies are presented in this thesis, which consists of five chapters. The first chapter of the thesis deals with the introduction of the topic chemical evolution and origin of life and up to date literature survey on inorganic minerals and clays which were efficient in concentrating the organic molecules on their surfaces and subsequently catalyzed aclass of prebiotic reactions during (iii) the course of chemical evolution. The present research problem has been posed in the light of work already done. The second chapter describes the experimental part and presents the synthesis of different metal cyanogen complexes, their characterization and method of analysis involved. A series of metal ferrocyanides, metal cobalticyanides and metal chromicyanides were synthesized and characterized using elemental analyses, infrared spectral studies, magnetic susceptibility measurements and TGA/ DTA as well as x-ray diffraction patterns. Proposed molecular formulae of the synthesized metal cyanogen complexes of different series are as follows: 1. Metal Ferrocyanides (i) Zn2[Fe(CN)6].3H20 (ii) Ni2[Fe(CN)6].5H20 (iii) Co2[Fe(CN)6].5H20 (iv) Cu2[Fe(CN)6].7H20 (v) Cd2[Fe(CN)6] 2. Metal Cobalticyanides (i) Co3[Co(CN)6]2.14H20 (ii) Cu3[Co(CN)6]2.14H20 (iii) Cd3[Co(CN)6]2.14H20 (iv) Sn3[Co(CN)6]2.12H20 (iv) 3. Metal Chromicyanides (i) Co3[Cr(CN)6]2.14H20 (ii) Cu3[Cr(CN)6]2.6H20 (iii) Cd3[Cr(CN)6]2.14H20 (iv) Sn3[Cr(CN)6]2.4H20 Adsorption of different substrates on metal cyanogen complexes was studied spectrophotometrically. The third chapter presents the results of studies on interaction of ribose nucleotides (5'-AMP, 5'-GMP, 5'-CMP, 5'-UMP) with cobalt and cadmium ferrocyanides and aromatic amino acids (tryptophan and phenylalanine) with metal ferrocyanides, namely, zinc-, nickel-, cobalt-, and copper ferrocyanides. Adsorption trend at neutral pH was found to follow Langmuir Adsorption Isotherm. Maximum adsorption was found for tryptophan on zinc ferrocyanides as 43.8% whereas, among ribose nucleotides, 5'-GMP on cobalt ferrocyanide was maximum as 37.3%. The metal ferrocyanides were found as more effective adsorbent towards amino acids than ribose nucleotides. Infra red spectral studies of the adsorption adducts showed that adsorption of ribose nucleotides and aromatic amino acids on metal ferrocyanides takes place due to interaction between adsorbate molecule and outer metal ion present in the lattice of metal ferrocyanides. The fourth chapter comprises the results of studies on the interaction of ribose nucleotides (5'-AMP, 5'-GMP, 5'-CMP, 5'-UMP) with stannous-, cobalt-, copper-, and cadmium cobalticyanides. The adsorption isotherms were found as (v) Langmurian in nature. Metal cobalticyanides were found as effective adsorbents towards ribose nucleotides. 5'-GMP showed maximum adsorption on all the four metal cobalticyanides studied. Maximum adsorption (60.6%) was found for 5'- GMP on Sn3[Co(CN)6]2.12H20 where as 5'-UMP on Cd3[Co(CN)6]2.14H20 showed minimum adsorption (30.7%). The fifth and the last chapter of the thesis presents results on the studies with metal chromicyanides, namely, stannous-, cobalt-, copper-, and cadmium chromicyanides. Interaction of metal chromicaynides with all the four ribose nucleotides has been studied. Again among nucleotides, 5'-GMP was found to adsorbed more on all the metal chromicyanides studies. Stannous chromicyanide was found as the best adsorbent among all the four metal chromicyanides. Further, adsorption of the two naturally occurring aromatic amino acids (tryptophan and phenylalanine) on metal chromicyanides has also been studied. Results on adsorption of these amino acids are similar to those with metal ferrocyanides. Out of all the metal chromicyanides studied, copper chromicyanide was found to catalyze some important reactions relevant to chemical evolution and origin of life. It was found to catalyze the conversion of cysteine to cystine, fructose to pyruvaldehyde and hydroquinone to benzoquinone. A detailed study of the above reactions catalyzed by copper chromicyanide has been done and results are presented.