Helix untwisting

X-ray measurements on LC elastomers have shown [6-8] that the reversible transition between a chiral smectic C phase with and without a helical superstructure can be induced mechanically. The helix untwisted state corresponds in this case to a polar ferroelectric monodomain. The piezoelectricity arising from this deformation of the helical superstructure (which does not require a complete untwisting) has been demonstrated [9] for polymers cross-linked by polymerization of pendant acrylate groups (Figure 15). 

The role of the boundary conditions was discussed theoretically in [37, 38]. When the helical axis is parallel to the limiting walls and the field E h, the threshold for the helix untwisting decreases when following from the planar to the homeotropic anchoring. For cell thickness a < y/2Pofn,... 

The linear-in-field fiexoelectric coupling, discussed in Section 4.5, for nematics can modity the threshold field for helix untwisting in cholesterics. The flexo-effect stabilizes the helical structure, thus, it results in a decrease of an apparent dielectric anisotropy [40]... 

Thus, the flexoelectric effect in cholesterics is as fast as the helix untwisting effect. By placing the sample between the cross polarizers and applying... 

Following formation of the polymerase-promoter complex, several turns of the DNA double helix untwist, forming a bubble and exposing 14 or so base pairs of the two strands. Appropriate ribonucleotides then line up by hydrogenbonding to their complementary bases on DNA, bond formation occurs in the... 

By increasing the lateral dipoteniomem at die chiral center (polymer 2b in Ihble 1), a slight increase of (he spontaneous polarization from 1 to 3 nC/cm was observed. In comparison with their monomers, the polymers exhibit a three to five times lower spontaneous polarization [9], More details on the physical investigation of those two polymers, especially concerning (be relaxation limes (of helix untwisting (CJoldstone mode) and tbe soft mode, are presented in Ref. 10 (see also Sec. n.I). 

It is evident from X-ray diffraction data that the base layers of DNA are, normally, making van der Waals contact with one another above and below, and that the distance between the centres of the atoms, in adjacent pairs on a strand, is 3.36 A. This means that another 3.36 A of space (6.72 A in all) must be provided to admit the aminoacridine molecules, which have exactly the same thickness as those of the purine and pyrimidine bases. This space could be provided, if the helix untwisted slightly, and that turned out to be exactly what happened. 

Armed with this information, the unwinding problem mentioned ear-lier can be reconsidered. By the alternating action of endonucleolytic and ligase activities, the unwinding of DNA could be reduced to an untwisting of only a small part of the double helix at any given time. Both activities are part of the enzyme called topo-isomerase 1. 

The mounting evidence in the case of the curdlan triple helix (13)is that the chains can untwist slightly (or twist tighter) under different conditions giving rise to quite different x-ray diffraction patterns as the symmetry is destroyed. These changes should not be confused with complete untwining of the chains. 

The conformation of the polytetrafluoroethylene molecule in the low temperature form (Phase II) has been determined to be 2.159 CF2 units per turn of the helix within the limits of experimental error. This conformation is slightly untwisted from the previously assigned 13/6 = 2.167 value but is substantially different from that for the 25°C form (Phase IV) in which the conformation is 15/7 = 2.143. By comparison, the planar zig-zag is 2/1 = 2.000. 

Figure 2 Phase diagram of a solution of twisted ribbons which form fibrils, where the relative helix pitch of isolated ribbons hribbom/a is plotted against the relative side-by-side attraction energy between ribbons "ri/ fibnitTibrii = (27r2b2/a2 ktwist. The areas divided by the thick lines reveal the conditions where ribbons, fibrils, and infinite stacks of completely untwisted ribbons are stable. The dotted lines are lines of stability for fibrils containing p ribbons (p are written on the lines) kbend/ktwist = 0.1 (Aggeli et al., 2001b).

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