Molecules, helical

A regular helical chain may be defined as a chain that possesses a screw axis, where 1. The operation corresponding to this is a rotation 

7 (a) The threefold helix for an isotactic vinyl polymer. The X group of the structural unit -fCHX—CH2T- is shown shaded. The carbon backbone forms a right-handed helix and the right-handed 3i and the left-handed 32 helices are indicated by the ribbons, (b) The 13e helix of PTFE, showing the twisted backbone, and a spacefilling model viewed from the side and viewed along the chain axis, ((a) reproduced from The Vibrational Spectroscopy of Polymers by D. I. Bower and W. F. Maddams. 

CF2 units in one translational repeat unit, as shown in fig. 4.7(b). This repeat unit corresponds to a twist of the backbone through 180°. In polyethylene a rotation of 180° is required to bring one CH2 group into coincidence with the previous one but, because of the twist, the angle required for PTFE to bring one CF2 group into coincidence with the next is 180° less 180°/13, or 12u/13, so that the structure is a 13g helix. 

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The DNA isolated from different cells and viruses characteristically consists of two polynucleotide strands wound together to form a long, slender, helical molecule, the DNA double helix. The strands run in opposite directions that is, they are antiparallel and are held together in the double helical structure through interchain hydrogen bonds (Eigure 11.19). These H bonds pair the bases of nucleotides in one chain to complementary bases in the other, a phenomenon called base pairing. 

Stnicture/chiroptics relationships of planar chiral and helical molecules, in particular of heteracyclophanes and heterohelicenes 98EJ01491. 

The chromosomes of Escherichia coli and other bacteria are single, double-stranded DNA molecules with a total length of more than 1,000 pm. Relaxed DNA exists as a helical molecule, with one full turn of the helix occurring approximately every 10.4 base pairs. This molecule must undergo several folding and compaction steps to fit into an E. coli cell which is only 1-3 pm long. Despite this enormous compaction, bacterial DNA must be accessible for the bacterial enzymes that catalize DNA replication and transcription... 

The trisulfane molecule exists as two conformers which have been termed as cis- and trans-HzSi. While the trans-form is a helical molecule of C2 symmetry with the motif ++ (or — for the enantiomer), the cfs-form is of Q symmetry with the motif +- (identical to -+). Both forms have been detected by rotational spectroscopy [17, 45, 46]. The motif gives the order of the signs of the torsion angles at the SS bonds. The geometrical parameters [17] are presented in Table 4. The trans-isomer is by only 1 kj mol more stable than the cfs-form but the barrier to internal rotation from tmns to cis is 35 kJ mor [46]. The dipole moments were calculated by ab initio MO theory at the QCISD/TZ+P level as 0.68 D (trans) and 2.02 D (cis) [46]. For geometrical parameters of cis- and trans-trisulfane calculated at the MP2/6-311++G> > level, see [34]. 

RNA exists as a single strand, whereas DNA exists as a double-stranded helical molecule. However, given the proper complementary base sequence with opposite polarity, the single strand of RNA—as demonstrated in Figure 35-7—is capable of folding back on itself like a hairpin and thus acquiring double-stranded characteristics. 

Fig. 6.14 A gramicidin channel consisting of two helical molecules in the head-to-head position. (According to V. I. 

For a general review of helical molecules in organic chemistry see Meurer, K. P., Vogtle, F. Top. Curr. Chem. 127, 1 (1985)... 

Suppose that one takes an electron micrograph of such a helical molecule at a resolution sufficient to reveal the helix structure. This micrograph will appear as a projection of the helix in two dimensions (Scheme 19a) and will still not allow one to distinguish between a dextro- and levorotatory helix positioned as mirror images as in Scheme 19a. An unequivocal solution to the problem may be achieved, in analogy to the optical solution illustrated in Section II, if a second micrograph is taken with the plane of the sample tilted in a known direction, yielding a new two-dimensional projection (Scheme 19b). 

Most DNA occurs in nature as a right-handed double-helical molecule known as Watson-Crick DNA or B-DNA (Fig I-1-9). The hydrophilic sugar-phosphate backbone of each strand is on the outside of the double helix. The hydrogen-bonded base pairs are stacked in the center of the molecule. There are about 10 base pairs per complete turn of the helix. A rare left-handed double-helical form of DNA that occurs in G-C-rich sequences is known as Z-DNA. The biologic function of Z-DNA is unknown, but may be related to gene regulation. 

DNA codes for its own synthesis at the time of cell division. Thus, DNA acts as the agent of inheritance. As is developed below, DNA is a double-stranded helical molecule—the famous double helix—in which the two strands are complementary. DNA is the repository of information that is expressed in synthesis of the proteins of the cell. Therefore, DNA acts as the determinant of the biochemical personality of the cell. ... 

Tropomyosin is a long helical molecule (70 kDa) which extends along the long axis of the actin filament (Figure 13.7). Each tropomyosin molecule covers seven actin monomers and plays a central role in the regulation of muscle contraction. 

A single a-helical molecule is stabilized by the intramolecular hydrogen bonding between the ( ) and ( + 4) residue along the peptide backbone, allowing for the formation of a right-handed helix with 3.6 amino acids per mm. However, a lone helix is rare. Typically, the helix must be stabilized by interactions with other portions of the protein or, in the case of coiled coils, it may be stabilized by interactions with additional helices. 

A review of the older literature on compounds with a stereogenic axis is available22, as are reviews on planar chiral molecular structures 23, on the stereochemistry of twisted double bond systems 24, on helical molecules in organic chemistry 25, and on the synthesis and stereochemistry of chiral organic molecules with high symmetry 26. 

One of the major features in the sequence of cell division is the formation of the mitotic spindle and the subsequent separation of chromosomes into their respective daughter cells. An important element of the spindle is the highly conserved, helical molecule tubulin. In addition to spindle formation and the segregation of chromosomes in cell division, alternating helices of a- and -tubulin form the microtubules that form part of the cytoskeleton and have active roles in cell organelle organisation. 

Meurer, K. P., and Vogtle, F. Helical Molecules in Organic Chemistry. 727, 1-76 (1985). Montanari, F., Landini, D., and Rolla, F. Phase-Transfer Catalyzed Reactions. 101, 149 200... 

DNA is a linear polymer of covalently joined deoxyribonucleotides, of four types deoxyadenylate (A), deoxyguanylate (G), deoxycytidy-late (C), and deoxythymidylate (T). Each nucleotide, with its unique three-dimensional structure, can associate very specifically but non-covalently with one other nucleotide in the complementary chain A always associates with T, and G with C. Thus, in the double-stranded DNA molecule, the entire sequence of nucleotides in one strand is complementary to the sequence in the other. The two strands, held together by hydrogen bonds (represented here by vertical blue lines) between each pair of complementary nucleotides, twist about each other to form the DNA double helix. In DNA replication, the two strands separate and two new strands are synthesized, each with a sequence complementary to one of the original strands. The result is two double-helical molecules, each identical to the original DNA. 

Crystallinity of polypropylene is usually determined by x-ray diffraction. Isotactic polymer consists of helical molecules, with three monomer units per chain unit, resulting in a spacing between units of identical conformation of 0.65 nm. 

Syndiotactic polypropylene also forms helical molecules however, each chain unit consists of four monomer units having a spacing of 0.74 nm. The unit cell is orthorhombic and contains 48 monomer units having a crystallographic density of 0.91 g/cmJ. 

Higgs, P. W. The vibration spectra of helical molecules infrared and Raman selection rules, intensities and approximate frequencies. Proc. roy. Soc. A220, 472—485 (1953). 

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