RESUMEN
We study how the three-dimensional shape of rigid filaments determines the microscopic dynamics and macroscopic rheology of entangled semidilute Brownian suspensions. To control the filament shape we use bacterial flagella, which are microns-long helical or straight filaments assembled from flagellin monomers. We compare the dynamics of straight rods, helical filaments, and shape-diblock copolymers composed of seamlessly joined straight and helical segments. Caged by their neighbors, straight rods preferentially diffuse along their long axis, but exhibit significantly suppressed rotational diffusion. Entangled helical filaments escape their confining tube by corkscrewing through the dense obstacles created by other filaments. By comparison, the adjoining segments of the rod-helix shape-diblocks suppress both the translation and the corkscrewing dynamics. Consequently, the shape-diblock filaments become permanently jammed at exceedingly low densities. We also measure the rheological properties of semidilute suspensions and relate their mechanical properties to the microscopic dynamics of constituent filaments. In particular, rheology shows that an entangled suspension of shape rod-helix copolymers forms a low-density glass whose elastic modulus can be estimated by accounting for how shear deformations reduce the entropic degrees of freedom of constrained filaments. Our results demonstrate that the three-dimensional shape of rigid filaments can be used to design rheological properties of semidilute fibrous suspensions.
RESUMEN
Recent advances in fluorescence super-resolution microscopy are providing important insights into details of cellular structures. To acquire three dimensional (3D) super-resolution images of DNA, we combined binding activated localization microscopy (BALM) using fluorescent double-stranded DNA intercalators and optical astigmatism. We quantitatively establish the advantage of bis- over mono-intercalators before demonstrating the approach by visualizing single DNA molecules stretched between microspheres at various heights. Finally, the approach is applied to the more complex environment of intact and damaged metaphase chromosomes, unravelling their structural features.