Multivalent ion driven condensation of DNA-actin polyelectrolyte mixtures

– Multivalent ions can induce condensation of like-charged polyelectrolytes into compact states, a process that requires different ion valence for different polyelectrolyte species. We have examined the trivalent ion induced condensation behavior in binary anionic polyelectrolyte mixtures consisting of DNA coils and F-actin rods, and observe a micro-phase separation between the two polyelectrolytes into coexisting finite-sized F-actin bundles and DNA toroids. Further, by increasing the DNA volume fraction in the mixture, condensed F-actin bundles can be completely destabilized, leading to only DNA condensation within the mixture. We examine a number of possible causes and propose a model based on polyelectrolyte competition for ions. Electrostatics in complex fluids often exhibits counterintuitive effects [1]. Multivalent ions can mediate attraction between like-charged polyelectrolytes in aqueous solution and induce condensation into compact states, in contradiction to all mean field theories [2–7]. Recent experimental examples include DNA, F-actin, microtubules, and filamentous viruses [8]. It is also known that ions of different valences are generally required to condense different polyelectrolytes. Trivalent ions are usually required to condense DNA, while only divalent ions are required to condense F-actin and viruses of the Ff family, and monovalent ions do not condense any of them [9]. What happens when multivalent ions of a given valence are used to condense a mixture of two different polyelectrolytes? This represents a broad class of problems with many combinations. Individual polyelectrolyte parameters, such as charge density, contour length, and flexibility, can be varied, and the ion valence can be chosen to condense either or both of the polyelectrolytes in the mixture. The organization of the polyelectrolyte mixture can have a wide range of structural possibilities, from ordered or disordered composite states to complete phase separation. The phase behavior of these mixed polyelectrolyte systems with multivalent ions has applications ranging from water purification to cystic fibrosis. (∗) E-mail: [email protected]