| Abstract
| - The interaction between DNA and alkyltrimethylammonium bromides of various chain lengths hasbeen investigated. It is known that these systems phase separate with the formation of a precipitate; thisimportant feature allows, for example, purification of nucleic acids. Phase maps were drawn for the aqueoussystems illustrating the associative phase separation. The boundary of the two-phase region for the dilutepart of the phase diagram was evaluated by turbidimetry, in both the absence and presence of salt. Theextension of the precipitate region increases strongly with the surfactant alkyl chain length, and weobserved no redissolution with an excess of surfactant. The addition of NaBr led to novel interestingfindings. The phase diagram studies were correlated with the single molecule conformational behavior ofthe same systems as studied for very diluted solutions by fluorescence microscopy. DNA exhibits a discretephase transition in the presence of cationic surfactants from coils to globules. Results demonstrate thatthe coil−globule coexistence interval is narrow for CTAB and becomes wider for the shorter-chainedsurfactant. The findings for flexible polyions of lower charge density differ qualitatively from what we findhere 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 surfactantaggregates among the different polyions. For DNA, the very low values of surfactant concentration atwhich 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.
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