We report on single-particle dynamics of strongly interacting filamentous fd virus particles in the liquidcrystalline columnar state in aqueous solution. From fluorescence microscopy, we find that rare, discrete events take place, in which individual particles engage in sudden, jumplike motion along the main rod axis. The jump length distribution is bimodal and centered at half-and full-particle lengths. Our Brownian dynamics simulations of hard semiflexible particles mimic our experiments and indicate that full-length jumps must be due to collective dynamics in which particles move in stringlike fashion in and between neighboring columns, while half jumps arise as a result of particles moving into defects. We find that the finite domain structure of the columnar phase strongly influences the observed dynamics. The interplay between ordering, fluctuations, and mobility of particles is a key ingredient in our current understanding of soft matter. The Lindemann criterium for crystal melting purports that a crystal can only be stable provided that fluctuations of the particles away from the equilibrium positions are less than about 1=8 of the lattice distance [1], which indeed is observed in real space in crystalline dispersions of spherical colloids [2]. In glassy dispersions of colloidal spheres, there is no long-range order but particle caging hinders free diffusion: breaking and reformation of cages dominates the dynamics [3]. The situation is more complex for elongated colloidal particles such as DNA, actin filaments, and filamentous viruses because of the additional orientational degree of freedom [4,5]. This gives rise to a sequence of mesophases with increasing packing fraction, from the (chiral) nematic where there is only orientational ordering to smectic and (hexatic) columnar liquid-crystalline phases characterized by one-and two-dimensional positional order [6,7].While self-diffusion in nematic [8,9] and smectic [10-14] phases has been investigated theoretically and experimentally, little is known about dynamics in the dense columnar phase, where particles are stacked into liquidlike columns that are organized in a hexagonal array. Perfect hexagonal ordering is frustrated for the chiral fd virus by the helical twist of the particles, spawning dislocations. This breaks long-range hexagonal translational order, giving rise to a hexatic columnar phase [7]. In this Letter, we report on the intriguing motion of particles in this highly viscous hexatic phase, using particle tracking of fluorescently labeled fd viruses. We observe rare and discrete jump events along the director, predominantly of half a particle length, while a significant fraction of events involves full-length jumps. A small fraction of the particles reorients during the jump, hinting at the presence of structural defects, such as dislocations or grain boundaries.Hoppinglike diffusion processes of one rod length have been observed before in the smectic of an fd virus [11,12]. It stems from the underlying lamellar structure of the smectic th...

By means of replica exchange molecular dynamics simulations we investigate how the length of a silk-like, alternating diblock oligopeptide influences its secondary and quaternary structure. We carry out simulations for two protein sizes consisting of three and five blocks, and study the stability of a single protein, a dimer, a trimer and a tetramer. Initial configurations of our simulations are β-roll and β-sheet structures. We find that for the triblock the secondary and quaternary structures upto and including the tetramer are unstable: the proteins melt into random coil structures and the aggregates disassemble either completely or partially. We attribute this to the competition between conformational entropy of the proteins and the formation of hydrogen bonds and hydrophobic interactions between proteins. This is confirmed by our simulations on the pentablock proteins, where we find that, as the number of monomers in the aggregate increases, individual monomers form more hydrogen bonds whereas their solvent accessible surface area decreases. For the pentablock β-sheet protein, the monomer and the dimer melt as well, although for the β-roll protein only the monomer melts. For both trimers and tetramers remain stable. Apparently, for these the entropy loss of forming β-rolls and β-sheets is compensated for in the free-energy gain due to the hydrogen-bonding and hydrophobic interactions. We also find that the middle monomers in the trimers and tetramers are conformationally much more stable than the ones on the top and the bottom. Interestingly, the latter are more stable on the tetramer than on the trimer, suggesting that as the number of monomers increases protein-protein interactions cooperatively stabilize the assembly. According to our simulations, the β-roll and β-sheet aggregates must be approximately equally stable.