Helix handedness

Statistical substitution between chains having the same helix handedness, but different orientations of the side groups ("up" and "down chains) was proved or at least suggested as probable for a variety of isotactic vinyl polymers. In particular we will mention here the cases of isotactic polypropylene (53), isotactic poly-w-butene-1 (54), isotactic polyaldehydes (55) and isotactic-1,2-poly-4-methylpentadiene (56). 

The threefold helices of cellulose are chiral, l.e., their left-and righthanded conformations are not Identical. It is not yet known whether the structure of Na-cellulose IIB Is characterized by one particular helix handedness, as both conformations are stable and of not very different conformational energy. The x-ray diffraction diagram (cf. Fig. 2B) Is rich In detail and It should be possible to determine the handedness of the Na-cellulose IIB helix from a detailed x-ray refinement. 

Although overcrowded afkenes are attractive due to the efficient switching of LC handedness, the delicate synthesis of overcrowded alkenes remains a limitation for their practical applications. Normally the separation of two enantiomers of crude racemic product to obtain optically pure isomer has to rely on the preparative high performance liquid chromatography (HPLC), which is not suitable for a large-scale preparation. Consequently, researchers are seeking for other types of photoresponsive molecules for the switching of helix handedness. 

The latter is energetically more favorable and has helical parameters which are very similar to that of the polypeptide alpha helix. As can be judged from Fig. 15 the two models have a distinctly different carbonyl orientation either perpendicular or parallel to the helix axis. Unlike the poly(S-lactide) where only one helix handedness is stable, both right and left-handed helices are equally stable in this calculation. This is evident from the symmetry in Fig. 14. 

Figure 4.24 Superhelix with helix angle < > governing handedness of packing between preferential helical main chains of contiguous polysilane chains for origin of Cotton effect of polysilane aggregates.

Using circular dichroism and nuclear magnetic resonance spectrometry, Veatch and coworkers1 established that four conformational species of gramicidin A exist in solution. Two were postulated to be helixes of opposite handedness. 

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