Phase Transition of Catenated DNA Networks in Poly(ethylene glycol) Solutions

Conformational phase transitions of macromolecules are an important class of problems in fundamental polymer physics. While the conformational phase transitions of linear DNA have been extensively studied, this feature of topologically complex DNA remains unexplored. We studied the polymer-and-salt-induced (psi) phase transition of kinetoplasts, using single-molecule fluorescence microscopy. A single kinetoplast molecule from Crithidia fasciculata is a giant planar network of thousands (~ 5000) of catenated circular DNAs. We observe that kinetoplasts can undergo a reversible phase transition from the flat to the collapsed phase in the presence of NaCl as a function of the concentration of crowding agent poly(ethylene glycol). The nature of this phase transition is tunable through varying ionic strength. For linear DNA, the co-existence of coil and globule phases was attributed to a first order phase transition, associated with a double well potential in the transition regime. In contrast, kinetoplasts navigate from the flat to the collapsed phase by passing through an intermediate regime, characterized by the co-existence of a thermodynamically stable multi-population with varying shapes and sizes. Long temporal stability of the multi-population in the transition regime suggests a rugged energy landscape.