A mechanism for the entrapment of DNA at an air-water interface.

Addition of the intercalating dye quinacrine to a low ionic strength solution of DNA in quantities sufficient to saturate the high affinity sites in the DNA will result in the accumulation of the DNA at the solution interface. This entrapment of DNA at the air-water interface has been assayed by the adsorption of DNA to untreated carbon-coated electron microscope grids touched to the solution surface. Other intercalating dyes can also bring about this entrapment, if they possess a side arm large enough to occupy one of the DNA grooves when the dye is intercalated into the DNA. The extension and unwinding of the DNA helix brought about by the intercalating chromophore of the dye molecules are not requirements for the entrapment process. Spermidine, a simple polyamine that will bind to the DNA minor groove but that has no intercalating chromophore, was found to bring about this entrapment. Even simple mono- and divalent cations in the absence of the above ligands were found to promote a low level of surface entrapment. A model for the entrapment of DNA at the air-water interface is proposed in which one (or both) of the hydrophobic grooves of the DNA becomes a surface-active agent as a consequence of the association of various ligands and charge neutralization.