Chains with preferred conformation

Fig. 11.16. The NMR stick spectra of s-PP predicted for the chains with preferred conformations. The chemical shift difference on the basis of the -y-effect is noted in the stick spectrum...

Thus, the enhancements in chlorine removal from W diads compared to EV diads and from m-W diads compared to r-W diads observed in the (n-Bu)3SnH reduction of DCP, TCH, and PVC are consistent with the free-radical chain reaction mechanism. Inductive effects produced by neighboring 7-Cl s tend to favor the reduction of W diads relative to EV diads and steric interactions resulting from different preferred conformations in each isomer favor the removal of Cl from m-W diads relative to r-W diads. 

These results are quite compatible with the preferred conformations of the two-chain carbonyl diacids in aqueous media mentioned above (Porter et al, 1986b, 1988). The meso-compounds preferred collinear conformation, which places the hydrogens at the asymmetric carbons in a nearly eclipsed position relative to each other (Fig. 44), is more stable than that of the ( )-diastereomer by about 1.2kcalmol 1. In this conformation, the two carboxylic acid groups at the ends of the chain can be attached to the water surface side-by-side. The entire molecule can then behave as a good-amphiphile whose structure is similar to a pair of single-chain fatty acid molecules bound side-by-side, each chain mirroring the other about the molecules plane of symmetry. 

Many conformations were sampled by the usual MC procedure. The result is of course that there is no preferred orientation of the molecule. Each conformation can, however, be characterised by an instantaneous main axis this is the average direction of the chain. Then this axis is defined as a director . This director is used to subsequently determine the orientational order parameter along the chain. The order is obviously low at the chain ends, and relatively high in the middle of the chain. It was found that the order profile going from the centre of the molecules towards the tails fell off very similarly to corresponding chains (with half the chain length) in the bilayer membrane. As an example, we reproduce here the results for saturated acyl chains, in Figure 10. The conclusion is that the order of the chains found for acyl tails in the bilayer is dominated by intramolecular interactions. The intermolecular interactions due to the presence of other chains that are densely packed around such a chain,... 

The influence of chain length and side-chain modifications of ACTH-derived peptides on active avoidance behaviour in rats will be discussed. H-Met(02)-Glu-His--Phe-D-Lys-Phe-OH (Org 2766) emerged from these studies as an orally active peptide with an increased potency and selectivity of action. Physico-chemical data (from the literature) on the reference peptide ACTH--(4-10) did not point to a preferred conformation in solution, whereas in the crystalline state an antiparallel 3-pleated sheet structure was found. At the receptor site we suggested an a-helical conformation in which the Phe and Met residues are close together. Additional support for this suggestion came from the behavioural activity of [des-Tyr", Met ]enkephalin and of cyclo--(-Phe-Met-cAhx-), eAhx merely serving as a spacer. 

For higher modes, the ratio xjxt becomes sensitive to the correlations. As p increases, tp/t, decreases, as shown by Eq. (38). For illustration, this ratio is plotted semilogarithmically in Figure 2 as a function of pjN for a chain with 104 beads and for P = 0, 0.2,0.5, and 0.9. It is seen that in this one-dimensional model the relaxation spectrum is broadened as the energetic preference for extended conformations (P > 0) is increased. In particular, the longest and shortest relaxation times are related by... 

In most cases, open-chain allylsilanes react with electrophiles with anti stereoselectivity156-162. The simple explanation for this observation follows from the probable conformation of the allylsilane. The preferred conformation 129 will have the small substituent H eclipsing the double bond. 

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