Semiflexible Polymers and Filaments

(F-) actin or microtubules belong to the class of semi-flexible polymers. The biological function of these polymers requires considerable mechanical rigidity. For example, actin filaments are the main structural element of the cytoskeleton which gives the cell unique mechanical properties as it forms a network rigid enough to maintain the shape of the cell and to transmit forces, yet flexible enough to allow for cell motion and internal reorganization in response to external mechanical stimuli. Another important class of semi-flexible polymers are polyelectrolytes where the electrostatic repulsion of the charges along the backbone can give rise to considerable bending rigidity depending on the salinity of the surrounding solution. The physics of semi-flexible polymers becomes fundamentally different from the physics of flexible synthetic polymers when their bending energy dominates over conformational entropy. The bending stiffness is characterized by the persistence length. On scales smaller than the persistence length, bending energy dominates and qualitatively new semi-flexible behaviour appears. Biopolymer persistence lengths range from 50nm for DNA to the 10μm-range for Factin or even up to the mm-range for microtubules and are thus comparable to typical contour lengths such that semiflexible behaviour plays an important role.