No abstract

Abstract. (C30H20N2)n, monoclinic, P2Jc, a = 12.865 (5), b = 4.907 (4), c = 17.403 (5) .h, fl = I08.3 (4) ° , Z = 2/n, M r = n(408.5), Dob s = 1.30, Dealt = 1.301 g cm -3. Intensity data collected on a twocircle diffractometer using graphite-monochromated Cu K~t radiation were refined by full-matrix least squares to an R factor of 0-086. Single-crystal structure analyses show that the polymer backbone has the alternating double-single-triple-single bonding pattern and is a planar conjugated system. The carbazolyl groups are planar and make angles of 43.3 and 81° with the b axis and the plane of the polymer backbone, respectively.Introduction. Intensity data were collected on a HewlettPackard-Canberra controlled Supper diffractometer of Weissenberg geometry, using graphite-monochromated Cu K~t (2 = 1.54178 ,~) radiation and a pulseheight analyser. A fixed-counter moving-crystal technique with variable step scans was used (Hanson & Nordman, 1975) (scan range = 9 to 12°; rate = 2 to 8 ° min-~; sin 20ma× = 1.0). 880 reflections were measured;

Articles you may be interested inStrain-induced disorder, phase transformations, and transformation-induced plasticity in hexagonal boron nitride under compression and shear in a rotational diamond anvil cell: In situ x-ray diffraction study and modeling The Schottky barriers produced by polymeric sulfur nitride on compound semiconductorsThe solid-state synthesis of a new phase of (SN)x is reported. X·ray diffraction results show that mechanical shear induces a transformation of the known, monoclinic phase (space group P2t/c) to a phase of orthorhombic symmetry (probable space group P2 1 2 1 2 1 ). The unit cell parameters of the shear-induced phase of (SN)x are a = 6.251, b = 4.429, and c = 4.807 A. Since the approximate chain repeat dimension (b) and the chain symmetry (2 1 ) are the same before and after the transformation, the chain geometries for the two phases of (SN)x are predicted to be quite similar, if not identical. However, important differences are expected for interchain interactions that have been previously shown to be essential in understanding various properties of the monoclinic phase of (SN)x' Both phases of (SN)x are shown to be stable with respect to thermally-induced interconversion for temperatures up to 175·C. Molecular packing calculations are used to predict structural aspects of the shear-induced phase and the relationship between parent and daughter phases. The latter results are consistent with the monoclinic-to-orthorhombic transformation being martensitic in character.The effect of mechanical shear and subsequent thermal annealing on the x-ray diffraction pattern of (SN)" is illustrated in Fig. 1. In this case the sheared (SN)" was produced by severely grinding phase I crystals with

Interpretation of a new electron-density map at 3.25-A resolution has led to a somewhat revised model for the free radical (semiquinone) structure of flavodoxin from Clostridium MP. Although the general conformation of the molecule is very similar to that of oxidized Desulfovibrio vulgaris flavodoxin, flavin mononucleotide-protein interactions are not identical in the two flavodoxins. In the Cl. MP semiquinone molecule, the isoalloxazine ring appears to retain the essentially planar conformation characteristic of oxidized flavins; within the limits imposed by the resolution of the data, the map shows no evidence for bending of the isoalloxazine ring about N5-N1O.Analysis of the structure of flavodoxin crystals is expected to provide a detailed picture of flavin mononucleotide-protein interactions, which account for the characteristic chemical properties of these model flavoproteins (1, 2). Earlier we reported an interpretation of an electron-density map of Clostridium MP flavodoxin semiquinone at 3.25-A resolution. The model constructed on the basis of that map was considered tentative in several regions, and the orientation of flavin mononucleotide could not be assigned unequivocally (3).Recently, we calculated another map of the semiquinone form of flavodoxin at 3.25-A resolution, using three heavyatom derivatives, Sm+3, Pt+2, and Au+', and giving more weight to anomalous scattering differences in the phasing. The resulting map had a much improved figure of merit, 0.80, and has proven easier to interpret. The new map clearly suggested a flavin mononucleotide position different from that proposed earlier and required some revisions in the chain tracing ( Fig. 1). At about the same time, Watenpaugh et al. (4) succeeded in determining the structure of the oxidized form of a very similar protein from Desulfovibrio vulgaris at 2.5-A resolution. Part of our current model of the semiquinone form was constructed after a preliminary sketch of the model of Watenpaugh et al. had been kindly sent to us by Dr. Jensen. We describe our new molecular model briefly here in order to facilitate comparison of the two structures (5).Despite the greater chain length of flavodoxin from D.vulgaris (about 10 additional residues), the three-dimensional structures of the two molecules are clearly homologous. The prosthetic group is similarly situated in both proteins, but the flavin mononucleotide-protein interactions appear to differ in certain respects. Some differences are not unexpected in view of the known dissimilarities in amino-acid composition (3, 6), in optical and circular dichroic spectra (7), and in affin'ty for modified flavins (7,8 Fig. 1. In the matching regions, the average displacement of a total of 113 alpha carbons in constructing the new model was 1.7 A; in the best-defined helix, HI, the change was only 0.5 A. However, altered chain connections in the vicinity of the Nterminus and the flavin mononucleotide have led to a quite different ordering of the amino-acid sequence along the model. In the interpretati...

No abstract