ABSTRACT
Acknowledgements............................................................................ 242
Glossary of Sym bols.......................................................................... 242
References........................................................................................... 244
1 Introduction
Semiflexible polymers are macromolecules with a stiffness intermediate between the two extreme cases of random coils and rigid rods. The stiffness is conveniently characterized by the persistence length £p, which is defined as the arc length or ‘chemical distance’, over which the tangential correlations of the contour decay to 1 ¡e. Only if this length is much smaller than the total contour length L and the typical length scale of observation A, can the polymer be regarded as a flexible chain (of Kuhn length 2ip) and can be described by the standard models for flexible polymers [21]. For many naturally occuring polymers £p is not very small with respect to L or A or both of them, so that the flexible chain models are not appropriate. On the other hand, molecular self-interactions, which complicate substantially the mathematical modeling for molecules with a total contour length more than hundred times larger than £p, can safely be neglected. Especially some thread-like biomolecules such as actin filaments, intermediate filaments, microtubuli, DNA, collagen etc., which play an important role in cell biology [44, 50, 78], are good examples of what physicists have in mind when they think of semiflexible polymers. Some of these molecules have persistence lengths that are by several orders of magnitude larger than their lateral diameter and are comparable to or even longer than their contour length. In many cases their huge dimensions (L can be of the order of several
micrometers) render their internal dynamics accessible to light scattering techniques and even to light microscopy. (To give the reader an impression of a typical experimental system, we have reproduced in Figure 1 an elec tron micrograph of a semidilute actin solution from Ref. [33].) Certain cylindrical assemblies of amphiphilic molecules, so called wormlike micelles, can also be regarded as semiflexible chains. And there is, of course, a variety of synthetic polymers to which the notion of semiflexibility applies. We will argue below that the failure of the common flexible chain models for semiflexible macromolecules becomes especially apparent in their dynamical properties. This is a consequence of the hydrodynamic interaction, which obeys a reciprocal distance law, giving strong weight to the small scale structure.
