Kornerupine: its crystal structure

Three-dimensional analysis of the crystal structure of kornerupine reveals the crystallochemical formula Mg(VI)(2)Mg(VI)AlVI(6)[Si(2)O(7)] [(Al,Si)(2) SiO(10)]O(4)(OH), with four formula units in the structure cell of a = 16.100 (2) A, b = 13.767(2) A, c = 6.735(2) A; space group, Cmcm. The unusual crystal structure includes walls of Al-O edge and corner-sharing octahedra, and chains of alternating Mg-O and Al-O octahedra fused to the walls by further edge-sharing to form dense slabs. These slabs are held together by [Si(2)O(7)] corner-sharing tetrahedral pairs and [(Al,Si)(2)SiO(10)] corner-sharing tetrahedral triplets.     

Tuhualite crystal structure

As judged by crystal structure analysis, the crystallochemical formula of tuhualite is (Na,K)(2)(VI) (Fe(+3))(2)(VI) (Fe(+2))(2)(VI) Si(12)O(30) . H(2)O, with four such formulas in the unit cell. The structure is characterized by six-repeat double chains of silicon tetrahedrons and chains of edge-sharing, alternating iron tetrahedrons and octahedrons. Chains of each kind are connected by corner sharing to four chains of the other type. The iron cations appear to be anomalously ordered with Fe(+2) in tetrahedral and Fe(+3) in octahedral sites.     

Modified spinel, Beta-manganous orthogermanate: stability and crystal structure

A new high-pressure polymorph with a modified spinel structure, beta-Mn(2)GeO(4), is stable in a pressure range intermediate between the field of the polymorph with the olivine structure and that of another high-pressure polymorph. Oxygen atoms are located approximately in cubic close packing with manganese and germanium atoms in octahedral and tetrahedral interstices, respectively, as in the spinel structure; however, germanium atoms form Ge(2)O(7) groups instead of isolated GeO(4) groups.     

The crystal structure of human Argonaute2

Argonaute proteins form the functional core of the RNA-induced silencing complexes that mediate RNA silencing in eukaryotes. The 2.3 angstrom resolution crystal structure of human Argonaute2 (Ago2) reveals a bilobed molecule with a central cleft for binding guide and target RNAs. Nucleotides 2 to 6 of a heterogeneous mixture of guide RNAs are positioned in an A-form conformation for base pairing with target messenger RNAs. Between nucleotides 6 and 7, there is a kink that may function in microRNA target recognition or release of sliced RNA products. Tandem tryptophan-binding pockets in the PIWI domain define a likely interaction surface for recruitment of glycine-tryptophan-182 (GW182) or other tryptophan-rich cofactors. These results will enable structure-based approaches for harnessing the untapped therapeutic potential of RNA silencing in humans.     

Valinomycin crystal structure determination by direct methods

The conformation of an uncomplexed form of the antibiotic valinomycin (C(54)N(6)O(18)H(90)) has been determined by direct methods including a novel technique for strong enantiomorph discrimination via the calculation and systematic analysis of cosine invariants of a special type. The intramolecular hydrogen bonding scheme and the isopropyl group stereochemistry of uncomplexed valinomycin are compatible with interpretations of spectral measurements for the complexed and uncomplexed molecule in solution but are different from any previously proposed structure. The simple conformational change of a hydrogen bond shift, which could be induced by the process of potassium ion complexing, transforms the uncomplexed into the complexed structure.     

Crystal and molecular structure of acetylselenocholine iodide

The structure of acetylselenocholine iodide has been determined by x-ray crystallographic analysis. The molecule is in the trans conformation about the C-C bond of the choline residue. This conformation appears to explain the molecule's inability to give a positive cholinergic response when added to an electroplax preparation.     

Diphenylhydantoin and diazepam: molecular structure similarities and steric basis of anticonvulsant activity

Diphenylhydantoin and diazepam are two useful antiepileptic drugs. Though not obviously related chemically, their molecular conformations exhibit marked similarities. These similarities indicate a steric basis for their anticonvulsant activity and lead to conclusions about the receptor sites for this type of pharmaceutical.     

Photoproduction of molecular hydrogen by a plant-algal symbiotic system

The rapidly growing water fern Azolla, which contains a nitrogen-fixing blue-green algal symbiont, has been studied as a possible system for photoproduction of molecular hydrogen. When this plant is grown on a combined nitrogen supply, photochemically generated hydrogen can be diverted through the algal nitrogenase system, which serves as a source of molecular hydrogen generated from water. This symbiosis has several advantages as a possible biological energy conversion system.     

Cyclic uridine-3',5'-phosphate: molecular structure

The crystal structure of the triethylammonium salt of cyclic uridine-3',5'-phosphate was solved by use of the tangent formula to refine phase angles based upon the positions of six of the atoms. The two independent uracil rings are planar and in the keto form. The base-sugar torsion angles are in the anti range. The sugar puckering is C3'-endo, and the ribose conformation about the C5'-C4' bond is transgauche.     

Termites: a potentially large source of atmospheric methane, carbon dioxide, and molecular hydrogen

Termites may emit large quantities of methane, carbon dioxide, and molecular hydrogen into the atmosphere. Global annual emissions calculated from laboratory measurements could reach 1.5 x 10(14) grams of methane and 5 x 10(16) grams of carbon dioxide. As much as 2 x 10(14) grams of molecular hydrogen may also be produced. Field measurements of methane emissions from two termite nests in Guatemala corroborated the laboratory results. The largest emissions should occur in tropical areas disturbed by human activities.     

Crystal and molecular structure of a thymine phototrimer

Thymine trimer was isolated from a frozen aqueous solution of thymine which was irradiated with ultraviolet light and was presumably formed through the rearrangement of an initial oxetane photoproduct. X-ray diffraction analysis of a single crystal has confirmed the trimeric diol structure and has established the stereoconfiguration of the molecule. The possible importance of the diol structure in photobiology is pointed out.     

Crystal and molecular structure of a thymine-thymine adduct

Thymine-thymine adduct is a product isolated from thymine irradiated with ultraviolet light in frozen aqueous solution. This compound is presumably formed through the rearrangement of an initial photoproduct. Single crystal x-ray diffraction analysis has confirmed the molecular formula of the adduct, 5-hydroxy-6-4'-(5'-methylpyrimid-2'-one)-dihydrothymine, except for the possibility of a hydrogen atom on the 3' nitrogren rather than the 1' nitrogen, and has established the stereoconfiguration of the molecule.     

一种新型二茂铁丁酸混配合物的合成及晶体结构

以二茂铁基丁酸(Fb)、4,4′-联吡啶(bpy)以及Zn(AC)2.2H2O为原料合成了一个新的混配合物Zn(Fb)2(bpy)2。X-射线单晶衍射分析表明,该化合物是一个四配体六配位单核配合物,并以氢键另连接两个二茂铁分子。     

The crystal structure of a mammalian fatty acid synthase

Mammalian fatty acid synthase is a large multienzyme that catalyzes all steps of fatty acid synthesis. We have determined its crystal structure at 3.2 angstrom resolution covering five catalytic domains, whereas the flexibly tethered terminal acyl carrier protein and thioesterase domains remain unresolved. The structure reveals a complex architecture of alternating linkers and enzymatic domains. Substrate shuttling is facilitated by flexible tethering of the acyl carrier protein domain and by the limited contact between the condensing and modifying portions of the multienzyme, which are mainly connected by linkers rather than direct interaction. The structure identifies two additional nonenzymatic domains: (i) a pseudo-ketoreductase and (ii) a peripheral pseudo-methyltransferase that is probably a remnant of an ancestral methyltransferase domain maintained in some related polyketide synthases. The structural comparison of mammalian fatty acid synthase with modular polyketide synthases shows how their segmental construction allows the variation of domain composition to achieve diverse product synthesis.     

Triiodothyronine: the 3'iodine is proximal to the -ring in crystal structure conformation

The crystal and molecular structure of the thyroid hormone L-triiodothyronine has been determined by x-ray diffraction. The two phenyl rings are almost perpendicular to each other, the acute angle between their normals being 82 degrees. The 3'-iodine is situated proximal to the alpha-ring, rather than distal as inferred from chemical studies. Theoretical calculations indicate this proximal conformation to be energetically favored over the distal one.     

Conserved folding in retroviral proteases: crystal structure of a synthetic HIV-1 protease

The rational design of drugs that can inhibit the action of viral proteases depends on obtaining accurate structures of these enzymes. The crystal structure of chemically synthesized HIV-1 protease has been determined at 2.8 angstrom resolution (R factor of 0.184) with the use of a model based on the Rous sarcoma virus protease structure. In this enzymatically active protein, the cysteines were replaced by alpha-amino-n-butyric acid, a nongenetically coded amino acid. This structure, in which all 99 amino acids were located, differs in several important details from that reported previously by others. The interface between the identical subunits forming the active protease dimer is composed of four well-ordered beta strands from both the amino and carboxyl termini and residues 86 to 94 have a helical conformation. The observed arrangement of the dimer interface suggests possible designs for dimerization inhibitors.     

Encapsulation of molecular hydrogen in fullerene C60 by organic synthesis

In spite of their importance in fundamental and applied studies, the preparation of endohedral fullerenes has relied on difficult-to-control physical methods. We report a four-step organic reaction that completely closes a 13-membered ring orifice of an open-cage fullerene. This process can be used to synthesize a fullerene C60 encapsulating molecular hydrogen, which can be isolated as a pure product. This molecular surgical method should make possible the preparation of a series of C60 fullerenes, encapsulating either small atoms or molecules, that are not accessible by conventional physical methods.