DNA phase behavior in the presence of oppositely charged surfactants

The interaction between DNA and alkyltrimethylammonium bromides of various chain lengths has been investigated. It is known that these systems phase separate with the formation of a precipitate; this important feature allows, for example, purification of nucleic acids. Phase maps were drawn for the aqueous systems illustrating the associative phase separation. The boundary of the two-phase region for the dilute part; of the phase diagram was evaluated by turbidimetry, in both the absence and presence of salt. The extension of the precipitate region increases strongly with the surfactant alkyl chain length, and we observed no redissolution with an excess of surfactant. The addition of NaBr led to novel interesting findings. The phase diagram studies were correlated with the single molecule conformational behavior of the same systems as studied for very diluted solutions by fluorescence microscopy. DNA exhibits a discrete phase transition in the presence of cationic surfactants from coils to globules. Results demonstrate that the coil-globule coexistence interval is narrow for CTAB and becomes wider for the shorter-chained surfactant. The findings for flexible polyions of lower charge density differ qualitatively from what we find here for DNA. For the first, large amounts of surfactant have to be added before phase separation occurs, and the change in the polyion extension is gradual, indicating an essentially uniform distribution of surfactant aggregates among the different polyions. For DNA, the very low values of surfactant concentration at which phase separation starts demonstrate a different binding interaction; as binding to a polyion starts, further binding is facilitated, and one DNA molecule is saturated before binding starts at another.