Dynamic structure factor of semiflexible macromolecules in dilute solution

The dynamic structure factor for molecular chains with variable stiffness in a dilute solution is investigated. In the limit of small scattering vectors q only the overall translational motion of the macromolecules contributes to the dynamic structure factor. The translational diffusion coefficient D exhibits a chain length dependence D∼1/√L for flexible chains and D∼ln L/L+const/L for rodlike chains. For flexible chains there is an intermediate scattering vector regime in which the decay rate or spectral linewidth of the dynamic structure factor is proportional to q3 indicating that stretching modes are dominant. Such an intermediate scattering vector regime cannot be observed for semiflexible or rodlike chains. At large scattering vectors q/2p≳1.5, where 1/2p is the persistence length of the macromolecules, the chain stiffness becomes important for any kind of molecules, i.e., even for very flexible ones. The dynamic structure factor and the decay rate are compared with experimental results of quasielastic neutron and light scattering experiments on different natural and synthetic macromolecules. These experimental results are in good agreement with the theoretical predictions. Furthermore, we determine the persistence length of F-actin from a dynamic light scattering experiment.