We have calculated at 5.0 A resolution an electron-density map of the large 50S ribosomal subunit from the bacterium Haloarcula marismortui by using phases derived from four heavy-atom derivatives, intercrystal density averaging and density-modification procedures. More than 300 base pairs of A-form RNA duplex have been fitted into this map, as have regions of non-A-form duplex, single-stranded segments and tetraloops. The long rods of RNA crisscrossing the subunit arise from the stacking of short, separate double helices, not all of which are A-form, and in many places proteins crosslink two or more of these rods. The polypeptide exit channel was marked by tungsten cluster compounds bound in one heavy-atom-derivatized crystal. We have determined the structure of the translation-factor-binding centre by fitting the crystal structures of the ribosomal proteins L6, L11 and L14, the sarcin-ricin loop RNA, and the RNA sequence that binds L11 into the electron density. We can position either elongation factor G or elongation factor Tu complexed with an aminoacylated transfer RNA and GTP onto the factor-binding centre in a manner that is consistent with results from biochemical and electron microscopy studies.

The 30S ribosomal subunit binds messenger RNA and the anticodon stem-loop of transfer RNA during protein synthesis. A crystallographic analysis of the structure of the subunit from the bacterium Thermus thermophilus is presented. At a resolution of 5.5 A, the phosphate backbone of the ribosomal RNA is visible, as are the alpha-helices of the ribosomal proteins, enabling double-helical regions of RNA to be identified throughout the subunit, all seven of the small-subunit proteins of known crystal structure to be positioned in the electron density map, and the fold of the entire central domain of the small-subunit ribosomal RNA to be determined.

Highly ordered, near-single-crystal lamellar films of a triblock copolymer (polystyrene−polybutadiene−polystyrene, PS/PB/PS) were used to study the deformation mechanism of a structure of alternating glassy−rubbery layers, at different orientations of the deformation axis relative to the layer normal. Synchrotron radiation was used for simultaneous in-situ deformation and small-angle X-ray scattering measurements. These were augmented with direct imaging of the structure by transmission electron microscopy. The deformation mechanism depends on the orientation of the force with respect to the structure. Loading parallel to the lamellae results in yielding by propagation of a stable macroscopic neck. The glassy PS layers break up at the neck front, releasing the rubbery layers to achieve high strain. The morphology that develops by deformation of the structure in other directions is an ensemble of new tilt boundaries oriented along the deformation axis. The lamellar normals tilt away from the deformation axis with increasing strain, keeping the lamellar spacing essentially constant. The effect of force applied perpendicular to the lamellae is to fold the layers into a “chevron” morphology, similar to other layered systems such as smectic liquid crystals. At high strain, plastic deformation and fracture of the glassy PS hinges of the “chevron” structure leads to symmetric kink boundaries parallel to the force axis. In addition, nucleation of kink bands around defects and propagation of the kink boundaries into adjacent regions can lead to a similar morphology. The lamellar spacing remains constant during perpendicular stretching, and the tilt angle of the lamellar normal follows the macroscopic deformation in an affine manner. Stretching at 45° forms asymmetric kink boundaries parallel to the force axis. The major limbs of the kink band tilt with increasing strain so that the angle between the lamellar normal and the force axis increases from its initial value of 45°, while the lamellar period remains constant. The minor limbs tilt in the opposite direction and exhibit dilation of the lamellar spacing. Eventually the layers rupture, forming voids at the kink-boundary interfaces. The tilt angle of the major-limb lamellae, as a function of strain, is less than predicted by the affine model. This study suggests a general deformation mechanism for a lamellar structure of alternating glassy and rubbery layers. The layered structure responds to deformation, in any direction other then parallel to the layers, by creating new internal tilt-grain boundaries parallel to the deformation axis. At higher strain the layers yield and subsequently fracture at the kink-boundary interfaces. With increasing strain the lamellar stacks between the kink boundaries tilt toward the deformation axis until they are nearly parallel to it. Since the main features of this mechanism are independent of the initial orientation angle of the layers relative to the deformation axis, it is relevant also to polygranular, globally unoriented lamellar st...

With access to whole genome sequences for various organisms and imminent completion of the Human Genome Project, the entire process of discovery in molecular and cellular biology is poised to change. Massively parallel measurement strategies promise to revolutionize how we study and ultimately understand the complex biochemical circuitry responsible for controlling normal development, physiologic homeostasis and disease processes. This information explosion is also providing the foundation for an important new initiative in structural biology. We are about to embark on a program of high-throughput X-ray crystallography aimed at developing a comprehensive mechanistic understanding of normal and abnormal human and microbial physiology at the molecular level. We present the rationale for creation of a structural genomics initiative, recount the efforts of ongoing structural genomics pilot studies, and detail the lofty goals, technical challenges and pitfalls facing structural biologists.

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The mechanical properties of the double gyroid (DG) cubic phase in glassy−rubbery block copolymer systems are examined. The stress−strain properties of an isoprene-rich polystyrene/polyisoprene/polystyrene (SIS) triblock and a polystyrene/polyisoprene (SI) starblock DG, both comprised of two separate interpenetrating glassy networks embedded in rubbery matrices, are compared to those of the sphere, cylinder, and lamellar morphologies. This 3-dimensionally interpenetrating periodic nanocomposite is found to have superior properties over those of its classical counterparts, attributable to the morphology rather than to the volume fraction of the glassy component, the architecture of the molecule, or the molecular weight. The DG is the only polygranular/isotropic thermoplastic elastomer morphology which exhibits necking and drawing and which requires considerably higher stresses for deformation up to 200% strain than any of the three classical microdomain morphologies. The deformation behavior of the DG is further investigated as a function of applied strain using in situ synchrotron small-angle X-ray scattering. Yielding and necking are observed at ∼20% strain, accompanied by sudden changes in the SAXS patterns:  the characteristic Bragg rings of the DG disappear and are replaced by a lobe pattern containing streaks and diffuse scattering. Analysis of the {211} reflection in the SAXS data indicates that PS networks play a large role in governing the deformation behavior. The necking behavior of the DG suggests a different deformation mechanism. The DG samples recover both microscopically and macroscopically upon unloading and annealing, indicating that the complex interconnected nanocomposite structure was not permanently damaged, even after having been stretched to 600% strain.

The relative positions of the centers of mass of the 21 proteins of the 30S ribosomal subunit from Escherichia coli have been determined by triangulation using neutron scattering data. The resulting map of the quaternary structure of the small ribosomal subunit is presented, and comparisons are made with structural data from other sources.

We have prepared and characterized photosensitized zinc oxide (ZnO) nanoclusters, dispersed in methanol, using carboxylated coumarin dyes for surface adsorption. Femtosecond time-resolved emission spectroscopy allows us to measure the photo-induced charge carrier injection rate constant from the adsorbed photosensitizer to the n-type semiconductor nanocluster. These results are compared with other photosensitized semiconductors. IntroductionPhotosensitized nanoclusters of wide bandgap n-type semiconductors are promising systems for efficient solar energy conversion. A photosensitizing dye adsorbed onto the surface of the semiconductor can inject an electron into the semiconductor, which could possibly be used directly in a solar photovoltaic device or stored in a battery. A system design goal is to select a dyehemiconductor pair for which the ground electronic state of the dye w i l l lie in the bandgap between valence and conduction bands of the semiconductor, while the excited state of this dye will have an energy slightly higher than the sum of the conduction band edge and interfacial electron transfer reorganization energies. Several dyes have been used as photosensitizers, including organic laser dyes, porphyrin derivatives, and inorganic complexes of ruthenium, osmium, and iron. A number of semiconductor materials have been studied for this application, including Ti02, Sn02, Sn&, and ZnO. For an overview of these studies of dye-sensitized electron-transfer at the electrode intedace, we refer to the chapter by by Gerischer and Tributsch?** Photosensitizers included riboflavin, eosin, rose bengal, rhodamine B, and cyanine dyes. Higher efficiency was achieved in similar systems by Tsubomma, et aL6p6 using sintered porous disks of 50. This preparation was used in order to achieve a sufficiently high surface coverage of Zinc oxide electrodes photosensitized by a number of dyes were fist reported adsorbed dye on the ZnO electrode. Similar results using Rhodamine B and RuIIpolypyridyl dye sensitized sintered-porous ZnO were also reported by Alonso, et d 7 T i 0 2 has also been extensively used as a photosensitizer, with the first report for single-crystal rutile photosensitized by Ru(bpy)$+ and its derivatives given by Clark and Sutin.8 A number of researchers have prepared and characterized nanocrystalline ZnO, including Spanhel and Anderson: Bahnemann,lO and Cavaleri, et al. l1 The results mentioned here provide the foundation for the present study.A number of different types of photosensitizers for metal-oxide semiconductors have been studied, including both organic and inorganic complex chromophores. For strong binding between the photosensitizer and the metal-oxide semiconductor surface, a carboxylate, phosphonate, or sulfate group on the photosensitizer molecule is neededl2-I4. In the experiments presented here, we use 7-aminOCO~~narin chromophores that have a carboxylate substituent at the 3-position. The carboxylate, perhaps in concert with the 2-position carbonyl oxygen, binds to one or more metal-cat...