STUDIES ON METAL FERROCYANIDES (II) AS PREBIOTIC CATALYST

Abstract

During the last few decades experimental and theoretical evidences suggest that life on earth might have evolved as a results of several complicated reactions that had occurred on earth's surface. Gradually the complexity of reactions increased and more and more complex molecules were formed which ultimately gave birth to the first living cells. It is thought that minerals containing active surface like clays, mixed valence hydroxides etc. could have played a pivotal role in the course of chemical evolution. The surface of these solids might have catalysed a class of prebiological reactions responsible for origins of life. Scientists have proposed the early oceans as the probable birth place of life on our planet. One of the considerations that strongly favours the oceans, as the primeval locale for chemical evolution is that organic compounds, once formed and settled in ocean would have been protected against the radiolysis caused by strong ultraviolet light on primitive earth. Further, the hypothesis of chemical evolution in primeval seas is preferred because the oceans would have provided a wealth of catalytic surface, source of transportation and mixing of intermediate products in addition to their protection from ultraviolet light. Cyanide ion reacts rapidly with the ferroferric end member (green rust) in pyroaurite series to form ferrocyanide, while ferrocyanide ion, presumably the most common cyanide species in anoxic ocean, forms a green rust interlayer complex of higher stability. In view of the perceived potential of inorganic minerals, metal ferrocyanide of (ii) general formula M2[Fe(II)(CN)6], where M is a transition metal, has been proposed as an important prebiotic catalyst. Accumulation of such metal ferrocyanide locally on the primitive earth has been proposed. Interaction and reaction of amino acids and nucleotides with zinc, nickel and copper ferrocyanides have been studied previously in our laboratory. A number of workers have proposed the formation of benzene and the compounds containing benzene ring i.e. phenyl acetylene, naphthalene, styrene, aromatic hydrocarbons and compounds containing nitrogen heterocyclic compounds etc. in prebiotic conditions. Therefore, one could reasonably postulate the presence of aromatic amines and pyridine rings on primitive earth environment. In the present work, interaction of some simple organic molecules e.g. aromatic amines (aniline, p-toluidine and p-chloroaniline) and aminopyridines (2-amino,3-amino and 4-aminopyridines) with several metal ferrocyanides (namely, manganese, cobalt, nickel, copper, zinc and cadmium ferrocyanides) has been studied. In the course of studies on the interaction of aromatic amines and aminopyridines with metal ferrocyanides, it was observed that all the metal ferrocyanides show high affinity for adsorption of aromatic amines as well as aminopyridines. While manganese ferrocyanide showed chemical reaction with both the class of compounds and gave coloured products. The results obtained are presented in the thesis, which contains six chapters. The first chapter of the thesis presents an uptodate survey of literature on the role of clays and other inorganic minerals in concentration and condensation of biomonmers to afford biopolymers (iii) in the course of chemical evolution. Also the problem of present research has been posed in the context of the cited work. The second chapter concerns with the preparation of different metal ferrocyanides using potassium ferrocyanide and salts of respective metals by standard methods reported in the literature. The metal ferrocyanides were characterized by various techniques like elemental analysis, spectral, magnetic moment and thermal studies. Different experimental methods employed in the research have also been written,in this chapter. The third chapter contains results of the studies on adsorption of aromatic amines on Co, Ni, Zn and Cd ferrocyanides. Adsorption of aromatic amines has been found to obey the Langmuir Adsorption Isotherm. Effects on the i.r. frequencies of absorbate as well as adsorbent after adsorption have also been studied. During the adsorption studies it was found that neutral pH (7.0) was suitable for adsorption studies. Acidic and basic medium were found to be less suitable for adsorption. The fourth chapter deals with the analysis of oxidation products formed due to reaction of aromatic amines with manganese ferrocyanide. Products formed are characterized as amine oligomers containing two to four units. Characterization of products has been done on the basis of spectral and GC-MS studies. Aniline gives a cyclic tetramer as oxidation product. p-Toluidine showed the formation of trimer whereas p-chloroaniline give dimer as oxidation product. The fifth chapter of the thesis incorporates the detailed adsorption studies of aminopyridines (2-amino, 3-amino and 4- (iv) aminopyridines) with metal ferrocyanides (namely; Co, Ni, Cu and Zn ferrocyanides). Adsorption in this case also follows the Langmuir Adsorption Isotherm. pH effect on the adsorption studies were also carried out and neutral pH (pH = 7.0) was found suitable for the studies. Shifts in typical i.r. bands of aminopyridine and ferrocyanides after adsorption has also been discussed. The sixth chapter consists of results on the oxidation products formed due to reaction of aminopyridines with manganese ferrocyanide. Oxidation products formed due to reaction of all the three aminopyridines showed a corresponding dimer (azopyridines). Characterization of all the products have been carried out with the help of spectral and GC-MS studies. The present research supports our view that metal ferrocyanides which were present on the primitive earth could have acted as a prebiotic catalyst.