The self-assembly kinetics for a norovirus capsid protein were probed by time-resolved small-angle X-ray scattering and then analyzed by singular value decomposition and global fitting. Only three species contribute to the total scattering intensities: dimers, intermediates comprising some 11 dimers, and icosahedral T = 3 capsids made up of 90 dimers. Three-dimensional reconstructions of the intermediate robustly show a stave-like shape consistent with an arrangement of two pentameric units connected by an interstitial dimer. Upon triggering of self-assembly, the biphasic kinetics consist of a fast step in which dimers are assembled into intermediates, followed by a slow step in which intermediates interlock into capsids. This simple kinetic model reproduces experimental data with an excellent agreement over 6 decades in time and with nanometer resolution. The extracted form factors are robust against changes in experimental conditions. These findings challenge and complement currently accepted models for the assembly of norovirus capsids.

The tunable structure of silica–latex nanocomposites made of silica nanoparticles (radius ≈ 80 Å) and a copolymer of methyl methacrylate and butyl acrylate–latex beads (radius ≈ 210 Å) has been studied by small-angle neutron scattering and transmission electron microscopy. An aggregation diagram as a function of the control parameterssilica volume fraction and precursor solution pHhas been established. In this aggregation diagram, isoaggregation lines have been identified. It was used to express the small-strain reinforcement factor measured with stress–strain isotherms as a function of volume fraction at fixed aggregation number in the range from 50 to 100. The large-strain properties have been rationalized using the energy needed to rupture samples, and this quantity has been found to present an optimum at intermediate volume fractions (15%). In order to understand the striking rheology of the system, a neutron contrast-matching study has been undertaken by adding deuterated polymer beads. The bead demixing kinetics during annealing has been used to characterize the dynamics in various environments defined by the hard silica structure. In particular, in nanocomposite samples containing 15 vol % of silica the dynamics is found to be blocked.

Abstract:The evolution of the polymer structure during nanocomposite formation and annealing of silica-latex nanocomposites is studied using contrast-variation small angle neutron scattering.The experimental system is made of silica nanoparticles (R si ≈ 8 nm) and a mixture of purpose-synthesized hydrogenated and deuterated nanolatex (R latex ≈ 12.5 nm). The progressive disappearance of the latex beads by chain interdiffusion and release in the nanocomposites is analyzed quantitatively with a model for the scattered intensity of hairy latex beads and an RPA description of the free chains. In silica-free matrices and nanocomposites of low silica content (7%v), the annealing procedure over weeks at up to T g + 85 K results in a molecular dispersion of chains, the radius of gyration of which is reported.At higher silica content (20%v), chain interdiffusion seems to be slowed down on time-scales of weeks, reaching a molecular dispersion only at the strongest annealing. Chain radii of gyration are found to be unaffected by the presence of the silica filler. Tables: 5 2 Figures: 7

This Letter reports on the remarkable selectivity of capsid proteins for packaging synthetic polyelectrolytes in viruslike particles. By applying the contrast variation method in small-angle neutron scattering, we accurately estimated the mean mass of packaged polyelectrolytes ⟨Mp⟩ and that of the surrounding capsid ⟨Mcap⟩. Remarkably, the mass ratio ⟨Mp⟩/⟨Mcap⟩ was invariant for polyelectrolyte molecular weights spanning more than 2 orders of magnitude. To do so, capsids either packaged several chains simultaneously or selectively retained the shortest chains that could fit the capsid interior. Our data are in qualitative agreement with theoretical predictions based on free energy minimization and emphasize the importance of protein self-energy. These findings may give new insights into the nonspecific origin of genome selectivity for a number of viral systems.

The SAXS measurements were performed with the GTPase domain of human Septin 2 (SEPT2G) at 0.5 and 1 mg/mL and temperatures from 4 to 45 C. At 4 C, our results demonstrate that SEPT2G is self-aggregated as a dimer at 0.5mg/mL, whereas dimers coexist with cylinder-like aggregates (36 nmlong and 12 nm-cross section) at 1mg/mL. At this temperature, the protein does not evolve over one hour of observation. As the temperature was increased to 15 C we verified that, initially coexisting with the protein dimer and cylinders, a small amount of larger aggregates is also present in solution. However, the number of very large aggregates increases with time concomitantly with the decrease of cylinder amount in the solution.Analyzing the samples at 37 C it's not possible to observe cylinders anymore and the amount of dimers decreases from 50% to 20% in less than 1 hour. For 45 C this effect is even more accentuated: the percentage of dimers is only 6% in solution.In conclusion, our results showed the coexistence of dimers of SEPT2G with small fibers and larger aggregates in solution that evolve not only with concentration and temperature but also with time. This work is supported by FAPESP and CNPq. The authors thank the LNLS SAXS beam line staff.