Spontaneous Condensation of RNA into Nanoring and Globular Structures.

The effect of polyvalent cations, like spermine, on the condensation of DNA into very well-defined toroidal shapes has been well studied and understood. A great effort has been made to obtain similar condensed structures from RNA molecules, but so far, it has been elusive. In this work, we show that single-stranded RNA (ssRNA) molecules can easily be condensed into nanoring and globular structures on a mica surface, where each nanoring structure is formed mostly by a single RNA molecule. The condensation occurs in a concentration range of different cations, from monovalent to trivalent, but at a higher concentration, globular structures appear. RNA nanoring structures were observed on mica surfaces by atomic force microscopy (AFM). The samples were observed in tapping mode and were prepared by drop evaporation of a solution of RNA in the presence of one type of the different cations used. As far as we know, this is the first time that nanorings or any other well-defined condensed RNA structures have been reported in the presence of simple salts. The RNA nanoring formation can be understood by an energy competition between the hydrogen bonding forming hairpin stems-weakened by the salts-and the hairpin loops. This result may have an important biological relevance since it has been proposed that RNA is the oldest genome-coding molecule, and the formation of these structures could have given it stability against degradation in primeval times. Even more, the nanoring structures could have the potential to be used as biosensors and functionalized nanodevices.

Influence of side chain isomerism on the rigidity of poly(3-alkylthiophenes) in solutions revealed by neutron scattering.

Using small-angle neutron scattering, we conducted a detailed conformational study of poly(3-alkylthiophene) solutions in deuterated dichlorobenzene. The focus was placed on addressing the influence of the spatial arrangement of side chain constituents on backbone conformation. We demonstrate that by introducing a branch point in the side chain, side chain steric interactions may promote torsional motion between backbone units, resulting in greater chain flexibility. Our findings highlight the key role of topological isomerism in determining the chain rigidity and throw new light on the debate about the effective approaches for optimizing the electronic properties of conducting polymers via side chain engineering.

Local elasticity in nonlinear rheology of interacting colloidal glasses revealed by neutron scattering and rheometry.

The flow of colloidal suspensions is ubiquitous in nature and industry. Colloidal suspensions exhibit a wide range of rheological behavior, which should be closely related to the microscopic structure of the systems. With in situ small-angle neutron scattering complemented by rheological measurements, we investigated the deformation behavior of a charge-stabilized colloidal glass at particle level undergoing steady shear. A short-lived, localized elastic response at particle level, termed as the transient elasticity zone (TEZ), was identified from the neutron spectra. The existence of the TEZ, which could be promoted by the electrostatic interparticle potential, is a signature of deformation heterogeneity: the body of fluids under shear behaves like an elastic solid within the spatial range of the TEZ but like fluid outside the TEZ. The size of the TEZ shrinks as the shear rate increases in the shear thinning region, which shows that the shear thinning is accompanied by a diminishing deformation heterogeneity. More interestingly, the TEZ is found to be the structural unit that provides the resistance to the imposed shear, as evidenced by the quantitative agreement between the local elastic stress sustained by the TEZ and the macroscopic stress from rheological measurements at low and moderate shear rates. Our findings provide an understanding on the nonlinear rheology of interacting colloidal glasses from a micro-mechanical view.

Communication: Probing the existence of partially arrested states in ionic liquids.

The recent predictions of the self-consistent generalized Langevin equation theory, describing the existence of unusual partially arrested states in the context of ionic liquids, were probed using all-atom molecular dynamics simulations of a room-temperature ionic liquid. We have found a slower diffusion of the smaller anions compared with the large cations for a wide range of temperatures. The arrest mechanism consists on the formation of a strongly repulsive glass by the anions, stabilized by the long range electrostatic potential. The diffusion of the less repulsive cations occurs through the holes left by the small particles. All of our observations in the simulated system coincide with the theoretical picture.

Scattering from Colloid-Polymer Conjugates with Excluded Volume Effect.

This work presents scattering functions of conjugates consisting of a colloid particle and a self-avoiding polymer chain as a model for protein-polymer conjugates and nanoparticle-polymer conjugates in solution. The model is directly derived from the two-point correlation function with the inclusion of excluded volume effects. The dependence of the calculated scattering function on the geometric shape of the colloid and polymer stiffness is investigated. The model is able to describe the experimental scattering signature of the solutions of suspending hard particle-polymer conjugates and provide additional conformational information. This model explicitly elucidates the link between the global conformation of a conjugate and the microstructure of its constituent components.

Phase behavior under a noncentrosymmetric interaction: shifted-charge colloids investigated by Monte Carlo simulation.

Using Monte Carlo simulations, we investigate the structural characteristics of an interacting hard-sphere system with shifted charge to elucidate the effect of the noncentrosymmetric interaction on its phase behavior. Two different phase transitions are identified for this model system. With increasing volume fraction, an abrupt liquid-to-crystal transition first occurs at a significantly lower volume fraction than in centrosymmetrically charged systems. This is due to the stronger effective interparticle repulsion caused by the additional charge anisotropy. Moreover, within the crystal state at higher volume fraction, the system further undergoes a continuous disorder-to-order transition with respect to charge orientation. Detailed analyses in this work disclose the nature of these transitions, and orientation fluctuation can cause noncentrosymmetric unit cells. The dependence of crystal formation and orientational ordering on temperature was also examined. These findings indicate that the noncentrosymmetric interaction in this work results in additional freedoms to fine-tune the phase diagram and increase the functionalities of materials. Moreover, these model studies are essential to advance the future understanding regarding the fundamental physiochemical properties of novel Janus colloidal particles and protein crystallization conditions.

Influence of Molecular Solvation on the Conformation of Star Polymers.

We have used neutron scattering to investigate the influence of concentration on the conformation of a star polymer. By varying the contrast between the solvent and the isotopically labeled stars, we obtain the distributions of polymer and solvent within a star polymer from analysis of scattering data. A correlation between the local desolvation and the inward folding of star branches is discovered. From the perspective of thermodynamics, we find an analogy between the mechanism of polymer localization driven by solvent depletion and that of the hydrophobic collapse of polymers in solutions.

Dynamical Threshold of Diluteness of Soft Colloids.

Soft colloids are hybrids between linear polymers and hard colloids. Their solutions exhibit rich phase phenomenon due to their unique microstructure. In scaling theories, a geometrically defined overlap concentration c* is used to identify the concentration regimes of their solutions characterized with distinct conformational properties. Previous experiments showed that the average size of soft colloids remains invariant below c* and varies characteristically above it. This observation reveals the causality between the conformational evolution and the physical overlap between neighboring particles. Using neutron scattering, we demonstrate that the competition between the interparticle translational diffusion and intraparticle internal dynamics leads to significant conformational evolution below c*. Substantial structural dehydration and slowing-down of internal dynamics are both observed before physical overlap develops. Well below c*, a new threshold of diluteness cD* emerges as the crossover between the characteristic times associated with these two relaxation processes. Below this dynamically defined cD*, the two relaxation processes are essentially uncoupled, and therefore, the majority of the soft colloids retain their unperturbed conformational dimensions. Our observation demonstrates the importance of incorporating dynamical degrees of freedom in defining the threshold of diluteness for this important class of soft matter.

Equilibrium structure of the multi-component screened charged hard-sphere fluid.

The generalized mean spherical approximation of the structural properties of the binary charge-symmetric fluid of screened charged hard-spheres of the same diameter, i.e., the screened restricted primitive model, is extended to include binary charge-asymmetric and multi-component fluids. Molecular dynamics simulation data are generated to assess the accuracy of the corresponding theoretical predictions.