No abstract

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.

Structural and phase behavior effects resulting from the addition of a polyelectrolyte to a solution of oppositely charged vesicles are investigated in this work. Two cationic polyelectrolytes derived from hydroxyethylcellulose were used: JR400, a homopolymer, and Quatrisoft LM200, a polymer modified with alkyl side chains. The vesicles are composed of mixed anionic surfactant (sodium dodecyl sulfate) and cationic surfactant (didodecyldimethylammonium bromide), bearing 29 mol % of the cationic amphiphile. The phase behavior for the two mixed polymer-surfactant systems was investigated for polymer concentrations between 0.001 and 3 wt%. Three main regions were found in the two-phase maps, upon polymer addition: (i) a bluish solution phase; (ii) a wide region of phase separation, containing a precipitate and a solution; and (iii) a polymer-rich gel region, forming upon charge reversal of the system. Cryo-TEM imaging of the solution phase shows the formation of faceted vesicles and disklike aggregates, upon addition of JR400. For the LM200 system, besides the formation of faceted vesicles, clusters of vesicles and other bilayer structures are imaged. In the polymer-rich phase of JR400, membrane fragments, disklike aggregates, and vesicles are also found. These bilayer aggregates are likely to be involved with the polymer in highly connected networks, giving rise to the observed bluish gels. Electrostatic interactions, reinforced by hydrophobic interactions in the case of LM200, are the main driving force for the structural transitions observed.