Adsorbing DNA to mica by cations: Influence of the 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+, 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 a 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+ to overcompensate the negative mica charge and induce long-ranged attractions. Mg2+ and Ca2+ to from a few specific and long-lived contacts to bind DNA with high affinity.