DNA bundle formation induced by cationic surfactants

We have evidenced bundle formation in DNA solutions, induced by the addition of a monovalent cationic surfactant to a solution of short DNA rods. The bundles are rod-like, with a large axial ratio ( ∼ 10). Their formation is very likely controlled not only by electrostatic interactions, but also by hydrophobic interactions between surfactant chains, once bound to the DNA. The bundles have diameters and lengths both remarkably well defined and possess internal organisation. The value of the diameter is close to that predicted by a model in which interactions are mediated by condensed counterions; the length might be limited by steric hindrance due to bundle overlapping or by intrinsic chain flexibility.