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The effect of adsorbed ions and pH on the adsorption of several purine and pyrimidine nucleotides on montmorillonite was studied. The cations used to prepare homoionic montmorillonite was Na+, Mn2+, Fe3+, Co2+, Ni+, Cu2+, and Zn2+. The nucleotides studied were 5'-,3'-, and 2'-AMP, and 5'-CMP in the pH range 2 through 12. The results show that preferential adsorption amongst nucleotides and similar molecules is dependent upon pH and the nature of the substituted metal cation in the clay. At neutral pH, it was observed that 5'-AMP was more strongly adsorbed than 2'AMP, 3'-AMP, and 5'-CMP. Cu2+ and Zn2+ clays showed enhanced adsorption of 5'-AMP compared to the other cation clays studied in the pH range 4-8. Below pH 4, the adsorption is attributed to cation and anion exchange adsorption mechanisms: above pH 4, anion exchange may also occur, but the adsorption (when it occurs) likely depends on a complexation mechanism occurring between metal cation in the clay exchange site the biomolecule. It is thus proposed that homoionic clays may have played a significant role in the concentration mechanism of biomonomers in the prebiotic environment, a prerequisite step necessary for the formation of biopolymers in the remaining steps leading to the origin of life.

The adsorption of 5'-AMP and 5'-CMP was studied in saturated solutions of several soluble mineral salts (NaCl, Na2SO4, MgCl2 X 6H2O, MgSO4 X 7H2O, CaCl2 X 2H2O, CaSO4 X 2H2O, SrCl2 X 6H2O, SrSO4, and ZnSO4 X 7H2O) as a function of pH, ionic strength, and surface area of the solid salt. The adsorption shows a pH dependence; this can be correlated with the charge on the nucleotide molecule which is determined by the state of protonation of the N-1 nitrogen of 5'-AMP or N-3 nitrogen of 5'-CMP and the phosphate oxygens. The adsorption which results from the binding between the nucleotide molecule and the salt surface is proposed as being due to electrostatic forces. It was concluded that the adsorption was reversible in nature. The adsorption shows a strong dependence upon ionic strength and decreases with increasing ionic strength. Surface area is shown to be an important factor in evaluating and comparing the magnitude of adsorption of nucleotides onto various mineral salts. The implications of the results of the study are discussed in terms of the importance of soluble mineral salts as adsorption sites in the characterization of the adsorption reactions of an adsorbed template in biogeochemical cycles.