Dynamic light scattering from semidilute actin solutions: A study of hydrodynamic screening, filament bending stiffness, and the effect of tropomyosin/troponin-binding

Quasi-elastic light scattering (QELS) is applied to investigate the effect of the tropomyosin/ troponin complex (Tm/Tn) on the stiffness of actin filaments. The importance of hydrodynamic screening in semidilute solutions is demonstrated. A new concentration dependent expression for the dynamic structure factor g(k, t) of semiflexible polymers in semidilute solutions is used to analyze the experimental QELS data. A concentration independent value for the bending modulus κ is thus obtained. It increases by 50% as a consequence of Tm/Tn binding in a 7:1:1 molar ratio of actin/Tm/Tn. In addition a new expression for the initial slope of the dynamic structure factor of a semiflexible polymer is used to determine the effective hydrodynamic diameter of the actin filament. Our results confirm the general relevance of the concept of (intrinsic) semiflexibility to polymer dynamics.