An isomorphous heavy-atom derivative of crystaline formylmethionine transfer RNA

An isomorphous osmium derivative of crystalline yeast initiator transfer RNA has been prepared and interpreted to 6-angstrom resolution. The coordinates of the heavy atoms have been determined by Patterson and "direct" methods applied to the difference coefficients of the centric projections, followed by least-squares refinement. There is one dominant site per asymmetric unit, consistent with the finding by neutron-activation analysis that there is approximately one osmium atom per molecule of transfer RNA. The osmium derivative appears to be a normal substrate for enzymatic aminoacylation.     

A functional model for O-O bond formation by the O2-evolving complex in photosystem II

The formation of molecular oxygen from water in photosynthesis is catalyzed by photosystem II at an active site containing four manganese ions that are arranged in di-mu-oxo dimanganese units (where mu is a bridging mode). The complex [H2O(terpy)Mn(O)2Mn(terpy)OH2](NO3)3 (terpy is 2,2':6', 2"-terpyridine), which was synthesized and structurally characterized, contains a di-mu-oxo manganese dimer and catalyzes the conversion of sodium hypochlorite to molecular oxygen. Oxygen-18 isotope labeling showed that water is the source of the oxygen atoms in the molecular oxygen evolved, and so this system is a functional model for photosynthetic water oxidation.     

液态Ca50Zn50合金快速凝固过程中微观结构的演变特性

采用分子动力学方法对液态Ca50Zn50合金的快速凝固过程进行了模拟研究.并采用双体分布函数、HA键型指数法、原子团类型指数法(CTIM)和可视化等方法对凝固过程中微观结构的演变特性进行了分析.结果表明：由于Ca,Zn原子的半径差异较大,导致总双体分布函数的第1峰明显分裂成为3个次峰,随着温度下降,半径较小的Zn原子互为近邻的几率明显增加.系统中1551键型所构成的二十面体基本原子团(12 0 12 0)的数量占绝对优势,对非晶态结构的形成起主导作用.系统中基本原子团的中心原子主要是原子半径较小的Zn原子.具有短程序的基本原子团之间大多以共顶点(VS),共棱线(ES),共面(FS)和相互交叉(IS)的方式相结合,在凝固形成非晶态的过程中,基本原子团大多以相互交叉(IS)的方式相结合,最后形成各种不同尺寸的、原子结合比较紧密、结构比较稳定的较大团簇.     

Ti8C12+-Metallo-Carbohedrenes: A New Class of Molecular Clusters?

During the course of studying the dehydrogenation reactions of hydrocarbons by titanium atoms, ions, and clusters, an exceptionally stable and abundant cluster which contains 8 titaniums and 12 carbons was discovered. "Titration" reactions with ND(3) reveal the uptake of eight molecules, pointing to the fact that the titanium atoms are at exposed positions of similar coordination. A dodecahedral structure of T(h) point group symmetry is proposed to account for the unusual stability of this molecular cluster. The Ti(8)C(12)(+) dodecahedron has 12 pentagonal rings and each of the rings is formed by two titanium and three carbon atoms, where each titanium is bound to three carbons. Based on the model, it is expected that neutral Ti(8)C(12) would be a stable metallo-carbododecahedral molecule and may comprise one member of a new class of molecules, namely metallo-carbohedrenes.     

Neutron diffraction of cell membranes (myelin)

Small-angle neutron diffraction (wavelength 4.05 angstroms) of human and rabbit sciatic nerve has been carried out by means of the Brookhaven high flux beam reactor with an automated slit camera. Most of the free water of the nerves was substituted in order to minimize incoherent scatter of hydrogen atoms. The differences in amplitude and phase shifts between neutrons and x-rays resulted in a neutron diffraction pattern that was completely different from the x-ray pattern. The neutron pattern consisted of a single peak of about 89-angstrom spacing in the region examined (up to 6-angstrom spacing). The strong third, fourth, and fifth order reflections (about 60, 45, and 36 angstroms) seen in the x-ray pattern were suppressed. The neutron data indicated a strong scattering from one portion of the membrane.     

Paramyosin: molecular length and assembly

Paratnyosin paracrystails formned with divalent cations have a 725-angstrom axial period and show simple negative staining patterns in the electron microscope. The structure of the aggregates is interpreted in terms of an array of polar molecules about 1275 angstroms long, with "gaps" and "overlaps" in the molecular assembly. Antiparallel relatiotns between molecules lead to the assembly of paracrystals with opposite polarity at either end. Implications of the in vitro structure for filaments containing paramyosin in muscle are discussed.     

Mariner 6: ultraviolet spectrum of Mars upper atmosphere

Emission features from ionized carbon dioxide and carbon monoxide were measured in the 1900- to 4300-angstrom spectral region. The Lyman alpha 1216-angstrom line of atomic hydrogen and the 1304-, 1356-, and 2972-angstrom lines of atomic oxygen were observed.     

X-ray diffraction from magnetically oriented solutions of macromolecular assemblies

A simple system was developed for obtaining x-ray diffraction patterns from magnetically oriented solutions of macromolecular assemblies. A small permanent magnet was designed that produces a magnetic field of 16 kilogauss in a volume of 1 cubic millimeter and is mountable on most x-ray cameras. Many subcellular structures have sufficient diamagnetic anisotropy that they exhibit orientation in dilute solution when placed between the poles of the magnet. Diffraction from solutions oriented in this magnet can provide substantially more structural information than small-angle scattering from isotropic solutions. In favorable cases, such as dilute solutions of filamentous bacteriophages, it is possible to produce oriented fiber diffraction patterns from which intensities along layer lines can be measured to 7-angstrom resolution. The magnetically induced birefringence observed in solutions of other macromolecular assemblies suggests that this technique may have broad applicability to subcellular structures.     

Subunit structure of aldolase

A new crystal form of rabbit muscle aldolase shows that the molecule has 222 symmetry to at least 4-angstrom resolution, and hence that the gross conformation of the four subunits is the same. Comparison of the new form with a previously reported form establishes the number of molecules per unit cell, n, in the older form. For an independent check, the "crystal-volume and protein-content method" was developed to determine n without directly measuring the water content of the crystals.     

The structure and catalytic cycle of a sodium-pumping pyrophosphatase

Membrane-integral pyrophosphatases (M-PPases) are crucial for the survival of plants, bacteria, and protozoan parasites. They couple pyrophosphate hydrolysis or synthesis to Na(+) or H(+) pumping. The 2.6-angstrom structure of Thermotoga maritima M-PPase in the resting state reveals a previously unknown solution for ion pumping. The hydrolytic center, 20 angstroms above the membrane, is coupled to the gate formed by the conserved Asp(243), Glu(246), and Lys(707) by an unusual "coupling funnel" of six α helices. Comparison with our 4.0-angstrom resolution structure of the product complex suggests that helix 12 slides down upon substrate binding to open the gate by a simple binding-change mechanism. Below the gate, four helices form the exit channel. Superimposing helices 3 to 6, 9 to 12, and 13 to 16 suggests that M-PPases arose through gene triplication.     

Sulfur Melting and Polymorphism under Pressure: Outlines of Fields for 12 Crystalline Phases

The polymorphism of sulfur has been investigated by static and dynamic methods up to 500 degrees C at 35 kilobars and up to 350 degrees C at 100 kilobars. The melting curve of sulfur to 31 kilobars and phase boundaries of the so-called "4.04-angstrom phase" have been determined. Evidence has been obtained for phase fields of nine new high-pressure forms of sulfur.     

Isolation of filaments from brain

A method is presented for the isolation of filaments of 90-angstrom diameter from the white matter of bovine brain by first floating the myelinated axons in a centrifugal field and then fractionating the axons on a series of density gradients. This results in a fraction that contains two types of bundles of filaments but few other constituents. The filaments are stable over a wide range of temperatures and at both low and high ionic strength. Their density and their resistance to digestion by ribonuclease and deoxyribonuclease indicate that they are primarily protein. The molecular weight of the subunit is approximately 60,000. The protein does not comigrate with microtubule protein and does not bind cholcicine or nucleotides.     

Dinoflagellate with blue-green chloroplasts derived from an endosymbiotic eukaryote

The dinoflagellate, Amphidinium wigrense, contains triple membrane-bound bodies we have termed "blue-green chloroplasts." We believe these chloroplasts were derived from a cryptomonad endosymbiont similar to that present in another blue-green dinoflagellate, Gymnodinium acidotum. These dinoflagellates provide evidence that a chloroplast has evolved from an endosymbiotic eukaryote.     

Far-ultraviolet photography of orion: interstellar dust

Wide-angle photography of Barnard's Loop Nebula suggests that the nebular emission is much less intense in the 1230- to 2100-angstrom spectral region than in the 2200- to 3200-angstrom near-ultraviolet region. This contrast may be due to differences in the absorption or scattering properties of the interstellar grains in these two wavelength regions.     

Cartilage collagen: a staggered substructure in reconstituted fibrils

In contrast to the typical transverse banding pattern of native and reconstituted skin collagen fibrils, reconstituted fibrils of cartilage collagen have an oblique banding pattern that results from a regular axial shift (89 angstroms) of component "subfibrils." The 89-angstrom shift may be related to the major helix of the collagen molecule.     

Mariner 9 ultraviolet spectrometer experiment: stellar observations

Photoelectric spectra have been obtained for a number of early-type stars in the 1100- to 2000-angstrom region with the Mariner 9 ultraviolet spectrometer. The resonance lines of H I, Si IV, and C IV are easily identified, as are features due to C II, C III, Si III, Fe II, N IV. The absolute energy distribution derived from the data lie about 20 percent below those of OAO-2 in the 1200- to 2000-angstrom region.     

Structure of the hydrophilic domain of respiratory complex I from Thermus thermophilus

Respiratory complex I plays a central role in cellular energy production in bacteria and mitochondria. Its dysfunction is implicated in many human neurodegenerative diseases, as well as in aging. The crystal structure of the hydrophilic domain (peripheral arm) of complex I from Thermus thermophilus has been solved at 3.3 angstrom resolution. This subcomplex consists of eight subunits and contains all the redox centers of the enzyme, including nine iron-sulfur clusters. The primary electron acceptor, flavin-mononucleotide, is within electron transfer distance of cluster N3, leading to the main redox pathway, and of the distal cluster N1a, a possible antioxidant. The structure reveals new aspects of the mechanism and evolution of the enzyme. The terminal cluster N2 is coordinated, uniquely, by two consecutive cysteines. The novel subunit Nqo15 has a similar fold to the mitochondrial iron chaperone frataxin, and it may be involved in iron-sulfur cluster regeneration in the complex.     

DNA structure: evidence from electron microscopy

The contour lengths of phiX174 DNA duplex and RNA-DNA hybrid molecules were measured by several commonly used electron microscopic techniques. The countour length of the hybrid molecules corresponds to a rise of 2.5 to 2.6 angstroms per base pair, as expected for the A conformation, while the length of phiX174 duplex DNA similarly measured corresponds to a 2.9-angstrom rise, very different from 3.4 angstroms of the classic B form. Thus any chromatin structure parameter based on electron microscopy and a rise of 3.4 angstroms must be reappraised. The possibility that DNA in dilute solution also has a rise of 2.9 angstroms and a screw of 10.5 base pairs per turn is discussed.     

Field-Induced Nanometer- to Atomic-Scale Manipulation of Silicon Surfaces with the STM

The controlled manipulation of silicon at the nanometer scale will facilitate the fabrication of new types of electronic devices. The scanning tunneling microscope (STM) can be used to manipulate strongly bound silicon atoms or clusters at room temperature. Specifically, by using a combination of electrostatic and chemical forces, surface atoms can be removed and deposited on the STM tip. The tip can then move to a predetermined surface site, and the atom or cluster can be redeposited. The magnitude of such forces and the amount of material removed can be controlled by applying voltage pulses at different tip-surface separations.