Aggregation and Adsorption at the Air-Water Interface of Bacteriophage ϕX174 Single-Stranded DNA

We study the phase behavior of phage ϕX174 single-stranded DNA in very dilute solutions in the presence of monovalent and multivalent salts, in both water (H2O) and heavy water (D2O). DNA solubility depends on the nature of the salts, their concentrations, and the nature of the solvent. The appearance of attractive interactions between the monomers of the DNA chains in the bulk of the solution is correlated with an adsorption of the chains at the air-water interface. We characterize this correlation in two types of aggregation processes: the condensation of DNA induced by the trivalent cation spermidine and its salting out in the presence of high concentrations (molar and above) of monovalent (sodium) cations, both in water and in heavy water. The overall solubility of single-stranded DNA is decreased in D2O compared to H2O, pointing to a role of DNA hydration in addition to electrostatic factors in the observed phase separations. DNA adsorption involves attractive van der Waals forces, and these forces are also operating in the bulk aggregation process.