Hierarchical structures of the actin/polycation complexes, investigated by ultra-small-angle neutron scattering and fluorescence microscopy

By employing a combined method of ultra-small-angle & small-angle neutron scattering (USANS&SANS) and fluorescence microscopy (FM), we investigated the solution mixtures of filamentous actin (F-actin) and the synthesized cationic polymer poly-N-[3-(dimethylamino) propyl] acrylamide methylchloride quaternary (PDMAPAA-Q). The combined USANS&SANS method, covering a wide range of length scales from 10 micrometres to nanometres, plays a crucial role in elucidating the hierarchical structure of the complex structure, as it is in an aqueous solution. FM determined that the complex of F-actin and PDMAPAA-Q appears with a finite size (> 10 mu m), referred to as a superbundle, and its morphology changes from a globular one to a stretched one by increasing the salt (KCl) concentration C-s from 0.01 to 0.3 M. USANS& SANS revealed that the superbundle is composed of a structural unit of a protobundle, in which PDMAPAA-Q binds F-actins in order to form a hexagonal lattice. The diameter of the protobundle (D-II), determined by USANS& SANS, increases from 40 to 290 nm as C-s increases from 0.01 to 0.3 M. In order to explain the finite-size & hierarchical condensation observed in the actin/PDMAPAA-Q solution, we employ a scenario of imperfect charge neutralization between F-actins and PDMAPAA-Q. Due to the chain connectivity of PDMAPAA-Q, a spatial distribution of positive charges around the F-actins becomes inhomogeneous, so that the repulsive electrostatic interaction appearing in the protobundle limits bundle formation with an infinite-size. The morphology of the superbundles is controlled by the bending rigidity due to individual protobundles, which significantly increases as D-II increases.