Inter-DNA attraction mediated by monovalent ions*

The compaction of negatively charged DNA is, in some cases, driven by crowding neutral polymers, while monovalent ions purportedly only act to screen inter-DNA repulsion. On the other hand, multivalent ions govern DNA condensation through strong electrostatic inter-DNA attraction. In the past, tantalizing hints implying monovalent ions induced DNA attraction was never proven due to the lack of a desirable technique. Here we unveil the existence of monovalent ions mediated effective attraction interactions between DNA molecules at high salt concentrations. We employ a new nanopore-based sensing technique to precisely quantify the knotting probability of long DNA chains, which is a very sensitive measure to inter-DNA interactions. Modelling effective DNA width from the knotting data, we demonstrate that the DNA-DNA interaction evolves from repulsive to attractive with increasing salt concentration, with the onset of attraction at the critical ionic concentration ranging between C_?????? = 1.5?? − 2??. The C_?????? corresponds to mean ionic distance approaching the hydration ion radius and scales well with the hydration radius of different salts indicating ion-induced correlations as the driving force for attraction. Our results provide valuable insights into behavior of DNA in the physiological environment and will be useful for the development of synthetic biological machinery in high salt monovalent ionic environments.