Aggregates formation between short DNA fragments and cationic surfactants

We have studied aggregate formation between dodecyltrimethylammonium bromide (DTAB) and monodisperse short fragments of DNA (50 nm length). DTAB is a cationic surfactant, able to bind to polyelectrolyte chains bearing electrical charges of opposite sign such as DNA, and to precipitate them out above a certain concentration. Light scattering was used to determine the size, shape and composition of the surfactant-polymer aggregates. The aggregates have a low polydispersity, contain several polymer chains and grow in a nonlinear manner with increasing binding ratio (as obtained with surfactant selective electrodes), when surfactant concentration increases. In addition, small DNA secondary structural changes upon surfactant binding were found (by using several spectroscopic methods). As surfactant concentration is increased further, phase separation is observed and X-ray scattering data for the DNA-rich phase shows that it has a liquid crystalline structure.