The Conformation of the Side Chain of 21-Alkylpregnanes

2003 ◽  
Vol 2003 (9) ◽  
pp. 556-558 ◽  
Author(s):  
James R. Hanson ◽  
Peter B. Hitchcock ◽  
Jorge A.R. Salvador
Keyword(s):  
Crystal Structures ◽  
Side Chain ◽  
Methyl Group ◽  
X Ray

The X-ray crystal structures of some 21-alkylpregnanes have been determined and the effects of a 16α,17α-epoxide and 21-methyl group on the conformation of the side chain are discussed.

Nitration of 2,3,4,6-Tetramethylphenol and 1,2,3,5-Tetramethylbenzene

1985 ◽  
Vol 38 (4) ◽  
pp. 587 ◽  
Author(s):  
MP Hartshorn ◽  
JM Readman ◽  
WT Robinson ◽  
J Vaughan
Keyword(s):  
Nitric Acid ◽  
Nitrogen Dioxide ◽  
Crystal Structures ◽  
Side Chain ◽  
X Ray ◽  
Fuming Nitric Acid

Nitration of 1,2,3,5-tetramethylbenzene (2a) with fuming nitric acid gives the tetramethylnitrobenzene (22), products of side-chain modification (23)-(27), the rearranged 6,6-dimethylcyclohexenones (8), (28), (29) and (30), and 2,3,4,6-tetramethyl ketone derivatives (10)- (13), (31) and (32). Reaction of 2,3,4,6-tetramethylphenol (7) with nitrogen dioxide gives the hydroxy dinitro ketone (9) in addition to the trinitrocyclohexenones (11)-(14) and (19). X-ray crystal structures are reported for compounds (11), (19), (28), (29), (30) and (32). 1H n.m.r ./stereochemistry correlations are reported for some 2,5-dinitro- and 2,5,6-trinitro-cyclohex-3-enones.

Twists and turns: a tale of two shikimate-pathway enzymes

2003 ◽  
Vol 31 (3) ◽  
pp. 543-547 ◽  
Author(s):  
K.A. Brown ◽  
E.P. Carpenter ◽  
K.A. Watson ◽  
J.R. Coggins ◽  
A.R. Hawkins ◽  
...  
Keyword(s):  
Crystal Structures ◽  
Shikimate Pathway ◽  
Structural Features ◽  
Side Chain ◽  
Computational Techniques ◽  
X Ray ◽  
X Ray Crystallography ◽  
X Ray Scattering ◽  
Presence And Absence ◽  
Energy Maps

We are studying two enzymes from the shikimate pathway, dehydroquinate synthase (DHQS) and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Both enzymes have been the subject of numerous studies to elucidate their reaction mechanisms. Crystal structures of DHQS and EPSPS in the presence and absence of substrates, cofactors and/or inhibitors are now available. These structures reveal movements of domains, rearrangements of loops and changes in side-chain positions necessary for the formation of a catalytically competent active site. The potential for using complementary small-angle X-ray scattering (SAXS) studies to confirm the presence of these structural differences in solution has also been explored. Comparative analysis of crystal structures, in the presence and absence of ligands, has revealed structural features critical for substrate-binding and catalysis. We have also analysed these structures by generating GRID energy maps to detect favourable binding sites. The combination of X-ray crystallography, SAXS and computational techniques provides an enhanced analysis of structural features important for the function of these complex enzymes.

Univalent Metal Complexes of (Perfluorophenoxy)acetic Acid. The Crystal Structures of (2,3,4,5,6-Pentafluorophenoxy)acetic Acid, Bis-μ-[(2,3,4,5,6-pentafluorophenoxy)acetato-O,O']-bissilver(I) and catena-Bis-μ-[(2,3,4,5,6-Pentafluorophenoxy)acetato-O]-bismercury(I)

1988 ◽  
Vol 41 (5) ◽  
pp. 683 ◽  
Author(s):  
TCW Mak ◽  
WH Yip ◽  
CHL Kennard ◽  
G Smith ◽  
EJ Oreilly
Keyword(s):  
Acetic Acid ◽  
Metal Complexes ◽  
Crystal Structures ◽  
Side Chain ◽  
X Ray Diffraction ◽  
Triclinic Space Group ◽  
Space Group ◽  
X Ray ◽  
A Cell ◽  
Oxyacetic Acid

The crystal structures of perfluorophenoxyacetic acid [(2,3,4,5,6- pentafluorophenoxy )acetic acid] and its anhydrous complexes with silver(I) and mercury(I) have been determined by X-ray diffraction and refined to residuals R = 0.046, 0.042 and 0.104 for 1235, 1789 and 2750 observed reflections respectively. The acid forms the usual hydrogen- bonded cyclic dimers [O---O, 2.705(3)Ǻ] with the oxyacetic acid side- chain synclinally oriented. Crystals age triclinic, space group Pī with Z 2 in a cell of dimensions a 7.479(1), b 7.592(2), c 8.463(2)Ǻ,α 80.15(2),β 85.34(2),γ 61.31(1)Ǻ. The silver complex, [Ag2L2],(1), has crystals with a 5.992(3), b 30.22(2), c 5.277(2)Ǻ, β 106.42(5)°, space group P21/a and Z 2. The bis-carboxylato (O,O′) dimers have Ag-Ag 2.943(1)Ǻ with Ag-O, 2.203, 2.230(3)Ǻ and O-Ag-O 161.6(2)°. The complex with mercury(I), [Hg2L2]n, (2), is centrosymmetric with the usual linear O-Hg-Hg-O-bonds [Hg-Hg, 2.514(1)Ǻ; Hg-O, 2.13(1)Ǻ; O-Hg-Hg, 173.9(2)°]. Longer peripheral bonds to Hg [Hg-O, 2.63(1)Ǻ] produce a polymeric structure. Crystals of (2) are triclinic, space group Pī with Z 1 in a cell of dimensions a 5.038(1), b 5.970(1), c 15.774(6)Ǻ, α 80.77(2),β 86.42(2),γ 84.38(2)°.

Syntheses and Crystal Structures of Bis[(trimethylsilyl)tetramethylcyclopentadienyl]titanium Dichloride and Monochloride

1996 ◽  
Vol 61 (9) ◽  
pp. 1307-1320 ◽  
Author(s):  
Michal Horáček ◽  
Róbert Gyepes ◽  
Ivana Císařová ◽  
Miroslav Polášek ◽  
Vojtech Varga ◽  
...  
Keyword(s):  
Crystal Structures ◽  
Electronic Effect ◽  
Absorption Bands ◽  
Trimethylsilyl Group ◽  
Methyl Group ◽  
X Ray ◽  
Titanium Dichloride ◽  
Hinge Position ◽  
Electron Donation ◽  
Trimethylsilyl Groups

(Trimethylsilyl)tetramethylcyclopentadiene (Cp'H) was obtained from the lithium salt of tetramethylcyclopentadiene and trimethylsilyl chloride. Bis[(trimethylsilyl)tetramethylcyclopentadienyl]titanium dichloride (1) and monochloride (2) were prepared by the generally known procedures. The X-ray crystal structures of 1 and 2 showed the placement of the bulky trimethylsilyl groups in side positions on opposite sides of the CE-Ti-CE planes. The steric congestion between the two chlorine atoms and two trimethylsilyl groups in 1 resulted in the nearly eclipsed conformation of the rings. Consequently, the steric hindrance between the methyl groups at the hinge position of the Cp' ligands led to a decrease in the angle between the Cp' ring planes by 4.8°, compared with the value of 44.6° in (C5Me5)2TiCl2. The titanocene skeleton in 2 is virtually the same as in (C5Me5)2TiCl. The electronic effect of the trimethylsilyl group upon various properties of 1 and 2 appeared to be ambiguous. Based on the position of λmax of electronic absorption bands of 1 the trimethylsilyl group exerted a stronger electron donation effect than the methyl group, however, the affinity of 2 to 2-methyltetrahydrofuran put the effect of the trimethylsilyl group close to that of hydrogen and the anisotropy of the EPR g-tensor of 2 close to that of the methyl group.

Transmission electron microscope studies in special ceramics

1978 ◽  
Vol 36 (1) ◽  
pp. 268-269
Author(s):  
A. Zangvil ◽  
L.J. Gauckler ◽  
G. Schneider ◽  
M. Rühle
Keyword(s):  
Phase Diagrams ◽  
Crystal Structures ◽  
Thin Foil ◽  
Limited Extent ◽  
Complex Materials ◽  
X Ray Diffraction ◽  
X Ray ◽  
Transmission Electron ◽  
Electron Microscopes ◽  
Lattice Resolution

The use of high temperature special ceramics which are usually complex materials based on oxides, nitrides, carbides and borides of silicon and aluminum, is critically dependent on their thermomechanical and other physical properties. The investigations of the phase diagrams, crystal structures and microstructural features are essential for better understanding of the macro-properties. Phase diagrams and crystal structures have been studied mainly by X-ray diffraction (XRD). Transmission electron microscopy (TEM) has contributed to this field to a very limited extent; it has been used more extensively in the study of microstructure, phase transformations and lattice defects. Often only TEM can give solutions to numerous problems in the above fields, since the various phases exist in extremely fine grains and subgrain structures; single crystals of appreciable size are often not available. Examples with some of our experimental results from two multicomponent systems are presented here. The standard ion thinning technique was used for the preparation of thin foil samples, which were then investigated with JEOL 200A and Siemens ELMISKOP 102 (for the lattice resolution work) electron microscopes.

Crystal Structures of Fragments D and Double-D from Fibrinogen and Fibrin

1999 ◽  
Vol 82 (08) ◽  
pp. 271-276 ◽  
Author(s):  
Glen Spraggon ◽  
Stephen Everse ◽  
Russell Doolittle
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Crystal Structures ◽  
Structure And Function ◽  
X Ray ◽  
Long Period ◽  
And Function

IntroductionAfter a long period of anticipation,1 the last two years have witnessed the first high-resolution x-ray structures of fragments from fibrinogen and fibrin.2-7 The results confirmed many aspects of fibrinogen structure and function that had previously been inferred from electron microscopy and biochemistry and revealed some unexpected features. Several matters have remained stubbornly unsettled, however, and much more work remains to be done. Here, we review several of the most significant findings that have accompanied the new x-ray structures and discuss some of the problems of the fibrinogen-fibrin conversion that remain unresolved. * Abbreviations: GPR—Gly-Pro-Arg-derivatives; GPRPam—Gly-Pro-Arg-Pro-amide; GHRPam—Gly-His-Arg-Pro-amide

Synthesis of Sulfoquinovose and Sulfoquinovosyl Diacylglycerides, and a Fluorogenic Substrate for Sulfoquinovosidases

2019 ◽  
Author(s):  
Yunyang Zhang ◽  
Janice Mui ◽  
Thimali Arumaperuma ◽  
James P. Lingford ◽  
ETHAN GODDARD-BORGER ◽  
...  
Keyword(s):  
Crystal Structures ◽  
Fluorogenic Substrate ◽  
Potassium Salts ◽  
X Ray ◽  
Sulfoquinovosyl Diacylglycerol ◽  
Sodium And Potassium

<p>The sulfolipid sulfoquinovosyl diacylglycerol (SQDG) and its headgroup, the sulfosugar sulfoquinovose (SQ), are estimated to harbour up to half of all organosulfur in the biosphere. SQ is liberated from SQDG and related glycosides by the action of sulfoquinovosidases (SQases). We report a 10-step synthesis of SQDG that we apply to the preparation of saturated and unsaturated lipoforms. We also report an expeditious synthesis of SQ and (<sup>13</sup>C<sub>6</sub>)SQ, and X-ray crystal structures of sodium and potassium salts of SQ. Finally, we report the synthesis of a fluorogenic SQase substrate, methylumbelliferyl a-D-sulfoquinovoside, and examination of its cleavage kinetics by two recombinant SQases.</p>

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