The behavior of multivalent ions in binary 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 investigate the condensation behavior of binary anionic polyelectrolyte mixtures consisting of DNA coils and F-actin rods in the presence of different multivalent ions (such as spermidine and tobramycin, a multivalent antibiotic). The mixture phase can separate 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.