Luchun Wang

Atomic structure and hierarchical assembly of a cross-β amyloid fibril.

The cross-β amyloid form of peptides and proteins represents an archetypal and widely accessible structure consisting of ordered arrays of β-sheet filaments. These complex aggregates have remarkable chemical and physical properties, and the conversion of normally soluble functional forms of proteins into amyloid structures is linked to many debilitating human diseases, including several common forms of age-related dementia. Despite their importance, however, cross-β amyloid fibrils have proved to be recalcitrant to detailed structural analysis. By combining structural constraints from a series of experimental techniques spanning five orders of magnitude in length scale—including magic angle spinning nuclear magnetic resonance spectroscopy, X-ray fiber diffraction, cryoelectron microscopy, scanning transmission electron microscopy, and atomic force microscopy—we report the atomic-resolution (0.5 Å) structures of three amyloid polymorphs formed by an 11-residue peptide. These structures reveal the details of the packing interactions by which the constituent β-strands are assembled hierarchically into protofilaments, filaments, and mature fibrils.

The structure and properties of nanosize crystalline silicon films

Nanosize crystalline silicon films are fabricated by using highly hydrogen‐diluted silane as the reactive gas and activated with rf+dc double‐power sources, in a conventional plasma‐enhanced chemical‐vapor‐deposition system. The structure of the deposited films as studied by means of high‐resolution electron microscopy, Raman scattering spectra, x‐ray‐diffraction pattern, IR transmission spectra, and ultraviolet ray analysis. The results show that there are many novel structural features and new physical properties for these nanosize crystalline silicon films. In particular, it is found that the optical‐absorption coefficient α is higher than that of a‐Si:H and μc‐Si:H films, the room‐temperature conductivity σd has the value of 10−3–10−1 Ω−1 cm−1, and the hydrogen content CH in nc‐Si:H films is higher than 30 at. %. The nc‐Si:H films have their peculiar features which are different from both a‐Si:H and μc‐Si:H films.

Structure of a type IV secretion system core complex

Type IV secretion systems (T4SSs) are important virulence factors used by Gram-negative bacterial pathogens to inject effectors into host cells or to spread plasmids harboring antibiotic resistance genes. We report the 15 angstrom resolution cryo–electron microscopy structure of the core complex of a T4SS. The core complex is composed of three proteins, each present in 14 copies and forming a ∼1.1-megadalton two-chambered, double membrane–spanning channel. The structure is double-walled, with each component apparently spanning a large part of the channel. The complex is open on the cytoplasmic side and constricted on the extracellular side. Overall, the T4SS core complex structure is different in both architecture and composition from the other known double membrane–spanning secretion system that has been structurally characterized.

Structure of the VirB4 ATPase, alone and bound to the core complex of a type IV secretion system

Type IV secretion (T4S) systems mediate the transfer of proteins and DNA across the cell envelope of bacteria. These systems play important roles in bacterial pathogenesis and in horizontal transfer of antibiotic resistance. The VirB4 ATPase of the T4S system is essential for both the assembly of the system and substrate transfer. In this article, we present the crystal structure of the C-terminal domain of Thermoanaerobacter pseudethanolicus VirB4. This structure is strikingly similar to that of another T4S ATPase, VirD4, a protein that shares only 12% sequence identity with VirB4. The VirB4 domain purifies as a monomer, but the full-length protein is observed in a monomer-dimer equilibrium, even in the presence of nucleotides and DNAs. We also report the negative stain electron microscopy structure of the core complex of the T4S system of the Escherichia coli pKM101 plasmid, with VirB4 bound. In this structure, VirB4 is also monomeric and bound through its N-terminal domain to the core’s VirB9 protein. Remarkably, VirB4 is observed bound to the side of the complex where it is ideally placed to play its known regulatory role in substrate transfer.

VISIBLE ELECTROLUMINESCENCE FROM NANOCRYSTALLITES OF SILICON FILMS PREPARED BY PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION

We have observed visible electroluminescence (EL) from silicon nanocrystallites which are embedded in a‐Si:H films prepared in a plasma enhanced chemical vapor deposition system. The EL spectra are in the range of 500–850 nm with two peaks located at about 630–680 and 730 nm, respectively. We found that the intensity of EL peaks is related closely to the conductivity of the deposited films. The carrier conduction path is discussed in terms of the material structural characteristics, and a tentative explanation of the light emission mechanism is proposed.

Photoluminescence of nanocrystallites embedded in hydrogenated amorphous silicon films

We have fabricated light‐emitting nanocrystallites embedded in an a‐Si:H matrix using a conventional plasma‐enhanced chemical‐vapor‐deposition system. It was found that the photoluminescence properties are directly related to the deposition parameters. The quantum size effect model is proposed to explain the photoluminescence. Two structural prerequisites are proposed for this kind of films to exhibit effective light emission: One is an upper limit for mean crystallite size of about 3.4 nm, the other is an upper limit for crystallinity of about 30%.

Microstructural investigation of as-deposited Co-Ag nano-granular films

The microstructures of as-deposited Co-Ag nano-granular films are characterized by X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy. In as-deposited films, there is a strong tendency for phase separation to occur; Co and Ag particles coexist in the films. The average size of Co grains varies with the Co concentration. Ag particles have the preferential lattice orientation and Co particles have lattice orientations , , , and in the direction of the film surface normal. There is a tendency for two connecting grains to have two groups of parallel lattice planes.

Growth morphology and characteristic structure in nanocrystalline Si film of high conductivity

A new type of hydrogenated nanocrystalline Si film with high electronic conductivity is investigated by means of transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). Nearly parallel columnar structures with growth orientation along the [110] zone axis of Si are found from cross‐sectional TEM images of the film. HRTEM observation reveals that these columns are composed of nanocrystallites (3–6 nm size) and dendritelike growth morphology, while in the region between columns the texture consists of smaller sized (<3 nm) grains embedded in a hydrogenated amorphous Si. The volume fraction of the crystalline component is about 58% measured by Raman spectroscopy. The conductivity of the film is very high, about 10−1 (Ω cm)−1. It is considered that this is directly related to the characteristic structure of the film.

Microstructures and characteristics of nano-size crystalline silicon films

Microstructures and characteristics of nano-size hydrogenated crystalline silicon films (nc-Si:H) have been studied by high-resolution electron microscopy (HREM), X-ray diffraction patterns and Raman spectroscopy. The microcrystalline grains in nc-Si:H films are about 3-5 nm in size and are separated by different characteristic boundaries. The volume fraction of the crystalline component is about 46%. Microdefects in nanocrystalline grains were also found. The electrical conductivities of the films were found by measurement to be about 10-3-10-2 ( Omega cm)-1. Analysis of the experimental results shows that the nano-size hydrogenated crystalline silicon films are in good agreement with the international definition of nanocrystalline material.

Microstructure and magnetoelastic properties of FeCo/Ag multilayers

There is growing evidence that in magnetic films of thickness less than 20 nm there can be significant change in the values of magnetic anisotropy constant and magnetoelastic coupling from those of bulk materials. While phenomenological models based on Neel’s idea of surface anisotropy may offer a partial explanation, it is vital to develop a more mechanistic understanding. Recently the potential contribution of interface and surface strains to the observed property change has been highlighted. Here we report the field emission gun TEM and electron spectroscopic images of the structure of the interface region between Fe50Co50 and Ag, the local crystallographic texture and the distribution of Ag in magnetostrictive Fe50Co50/Ag multilayers. The results are correlated with the bulk measurements of coercivity and the saturation magnetostriction constant. For the first time it is possible to provide a degree of microstructural interpretation of the magnetic data.