Internal motions of actin characterized by quasielastic neutron scattering.

Quasielastic neutron scattering (QENS) experiments were carried out on powders of F-actin and G-actin hydrated with D(2)O to characterize the internal dynamics on the picosecond time scale and the Ångstrom length scale. To investigate the effects of hydration, the measurements were done on samples at hydration ratio (h) of 0.4 (mg D(2)O/mg protein), containing only the first layer of hydration water, and at h = 1.0, containing more layers of water. The QENS spectra, obtained from the measurements at two energy resolutions of 110 and 15 μeV, indicated that the internal motions of both F-actin and G-actin have distributions of motions with distinct correlation times and amplitudes. Increasing hydration changes relative populations of these distinct motions. The effects of hydration were shown to be different between F-actin and G-actin. Elastic incoherent neutron scattering measurements provided the concerted results. The observed effects were interpreted in terms of the dynamical heterogeneity of the actin molecule: in G-actin, more surface loops become flexible and undergo diffusive motions of large amplitudes, whereas in F-actin the molecular interactions that keep the polymerized state suppress the large motions of the surface loops involved with polymerization so that the population of atoms undergoing large motions can increase only to a lesser degree.