Hydrodynamic properties of DNA and DNA–lipid complex in an elongational flow field

The aim of this study was to determine the difference between hydrodynamic properties of DNA–cetyltrimethylammonium (CTA) complex and those of DNA, which may be related to the difference in fibre-forming ability of DNA–CTA from that of DNA. Responses of DNA and DNA–CTA complex to an elongational flow field were investigated. In both solution systems, results suggesting a coil–stretch transition were obtained. From a critical strain rate value, the radius of gyration of DNA–CTA molecules in ethanol–glycerol solution was revealed to be 0.3–0.5 times of that of DNA in aqueous NaCl solution. Shear viscosity of DNA–CTA solution was much smaller than that of DNA solution, also suggesting a smaller size of DNA–CTA in ethanol–glycerol solution than that of DNA in aqueous NaCl solution. The plateau birefringence value of the DNA–CTA system, a parameter that indicates the local molecular conformation and the molecular arrangement, was only about 1/10 of that of the DNA system. There is an empirically determined molecular model of DNA–CTA complex in which a DNA molecule is sheathed by a cylindrical crust made of CTA chains. This structure reduces the DNA molecular density in a pure elongational flow field region but cannot explain the observed reduction of birefringence intensity. The small plateau birefringence value of DNA–CTA compared with that of DNA was attributed to the reduced molecular polarizability by the particular conformation of DNA molecules and CTA chains in the DNA–CTA system such as that expected by the conformational models.