Torsional freedom

Considering first Table 5, it can be seen that Tg values for the reported poly(organophosphazenes) spanned from very low (-105 °C in the case of poly[bis(n-butoxy)phosphazene]) up to very high (-1-220 °C for poly[tris(2,2 -dioxy-l,T-binaphthyl)phosphazene]), covering almost all the intermediate temperatures between these two limits. Low TgS are indicative of very high torsional freedom of the polyphosphazene chain, which is manifested clearly when flexible substituents of reduced bulkiness are used in the substitution... 

Fig. 3 Important 19F-labelled amino acids, (a) Compounds that are wo-steric to native amino acids can be incorporated into proteins biosynthetically, but they possess too many degrees of torsional freedom to be useful for ssNMR structure analysis, (b) In these artificial amino acids the 19F-reporter group is rigidly attached to the peptide backbone. They can be incorporated by solid-phase peptide synthesis, but some problems can arise due to racemisation (4F-Phg, 4CF3-Phg), steric hindrance of coupling (F3-Aib) or HF elimination (fluoro-Ala, F3-Ala). 4F-Phg is additionally problematic due to an ambiguity of the side-chain rotamer. The preferred 19F-labels for ssNMR structure analysis are CF3-Bpg and CF3-Phg (as suitable substitutes for Leu, lie, Met, Val and Ala), as well as F3-Aib and CF3-MePro...

The absence of Bohlmann bands in the infrared (IR) spectrum indicates that the nitrogen lone pairs of the di-iV-nitroso-l,4-diazepines 23 are delocalized <1995JOC7461>. While the absence of an amide moiety in the ring confers torsional freedom, the m-2,7-di-Ph groups reduce ring flexibility and the 111 NMR data are consistent with major and minor families of four twist chair conformers that project the C-7 Ph and C-6 R -substituents equatorially These interconvert by a pseudorotation that allows the C-2 phenyl and C-3 R substituents to adopt a pseudoaxial orientation in the preferred conformer family. 

In order to obtain molecular systems in which the internal motion is easier to study, it is customary to introduce halogen atoms in the molecules because of the enhanced scattering power of these atoms. On the other hand, the larger halogen atoms restrict the internal motion more than is the case in unsubstituted molecules. Halogen substitution thus leads to systems with less torsional freedom than the parent hydrocarbons. 

Global searching is more difficult for carbohydrates than for peptides, nucleosides and nucleotides. The C-OH bond has torsional freedom and therefore every C-OH- -O hydrogen bond included in the model introduces an additional parameter and increases the number of local minima in the potential energy hypersurface. This is not so for )NH- -0=C and )NH - N bonds, where there is no torsional freedom of the N-H bond and the hydrogen atomic position can be derived with reasonable precision from those of the nonhydrogen positions. 

Figure 2 Properties in polyphosphazenes are determined hy (1) the backbone bonds that control the inherent flexibility of the polymer via their influence on bond torsional freedom, and also provide photo-and thermo-oxidative stahihty (2) the side groups control polymer solubility, reactivity, thermal stability, crystallinity, cross-linking, and (indirectly) polymer flexibility (3) free volume between the side groups affects polymer motion, solvent penetration, membrane behavior, and density (4) functional groups (usually introduced hy secondary reactions) affect soluhihty, biological behavior, proton conduction, cross-hnking, and many other properties...

Four compounds 5-XXII-5-XXV with determined, but unpublished structures were chosen for testing with the following restrictions Z = 1 the space groups were not specified, but were limited to one of the more common ones, noted earlier. The molecules are essentially small and rigid, although two contain less common elements and 5-XXIV has some torsional freedom. 

This study is the first step towards a quantitative prediction of chain flexibility based on conformational analysis. Torsional relaxation of the adjacent bonds is very important. The present approach differentiates between the polymer relaxations in the glassy state and in the melt state. It provides insight into the crystallinity of a system and succeeds in explaining the isomorphic transformations of PDES. Chain flexibility is influenced by at least two types of factors the number of isomeric states available and the torsional freedom in a given state, which is determined by the shape of the potential well. 

Table 3.1 Degrees of Torsional Freedom to Specify ACE Active Site Geometry...

Experiments with glyceraldehyde-3-phosphate dehydrogenases from M. fervidus and P. woesei as well as with different mutant enzymes [29] indicate that the peptide chain is largely protected from covalent modification in the native protein conformation. It would appear that the native conformation protects the weak links in the chain against attacking water molecules by steric hindrance and by restricting the torsional freedom of the chain. 

The build-up approach has also been used to predict DNA structures. The number of degrees of torsional freedom is much greater for the nucleic acid subunits than for amino acids and currently requires access to supercomputer resources in order to surmount the vast combinatorial explosion. The building blocks used in the reported work are the 16 deoxydinucleoside monophosphates (Figure 6), which were combined to give a variety of molecules, the largest of which was a d(CG)6 d(CG)e dodecamer. In this case the lowest energy structures were the B and Z forms. 

Example The presence of the Zn-atom vis-a-vis the careful introduction of relatively more degrees of torsional freedom so as to contain effectively the most probable strategical positioning of the Zn-atom relative to ACE inhibitors viz., captopril, as illustrated in Figure 3.10. ... 

One of the unusual properties of (PNF2) is its low-tanperature elasticity, which reflects a relatively high degree of torsional mobility of the chain. Of the three polymeric phosphazene halides, the fluoride has the lowest glass transition temperature, which is in accord with it having least interchain interaction and highest torsional freedom at low tanperatures (Table 12.37). 

The P-N bond distance in polyphosphazenes (av. 1.59 A) is longer than C-C bond distances (1.54 A) found in organic polymers. The longer bond distance would allow a greater torsional freedom around it which would make the polymers flexible. 

The parent polyphosphazenes [NPF2]n and [NPCyn, have extremely low glass transition temperatures of -95 °C and -66 °C respectively. This data suggests that these polymers have a lot of torsional freedom or skeletal flexibility. Only below the glass transition temperatures all the torsional motion of the polymers is arrested. 

The most well-studied polysiloxane viz., [Mc2SiO]n has a Tg of -123 °C [1]. This is one of the lowest values for any polymer. As mentioned earlier in Chapter 2 the glass-transition temperature of a polymer corresponds to a description of its amorphous state. The Tg of a polymer can be taken as a measure of the torsional freedom of polymer chain segments. Above its Tg a polymer has reorientational freedom of motion of its chain segments, while below its Tg this is frozen. Usually elastomeric materials have low glass-transition temperatures. For example, natural rubber has a glass-transition temperature of -72 °C. Similarly, polyisobutylene has a ass-transition temperature of -70 °C. Thus, poly(dimethylsiloxane) has consid-... 

---