The self-assembly, elasticity, and dynamics of cardiac thin filaments^☆

Solutions of intact cardiac thin filaments were examined with transmission electron microscopy, dynamic light scattering (DLS), and particle-tracking microrheology. The filaments self-assembled in solution with a bell-shaped distribution of contour lengths that contained a population of filaments of much greater length than the in vivo sarcomere size (∼1 μm) due to a one-dimensional annealing process. Dynamic semiflexible modes were found in DLS measurements at fast timescales (12.5 ns–0.0001 s). The bending modulus of the fibers is found to be in the range 4.5–16 × 10<sup>−27</sup> Jm and is weakly dependent on calcium concentration (with Ca<sup>2+</sup> ≥ without Ca<sup>2+</sup>). Good quantitative agreement was found for the values of the fiber diameter calculated from transmission electron microscopy and from the initial decay of DLS correlation functions: 9.9 nm and 9.7 nm with and without Ca<sup>2+</sup>, respectively. In contrast, at slower timescales and high polymer concentrations, microrheology indicates that the cardiac filaments act as short rods in solution according to the predictions of the Doi-Edwards chopsticks model (viscosity, η ∼c<sup>3</sup>, where c is the polymer concentration). This differs from the semiflexible behavior of long synthetic actin filaments at comparable polymer concentrations and timescales (elastic shear modulus, G′ ∼ c1.4, tightly entangled) and is due to the relative ratio of the contour lengths (∼30). The scaling dependence of the elastic shear modulus on the frequency (ω) for cardiac thin filaments is G′ ∼ω<sup>3/4 ± 0.03</sup>, which is thought to arise from flexural modes of the filaments

The internal dynamic modes of charged self-assembled peptide fibrils

Photon correlation spectroscopy is used to study the internal dynamics of self-assembled charged peptide fibrils. Short neutral and charged polymeric aggregates have diffusive modes due to whole macromolecular motion. For long semiflexible fibrils the logarithm of the intermediate scattering function follows a q2t3/4 scaling at long times consistent with a Kratky-Porod free energy and preaveraged Oseen hydrodynamics. Persistence lengths on the order of micrometers are calculated for the peptide fibrils consistent with estimates from the liquid-crystalline phase behavior. Fibril diameters (5-35 nm) calculated from the initial decay of the correlation functions are in agreement with transmission electron microscopy measurements