Order-Order Transition of Size-mismatched Ions on F-actin Polyelectrolytes

Multivalent ions induce condensation of like-charged F-actin polyelectrolytes into close-packed bundles, in which multivalent ions organize into 1-D density waves. We examine the condensation behavior of anionic F-actin using multivalent cations with a large size mismatch, Ba$^{2+}$ and lysozyme(+9), a small globular protein (2.5nm x 2.5nm x 4.5nm). An unexpected first-order phase transition on the F-actin surface between a Ba$^{2+}$ counterion charge density wave state and 1-D close-packed lysozyme chains is found as the lysozyme-actin ratio is varied. By comparing wild-type lysozyme with genetically-engineered lysozyme with reduced charge, we show that this transition shifts with the actin-lysozyme isoelectric point.