Helical jumps

Figure 11.15 Molecular chain simulation of (a) perfect 3i PP helix and (b) 3i PP helix with two 120° helical jumps at the central position the helical hand changes at every helical jump propagation of the defect along the chain will reverse the helical hand. (From Reference 48 with permission from Elsevier.)...

In the previous section the helical jumping of the molecules in the crystallites of POM was discussed. Similar motions have been observed in a number of other crystalline polymers, including poly(ethylene oxide) (PEO), isotactic polypropylene (IPP) and polyethylene. At 240 K each helical chain in PEO makes about three jumps per second. In the crystalline regions of IPP helical jumps along the 3i helix are observed at temperatures in the range 340 380 K, whereas in syndiotactic polypropylene, which has a more complicated 4j helix, no jumps are observed. 

The (3 relaxation in polyethylene, which is most prominent in the low-crystallinity LDPE, is associated with the amorphous regions and almost certainly corresponds to what would be a glass transition in an amorphous polymer a diiference in its position in mechanical and dielectric spectra is therefore not surprising. The a relaxation, as discussed in section 7.6.3, is associated with helical jumps in the crystalline regions and, provided that the lamellar thickness is reasonably uniform, might be expected to correspond to a fairly well-defined relaxation time and to a narrow peak in the relaxation spectrum. The dielectric peak is indeed quite narrow, because the rotation of the dipoles in the crystalline regions is the major contribu-... 

In this case, we point to the fact that a fast (r < 5 s) and a slow phase have been observed in temperature-jump experiments also with the peptide Col 1-3. The slow phase - as already mentioned - has been associated with the cis-trans isomerism of peptide bonds in the direct neighborhood of the helical part. Only peptide bonds to which proline or hydroxyproline contribute their secondary nitrogen are able to assume a cry-configuration at equilibrium (cis to trans ratios of 1 40 to 1 l)l45). Therefore, the fast... 

For activity of ACTH-derived peptides at the receptor for pole-jumping activity, the basic requirement seems to be the presence of a Phe and Met residue in close proximity. It is interesting to see that Phe and Met are close together in an a-helical structure in ACTH peptides (and as intra-chain neighbours in Met-enkephalin) and in the crystalline state in ACTH-(4-10) as a 3-pleated sheet and in ACTH-(4-7) in the form of a horseshoe this close proximity is in line with the results of a Free-Wilson type of analysis. 

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