Conformational versatility

Compared to the parent a-amino acids, /9-amino acids are characterized by a much greater chemical diversity (five substitution positions versus three for a-ami-no acids) and conformational versatility. With the extra carbon atom in the backbone, the number of possible configurational isomers for / -amino acids increases dramatically (eight possible configurations versus two for a-amino acids) (Fig. 2.5). 

In the fiber structure, molecular chains in the crystalline region are highly oriented parallel to the fiber axis and a majority of molecular chains in the noncrystalline region are also highly oriented, so that both the conformational versatility and the mobility of molecular chains are much limited. But even in highly drawn fibers a significant, albeit minor, amount of amorphous material with a liquidlike mobility still exists. 

Such increases of y- and -relaxation temperatures with increasing draw ratio are thought to be due to the limited mobility of molecular chains in the noncrystalline region. As pointed out in the foregoing section, the amorphous molecular chains align fairly well parallel to the drawing direction. Therefore, the conformational versatility and mobility of such molecular chains are considered to be much restricted. 

Poly(mono-cyclohexylitaconate) (PMCHI) (see Scheme 2.16) is a polymer that present several interesting characteristis. By one hand is a polymer containing a cyclohexyl group what, as it was discussed in previous sections, is a chemical structure that provide several relaxational responses due to the conformational versatility of the saturated cyclic side chain. 

The results in Table VII depict how the band structure of carbyne [-CaC] (corresponding to Y - in Table VII) is affected by periodic inclusion of selected atoms or groups which, by introducing a kink into the otherwise rectilinear chain of cart e, would provide greater conformational versatility and therefore possibly improve processability. The chains were considered in their trans planar zig-zag conformation. It is seen that any of the modifica-tions to carb e indicated in Table VII produces an increased E value and a decreased BW value. That electrical conductivity would be adversely affected is reasonable given the disruption of the conjugated system caused 1 such modifications. As yet, no clear relationship is apparent between the specific molecular nature of Y and the effect of its inclusion on E and BW the relatively minor effect for the case Y - CH2 is notewSrthy and under continued study. 

Studies with other poly(/3-i -aspartate)s demonstrate that these polymers not only adopt conformational patterns that are similar to poly(a-amino acids), but that they exhibit greater conformational versatility. The range of conformations now include extended chain structures, arranged as antiparallel packings that come about by stretching poly(a-methyl-/3-L-aspartate) films in boiling water.209 In solution, the helix—coil conformational transition is a phenomenon common to the whole family of poly-(a-alkyl-/3-L-aspartates).210 The ordered conformation is responsive to environmental factors such as temperature and solvent in much the same way as for poly(a-peptides). 

Y. R. Huang, S. Knippenberg, B. Hajgato, J.-P. Francois, J. K. Deng, and M. S. Deleuze, Imaging momentum orbital densities of conformationally versatile molecules A benchmark theoretical study of the molecular and electronic structures of dimethoxymethane, J. Phys. Chem. A, 111 (2007) 5879-5897. 

These findings suggested homooligo-alanine to be of high conformational versatility. 

The variable co-ordination number of phosphorus leads to considerable conformational versatility. This, and the possibility of 7r-bonding, make the study of phosphorus compounds much more interesting than the frugal amount of data available in this section suggests. Phosphorus molecules could dearly be a very fruitful and rewarding area of structural research. 

Consequently, a higldy ordered chain structure is evolved with the concomitant loss of the conformational versatility that characterizes the disordered chain and the liquid state. For example, the trans state represents the bond orientation with the lowest energy in polyethylene. When successive bonds in the chain assume this orientation, a fully extended planar zigzag conformation results, as is illustrated in Fig. 1.2. From the multitudinous number of conformations available to the chain in the liquid state only one ordered structure survives that is characteristic of the crystal. An extended planar, or nearly planar, ordered conformation is characteristic of many polymers including polyamides, polyesters, cellulose derivatives, polydienes, and one of the low energy forms of the polypeptides. 

In analogy to monomers, the position of the ring substituent dramatically influences the location of T. For example, Tm of poly(tetramethylene isophthalate) is about 80 °C less than the corresponding polyester based on terephthahc acid. This decrease in Tm is accompanied by a 50% increase in A7/u. The difference in melting points is due to an increase in AS and reflects the difference in shape and conformational versatility of the two isomers. This difference in melting temperature is also found in other aromatic poly(esters).(73) Poly(decamethylene terephthalate) melts 100 °C higher than poly(decamethylene isophthalate) the difference between poly(ethylene terephthalate) and poly(ethylene isophthalate) is about 130 °C. 

Design and synthesis of conformationally fixed amino acids of heterocyclic series as versatile building blocks 97T12789. 

The United States refining industry is undergoing a restructuring phase. Refiners will continue to be under pressure and only the most efficient and profitable operations are going to survive. The survivors will be those who have some niche in the market place, have the versatility to handle low-cost crude, meet product demand, and conform to environmental regulations. 

Organic chemists who read this book and do the problems as they occur in the text will be rewarded with a functional understanding of NMR spectroscopy at a level that will allow them to make full use of this most versatile spectroscopic method for investigating the structures, stereochemistries, and conformations of organic molecules. 

A different synthetic access to a 1 -metallacyclopropene, which can be a versatile organometallic synthon, is displayed in Scheme 33. The mono-alkyne derivatives of W(IV)-calix[4]arene are easily accessible through the thermal displacement of cyclohexene from 32 using the appropriate acetylenes. The reaction led to complexes 34 and 172-174. The proposed 3-metallacyclopropene has been confirmed from the spectroscopic and the X-ray data. The H NMR data reveal a cone conformation of the calixarene with a four-fold symmetry, for which the... 

Calixarenes, when in their cone-conformation (54), represent versatile host systems for metalated container molecules and many examples have been reported in the literature (55-61). Reinaud and coworkers have carried out extensive work concerned with calix[6]arenes that are functionalized at the small rim by nitrogen arms (62), aiming to reproduce the hydro-phobic binding site of mononuclear zinc and copper metalloen-zymes. A recent example is the calix[6]arene ligand L1 (Fig. 3), in which a tris(2-methylpyridyl)amine unit covalently caps the calixarene small rim (63). The ligand forms copper complexes of... 

Fig. 9.3. Several mechanisms underlie the functional versatility of the ER. The different distributions of the alpha and beta isotypes of ER conditions a first step that warrants distinct functional profiles depending on the higher or lower affinity of the ligand for one or another isotype (upper panel). Then, differentligands generate distinct 3D conformations in the ligand-receptor complex that condition different interaction profiles with the promoters of target genes (lower panel)...

Gotte, G., Bertoldi, M., and Libonati, M. (1999). Structural versatility of bovine ribonu-clease A. Distinct conformers of trimeric and tetrameric aggregates of the enzyme. Eur.J. Biochem. 265, 680-687. 

Many polymer conformations can be described in terms of atoms regularly spaced along helices. Methods for analysing the diffraction patterns from helical structures provide a highly versatile technique for the determination of the repeat unit in such polymers. 

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