Semi-flexible Polymers and Filaments

  • The filaments of the cytoskeleton represent very thin nanorods: Actin filaments have a helical structure with a thickness of 8 nm, microtubuli resemble hollow cylinders with a thickness of 25 nm. Because of their bending stiffness, these filaments assume essentially straight conformations at low temperatures and in the absence of external forces. However, they undergo pronounced thermally excited shape fluctuations at room temperature and can be bent by relatively small external forces. Thus, as far as their elastic properties are concerned, cytoskeletal filaments behave in the same way as semi-flexible polymers, which may be regarded as "one-dimensional membranes".

  • Both nature and chemical synthesis provide a large variety of such polymers. In contrast to cytoskeletal filaments, which are formed by the self-assembly of proteins in aqueous solution, semi-flexible polymers are chain-like macromolecules, in which the monomers or subunits are connected by covalent bonds. Two prominent examples for such polymers are DNA molecules and polysaccharides.

  • The interplay between elastic properties, external forces, and thermal fluctuations of semi-flexible polymers and filaments involves the following aspects:

  • For reviews of these and related aspects, see Modelling semi-flexible polymers and Buckling, Bundling, and Pattern formation.