Qichen Huang

Fluorine-containing donor-acceptor complexes: crystallographic study of the interactions between electronegative atoms (N, O, S) and halogen atoms (I, Br)

On the basis of our results, which concern sp3N⋯Br–Rf [Rf=per(poly)fluoroalkyl], sp3O⋯I–Rf, sp2O⋯I–Rf, sp3N⋯I–Rf, sp2N⋯I–Rf and interactions of sp2N⋯I–Rf with different substituents, a brief profile of the interactions between electronegative atoms (N, O, S) and halogen atoms (I, Br) of fluorine-containing alkylates is determined. The first example of an aliphatic fluorine-containing donor-acceptor supramolecule that is based on the N⋯Br–Rf interaction is reported in this paper. Our X-ray structure shows that 1,2-dibromotetrafluoroethane and 1,4-dimethylpiperazine alternately form endless chains depending on the N⋯Br–Rf interactions. The distance between Br and N is 2.864(3) A, which is considerably longer than the average covalent bond between N and Br, but it is also definitively shorter than the sum of the corresponding van der Waals radii of N and Br. From two other crystals, we successfully obtained precise data on the sp3O⋯X–Rf and sp2O⋯X–Rf interactions. Furthermore, an investigation of the substituent group effect is presented. We also report a valuable method to recrystallize and collect X-ray data of co-crystals that are easily disordered.

X-ray studies on cross-linked lysozyme crystals in acetonitrile-water mixture.

Tetragonal crystals of hen egg white lysozyme were cross-linked and subjected to X-ray diffraction study in acetonitrile-water media with different acetonitrile concentrations. Crystals in neat acetonitrile did not scatter X-ray well. Structures of crystals in neat water, in 90% and 95% acetonitrile, and crystal back-soaked from acetonitrile to water, were determined to about 2 A resolution. For crystals in both 90% acetonitrile, and crystal back-soaked from acetonitrile to water, were determined to about 2 A resolution. For crystals in both 90% and 95% acetonitrile, only one protein-bond acetonitrile molecule is found in the active site cleft, and its location and binding-protein mode is similar to the C subunit of polysaccharide. The alteration in conformation and hydrogen-bond pattern involving water as solvent causes the reduction of the proteins flexibility in organic media. The back-soaked crystal regained its ordinary three-dimensional structure in water.

Cycloaddition reactions of β-trifluoroacetylvinyl ethers

β-Trifluoroacetyl vinyl ethers ROCHCHCOCF 3 react smoothly as dienophiles with α,β-unsaturated carbonyl compounds to give the unexpected 2-alkoxyl-5-trifluoroacetyl-3,4-dihydro-2 H pyrans. These products are formed by elimination and addition of the alcohol to the products of the normal hetero Diels–Alder reaction (2-alkoxyl-3-trifluoroacetyl-2,3-dihydro-2 H pyrans). In contrast, 1,3-dipolar cycloaddition of ROCHCHCOCF 3 with ArCHN(O)Me proceeds via a Z - endo transition state to give regio- and stereospecific 4-trifluoroacetyl substituted isoxazolidines and their derivatives.

X-RAY STUDIES ON TWO FORMS OF BOVINE BETA -TRYPSIN CRYSTALS IN NEAT CYCLOHEXANE

Two orthorhombic forms (Vm values are 2.3 and 3.0 A3/Da) of bovine beta-trypsin crystals in neat cyclohexane were determined to 1.93 A resolution, by X-ray diffraction. Both structures in organic solvent are similar to those in aqueous solution. In the high packing density form, one cyclohexane molecule is found in a hydrophobic site near the active center. One sulfate locates at the active site with hydrogen or salt bond to the Ser-His catalytic diad, and five more sulfates bind on the molecular surface. The conformation of the side chains near the sulfates changed greatly. In the low packing density form, one cyclohexane and three sulfates are found. In both structures, one benzamidine molecule locates at the hydrophobic pocket of the active center. Most water molecules on the enzyme surface are retained except some with high temperature factors.

Reactions of fluoroalkanesulfonyl azides with trimethylsilyl enol ethers

Reactions of per(poly)-fluoroalkanesulfonyl azides R=fSO=2N=3 with trimethylsily enol ethers RCH=C(H)OSiMe=3 (R=1=tBu, Ph)afforded α－[N-(fluoroalkanesulfonyl)] amino ketones RC(O)CH=2NHSO=2R=f in good yields. The cycloanalog 1-cyclohexenyl trimethylsilyl ether reacted with the azides to give one-carbon ring-contracted product N-fluoroalkanesulfonyl cyclopentanecarboxamides, c -C=5H=11CONH-SO=2R=f. In the case of 1-cyclooctenyl trimethylsilyl ether, however, two products α－[N-(fluoroalkane sulfonyl)amine cyclooctanones and c-C=7H=13CONHSO=2R=f were obtained.

An efficient synthesis of 3-trifluoromethylated 8-oxabicyclo[3.2.1]octa-2,6-dienes

Abstract The rhodium(II) acetate catalyzed decomposition of 3-trifluoromethylated vinyldiazomethanes 3 in the presence of furans resulted in the formation of a series of 3-trifluoromethylated 8-oxabicyclo[3.2.1]octa-2,6-dienes. The 4-substituent on the vinyldiazomethanes had great effects on the product distribution and the stereo- and regiochemistry of the [3+4] annulation products. The rhodium(II) acetate catalyzed reaction of 4-carbonyl substituted vinyldiazomethanes 3a – c with furans resulted in cyclopropenes and [3+4] annulation products, while in the case of cyano-substituted vinyldiazomethanes 3d , only cycloaddition products were obtained. The reaction was presumed to follow a tandem cyclopropanation/Cope rearrangement mechanism.

Crystal Structure of the Complex of Concanavalin A and Hexapeptide

The X-ray structure analysis of a cross-linked crystal of concanavalin A soaked with the tripeptide molecule as the probe molecule showed electron density corresponding to full occupation in the binding pocket. The site lies on the surface of concanavalin A and is surrounded by three symmetry-related molecules. The crystal structure of the tripeptide complex was refined at 2.4-Å resolution to an R-factor of 17.5%, (Rfree factor of 23.7%), with an RMS deviation in bond distances of 0.01 Å. The model includes all 237 residue of concanavalin A, 1 manganese ion, 1 calcium ion, 161 water molecules, 1 glutaraldehyde molecule, and 1 tripeptide molecule. This X-ray structure analysis also provides an approach to mapping the binding surface of crystalline protein with a probe molecule that is dissolved in a mixture of organic solvent with water or in neat organic solvent but is hardly dissolved in aqueous solution.

Synthesis of N-(1,2,3,6-tetrahydropyridylidene)-fluoroalkanesulfonylamides from reactions of per(poly)fluoroalkanesulfonyl azides with 1,2-dihydropyridines

Abstract The 1,3-dipolar cycloaddition reaction of per(poly)fluoroalkanesulfonyl azides with 1,2-dihydropyridines gave N -(1,2,3,6-tetrahydropyridylidene)-fluoroalkanesulfonylamides as characterized by spectral methods and X-ray diffraction. The reaction mechanisms are discussed.

Refined 1.8Å resolution crystal structure of the porcine ϵ-trypsin☆

Abstract Porcine ϵ-trypsin is a three-chain inactivated trypsin from the limited autolysis of porcine β-trypsin. It is cleaved at positions Lys 60 -Ser 61 and Lys 145 -Ser 146 . The crystal structure has been determined by using the molecular replacement method, and refined at 1.8 A resolution. The R -value of final model is 0.184. Comparison with the electron density map of porcine β-trypsin (PTRY) in complex (BBIT), and with that of native bovine β-trypsin (HTNA), revealed no obvious changes except at the autolysis positions, and no changes at the active center were observed. The autolysis at positions Lys 60 -Ser 61 and Lys 145 -Ser 146 does not affect the conformation of His-57 in the active center and therefore cannot explain for a loss in porcine ϵ-trypsin activity.

CRYSTAL STRUCTURE OF THE COMPLEX FORMED BETWEEN BOVINE BETA -TRYPSIN AND MCTI-A, A TRYPSIN INHIBITOR OF SQUASH FAMILY, AT 1.8-A RESOLUTION

The stoichiometric complex formed between bovine β-trypsin and Momordica charantia, Linn. Cucurbitaceae trypsin inhibitor A (MCTI-A) was crystallized and its X-ray crystal structure was refined to a final R value of 0.179 using data of 7.0- to 1.8-Å resolution. Combination with results on the complex of MCTI-A with porcine trypsin gives the sequence of MCTI-A definitely, of which 13 residues are conserved compared with other squash family trypsin inhibitors. Its spatial structure and the conformation of its primary binding segment from Cys3I (P3) to Glu7I (P3′), which contains a reactive scissile bond Arg5I C–Ile6I N, were found to be very similar to the other squash family proteinase inhibitors.