Counterion induced bundle formation of rodlike polyelectrolytes

Abstract A number of filamentous biopolymers including actin filaments (F‐actin), microtubules (MT), tobacco mosaic virus (TMV), and the filamentous bacteriophage fd form bundles under well‐defined conditions. All of these macromolecules are negatively charged rodlike assemblies, and lateral association is induced by a number of cations such as divalent and trivalent metal ions and homopolymers of basic peptides. The general features of bundle formation are largely independent of the specific structure of the biopolymers and the bundling agent used. They are also approximately independent of the concentration of macromolecules. However, a threshold concentration of bundling agent is required in order to form large lateral aggregates, detected by a sharp increase in light scattering and by electron microscopy. The threshold concentration varies strongly with the valence of the cations and increases with the ionic strength of the solution. The formation of bundles is reversible by polyanions such as nucleoside phosphates. This overall behavior is similar to the phenomenon of DNA condensation and can be explained by applying polyelectrolyte theories, including the concept of counterion condensation. Our results provide quantitative comparisons with a number of predictions of polyelectrolyte theories, including the calculation of apparent binding affinity and its dependence on ionic strength. They also support the prediction of an attractive interaction between the neighboring filaments due to sharing of counterion clouds. In addition, the simply detectable, reversible assembly of macromolecules in solution serves as a sensitive indicator of the active association of small ions and the formation of ion clusters. Association constants of Co(NH 3 ) ‐ATP and Lys 18 ‐ATP were determined by application of this model.