Antiparallel orientation

Vani and Vijayan [121] investigated the crystal structure of 7M-p-methoxy-benzylidene-p-phenyl-azoaniline (MBPAA). The two crystaUographically independent molecules are oriented antiparallel to each other. The packing of the molecules shows a herringbone arrangement. 

It may be noted that simple MOPAC AMI calculations suggest that the dipole moment of NOBOW is oriented antiparallel to the molecular arrow. As indicated in Figure 8.25, this means that for an up field, the molecular arrows are pointing down. Given the definition of the sign of P in FLCs, this also means that domains of the ShiCaPa phase with positive chirality have negative ferroelectric polarization, and vice versa. 

A coupling constant (1.5 Hz) is observed between F-3e and that H-6 atom which resides for most of the time in the orientation antiparallel to the C-5-0-5 bond in l,2,4,6-tetra-0-acetyl-3-deoxy-3-fluoro-/3-D-glucopyranose (126), and it was suggested296 that the favored pathway for 19F- H coupling to occur over five bonds would approximate such... 

A counter-clockwise rotation would bring the CF bond synclinal to the sp2 lone pair. The synclinal overlap is weaker than the anticlinal,28 because of the negative overlap between sp2 and the fluorine AO in o CF. We have used this argument in our treatment of the Walden inversion (p. 179). Pushing the analogy a little further, we can equate the sp2-o CF interaction to an SN2 attack of the sp2 orbital on the CF bond. This attack must proceed with inversion (because the reaction occurs at a carbon center), so the sp2 orbital will be best oriented antiparallel to CF. 

Br bond dipoles are oriented antiparallel and thus able to avoid repulsion). If this chair conformer eliminates HBr, the dihedral angle between the C-H and the SC-Br that are to be broken on the way to 3-bromocyclohexene measures a favorable 180°. the dihedral angle between the H and Br atoms to be eliminated to afford 1 -bromocyclohexene is an unfavorable 60°. As long as the reaction temperature is not too low and/or the potassium alkoxide not too undemanding sterically so that ether formation by the SN2 mechanism becomes competitive, the 3-bromocyclohexene eliminates a second equivalent of HBr in another 1,2-elimination, forming 1,3-cyclohexadiene. 

For nuclei for which I =Vi the nuclei with spin ni = I Vi are oriented parallel to the external magnetic field and they have a lower energy than the nuclei with spins mt = Zi, that are oriented antiparallel to the magnetic field there states are labelled a-state and /j-state, respectively. The energy difference between the two nuclei is... 

Because the glycosyl links and the associated sugar-phosphate backbone are related, in the Watson-Crick duplex, by a pseudodyad perpendicular to the helix axis, the two strands are oriented antiparallel. They are parallel in the Hoogsteen duplex because the glycosyl links are related by rotation about the helix axis. 

Now consider the pair of electrons in the bond between A and X. By the Pauli exclusion principle, these electrons are oriented antiparallel to each other, as... 

In the antiferromagnetic phase the migration of excitation energy is slowed down, because nearest neighbor Gld (or Tb ) ions are oriented antiparallel, which makes energy transfer by exchange interaction impossible. 

The antiferromagnetic ordering below was proved by Cox et al. (1986). They found a simple arrangement of the U moments of (0.8 0.1 )/r n (at 5 K). These are oriented antiparallel to their nearest-neighbours within the basal planes and to the near-neighbours along the c-axis. The magnetic unit cell is identical with the chemical unit cell. 

In the peroxisomal importer protein, PEX5, the seven consecutive TPRs do not form an extended superhelix but separate into two TPR arrays, each composed of three repeats. These arrays are oriented antiparallel to one... 

Fig. 5.3 Vibrational modes of the (Raman active) phonons Ag and Bg, schematic representation, b is the twofold axis of the monoclinic crystal. For the Bg phonons the axes of rotational symmetry of the two molecules in the unit cell are parallel, while for the Ag phonons, they are oriented antiparallel. K=0.

Fig. 5.12. Epitaxial film for bubble memory (a) no applied field (b) small field (oriented antiparallel to the black domains) (c) by saturating and decreasing the field, cyclindrical (bubble) domains are formed.

From WAXS and SAED data of both ProNectin F lyophilized powder and sprayed fibrils, the current model indicates that ProNectin F crystallizes into a chain folded pleated sheet of beta strands (Anderson et a/. 1994). The strands are oriented antiparallel. The beta strands are not fully extended, but have a more compressed crankshaft conformation. This conformation agrees with the predicted conformation of unoriented silk fibroin protein, the Silk I structure (Lotz and Keith 1971). The crystal dimension in the c direction (along the peptide backbone) is consistent with a theoretical length of 11.6 nm for nine SEP blocks (54 amino acids) in this conformation. This predicts that the width of the ProNectin F tile is controlled at least in part by the number of amino acids in the silklike block domains. 

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