pH profile of the adsorption of nucleotides onto montmorillonite
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 were Na+, Mn2+, Fe3+, Co2+, Ni2+, 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 and 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.