Adsorbing DNA to Mica by Cations: Influence of Valency and Ion Type

Ion-mediated attraction between DNA and mica plays a crucial role in biotechnological applications and molecular imaging. Here, we combine molecular dynamics simulations and single-molecule atomic force microscopy experiments to characterize the detachment forces of single-stranded DNA at mica surfaces mediated by the metal cations Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺. Ion-specific adsorption at the mica/water interface compensates (Li⁺ and Na⁺) or overcompensates (K⁺, Cs⁺, Mg²⁺, and Ca²⁺) the bare negative surface charge of mica. In addition, direct and water-mediated contacts are formed between the ions, the phosphate oxygens of DNA, and mica. The different contact types give rise to low- and high-force pathways and a broad distribution of detachment forces. Weakly hydrated ions, such as Cs⁺ and water-mediated contacts, lead to low detachment forces and high mobility of the DNA on the surface. Direct ion-DNA or ion-surface contacts lead to significantly higher forces. The comprehensive view gained from our combined approach allows us to highlight the most promising cations for imaging in physiological conditions: K⁺, which overcompensates the negative mica charge and induces long-ranged attractions. Mg²⁺ and Ca²⁺, which form a few specific and long-lived contacts to bind DNA with high affinity.