ROLE OF METAL FERROCYANIDES IN CHEMICAL EVOLUTION

Abstract

Recent years have been marked by significant progress m the studies related to chemical evolution and origin of life. It is widely accepted that the origin of life processes began with the formation of important biomonomers from simpler molecules present in the prebiotic environment and their subse quent condensation to biopolymers. Bernal was the first to put forward the hypothesis that the clay minerals might have Played an important role in,concent rating monomeric molecules. Due to ease of formation of cyanide under prebiotic conditions abundance of cyanide is very likely on the primitive earth. It is assumed that during the course of chemical evolution cyanide might have formed some insoluble metal ferrocyamd with the abundant transition metal ion ies earth. It s present on the primitive is also proposed that these metal ferrocyanides settled at the shore of primitive ocean could have adsorbed biomonomers and may have been one of the important factors creating conditions for further structural complication of molecules in chemical evolution. No studies, so far have been made to utilise these compounds in the field of chemical evolution. In recent years considerable work has been done on the adsorption and reactions of biomonomers. Sorption of < -amino acids, by copper montmorillonite has been reported by Bodenheimer and Heller. Detailed studies on adsorption of 15 proteinous (iii) amino acids from dilute (10 uM) distilled water solutions onto kaolinite and montmori1lonite have been recently done by Hedges and Hare. Lawless and Levi have synthesized polypeptides conta ining a maximum of eight amino acid molecules through adsorption process under wetting and drying cycles using clays such as montmorillonite, kaolinite, and bentonite coordinated with divalent cations, viz., Zn2+, Cu2+, and Mg2+ etc. The discovery that RNA can catalyze chemical transformat ions suggested that RNA could have served both as catalyst and repository for genetic information in primitive life. Many of the researchers working on formation of different biomolecules concentrated their studies on nucleotide polymerization only. Recently adsorption and polymerization of nucleotides on clay surface have been extensively studied by Ferris and his coworkers where several 3'-5' linked polynucleotides from simple monomers have been demonstrated in the presence of Na+-montmorillonite. The results of our studies presented in the thesis are based on the adsorption of oC -amino acids, ribose 5'-mononucleo tides, and 2'-deoxyribose 5'-mononucleotides on zinc- and copper ferrocyanides. A separate chapter is devoted on studies on the effect of copper- and^ zinc ferrocyanides on condensation of 2'-deoxy 5'-mononucleotides using a water soluble condensing agent, l-ethyl-3(3-dimethylaminopropyl ) carbodiimide,EDAC. The work embodied in thesis is divided into six chapters and the summary of each chapter is described as below : First Chapter of the thesis presents an uptodate survey of literature on the role of clays and other inorganic minerals (iv) in concentration and condensation of biomonomers to afford biopolymers in the course of chemical evolution. In the light of the work already done statement of the problem of present research has been made. Second Chapter deals with the experimental techniques involved in the subsequent studies on the adsorption of amino acids and nucleotides and also the experimental details on the condensation reactions of 2'-deoxymononucleotides. The procedural details of the separation and characterization of oligomers of 2'-deoxynucleotides using HPLC is also described. Third Chapter describes details of the results on the adsorption of aspartic acid, histidine, and glycine each being representative of acidic, basic and neutral amino acids respect ively on copper- and zinc ferrocyanides at pH 7.01. All the three amino acids showed higher adsorption on zinc ferrocyanide than the copper ferrocyanide. Studies on the effect of pH on adsorption suggested that all the three amino acids adsorbed in the form of anions on both the metal ferrocyanides as adsorp tion was found to be maximum above the isoelectric points of the amino acids. Neverthless, adsorption of histidine on zinc ferrocyanide occured below its isoelectric point. Adsorption phenomenon of amino acids on both copper- and zinc ferrocyanide followed Langmuir Adsorption Isotherm. Kr and V have been L m calculated and variations in their values have been suitably interpreted. Fourth and Fifth Chapters represent the results of studies on adsorption of ribose 5'-mononucleotides and 2'-deoxyribose (v) 5'-mononucleotides,respectively on zinc- and copper ferrocyanides at pH 7.01. All the nucleotides of ribose as well as 2'-deoxyribose (except pT) followed Langmuir Adsorption Isotherm on both the metal ferrocyanides studies. Adsorption of pT does not show the saturation phenomenon. Nature of adsorption was mostly irreversible as extraction of nucleotides from either of the metal ferrocyanides with 0.1M NH40H or 0.1M NH.Cl was unsuccessful. It was also observed in both ribose- or 2•-deoxyribose nucleotides that purine containing nucleotides were adsorbed quantitatively more than the pyrimidine containing nucleotides. A general explanation could be because complexation of the pyrimidine nucleotides can occur only through the phosphate moiety, while the significantly larger adsorption •of purine nucleotides at neutral pH may be due to its ability to form a bridging complex with the metal cations which involves the available N-7 purine ring position. In the Sixth and last chapter of the thesis results on oligomerization of 2'-deoxy 5'-mononucleotides in the presence of copper- and zinc ferrocyanides have been presented. A water soluble condensing agent l-ethyl-3-(3-dimethylaminopropyl) carbodiimide was used. Condensation reactions were carried out at two pHs viz., 6.5 and 8.5. in most of the cases nucleotides alongwith the oligomerized products if any formed were adsorbed on the metal ferrocyanides as HPLC analysis of the supernatant of the reaction mixtures did not give any peak of the product. However, in two instances remarkable results were obtained. Firstly, oligomerization of 2'-dpA with EDAC in the presence (vi) of copper ferrocyanide at pH 6.5 afforded 6.1 °/e of 3•:5•cdpA from a lower of 0.1% in its absence. Secondly, condensation of 2' dpc with EDAC in the presence of zinc ferrocyanide produced 60.8% of 3':5' cdpG. These two observations are remarkable as in none of the two reactions, pyrophosphate (5'-5' linked) products were formed.