Salt-controlled intrachain/interchain segregation in DNA complexed with polycation of natural origin

A coil-globule transition in individual double-stranded DNA triggered by chitosan polycations was directly observed by fluorescence microscopy. It was shown that the character of the conformational transition depends essentially on the amount of added low-molecular-weight salt. At low salt content the collapse proceeds via the formation of an intermediate necklace structure, which is stabilized by Coulomb repulsion between uncompensated charges of DNA chains. By contrast, at high salt content, when the electrostatic repulsion is sufficiently screened, the collapse proceeds directly between coil and globule states without intermediate structures. Also, salt affects the collapsed state of chitosan/DNA complexes: it decreases the size of single globules and favors the interglobular aggregation.