Search

TRACKING CRYSTAL GROWTH PATTERNS ATOM BY ATOM Scientists have discovered a new mode of crystal growth at the atomic level. Researchers at the DOE's Argonne National Laboratory (ANL), Lemont, Ill., [...]

Septins constitute a group of GTP-binding proteins involved in cytokinesis and other essential cellular functions. They form heterooligomeric complexes that polymerize into nonpolar filaments and are dynamic during different stages of the cell cycle. Posttranslational modifications and interacting partners are widely accepted regulators of septin filament function, but the contribution of nucleotide is undefined due to a lack of detailed structural information. Previous low-resolution structures showed that the G domain assembles into a linear polymer with 2 different interfaces involving the N and C termini and the G binding sites. Here we report the crystal structure of SEPT2 bound to GppNHp at 2.9 A resolution. GTP binding induces conformational changes in the switch regions at the G interfaces, which are transmitted to the N-terminal helix and also affect the NC interface. Biochemical studies and sequence alignment suggest that a threonine, which is conserved in certain subgroups of septins, is responsible for GTP hydrolysis. Although this threonine is not present in yeast CDC3 and CDC11, its mutation in CDC10 and CDC12 induces temperature sensitivity. Highly conserved contact residues identified in the G interface are shown to be necessary for Cdc3-10, but not Cdc11-12, heterodimer formation and cell growth in yeast. Based on our findings, we propose that GTP binding/hydrolysis and the nature of the nucleotide influence the stability of interfaces in heterooligomeric and polymeric septins and are required for proper septin filament assembly/disassembly. These data also offer a first rationale for subdividing human septins into different functional subgroups. filament | GTP-binding protein | X-ray crystallography

The mineral eskolaite ([Cr.sub.2][O.sub.3]) has been discovered in association with natural diamond from the Udachnaya pipe in Yakutia, where it is intergrown with an octahedral diamond, mostly as an inclusion in the diamond, but also exposed at its surface. A detailed study was performed on fragments extracted from the outer surface of the diamond, using single-crystal X-ray diffraction (XRD), high-resolution electron microscopy (HRTEM), analytical electron microscopy (AEM), including line-scan and elemental-mapping, EMP, and SIMS. These applied techniques confirmed the nature of the eskolaite with 86.8 wt% [Cr.sub.2][O.sub.3] and notable impurities of Ti[O.sub.2] (3.99 wt%), [Al.sub.2][O.sub.3] (2.00 wt%), [Fe.sub.2][O.sub.3] (5.83 wt%), and MgO (1.11 wt%). Trace elements, including V (4900 ppm), Mn (129 ppm), Zr (56 ppm), and Nb (32 ppm) were also detected. The entire range of REE is just at or below the limits of detection. A small picrochromite inclusion ([X.sub.Mg] 81.2; [Y.sub.Cr] 94.7) was detected in the eskolaite; its chemistry is typical of chromite diamond inclusions. It also contains minute inclusions of perovskite, corundum, and an unidentified Ti-phase. Nano-sized cavities in picrochromite were determined to consist of carbonate and quench products, including Si, Mg, Ca, P, K, and Cl. This may represent relics of the diamond-forming metasomatic fluids. The eskolaite, containing a picrochromite inclusion, was formed at high pressure within the diamond stability field from C-O-H-bearing fluids containing Ca, K, Cl, P, and possibly even peridotitic (U-type) oxides and silicates. Keywords: Diamond association, intergrowth, eskolaite, picrochromite, Udachnaya kimberlite pipe, Yakutia, Russia

Here we report a normal-mode-based protocol for modeling anisotropic thermal motions of proteins in x-ray crystallographic refinement. The foundation for this protocol is a recently developed elastic normal mode analysis that produces much more accurate eigenvectors without the tip effect. The effectiveness of the procedure is demonstrated on the refinement of a 3.42-[Angstrom] structure of formimino-transferase cyclodeaminase, a 0.5-MDa homooctameric enzyme. Using an order of magnitude fewer adjustable thermal parameters than the conventional isotropic refinement, this protocol resulted in a decrease of the values of [R.sub.cryst] and [R.sub.free] and improvements of the density map. Several poorly resolved regions in the original isotropically refined structure became clearer so that missing side chains were fitted easily and mistraced backbone was corrected. Moreover, the distribution of anisotropic thermal ellipsoids revealed functionally important structure flexibility. This normal-mode-based refinement is an effective way of describing anisotropic thermal motions in x-ray structures and is particularly attractive for the refinement of very large and flexible supramolecular complexes at moderate resolutions. anisotropic temperature factor | conformational flexibility | elastic normal mode analysis | tip effect | x-ray crystallographic refinement

Type III secretion systems (T3SS), found in several Gram-negative pathogens, are nanomachines involved in the transport of virulence effectors directly into the cytoplasm of target cells. T3SS are essentially composed of basal membrane-embedded ring-like structures and a hollow needle formed by a single polymerized protein. Within the bacterial cytoplasm, the T3SS needle protein requires two distinct chaperones for stabilization before its secretion, without which the entire T3SS is nonfunctional. The 2.0-[Angstrom] x-ray crystal structure of the PscE-[PscF.sup.55-85]-PscG heterotrimeric complex from Pseudomonas aeruginosa reveals that the C terminus of the needle protein PscF is engulfed within the hydrophobic groove of the tetratricopeptide-like molecule PscG, indicating that the macromolecular scaffold necessary to stabilize the T3SS needle is totally distinct from chaperoned complexes between pilus- or flagellum-forming molecules. Disruption of specific PscG-PscF interactions leads to impairment of bacterial cytotoxicity toward macrophages, indicating that this essential heterotrimer, which possesses homologs in a wide variety of pathogens, is a unique attractive target for the development of novel antibacterials. bacterial pathogenicity | chaperones | x-ray crystallography

Histone demethylases, members of an emerging transcription-factor family, remove methyl groups from the lysine residues of the histone tails and thereby regulate the transcriptional activity of target genes. Structures of the catalytic-core domain of jumonji-domain-containing histone demethylase 1 (JMJD1) with and without alpha-ketoglutarate in the presence of (Fe.sup.2+) are determined by X-ray crystallography.

Hereditary coproporphyria is an autosomal dominant disorder resulting from the half-normal activity of coproporphyrinogen oxidase (CPO), a mitochondrial enzyme catalyzing the antepenultimate step in heme biosynthesis. The mechanism by which CPO catalyzes oxidative decarboxylation, in an extraordinary metal- and cofactor-independent manner, is poorly understood. Here, we report the crystal structure of human CPO at 1.58-A resolution. The structure reveals a previously uncharacterized tertiary topology comprising an unusually flat seven-stranded [beta]-sheet sandwiched by u-helices. In the biologically active dimer ([K.sub.D] = 5 x [10.sup.-7] M), one monomer rotates relative to the second by [approximately equal to] 40 [degrees] to create an intersubunit interface in close proximity to two independent enzymatic sites. The unexpected finding of citrate at the active site allows us to assign Ser-244, His-258, Asn-260, Arg-262, Asp-282, and Arg-332 as residues mediating substrate recognition and decarboxylation. We favor a mechanism in which oxygen serves as the immediate electron acceptor, and a substrate radical or a carbanion with substantial radical character participates in catalysis. Although several mutations in the CPO gene have been described, the molecular basis for how these alterations diminish enzyme activity is unknown. We show that deletion of residues (392-418) encoded by exon six disrupts dimerization. Conversely, harderoporphyria-causing K404E mutation precludes a type I [beta]-turn from retaining the substrate for the second decarboxylation cycle. Together, these findings resolve several questions regarding CPO catalysis and provide insights into hereditary coproporphyria. coproporphyrinogen oxidase | oxidative decarboxylation | mitochondria | x-ray crystallography

The iridium(perfluoropropyl)(vinyl) complex Cp(super *)Ir(PMe3)(n-C3F7)(CH=CH2) was prepared. It is characterized by X-ray crystallography, and its ground state conformation in solution was determined by (super 19)F{(super 1)H} HOESY NMR studies.

C3 exoenzymes (members of the ADP-ribosyltranferase family) are produced by Clostridium botulinum (C3bot1 and -2), Clostridium limosum (C3lim), Bacillus cereus (C3cer), and Staphylococcus aureus (C3stau1-3). These exoenzymes lack a translocation domain but are known to specifically inactivate Rho GTPases in host target cells. Here, we report the crystal structure of C3bot1 in complex with RalA (a GTPase of the Ras subfamily) and GDP at a resolution of 2.66 [Angstrom]. RalA is not ADP-ribosylated by C3 exoenzymes but inhibits ADP-ribosylation of RhoA by C3bot1, C3lim, and C3cer to different extents. The structure provides an insight into the molecular interactions between C3bot1 and RalA involving the catalytic ADP-ribosylating turn-turn (ARTT) loop from C3bot1 and helix [alpha]4 and strand [beta]6 (which are not part of the GDP-binding pocket) from RalA. The structure also suggests a molecular explanation for the different levels of C3-exoenzyme inhibition by RalA and why RhoA does not bind C3bot1 in this manner. ADP-ribosylation | protein-protein interaction | x-ray crystallography

A convenient approach to the synthesis of highly enantiomerically enriched dihydrofuran compounds via a novel retro-claisen photoarrangement is described. The scope and generality of this interesting and unusual transformation is being explored.

A unique and foremost example of a 2:1 cocrystal of mixed Cu(I)/Cu(II) complexes and a novel ferromagnetic bis(mu-hydroxo)dicopper(II) complex is presented. X-ray crystallographic techniques establish the composition of these complexes.

A total of 39 geminal dicationic ionic liquids possessing many interesting crystalline and physicochemical properties ionic liquids were synthesized. The effects of the structural variations on the physicochemical and solvation properties of the unusual class of ionic liquids are examined.

Structure of the unusual anthelmintic pyrrolobenzoxazine terpenoid natural product CJ-12662 was confirmed by X-ray crystallography and partial synthesis from 2-chloronitrobenzene. The pyrrolobenzoxazine moiety was synthesized using dye-sensitized photooxygenation of a tryptophan derivative.

2,4,6-Triphenylbenzaldehyde 1 undergoes a condensation reaction with 2-aminophenol to give N-(2',4',6'triphenylbenzylidene)-2-iminopbenol(TPIP) 2. The imine 2 can be reduced with NaB[H.sub.4] in ethanol to form N-(2',4',6'triphenylbenzyl)-2-aminophenol(TPAP) 3. Addition of trimetbylaluminum to 2 or 3 results in the formation of the complexes TPIP-Al[Me.sub.2]*AlMe.sub.3] (4) or TPAP-Al[Me.sub.2] (5). Compounds 2, 3, and 4 have been crystallographically characterized. Key words. N,O ligands, aluminum, m-terphenyl, Schiff bases, X-ray crystallography.

The thermal decarboxylation of 2-methoxycarbonyl-5-(4'-nitrophen-oxy)tetrazole (la) with the electron-rich, aromatic compounds (anisole, N,N-dimethylaniline, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene), neat or in polar solvents (DMSO, DMF, and C[H.sub.3]CN), is investigated. The solid phase thermal decomposition of a mixture of 1a, 1,3,5-trimethoxybenzene, and a Lewis acid (Alp[Cl.sub.3]) produces methyl-2,4-dimethoxysalicylate (8) in good yield, instead of the expected 2,4,6-trimethoxybenzoic acid. The X-ray structure of 8 shows intramolecular hydrogen bonds between the carbonyl oxygen and hydrogens of Me and OH groups. A measured p[K.sub.a] value of 6.8 compares well with a value of 6.4 estimated using the [C=O***H***O] hydrogen bond distances. Key words: anti-tumor, novel synthesis, X-ray structure, hydrogen bonds, computational analysis. On a etudie la decarboxylation thermique du 2-methoxycarbonyl-5-(4'-nitrophenyl)tetrazole (la) en presence de composes aromatiques riches en electrons (anisole, N,N-dimethylaniline, 1,4-dimethoxybenzene et 1,3,5-trimethoxybenzene) a l'etat pur ou dans des solvants polaires (DMSO, DMF et C[H.sub.3]CN). La decomposition thermique en phase solide d'un melange de 1a, de 1,3,5-trimethoxybenzene et d'un acide de Lewis (Al[Cl.sub.3]) conduit a la formation du 2,4-dimethoxysalicylate de methyle (8) avec un bon rendement au lieu de l'acide 2,4,6-trimethoxybenzoique attendu. La structure du compose 8 telle que determinee par diffraction des rayons X montre la presence de liaisons hydrogenes intramoleculaires entre l'oxygene du carbonyle et les atomes d'hydrogene des groupes methyle et hydroxyle. La valeur mesuree de 6,8 pour le p[K.sub.a] se compare bien avec la valeur de 6,4 evaluee sur la base des longueurs des liaisons hydrogenes [C=O***H***O]. Mots cles : antitumoral, synthese nouvelle, structure par diffraction des rayons X, liaisons hydrogenes analyse conformationnelle. [Traduit par la Redaction]

One pair of diagonal nitroxides on the calix[4]arene upper rim was found to form a strong antiferromagnetic, intramolecular dimer in solid state. Such intramolecular dimers of nitroxides could serve as a antiferromagnetic coupling unit and a building block for exchange-coupled macromolecules.

Tetrameric hemoglobins are the most widely used systems in studying protein cooperativity. Allosteric effects in hemoglobins arise from the switch between a relaxed (R) state and a tense (T) state occurring upon oxygen release. Here we report the 2.0-[Angstrom] crystal structure of the main hemoglobin component of the Antarctic fish Trematomus newnesi, in a partial hemichrome form. The two [alpha]-subunit iron atoms are bound to a CO molecule, whereas in the [beta] subunits the distal histidine residue is the sixth ligand of the heme iron. This structure, a tetrameric hemoglobin in the hemichrome state, demonstrates that the iron coordination by the distal histidine, usually associated with denaturing states, may be tolerated in a native-like hemoglobin structure. In addition, several features of the tertiary and quaternary organization of this structure are intermediate between the R and T states and agree well with the R [right arrow] T transition state properties obtained by spectroscopic and kinetic techniques. The analysis of this structure provides a detailed pathway of heme-heme communication and it indicates that the plasticity of the [beta] heme pocket plays a role in the R [right arrow] T transition of tetrameric hemoglobins. protein denaturation | protein function | x-ray structure | Antarctic

Scientists in the Crystallography Section at NIST/NBS made several outstanding contributions which greatly promoted the development and advancement of high pressure x-ray crystallography during the second-half of the 20th century. […]

Research investigating three types of mixed lithium-heavier alkali metal tert-Butoxides is presented. Particular attention is given to X-ray crystallographic techniques applied to analyse the structure of of heterometallic compounds.

The synthesis of a novel strained compound by the reaction of dichlorotetramethyldisilane and 1,2-dilithiocarborane is reported. The synthesized compound, 1,2-(1',1',2',2'-tetramethyldisilane-1,2')carborane, was characterized with the use of solution methods and its structure ascertained by single-crystal x-ray diffraction. The general chemistry of the novel strained compound is described along with the results of polymerization attempts conducted.

We have developed an improved x-ray imaging system based on spherically curved crystals. It is designed and used for diagnostics of targets ablatively accelerated by the Nike KrF laser. A spherically curved quartz crystal (2d = 6.687 [Angstrom] R = 200 mm) has been used to produce monochromatic backlit images with the He-like Si resonance line (1865 eV) as the source of radiation. The spatial resolution of the x-ray optical system is 1.7 [[micro]meter] in selected places and 2-3 [[micro]meter] over a larger area. Time-resolved backlit monochromatic images of polystyrene planar targets driven by the Nike facility have been obtained with a spatial resolution of 2.5 [[micro]meter] in selected places and 5 [[micro]meter] over the focal spot of the Nike laser.

We are developing large-area, thick, structured CsI(T1) imaging sensors for a wide variety of X-ray imaging applications. Recently we have fabricated structured CsI(T1) scintillators ranging from 30 [[micro]meter] (16 mg/[cm.sup.2]) to 2000 [[micro]meter] (900 mg/[cm.sup.2]) in thickness and up to 15 x 15 [cm.sup.2] in area. Even 2000-[[micro]meter]-thick film showed well-controlled columnar growth throughout the film. Material characterization confirmed that the film is crystalline in nature and that the stoichiometry is preserved. To improve the spatial resolution of thick films, post-deposition treatments were performed. The effect of these treatments on film characteristics was quantitatively evaluated by measuring signal output, modulation transfer function [MTF(f)], noise power spectrum [NPS(f)], and detective quantum efficiency [DQE(f)]. The data show that by proper film treatments, the film DQE(f) can be improved.

An explanation is offered regarding the differences in the ability of a macrocyclic inhibitor and its two acyclic derivatives to bind to the fungal aspartyl proteinase known as penicillopepsin. The binding affinity of the three inhibitors to the enzyme was found to be inversely related to the predicted degree of conformational flexibility. One bound acyclic derivative adopts a more open conformation compared to the macrocyclic inhibitor. The other acyclic linear inhibitor differs from the macrocycle by removal of a methylene group on one side of the naphthalene ring.

The electronic structure of the Ni-X center in D. gigas hydrogenase redox forms were analyzed using a combination of dosimetric and spectroscopic techniques. Results reveal that the dinuclear cluster is essential in a Ni-Fe center shown in the X-ray crystal study. This suggest that D. gigas is unexpected in a strong antiferromagnetically coupled Ni-Fe center measured in Ni(II)(S1 = 1) - Fe(III) (S2 = 5/2) complex.

The highly selective photochemical solid state reaction of 1-(4'-biphenylyl)-2-phenyl-diazopropane led to the synthesis of (Z)-1-(4'-biphenylyl)-2-phenylpropene. The synthetic mechanism involves the synthesis of a carbene intermediate and is characterized by a highly stereoselective 1,2-H shift. Furthermore, the crystalline solid state and conformational factors that determine orbital alignment control the reactivity of the aryldiazoalkanes.

The structure of [AsPh4][EtSCo(DH)2(CH3)] was analyzed by X-ray crystallography to determine the characteristics of the organocobalt complex. The organocobalt complex is composed of a unidendate coordinated thiolate with a long C-S bond due to the weak trans influence of the thiolate to the Co-C bond. Furthermore, the alkanethiolate ligand is comparable to alkylphosphines in its affinity for axial coordination to B12 models.

Furamidine is a minor groove binding drug that has been demonstrated to be more potent than pentamidine against the AIDS-associated pathogen Pneumocystis carinii. Its structure is similar to that of the antitrypanosomal drug berenil, the main difference between the two being the substitution of a triazene linker with a furan ring. Crystallographic and spectroscopic analysis of the two drugs are conducted to explain their interaction with the DNA sequence d(CGCGAATTCGCG)2. Results are then used to explain the differences between the two ligands.

A diversity of twinning and exsolution textures has been observed by transmission electron microscopy in natural kalsilite from volcanic and metamorphic rocks (Mount Nyiragongo, Zaire; Alban Hills, Italy; Kerala, southern India). The phase transitions responsible for these microstructures were examined by high-temperature powder and single-crystal X-ray diffraction. Pure kalsilite has [P6.sub.3]mc symmetry at high temperatures but transforms to its [P6.sub.3] state through a two-phase field, between [approximately]870 and [approximately]920 [degrees] C. In this two-phase field, the low form coexists with a structure that has a sixfold repeat of its a unit-cell dimension. For [approximately][Ks.sub.88], the high-temperature state is high tetrakalsilite with possible space group [P6.sub.3]mc; a two-phase field between [approximately]890 and [approximately]930 [degrees] C involves the coexistence of this phase with [P6.sub.3] kalsilite. High tetrakalsilite with composition [approximately][Ks.sub.74] reverts to low tetrakalsilite on quenching to room temperature from 950 [degrees] C. Lattice-parameter variations, and estimates of the Al-O-Si angles that can be derived from them, imply that the stability limit for the high structural states occurs when the angles for apical and basal O atoms converge. At lower temperatures, pure [P6.sub.3] kalsilite appears to have transformed during a metamorphic evolution in nature to an intergrowth of the [P6.sub.3] structure and a structure for which P31c symmetry is proposed. The latter can be thought of as a polytype of kalsilite, with (001) layers stacked in an eclipsed array rather than in the staggered array of normal low kalsilite. In this regard, KAlSi[O.sub.4] seems to be remarkably similar to KLiS[O.sub.4]. If [approximately]3-12 mol% NaAlSi[O.sub.4] is present in solid solution, a [P6.sub.3] structure with an a parameter [square root of 3] greater than normal low kalsilite develops. An anomaly in the lattice parameters of [Ks.sub.88] suggests that the transition temperature below which this superstructure develops may be [approximately]500 [degrees] C. Natural nepheline exsolved from kalsilite displays merohedral twinning, which can be accounted for by a [P6.sub.3]mc [approaches] [P6.sub.3] transition during cooling. Transformation behavior in the Ne-Ks system may be explained, at least qualitatively, in terms of Na-K ordering between cavity sites, ordering of basal O atoms between two sites on either side of the mirror plane parallel to the c axis of [P6.sub.3]mc structures, ordering of apical O atoms between three sites, and coupling between all these processes.

The structural phase transitions of monoclinic tridymite that occur at elevated temperatures, as well as the structure of the high-temperature modifications, have been studied by 29Si MAS NMR. The sample, originating from the surface of a refractory silica brick, displays sharp discontinuities in the first moment ([M.sub.1]), second moment ([M.sub.2]), or both, of the 29Si MAS NMR spectrum at the following phase transitions: 108 [+ or -] 1 [degrees] C for the monoclinic (MC) to orthorhombic (OP) transition, 165 [+ or -] 5 [degrees] C for the transition from OP to a second orthorhombic phase (OS), and 206 [+ or -] 4 [degrees] C for the transition from OS to a third orthorhombic phase (OC). [M.sub.1] for the OC phase becomes more negative with increasing temperature up to [approximately]323 [degrees] C, at which there is also a small discontinuity in [M.sub.2], indicating a transition from OC to a hexagonal phase (LHP). The spectrum of the MC phase consists of 12 narrow lines of equal area, consistent with the known crystal structure. The spectrum of the OP phase is broad and asymmetric, indicating a distribution of local environments at the Si sites. It can be simulated assuming an incommensurate structural modulation with a maximum amplitude of [approximately]5 x [10.sup.-2] [Angstrom], but it is also consistent with a recent domain-formation model, provided that most disorder occurs at the Si6 site. The spectrum of the OS phase is asymmetric, indicating a nonlinear incommensurate modulation with a maximum amplitude of [approximately]2 x [10.sup.-2] [Angstrom]. The spectra of higher temperature phases are consistent with the single Si site of the known crystal structures, but [M.sub.1] values imply significantly smaller mean Si-O-Si angles e.g., = 152.6 [degrees] at 220 [degrees] C from NMR, compared with 168.0 [degrees] from X-ray diffraction data. This difference is consistent with disorder of O atoms around Si-Si axes.

Glycosylation-inhibiting factor (GIF) is a cytokine that is involved in the regulation of IgE synthesis. The crystal structure of recombinant human GIF was determined by the multiple isomorphous replacement method. The structure was refined to an R factor of 0.168 at 1.9 [Angstrom] resolution. The overall structure is seen to consist of three interconnected subunits forming a barrel with three 6-stranded [Beta]-sheets on the inside and six [Alpha]-helices on the outside. There is a 5-[Angstrom]-diameter 'hole' through the middle of the barrel. The barrel structure of GIF in part resembles other 'trefoil' cytokines such as interleukin 1 and fibroblast growth factor. Each subunit has a new class of [Alpha] + [Beta] sandwich structure consisting of two [Beta]-[Alpha]-[Beta] motifs. These [Beta]-[Alpha]-[Beta] motifs are related by a pseudo-twofold axis and resemble both interleukin 8 and the peptide binding domain of major histocompatibility complex protein, although the topology of the polypeptide chain is quite different.

The synthesis of tellurium diimide dimers of the type RNTe(micro-NR')eTNR is presented. The scheme involves the reactions of the PN2Te heterocycle Ph2PN(SiMe3)2Te(Cl)NR (R= PPh2NSiMe3) with R'NHLi (R = tBu, tOct) and TeCl4 with tBuNHLi in a 1:4 molar ratio. Structural studies showed that the crystals of these dimers consist of a four-membered Te2Ne2 ring with the short, exocyclic Te=NR bonds arranged in either a cis or trans configuration.

The class II associated invariant chain peptide (CLIP) fragment binds to DR3 in a manner similar to that in which antigenic peptides couple to class II histocompatibility glycoproteins. The complex between HLA-DR3 and CLIP, obtained from a human cell line was analyzed using X-ray crystallography. The structure appears to be that of a substrate for the DM exchanger. The binding between DR3 and CLIP indicates that CLIP motif of Ii stabilizes class II molecules. The CLIP attached to DR3 has a half life of 4 hours.

The x-ray structure of clotting factor IXa reveals an atomic source of hemophilia B and the spatial distribution of mutation sites describes a structural model for factor X-stimulation by phospholipid-attached fIXa and cofactor VIIIa. The strained conformation of the N-terminal Gla module may result in incomplete active site formations and thereby may decrease the catalytic activity of fIXa. The structure of fIXa-beta is similar to that of a tulip with the catalytic module forming the two interlaced EGF domains and the N-terminal Gla forming the bulb.

X-ray crystallographic studies of the binding of two beta-lactam antibiotics, cephalothin and cefotaxime, with the Ser residue at position 62 of the D-alanyl-D-alanine carboxypeptidase/transpeptidase (DD-peptidase), reveal that a water molecule near the acylated beta-lactam carbonyl carbon atom is not activated enough for nucleophilic attack and deacylation. Beta-lactam antibiotics attack DD-peptidases, which catalyze the last step in the cross-linking of bacterial cell wall peptidoglycan. Results highlight the significance of a side chain rotation in residue Thr301 in cefotaxime.

Binding of monovalent cations produces conformational changes affecting the functional properties of tryptophan synthase alpha2beta2 complex from Salmonella typhimurium, and the functional changes are retained in the crystalline state. In the presence of cations such as lithium, sodium or ammonium, the phosphorescence decay of the tryptophan-177 beta residue situated near the active site on the beta region between the alpha and beta subunits is biphasic and there is an increase in the lifetime of the enzyme. The role of the cations appears to be structural rather than catalytic. They may act to stabilize conformations of enzymes and intermediates.

The crystal structure of urease from Klebsiella aerogenes has been determined at 2.2 A resolution and refined to an R factor of 18.2 percent. The enzyme contains four structural domains: three with novel folds playing structural roles, and an [(αβ).sub.8] barrel domain, which contains the bi-nickel center. The two active site nickels are 3.5 A apart. One nickel ion is coordinated by three ligands (with low occupancy of a fourth ligand) and the second is coordinated by five ligands. A carbamylated lysine provides an oxygen ligand to each nickel, explaining why carbon dioxide is required for the activation of urease apoenzyme. The structure is compatible with a catalytic mechanism whereby urea ligates Ni-1 to complete its tetrahedral coordination and a hydroxide ligand of Ni-2 attacks the carbonyl carbon. A surprisingly high structural similarity between the urease catalytic domain and that of the zinc-dependent adenosine deaminase reveals a remarkable example of active site divergence., Urease (urea amidohydrolase; E.C. 3.5.1.5), a nickel-dependent metalloenzyme, catalyzes the hydrolysis of urea to form ammonia and carbon dioxide (1) with a rate approximately [10.sup.14] times the rate of the [...]

The high oxygen affinity of Ascaris hemoglobin domain I (D1) appears to be associated with its structural similarity to myoglobin. The x-ray-derived crystal structure shows a strong hydrogen bond between tyrosine at position 10 in the B helix of D1 and the distal oxygen of the ligand, in combination with a weak hydrogen bond between glutamine-E7 and the proximal oxygen. These bonds hold the ligand in the binding pocket. A third hydrogen bond between these two amino acids gives stability to the structure.

The compound tetramethylpiperazinium pentachloroferrate(III), C8H20Cl5N2Fe, crystallizes in the monoclinic space group C2/c, and a five-coordinate trigonal bipyramidal (FeCl5)2- species is present in an ionic crystal lattice, as reflected by the single-crystal X-ray diffraction study. A three-dimensional antiferromagnetic ground state in a long range magnetic order between 6 and 7 Kelvin is observed by Fe(super 59) Mossbauer spectroscopy and a.c. susceptometry.

Crystallization and x-ray diffraction analysis of the enzyme NADPH-cytochrome p450 reductase (CPR) from Escherichia coli is possible. CPR is a membrane protein and is a necessary component of the microsomal cytochrome p450 monooxygenase system that catalyzes the transfer of electrons from NADPH to cytochrome p450. It is a rare protein as it is one of the two mammalian enzymes that contain both flavin adenine dinucleotide and flavin mononucleotide.

A 2.2 Angstrom resolution X-ray crystal analysis reveals the three-dimensional structure of heterotrimeric (alpha,beta,gamma) G protein's alpha-subunit. The alpha-subunit is a transducin-alpha complex which contains GTP-gamma-S. GTP's gamma-phosphate conveys information regarding conformational changes. All G-proteins, coupled with a specific seven-helix receptor, share a common receptor-induced information-transducer mechanism.

The lattice motions and related displacements in the partially reacted crystal of 2,5-distyrylpyrazine are analyzed and associated to the topochemical postulate and the mechanism of phonon assistance. Lattice dynamical computations, the assignment of modes and the structure of the partially reacted crystal are used for this purpose. The topochemical postulate and phonon assistance models are used for interpreting solid-state reactions. A concerted X-ray crystallographic and polarized, single-crystal Raman scattering study evaluates the models' applicability in a partially reacted crystal of 2,5-distyrylpyrazine, a prototypical reacting organic solid.

Gold and platinum complexes were synthesized and their structures reported. CEP was added to (THT)AuCl to produce (CEP)-AuCl and ((CEP)2Au)Cl. The structure of the latter compound showed Au, P and Cl on a 3-fold rotational axis with Au at the inversion center. The Cl was on a position with 32 symmetry between two cations with S6 symmetry. Six -CN groups encapsulating the gold atom were separated by a length of 3.579 Angstrom. This encapsulation results in a two-coordinate, linear P-Au-P geometry and eliminates the luminescence that is generally observed in three-coordinate species.

The solid state structures of n-butyllithium-dimethoxyethane (n-Buli-DME), n-butyllithium-N,N,N',N'-tetramethylethylenediamine (n-Buli-TMEDA) and n-butyllithium-tetrahydrofuran (n-Buli-THF) are presented and compared with previously calculated structures. Findings show that the X-ray crystal structures of n-Buli-DME, n-Buli-THF and n-Buli-TMEDA aggregates are solid state representations of dimeric and tetrameric structures observed in solution. The x-ray crystal structures of n-Buli complexes are also determined.

A study on the high-pressure crystal chemistry of cubanite was discussed. Single-crystal X-ray diffraction was used to study a single crystal of cubanite placed in a diamond-anvil cell at room temperature. Results indicate that Cu-S bonds in the crystal structure of the cubanite were shortened when subjected to pressure. However, the high pressure did not affect the electron transfer process of the cubanite, as indicated in the constant Fe-S distance.

Astudy on the chemical chemistry of four vesuvianite crystals was discussed. Single-crystal X-ray diffraction was used together with the EXAFS method in refining the vesuvianites' crystal structures. The site preferences of ions along the fourfold rotational axis were the parameters used in the experiment. The correlation in the distances between the B-O10 and O10-O10 was demonstrated.

The crystal structure of calcium-bound calmodulin (Ca sup 2+ -CaM) bound to a peptide analog of the CaM-binding region of chicken smooth muscle myosin light chain kinase has been determined and refined to a resolution of 2.4 angstroms ([Angstroms]). The structure is compact and has the shape of an ellipsoid (axial ratio 2:1). The bound CaM forms a tunnel diagonal to its long axis that engulfs the helical peptide, with the hydrophobic regions of CaM melded into a single area that closely covers the hydrophobic side of the peptide. There is a remarkably high pseudo-twofold symmetry between the closely associated domains. The central helix of the native CaM is unwound and expanded into a bend between residues 73 and 77. About 185 contacts ( 80% of this total., Calmodulin (CaM) is a 148-amino acid protein present in all eukaryotic cells that serves as the primary receptor for intracellular [Ca.sup.2t][1]. The three-dimensional structure of native CaM with four bound [...]