Helical structures pendants

The helical structure is largely caused by crowding of the bulky lateral substituents, pendant on each backbone carbon atom. In addition, if not relieved by tautomeric structural changes, the accumulation of vicinal dipoles, as are known to destabilize vicinal triketones, should cause a tendency for a molecule to assume a progressively twisted conformation that would allow an orthogonal rather than a parallel situation of neighboring dipoles. The core, then, of the cylinder is made up... 

From these facts, it is concluded that the polymer complexes are formed by specific base pairing between pendant adenine and thymine or uracil units of poly-L-lysine derivatives retaining their helical conformations (Fig. 26). The lowering of the base content in the polymers results in the decrease of the helical structure and also in the decrease of interactions with the complementary polymer. 

The chiral groups substituted p-conjugated polyacelylenes form helical structure with a predominantly one-handed screw sense exhibits simultaneous changes in both optical and chiroptical properties [125, 126]. For example, the pendant chiral oxazoline moiety in poly(phenylacetylene) [127] induces an excess one handedness helical conformation in the main chain of the polymer. 

AFM and MM studies showed that these polymers in CHCI3 presented identical handedness for the internal (polyene backbone) and the external (pendants) helices (3/1 helix), whereas in THF the internal and external helices (2/1 helix) presented opposite helical senses. DSC traces supported the cis-cisoidal and cis-transoidal helical structures associated with those structural features. 

It must be remembered that the secondary structure of both the amorphous and crystalline regions typically tends toward a helical arrangement of the backbone for most polymers but not for PE, which forms a crank-shaft structure because of the lack of steiic restraints (i.e., lack of pendant groups off the backbone). 

Upon complexation with a lanthanide ion, these complexes may form square antiprism or twisted square antiprism (TS APR) structures with a vacant coordination site in the cap position, which is assumed to be occupied by a solvent molecule. Just as in the chelated complexes described previously, two distinct types of chiral stereochemistry are present. In analogy with OC-6 species, the sense of rotation of the pendant arms is denoted as A or A depending upon if the arms rotate clockwise (A) or counterclockwise (A) as one proceeds down the direction of the C4 axis. There is also chirality (or helicity) associated with the nonplanar 12-membered ring. If one looks along the skew-line connecting the coordinated nitrogens, the carbon atoms... 

The amidic bonds within amino acids can be also used to effect the organization of polymers into superstructures (Fig. 10). Thus, the formation of artificial helices on the basis of assembling polymers has been described by use of poly(acetylenes) bearing pendant L-valine side-chains. [71,72] Two effects are important for the association of these ladder-type polymers into double-stranded helices (a) the reduction of conformational freedom by the poly(acetylene) chain with respect to a conventional alkyl-chain and (b) the selective association of the L-valine residues by specific hydrogen bonding. An AFM image of the associates on a fiat surface demonstrates the presence of a string-pearl structure reminiscent of natural DNA. 

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