Crystalline Sheets Research Articles

Structure and Field Electron Emission of Carbon Nanotubes Dependent on Growth Temperature

AbstractThis present work deals with the temperature dependence on the growth and structure of CNTs grown by thermal CVD. The vertically aligned CNTs are synthesized on iron (Fe)-deposited silicon oxide (SiO2) substrate by thermal CVD using acetylene gas at temperatures in the range 750-950°C. Configuration and structural characteristics of CNTs have been investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As the growth temperature increases from 750 to 950°C, the growth rate and the average diameter increase while the density decreases by a factor of about 2. TEM images show that the relative amount of crystalline graphitic sheets increases with increasing the growth temperature and a higher degree of crystalline perfection can be achieved at 950°C. The HRTEM images reveal consistently that the degree of crystalline perfection increases progressively as the growth temperature increases. This result demonstrates that the growth rate, diameter, density, and crystallinity of carbon nanotubes can be controlled with the growth temperature.

Temperature effect on the growth of carbon nanotubes using thermal chemical vapor deposition

Vertically aligned carbon nanotubes (CNTs) are grown on iron-deposited silicon oxide substrates by thermal chemical vapor deposition (CVD) of acetylene gas at the temperature range 750–950°C. As the growth temperature increases from 750°C to 950°C, the growth rate increases by four times and the average diameter also increases from 30 nm to 130 nm while the density decreases by a factor of about two. The relative amount of crystalline graphitic sheets increases progressively with the growth temperature and a higher degree of crystalline perfection can be achieved at 950°C. This result demonstrates that the growth rate, diameter, density, and crystallinity of CNT can be controlled with the growth temperature.

Prevalence of the external surface over the internal pores in the spontaneous generation of tetrathiafulvalene radical cation incorporated in the novel delaminated ITQ-2 zeolite

ITQ-2 is a novel delaminated zeolite whose crystalline sheets (2.5 nm depth) define a large external surface (∽720 m2 g−1) having 12-membered ring (MR) open cups together with a reduced microporosity due to a 10 MR channel system. This feature is the reverse of that of classical zeolites (i.e., much larger internal area compared to the external surface). Herein, we have used tetrathiafulvalene and triphenylamine to probe the capability of layered ITQ-2 zeolite to generate persistent organic radical cations compared to conventional Y, mordenite and ZSM-5 zeolites as well as to Al/MCM-41 (3.2 nm diameter) and amorphous silica–alumina. By selective silylation of the external cups, it has been found that formation of a significant concentration of persistent organic radical cations in ITQ-2 is a property that occurs within the open cups of the external surface rather than inside the 10 MR pores.

Surface Freezing on Patterned Substrates

We show that the structure of a substrate pattern drastically influences the nature of surface freezing. By using phenomenological theory and computer simulations of a hard sphere fluid next to a substrate formed by a periodic array of fixed spheres, we find that a pattern which is commensurate with the bulk crystal induces complete surface freezing through a cascade of layering transitions. A rhombic pattern, on the other hand, either generates a crystalline sheet which is unstable as a bulk phase or prohibits surface freezing completely.

Surface Tongue-and-groove Contours on Lens MIP Facilitate Cell-to-cell Adherence

The lens major intrinsic protein (MIP, AQP0) is known to function as a water and solute channel. However, MIP has also been reported to occur in close membrane contacts between lens fiber cells, indicating that it has adhesive properties in addition to its channel function. Using atomic force and cryo-electron microscopy we document that crystalline sheets reconstituted from purified ovine lens MIP mostly consisted of two layers. MIP lattices in the apposing membranes were in precise register, and determination of the membrane sidedness demonstrated that MIP molecules bound to each other via their extracellular surfaces. The surface structure of the latter was resolved to 0.61 nm and revealed two protruding domains providing a tight “tongue-and-groove” fit between apposing MIP molecules. Cryo-electron crystallography produced a projection map at 0.69 nm resolution with a mirror symmetry axis at 45 ° to the lattice which was consistent with the double-layered nature of the reconstituted sheets. These data strongly suggest an adhesive function of MIP, and strengthen the view that MIP serves dual roles in the lens.

Synthesis of bamboo-shaped multiwalled carbon nanotubes using thermal chemical vapor deposition

The vertically aligned multiwalled carbon nanotubes (CNT) are synthesized in high density on a large area of cobalt deposited silicon oxide substrate, by thermal chemical vapor deposition using C 2H 2 gas, at 950°C. The diameter of CNTs is distributed in the range of 80–120 nm and the length is about 20 μm. High-resolution transmission electron microscopy analysis reveals that the multiwalled CNTs have the crystalline graphite sheets and the bamboo structure that the tube consists of hollow compartments separated with graphite layers. A possible growth mechanism is suggested to explain the structure of CNTs. The emission current density is 1.1 mA cm −2 at 4.5 V μm −1, showing the Fowler–Nordheim behavior.

Powder design for UV-attenuating agent with high transparency for visible light

To design high-performance ultraviolet (UV)-attenuating agent for dispersion type of functional paints and/or cosmetic powders, nano-meter size of zinc oxide (ZnO) powder was immobilized onto the surface of 4 μm square size of crystalline sheet (H-ilerite: synthetic polysilicic acid (PSA)). A high-speed elliptical-rotor-type powder mixer (HEM) was applied for exfoliating in a sheet one by one from the PSA agglomerates without breakage of the original shape and for arranging the ZnO in the mono-particle layer on the sheet during dispersing the self-coagulative ultra-fine ZnO. The critical operating conditions of the HEM were determined by an observation of the morphological change of PSA. Optical characteristics of the compounded powder (ZnO-PSA) were evaluated in the form of suspension dispersed into polymer medium. The UV-attenuating effect was confirmed in the UVA region (320–400 nm) and the heat treatment after immobilization was one of the effective methods to get higher transparency for visible light.

Mass Determination of the Unit Cell of the Innermost Chorionic Layer in Drosophilidae by Scanning Transmission Electron Microscopy

The innermost chorionic layer (ICL) in eggshells of Drosophila melanogaster is a naturally occurring patchwork of thin three-dimensional crystalline plates located between the inner endochorion and the vitelline envelope. The mass-per-unit area of the ICL has been measured from scanning transmission electron microscope images of isolated unstained material and it was possible to distinguish up to four layers with the majority of the crystalline sheets being one to three layers thick. Taking into account the unit cell areas for the different crystals, we have estimated the mean ICL subunit sizes to be 36 kDa for Drosophila melanogaster, 35 kDa for Drosophila auraria, and 33 kDa for Drosophila teissieri. The results suggest that the three different Drosophilidae species have very similar average subunit masses.

Undecagold Cluster Labeling of Proteins at Reactive Cysteine Residues

Undecagold cluster labeling of reactive cysteine residues in numerous proteins has allowed the labeled sites to be identified by electron microscopy, providing high-resolution information on the location and orientation of subunits in oligomeric enzymes, virus capsids, crystalline sheets of membrane proteins, and muscle thin filaments. The range of applications of undecagold cluster labeling has been greatly extended by the availability of site-directed mutagenesis to introduce cysteine residues at sites of interest. In this paper I discuss factors that can influence the extent and specificity of labeling, methods for biochemical analysis of undecagold-labeled proteins, and the effects of undecagold cluster labeling on biological activity.

Tubulin Structure and the Development of Electron Crystallography

Abstract The identification of tubulin as the major protein component of microtubules (1) occurred at about the same time, a little over thirty years ago, as the first demonstration of the ability to obtain 3- dimensional data from electron micrographs. DeRosier and Klug's paper on image reconstruction of the bacteriophage tail (2) marks what many consider the beginning of electron crystallography - determining protein structures by electron microscopy. Microtubules have long been the target of active study by light and electron microscopy, and even before the discovery of tubulin played a significant role in driving technical developments in EM. The demonstration that tubulin could form two-dimensional crystalline sheets (3) seemed to pave the way for high resolution.structural studies. The discovery of the sheet polymer of tubulin came shortly after Unwin and Henderson's first publications on bacteriorhodopsin showing secondary structure resolved by EM (4). Application of their methods to tubulin in the late 1970's resolved low resolution features of the protein, but technical problems prevented achieving much more than showing the molecular envelope. Many developments in both tubulin studies and electron crystallography followed this work over the next 15 years. As techniques were refined for purifying, labeling and visualizing tubulin and microtubules, the dynamic properties of microtubules and their role in the cell cycle were characterized, and some hints about the mechanism of polymerization and disassembly were derived from biophysical and microscopic investigations.

Basement influence on foreland thrust and fold belt development: an example from the southern Urals

The reactivation of two sets of pre-existing basement faults provided strong control on the geometric and kinematic evolution of the Paleozoic south Urals foreland thrust and fold belt. Pre-existing basement faults oriented subparallel to the Uralide structural grain were related to several stages of Precambrian deformation. Reactivation focused the deformation along these earlier faults, resulting in the subsequent incorporation of crystalline basement thrust sheets in the thrust belt at an early stage of its evolution. Pre-existing basement faults oriented subperpendicular to the developing thrust belt structural grain were related to aulacogens in the Archean crystalline basement that were filled by variable thickness of Riphean-aged sediments. These influenced the localisation and development of numerous structures in the thrust belt that caused along-strike structural changes related to lateral ramps. Recognition of these features and their influence on the geometric and kinematic evolution of the south Urals foreland thrust and fold belt are important for understanding seismic reflection fabrics in the south Urals thrust belt, and for potential field, 3-D geological, and kinematic modelling.

A structural transect across the coastal mobile belt in the Brazilian Highlands (latitude 20°S): the roots of a Precambrian transpressional orogen

We present results of a detailed structural analysis from a 250 km-long, east–west-trending transect crossing the Coastal Mobile Belt, a part of the Precambrian orogen which lies between the eastern edge of the São Francisco craton and the Atlantic coast. The region exposes amphibolite–granulite grade metamorphic rocks and migmatites which formed at mid-lower crustal depths during the Brasiliano orogeny (0.63–0.52 Ga). This event marked the closure of the northernmost Adamastor Ocean and collision between the São Francisco and Congo Cratons during West Gondwana assembly. Brasiliano deformation resulted in W-vergent structures including thrust faults which accommodated kilometer-scale transport of crystalline basement, overturned kilometer-scale folds, sheath folds and penetratively developed gneissosity and schistosity. Isolated relics of an older folded fabric occur locally and may represent Transamazonian (2.2–2.0 Ga) deformation. The orogen is kinematically partitioned with the eastern 175 km dominated by moderately- to steeply dipping north-trending dextral strike-slip and oblique-slip faults and associated flower structures, whereas the western 75 km is dominated by W-vergent shallowly- to moderately east-dipping thrust faults. The boundary between these two provinces may mark a Brasiliano suture. Throughout the transect, quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets. The granulite-cored Serra do Caparaù massif, the highest mountains in South America outside of the Andes and Guyana shield, occupies a restraining bend between two Brasiliano dextral shear zones. The W-vergent Coastal Mobile Belt formed contiguously with the E-vergent Pan-African West Congo orogen now exposed along the conjugate margin of Africa. Thus an important late Precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent. Cretaceous opening of the South Atlantic and separation of the West Congo belt from the Coastal Mobile Belt may have been structurally influenced by this boundary.

Two-Dimensional Crystallization of Ca-ATPase by Detergent Removal

By using Bio-Beads as a detergent-removing agent, it has been possible to produce detergent-depleted two-dimensional crystals of purified Ca-ATPase. The crystallinity and morphology of these different crystals were analyzed by electron microscopy under different experimental conditions. A lipid-to-protein ratio below 0.4 w/w was required for crystal formation. The rate of detergent removal critically affected crystal morphology, and large multilamellar crystalline sheets or wide unilamellar tubes were generated upon slow or fast detergent removal, respectively. Electron crystallographic analysis indicated unit cell parameters of a = 159 Å, b = 54 Å, and γ = 90° for both types of crystals, and projection maps at 15-Å resolution were consistent with Ca-ATPase molecules alternately facing the two sides of the membrane. Crystal formation was also affected by the protein conformation. Indeed, tubular and multilamellar crystals both required the presence of Ca 2+; the presence of ADP gave rise to another type of packing within the unit cell ( a = 86 Å, b = 77 Å, and γ = 90°), while maintaining a bipolar orientation of the molecules within the bilayer. All of the results are discussed in terms of nucleation and crystal growth, and a model of crystallogenesis is proposed that may be generally true for asymmetrical proteins with a large hydrophilic cytoplasmic domain.

Break-up of the Greater Indo-Australian Continent and accretion of blocks framing South and East Asia

The Tibetan and ‘Sibumasu’ continental blocks rifted apart from the northern margin of Gondwanan Indo-Australia during the Permo-Carboniferous whereas, the IndoBurma-Andamans (IBA), Sikuleh, Lolotoi micro-continents did so during the Late Jurassic. This continental margin experienced glacial or cool conditions during the Permo-Carboniferous. The Tibetan and Sibumasu blocks drifted northward during mid-late Permian initiating opening of the Neo-Tethys. The arm of the Palaeo-Tethys located to the north of these blocks closed as these blocks were accreted to the South China and Indochina blocks which had separated from the same Gondwanan margin during early Palaeozoic. All these blocks were amalgamated to form the Cathaysialand. The Sibumasu block was accreted to the Indochina block, and the Tibetan Changtang block to Eurasia during the late Permian-mid Triassic. Contemporaneously Mesozoic Neo-Tethys expanded between the Indian, Lhasa and Changtang blocks. The Indian and Australian continents separated during the Cretaceous leading to the opening up of the Indian Ocean and closing of the Tethyan ocean. The Palaeo- and Neo-Tethyan sutures in Tibet, Yunnan, Myanmar, Laos-Thailand and Vietnam reveal the complex opening and closing history of the Tethys. IBA rotated clockwise from its earlier E-W orientation, because of dextral transcurrent fault movements which ensured faster northward movement of the Indian plate relative to Australia during late Cretaceous-early Eocene. Contemporaneous to India-Tibet terminal collision during early-mid Eocene there was thloeiitic-alkalic foreland volcanism (Abor and equivalents) at the leading edge of the Indian continent. Sustained post-collisional movement of the Indian plate, caused southward propagation of the Himalayan crystalline and frontal foreland thrust sheets. It also produced E-W trending folds and thrusts even in distal Central Indian Ocean areas as well as a clockwise rotation of the amalgamated SE Asian Cathaysian composite block. Dislocations of the 90 °E Ridge, indicate that the main compression occurred during early Eocene which was followed by movement since late Miocene. The ophiolite trail on IBA does not represent the eastern suture of the Indian continent. As a result of Late Oligocene oblique terminal collision between Sibumasu and 1BA, Mesozoic early Eocene ophiolites. their mid-Eocene cover and trench sediments occur as klippen on IBA. Oblique convergence between IBA and the Indian plate in turn produced an active subduction regime along the western margin of the Indo-Burma mobile belt and the Andaman-Java trench. The activity produced Neogcne-Quaternary volcanism. dextral strike-slip movement in Central Burma-Andaman-Sumatran region, and opened up the Andaman sea. Northward, the Shillong massif, representing NE prolongation of the Indian sub-continent, is technically juxtaposed against the northern end of IBA. The collision of these blocks might have uplifted the Shillong massif during Mio-Pliocene. Far eastward, the convergence was orthogonal between the Australian continent and the Indonesian Arc resulting emplacement of the Lolotoi continental rocks. Maubissi exotic blocks and ophiolitic rocks as nappe over the Timor shelf which possibly remained attached to the Australian continent.

Projection map of the reaction center-light harvesting 1 complex from Rhodopseudomonas viridis at 10 Å resolution

The photosynthetic reaction center-light harvesting 1 complex from Rhodopseudomonas viridis was purified and reconstituted into two-dimensional crystals. The single-layered crystalline sheets with lattice parameters a= b=133.3 Å and γ=120° were investigated by electron cryo-microscopy and the projection map at 10 Å resolution was calculated. The opening diameter of the light-harvesting ring of 72 Å is sufficient to allow slight movement of the reaction center within the ring. Based on characteristic features observed in the projection map, the mechanism of energy transfer from the light-harvesting 1 complex to the reaction center was discussed.

Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal

Sedimentological and provenance data from the lower Miocene–Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87 Sr/ 86 Sr record of the ocean. In western Nepal, the Dumri Formation is an ∼750–1300-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2–12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene–Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at ∼10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene–Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group (∼15–11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From ∼11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (∼4–5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460–530 Ma, ∼850–1200 Ma, ∼1.8–2.0 Ga, and ∼2.5 Ga. An abundance of Cambrian–Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87 Sr/ 86 Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian–Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87 Sr/ 86 Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

MEASUREMENT OF SURFACE STRESS: DESCRIPTION OF AN INTERFEROMETRIC METHOD

In a recent publication, we proposed a new experimental method for measuring the absolute surface stress of a thin crystalline sheet based on the deformation of very thin sheets (thickness e<10 μm) submitted to disymmetrical surface stresses (S1 on one side of the sheet, S2 on the other). This paper demonstrates that Michelson interferometry can be used to measure a surface stress change of 2–3 10-3 J·m -2 if the thickness and the Young modulus of the sheet are accurately known. Using a silicon substrate sample we found that one fourth of a monolayer of silver reduces the surface stress by 0.45 J·m -2.

Chronology of Paleozoic metamorphism and deformation in the Blue Ridge thrust complex, North Carolina and Tennessee

The Blue Ridge province in northwestern North Carolina and northeastern Tennessee records a multiphase collisional and accretionary history from the Mesoproterozoic through the Paleozoic. To constrain the tectonothermal evolution in this region, radiometric ages have been determined for 23 regionally metamorphosed amphibolites, granitic gneisses, and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone. Samples were collected along a northwest-southeast traverse across three crystalline thrust sheets located west of the Grandfather Mountain window. Ordovician metamorphic mineral ages (Llanvirnian to Caradocian) are recorded in the highest thrust sheets within the Blue Ridge thrust complex. Six samples of metapelite and amphibolite from the Spruce Pine thrust sheet and five samples of basement gneiss and metagabbro from the Pumpkin Patch thrust sheet yield Sm-Nd and Rb-Sr garnet ages of 460 to 450 Ma and hornblende ages of 472 to 451 Ma. In addition, multiple mineral isochron ages of 457+ or -2 and 458+ or -17 Ma were obtained from a Pumpkin Patch metagabbro. The garnet ages from the Pumpkin Patch thrust sheet (458, 455, and 451 Ma) are similar to those from the structurally overlying Spruce Pine thrust sheet (460, 456, 455, 455, and 450 Ma). Both thrust sheets exhibit similar upper amphibolite-facies conditions. Because of the high closure temperature for garnet, the garnet ages are interpreted to date growth at or near the peak of Taconic metamorphism. Devonian metamorphic ages are recognized in the Spruce Pine thrust sheet, where Sm-Nd and Rb-Sr garnet ages of 386 and 393 Ma and mineral isochron ages of 397+ or -14 and 375+ or -27 Ma are preserved. Hornblendes record similar 40 Ar/ 39 Ar, Sm-Nd, and Rb-Sr ages of 398 to 379 Ma. Devonian 40 Ar/ 39 Ar hornblende ages are also recorded in the structurally lower Pumpkin Patch thrust sheet. The Devonian mineral ages are interpreted to date a discrete tectonothermal event, as opposed to uplift and slow cooling from an Ordovician metamorphic event. This interpretation is based in part on the fact that Devonian and Ordovician mineral ages are intermixed spatially within the Spruce Pine and Pumpkin Patch thrust sheets. 40 Ar/ 39 Ar muscovite ages from regionally metamorphosed rocks indicate that much of the Blue Ridge cooled slowly to temperatures of about 375 degrees C by the Late Mississippian. White micas from mylonite zones separating thrust sheets record 40 Ar/ 39 Ar plateau ages of 323, 327, 328, and 336 Ma and Rb-Sr ages of 324, 324, 326, and 326 Ma. These reflect the age of Late Mississippian mylonitization (mica growth below or near respective Sr and Ar closure temperatures). The Mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the Alleghanian orogeny. The composite thrust stack of the Blue Ridge complex was subsequently thrust northwestward along the Linville Falls fault during middle Alleghanian orogeny (about 300 Ma).

Staphylococcus aureusα-Toxin: Characterization of Protein/Lipid Interactions, 2D Crystallization on Lipid Monolayers, and 3D Structure

Staphylococcus aureusα-toxin was characterized with respect to surface activity and its interaction with lipid monolayers. The protein alone had a detergent-like behavior at the air/water interface. Its affinity was higher for negatively charged than for neutral phospholipids. The interaction was pH dependent, showing a maximum increase at pH 7.0. Only a small part of the protein oligomer appeared to be inserted into the monolayers. Crystalline sheets of α-toxin were formed using negatively charged phospholipids. Electron microscopy of such areas, at different tilt angles, allowed reconstruction of a three-dimensional model following image processing. The sheets analyzed consisted of two protein layers arranged on a tetragonal lattice. Under the conditions used to grow the crystals the toxin formed 90-Å-wide cylinders with a height of 70 Å. One of the imposed fourfold axes running perpendicular to the plane of the crystalline layer is positioned at a protein-deficient region which forms a 25-Å-wide pore through the oligomer.

Visualizing the Secondary Structure of Tubulin: Three-Dimensional Map at 4 Å

We are in the process of determining the structure of tubulin using electron crystallography of zinc-induced, crystalline sheets. We have now extended the resolution to 4 Å, and there are many features in the map that appear to show details of the secondary structure. X-ray crystallographers are well aware of the problems of interpreting maps with such limited resolution, and the additional problem of the missing cone of data inherent in electron crystallography may make interpretation even more difficult. To investigate how reliably these maps can be interpreted, we have calculated density maps of a known structure, actin, under conditions similar to those of the tubulin map. Results of these simulations support the limited interpretations we made previously in the 6.5-Å maps and the more extensive interpretations we make here in the 4-Å map. Most of the secondary structure of the tubulin dimer can now be identified.