Twofold terminations

In the case of twofold terminations, we consider a layer of four atomic planes with spacings (aZi )i-plane, 1.48A, (Ii )i-plane, 0.92A, (Zi )2-plane, 1.48A, abq)2-plane. abq)-p me. implies that the twofold plane contains atomic positions of all classes, a, b and q, see Tables 12-1 and 12-2. (feg)-plane implies that the plane contains atomic positions of only classes a and b. For a bundle we define an effective (averaged) density of planes P2/(zi)... 

For this thick, 3.88 A broad twofold layer the maximum of P2/(zx) is a perfectly flat plateau (Figure 12-13). The height of the plateau defines the effective density of terminations to be 0.086A (Table 12-4). The support of the width of the plateau equals W = (l/2) (see Figure 12-13) and encodes the Fibonacci sequence of twofold terminations with terrace heights S = y = 6.3 A and L = tS= 0.2 A. 

In general, the Mizoroki-Heck coupling of aryl halides with both electron-rich and electron-poor terminal alkenes affords monoarylated products. With electron-deficient alkenes under selected reaction conditions, such as with excess of the aryl halide, with special catalysts at high temperatures or under high pressure, a twofold terminal arylation to give 1,1-diarylalkene derivatives may occur (Figure 3.41) [103]. Triple arylations... 

The oligomeric structures of chemokines vary. CXCL8 (IL-8), the prototypical CXC chemokine, is dimeric in solution (26) and in its crystalline form (due to twofold crystallographic symmetry) (27). The three-stranded P-sheet of each subunit joins to form a six-stranded P-sheet (Figure IB). The two C-terminal... 

Fig. 16. Parallel superpleated /J-structure proposed for Ure2p amyloid-like fibrils, from Kajava et al. (2004). (A) Ribbon diagram o( a dimer of Saccharnmyces cermnsiae l Jre2p C-terminal domains (PDB ID code 1G6W), generated with Pymol (DeLano, 2002). The monomers are colored in light and dark gray and are viewed down the twofold symmetry axis. The N- and C-termini are indicated, and residue 137 is denoted by an open arrow.

The structure of PaFd was the first to be crystallographically determined (Adman et ai, 1973). The basic fold of the protein may be described as a pair of two stranded antiparallel )3 sheets. The two 4Fe 4S clusters are sandwiched between these /3 strands on one side and several helical segments on the other side. The clusters are packed in a predominantly hydrophobic environment. The internal sequence homology is clearly reflected in the structure the two clusters and much of the polypeptide chain are related by an approximate internal twofold rotation axis. The two clusters are ligated by the two sets of four cysteines in the two halves of the molecule. Surprisingly, each cluster is liganded by cysteines from both halves of the sequence, rather than cysteines from only one half (which are adjacent in the sequence). Cluster 1 is coordinated by Cys-8, -11, and -14 in the amino-terminal half and Cys-45 of the carboxy-terminal half, while cluster 2 is coordinated by Cys-35, -38, and -41 of the carboxy-terminal half and Cys-18 of the amino-terminal half. 

Scheme 15 Twofold Heok oouplings on acceptor-substituted terminal alkenes to yield geminally diarylated alkenes. ...

Kury et al (Ref 7) on the basis of theoretical considerations of 2-D detonations in metal cylinders, conclude that the terminal cylinder expansion velocity is reached after about a twofold radial expansion. Their exptl results (Refs 7 13) for Comp B, HMX, and many HMX mixts justify this theoretical conclusion... 

Figure 7-17 The structure of insulin. (A) The amino acid sequence of the A and B chains linked by disulfide bridges. (B) Sketch showing the backbone structure of the insulin molecule as revealed by X-ray analysis. The A and B chains have been labeled. Positions and orientations of aromatic side chains are also shown. (C) View of the paired N-terminal ends of the B chains in the insulin dimer. View is approximately down the pseudo-twofold axis toward the center of the hexamer. (D) Schematic drawing showing packing of six insulin molecules in the zinc-stabilized hexamer.

Figure 28-13 (A) Stereoscopic ribbon drawing of the phyloge-netically conserved 180-residue C-terminal portion of the TATA-binding protein (TBP) from Arabidopsis thaliana. The sequence consists of two direct repeats, giving the protein an approximate twofold symmetry. From Nikolov et al.337 (B) Structure of the corresponding C-terminal core (residues 155-335) of the human TATA-binding protein (TBP) bound to the TATA sequence of a promoter in adenovirus DNA. From Nikolov et al.327 (C) Structure of human transcription factor IIB bound to a TBP from Arabidopsis thaliana, which, in turn, is bound to an adenovirus TATA sequence. Hypothetical B DNA extensions have been modeled at both ends of the DNA segment. The +1 at the left end is the transcription start site and the —43 upstream end is to the right. From Nikolov et al.338 Courtesy of Stephen K. Burley.

Convergent ligation of cyclic peptide 86b (1.6 mmol) in twofold excess (8 equiv) of 93 (0.1 mmol) with a final concentration of 1-2 mM was performed in H20 at pH 8 with MeCN (50%) as a cosolvent to increase the solubility of 93. The ligation yielded >80% of the four-branch peptide dendrimer 94 (MALDI-MS mJz - calcd 8685.0 found 8685.7) together with <15% of the two- and three-branch dendrimers (MALDI-MS for three-branch dendrimer m/z calcd 6602 found 6604.0). After 45 min, the reaction was terminated and purified by RP-HPLC isolated yield 58%. 

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