The symmetric diglyceride of 11-bromoundecanoic acid, C25H46Br205 (DBU), was crystallized from chloroform showing a tabular monoclinic aspect and having space group C2/c, with unit-cell dimensions a = 9.36, b = 5-58, c = 54.53 ~, fl = 90"40 ° and with Z= 4. The structure was determined by a three-dimensional X-ray diffraction analysis. Data were collected with a General Electric XRD-6 manual diffractometer and nickel-filtered Cu K~ radiation. The structure was solved by the heavy atom method. Block-diagonal least-squares refinement led to an R of 0.09. A pseudo twofold axis of molecular symmetry is utilized as a statistical axis of symmetry by the space group. The two hydrocarbon chain tails in the molecule point in opposite directions and are packed in layers with the chain axis parallel. The direction of the chain tilt, however, alternates in successive layers.
An analysis of thickness variances from an electron-microscope cross-section of myelin sheath shows that the standard deviation varies proportionally to the square root of the number of intervening lamellae. This result implies that the lamellar irregularities are additive and that a paracrystalline stacking model is an appropriate representation for the myelin structure. The X-ray diffraction data from myelin exhibit the characteristics of a paracrystalline material. Both electron microscope and X-ray diffraction evidence suggest that myelin is paracrystalline rather than crystalline.
The crystal structure of 2,4,7-trinitro-9-fluorenone, Cx3HsN307, has been determined. The crystals are monoclinic, space group P2t/n; cell dimensions are a=22.470 (3), b= 5.652 (1), c= 10.278 (2)/~, fl= 105-51 (I) °, Z=4; D,,= 1.692 and Dx= 1.675 g.cm -3. An initial set of three-dimensional X-ray data was obtained using Cu radiation with a manually operated General-Electric diffractometer; a second set was measured using Mo radiation with a computer controlled Picker diffractometer. The structure was solved with direct methods; refinement by full-matrix least-squares calculations gave an R index of 0.057 (weighted R= 0.045) for the Mo data. Anisotropic temperature factors were used for the carbon, nitrogen and oxygen atoms while the hydrogen atoms were refined with isotropic terms. The fluorene nucleus is planar. The bond lengths indicate that there is little interaction between the benzene rings, or between the rings and the carbonyl group. Because of steric crowding, the C(4) nitro group is twisted oUt of the C(1)-C(2)-C(3)-C(4)-C(4a)--C(9a) benzene ring plane by 34 °. A 23 ° r.m.s, rigid body libration of the C(7) nitro group about the C(7)-N bond was found.
The single crystal x-ray structure of racemic glycerol 1,2-(di-11bromoundecanoate)-3-(p-toluenesulfonate), a sulfolipid analogous to the membrane phospholipids, reveals a folded conformation.
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