Side-chain dipole orientation

Wada (109) also reported that nb of PBLG decreased as the solvent became less polar and could be extrapolated to a value of about 2.3 to 2.5 D at the limit of vacuum solvent for which the dielectric constant is unity. On a theoretical estimate (109) that the dipole moment of the polypeptide backbone would be 4 to 5D, he interpreted this limiting value of fih in terms of some regular orientations of side-chains occurring in such a way that the side-chain dipoles partially cancel out the backbone dipoles. 

This can be explained by the fact that in a polymer molecule (Fig. 78b) the longitudinal components of monomer unit dipoles mh are mutually compensated and the main part in the observed EB is played by normal components of monomer unit dipoles, mi, which can be parallel to the main chain of the macromolecule owing to its comb-like structure. In other words, in molecules of comb-like polymers containing mesogenic side chains, the orientations of the Mi components of the side group dipoles are correlated with each other. As a result, the macromolecule as a whole or part of it can exhibit a considerable dipole moment m in the direction of the main chain L (Fig, 78b). The existence of this dipole accounts for the orientation of the main chain in the field direction leading to negative EB. 

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

Polypeptides are electrically polar, carrying permanent dipoles at the planar CO-NH groups of the backbone chain and generally at some atomic groups of the side-chains. Because of the vector nature of dipoles, we must speak of the mean-square dipole moment, averaged over all possible conformations of the backbone chain and all accessible orientations of the side-chains when the dipolar nature of a polypeptide in solution is considered. The of a polypeptide thus may depend on what conformation the molecule assumes in a given solvent. 

Figure 17 Orientation of the side chains of the left- and right-handed a-helices of poly( -chlorobenzyl-L-aspartate). The solid arrows represent the direction of the C—Cl, ester, and amide dipoles, respectively.10...

However this may be, it does not help to explain the course of the alkali-catalysed reaction of the lya-hydroxy isomer (2). The alkali-catalysed reactions are formulated [204] as rearrangements of the i7 alkoxy anions, and the stereochemistry found in the products corresponds to reactions in the side chain conformation with the C(i7) and C(20) oxygen functions ri s-orientated by the mutual interactions of the anionic charge and the electric dipole of the carbonyl function. Both isomers react by migration of C i3), leading to the lya-epimeric-17-keto derivatives. In the ly/if-hydroxy isomer (i), this mode of reaction is the one favoured by a conformational preference for the chair-like transition state (E), but as Wendler has emphasised, the lycc-hydroxy-ao-ketone (2) appears to react through a transition state (G) of boat-like structure. Indeed,... 

PIG. 4.10 Schematic drawing of the hairy-rod polyglutamate douWe-tayer structure. P indicates the structure s periodicity. The trans isomer can be represented by a highly anlsometric, i.e., nondegenerate, transition dipole oriented along the alkyl side chains. 

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