Screw sense, helical ordering

Preferential Screw Sense Helical Ordering of Polysilanes... 

On the basis of the difference in optical rotation between copolymer and homopolymeric mixture having the same composition the molar optical rotation referred to one monomer residue of 4-methyl-l-pentene has been evaluated (Table XIX). This last increases in absolute value with increasing content of units derived from the optically active monomer and reaches an asymptotic value when the above content is larger than 50%. The optical rotation of 4-methyl-l-pentene units thus obtained can be of the same order of magnitude as that evaluated according to Brewster, assuming that these units are inserted in one screw sense helical chain. 

The preparation of helically well-ordered polymers with stable screw-sense, which is able to be transmitted to newly formed polymer main-chains effectively, is highly desired for the development of new methodology for the synthesis of optically active helical polymers. An aromatizing polymerization of 1,2-diisocyanobenzenes is promoted by methylpalladium(II) complexes, producing poly(quinoxaline-2,3-diyl)s.146-148 The polymerization proceeds with successive insertion of the two isocyano groups of the diisocyanobenzene to the carbon palladium bond of... 

In order to discuss the preferential screw sense capability qualitatively in a series of 44 and 45 helical copolymers with chiral-achiral side chains, we compared the UV and CD spectral characteristics of 44 and 45 as a function of the chiral molar composition in detail. 

Homopolymers such as poly[(V)-3,7-dimethyloctyl-2-methylpropylsilylene], 117, were initially studied, and the helix-helix transition was discussed in terms of an entropically driven phenomenon in which at temperatures below Tc the side chains of the helical polymer are in a very ordered state and enforce a particular screw sense, whereas above Tc, the side chains become disordered such that the main chain can relax into the opposite screw sense.314 This concept is expressed in Figure 47. 

Because of this principal structural divergence from coded peptides, peptoids lack amide protons. This property precludes the formation of the intramolecular H-bonds that contribute largely to the stabilization of the most common helical structures of a-peptides. Unlike that of a-peptides, the peptoid backbone is inherently achiral (as it is based on Gly residues). However, it has been reported that sufficient bias to form stable helices of a specific screw sense can be provided by side chains with an a-chiral carbon atom (that is linked directly to nitrogen) (32, 33). The extraordinary resistance of these helices to loss of their ordered secondary structure is another intriguing property of peptoid molecules. 

Such a compensated mixture provides an environment which exhibits both anisotropy and helicity. The two isomers have somewhat different order parameters in a solvent with a local screw sense and correspondingly exhibit different values of the direct coupling constants. In an ordinary nematic solvent (such as n-hexyloxyazoxybenzene) the two isomers give identical spectra. [56]. 

In order to explain the exciton splitting of the B electronic transition, coupling must be assumed to occur between aromatic residues which are in a fixed suitable stereochemical position [9]. The latter is reached when the macromolecules are in a helical conformation, the prevailing screw sense being reflected in the sign of the CD couplet observed. 

The flow in an anchor agitated vessel has been studied in detail by Peters and Smith . The impeller promotes fluid motion close to the vessel wall but the region near the shaft may be relatively stagnant. In addition, there is little top-to-bottom turnover. In order to promote a top-to-bottom motion a helical ribbon may be used and often a screw is added to the shaft to obtain motion in the central regions of the vessel. This combined impeller system would have a ribbon pumping upward near the wall of the vesel with the screw, twisted in the opposite sense, pumping downwards near the shaft. 

Fig. 26d, the width and screw pitch of the assemblies are similar to those observed in SEM images. The orientation of the height contour indicates that these superhelices have a right-handed sense. Many possible interacticMis, including hydrophobic, dipolar rt-jr interactions, and ordered packing tendency of a-helical polypeptide segments, are believed to be responsible for the formation of super-helical structures. 

---