Native silk fibers exhibit strength and toughness that rival those of the best synthetic fibers. Despite significant research, further insight is still needed to understand the mechanisms by which silkworms are capable of spinning such tough fibers. Here we propose that π-π and π-OH group interactions of tyrosine side chains provide templating effects, such that the crystal-forming domains are in registration, thereby fostering the self-assembly of the spinning dope. Intrinsic fluorescence measurements, in conjunction with circular dichroism, showed that during self-assembly of regenerated silk solutions, the tyrosine residues were localized in a more hydrophobic local environment, suggesting preferential assembly. In situ Fourier transform infrared spectroscopy indicated that cross-linking of the tyrosine residues resulted in the development of extended β-sheet structure. Additionally, control of cross-link density directly influenced the degree of crystallinity upon drying. Molecular dynamics simulations were performed on silk mimetic peptides in order to more thoroughly understand the role of tyrosines. The results indicated that tyrosine residues tended to transiently colocate in solution due to π-π interactions and hydrogen bonds with adjacent residues and with the peptide backbone. These more stable tyrosine interactions resulted in reduced lateral chain fluctuations and increased incidence of coordinated intrachain association, while introduction of a dityrosine bond directly promoted the formation of β-sheet structures. In total, the experimental and modeling data support a critical role for tyrosine-tyrosine interactions as a key early feature in the self-assembly of regenerated silk protein chains and therefore are important in the robust and unusual mechanical properties ultimately achieved in the process.

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Using a π-orbital tight-binding (TB) model within a perturbative formalism, the effects of substitutional impurities on the conductance of infinite metallic single-wall carbon nanotubes (MSWCNTs) are studied. The perturbative scheme is based on the energy dissipation of electrons travelling through the nanotube. A general expression for the differential conductance (DC) is presented, and scattering processes are investigated. It is demonstrated how the DC depends sensitively on the nature of the electronic band structure and velocity of carriers moving in the nanotube. We have shown that the quantum interference (QI) of electronic waves scattered by impurities plays a meaningful role. In particular, for the case of a couple of impurities the DC exhibits periodic oscillations comprising both positive and negative values. The negative differential conductance (NDC) stemming from the QI and rotational symmetry selection rule is very sensitive to the relative distance and symmetry of two impurities. This signature is absent for the case of a single impurity. In fact, the NDC can be attributed to the zero-temperature/elastic weak-localization correction to the conductance. As a result, the faster/higher and slower/shorter oscillations can then effectively be achieved by metallic zigzag and armchair nanotubes, respectively. A corrigendum for this article has been published in 2007 J. Phys.: Condens. Matter 19 469001

Motivated by innovative progresses in designing multi-layer graphene nanostructured materials in the laboratory, we theoretically investigate the Dirac plasmon modes of a spatially separated double-layer graphene nanoribbon system, made up of a vertically offset armchair and metallic graphene nanoribbon pair. We find striking features of the collective excitations in this novel Coulomb correlated system, where both nanoribbons are supposed to be either intrinsic (undoped/ungated) or extrinsic (doped/gated). In the former, it is shown the low-energy acoustical and the high-energy optical plasmon modes are tunable only by the inter-ribbon charge separation. In the later, the aforementioned plasmon branches are modified by the added doping factor. As a result, our model could be useful to examine the existence of a linear Landau-undamped low-energy acoustical plasmon mode tuned via the inter-ribbon charge separation as well as doping. This study might also be utilized for devising novel quantum optical waveguides based on the Coulomb coupled graphene nanoribbons.

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