Transition Metals Enhance the Adsorption of Nucleotides onto Clays: Implications for the Origin of Life

The chemical evolution of early life requires the concentration of monomers to polymerize from the diluted primordial ocean. Transition metals such as Fe, Mn, and Zn, could have reached considerable levels in the early seawater and/or hydrothermal fluid but their influences on the adsorption of biomolecules have not been clearly addressed yet. In this study, we conducted batch adsorption experiments to explore effects of various metal cations (Li, Mg, Ca, Zn, Ni, and Mn) on the adsorption of selected nucleotides (dGMP, dAMP, and AMP) and adenosine onto nontronite and montmorillonite. We also varied the concentration of the cations and pH of the solutions to evaluate their effects. Our results show that Zn and to some extent Ni increase the adsorption of nucleotides and adenosine compared with Na, Mg, and Ca which are major salts in modern seawater. This increased adsorption is primarily attributed to the mediating role of transition metals between the clays and nucleotides and adenosine. The enhancing effect depends little on salt concentration, but strongly varies as the pH of the solution changes. Presence of transition metals reverses the declining trend of the adsorption of dGMP as the elevation of pH and strongly favors adsorption of dGMP at alkaline pH presumably through precipitation of metal-hydroxides on the clay surface. Enhanced adsorption amount of biomolecules mediated by transition metals would potentially ease the origin of life in two aspects: concentration of simple organics for polymerization and protection of early biomolecules against UV radiation and heating in early seawater.