Polyvalent Metal Ion Promoted Adsorption of DNA Oligonucleotides by Montmorillonite

Montmorillonite (MMT) is a two-dimensional (2D) clay material. Its abundance on the early earth has attracted studies for its role in prebiotic reactions, and adsorption of DNA to MMT is potentially important for understanding the origin of life. Although several possible models of DNA adsorption on MMT have been established, a consensus on the adsorption mechanism has yet to be formed, thereby a fundamental adsorption study is performed here. Adding up to 300 mM NaCl failed to promote DNA adsorption on MMT, Al₂O₃, or SiO₂ nanoparticles. For polyvalent metals, DNA adsorption was achieved following the order Ce³⁺ > Cu²⁺ > Ni²⁺ > Zn²⁺. Among them, Ce³⁺ and Cu²⁺ inverted the surface charge of MMT to positive. In addition, using washing experiments, Cu²⁺- and Ce³⁺-mediated adsorption mainly depended on the DNA phosphate backbone, while Ni²⁺ and Zn²⁺ interacted with the backbone phosphate groups and adenine bases of DNA. Overall, these polyvalent metal ions promoted DNA adsorption via a cation bridge model. This research provides new insights into the surface interactions of MMT and DNA, which is conducive to future work on the interaction between clays and biopolymers.