Helix, sulfur

A. Rajca, H. Wang, M. Pink, S. Rajca, Annelated Heptathiophene a Fragment of a Carbon-Sulfur Helix, Angew. Chem. Int. Ed. 2000, 39, 4481-4483. 

Figure I. (above) Electronic configuration of a sulfur atom in the ground state (below) two-dimensional representation of a three-dimensional sulfur helix...

Fig. 16 View of the sulfur helix present in the fibrous allotrope. Left-handed helix (top) and right-handed helix (bottom) together with characteristic parameters are shown (cf. Fig. 17 and Table 19)...

On the other hand, the structure of Sa,2 was reported as tetragonal with the helices perpendicular to the c-axis of the unit cell (Table 20). The only reasonable possibility in structure determination was the one in which adjacent layers of hehces are oriented perpendicular to each other. The hehces of the layers have the same molecular structure and all the hehces in one layer have the same handedness. The closest packing of the helices is achieved if the molecules of parallel layers have opposite turns (see Fig. 19). This structure is unusual but not unlikely since Ae pitch of Ae sulfur helix is Ae same as Ae intermolecular Astance of neighboring helical axes, about 460 pm (see Table 20). The structure of Sa,2 is a cross-grained plywood structure . This peculiarity has caused Tuinstra to label this modification laminar sul-fiur (S ). 

Figure 4(b) - Projection of the structure of Ta4P4S29 onto a plane perpendicular to the direction of the helix. It shows clearly the sulfur helix inside the bigger one. 

Fig. 14. Choleic acid inclusion chemistry (a) crystal stmcture of DCA inclusion compound with phenanthrene (b) view along a DCA inclusion helix accommodating DMSO and water guest molecules (oxygen and sulfur atoms and methyl groups are represented by open circles and large and small black...

Many of the polysulfides described above have been investigated by X-ray diffraction on either powders or single crystals. In all cases the more sulfur-rich anions (n>3) form unbranched chains the symmetry of which varies between Ci, C2, and Cs. According to Fig. 1 the symmetry C2 results if all torsion angles have the same sign (right-handed helix + + +... left-handed helix ----...). If the different torsion angles of the anion vary between + and... 

Fia. 4-7.—The helix with seven atoms in two turns, proposed as the repeating unit in the chains of fibrous sulfur. 

Figure 18-8 Stereoscopic ribbon diagrams of the chicken bc1 complex (A) The native dimer. The molecular twofold axis runs vertically between the two monomers. Quinones, phospholipids, and detergent molecules are not shown for clarity. The presumed membrane bilayer is represented by a gray band. (B) Isolated close-up view of the two conformations of the Rieske protein (top and long helix at right) in contact with cytochrome b (below), with associated heme groups and bound inhibitors, stigmatellin, and antimycin. The isolated heme of cytochrome c, (left, above) is also shown. (C) Structure of the intermembrane (external surface) domains of the chicken bcx complex. This is viewed from within the membrane, with the transmembrane helices truncated at roughly the membrane surface. Ball-and-stick models represent the heme group of cytochrome cy the Rieske iron-sulfur cluster, and the disulfide cysteines of subunit 8. SU, subunit cyt, cytochrome. From Zhang et al.105...

Schematic representation of the C2-H2 zinc finger found in Xfin from Xenopus laevis. (Adapted from M. S. Lee et al., Science 245 645, 1989.) The recognition helix is stabilized by a complex involving zinc. Cysteine sulfurs are in yellow, and histidine nitrogens are blue.

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