Theoretical conformation, poly

The characteristic ratios of poly(pro-gly), poly(hyp-gly), poly(gly-gly-pro-gly), poly(gly-gly-hyp-gly), and poly(pro-ale) are determined in water, The results confirm the main features of the theoretical conformational maps derived by Rory and coworkers for glycine followed by either l-proline or a nonproline residue. Small adjustments, well within the uncertainty described by Schimme and Rory, are suggested In the conformational map for L-proline followed by glycine. The constants for the Lennard-Jones functions of Scheraga and coworkers, as used by Madison and Scheliman, produce a conformational map for L-proline followed by a nonproline residue which is in somewhat poorer agreement with experiment. The two sets of modified constants introduced by Madison and Scheliman fall to predict the conformational properties of these sequential copolypeptides. 

The validity of this dynamic model can be justified by the fact that in polystyrene and other carbon-chain polymers of the type (—CH2-CHR—)n(poly(p-chlorostyrene), poly(methyl acrylate) or poly(vinyl acetate)), containing no methyl groups bonded directly to the main chain, rotational isomerization with U 5—6 Kcal/mol (in the time interval of t 10 — 10 s in solvents with tj 0.01 P) can occur l This conclusion is also confirmed by theoretical conformational calculations of these polymers. 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

Poly(aminoborane) has a relatively simple structure, as shown in Figure 9. Jacquemin and co-workers [63] performed a comprehensive set of calculations to investigate the infrared spectra, vertical excitation energies, geometries, atomic charges and dipole moments as a function of conformation. A variety of theoretical methods were employed, including Hartree-Fock, MP2 and... 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

Figure 6 shows how the transition of poly(e-carbobenzoxy L-lysine) (PCBL) in m-cresol is affected by the chain length of the sample (23). The trend displayed here is general and conforms to the theoretical prediction deduced from Fig. 1 the transition curve becomes sharper as chain length increases. Comparison with Fig. 5 indicates that in the same solvent, m-cresol, the direction of transition of PCBL is opposite to that of PBLA and the two transitions differ markedly in sharpness. This again illustrates the crucial importance of polymer-solvent interactions in the transitions of polypeptides. 

Birshtein and Luisi (17) made attempts to prove the theoretical validity of the above model by applying the theory of VOLKENSTEIN (137, 138, 139), Birshtein and Ptitsyn (16) on the conformation of isotactic macromolecules, to optically active poly-a-olefins. From their results it is possible to conclude that the presence of asymmetric carbon atoms in the lateral chains might cause the prevalence of main chain sections spiraled in one of the two possible screw senses, mainly for entropic reasons. 

Another class of soluble polysilylenes exhibits essentially no or very weak thermochromism. This class includes poly(cyclohexylmethyl- 15, 38), poly(phenylmethyl- 15, 38), (polytrimethylsilylmethyl- 15), and poly(diarylsilylenes) 46), all of which appear to be conformationally locked over a wide range of temperatures. In terms of our theoretical perspective, this behavior would arise from the steric effects of bulky substituents, which imply a large value of e and, hence, a small coupling constant Vj /e. For aryl-substituted polysilylenes, the proximity of an aromatic group to the backbone could also stabilize a highly ordered rodlike conformation via enhanced dispersion interactions. 

The local-density functional approach was used to compare the band structures of the sW-trans conformation of unsubstituted polysilane with a 4/1 helical conformation and with an dll-trans conformation of dimethyl-substituted poly silane. In line with previous theoretical studies, the electronic wave functions in the vicinity of the Fermi level are primarily silicon-back-bone states, with the major effect of methyl substitution being a decrease in the gap. The predicted absorption spectra for the dll-trans conformations of unsubstituted and dimethyl-substituted polysilane are similar for nearthreshold absorption. Given this similarity, we believe that the shift in energy and strong anisotropy of threshold absorption that we predict for the two extremes of the dll-trans conformation and the dll-gauche model will also occur in alkyl-substituted systems, which are currently under investigation. 

Incidental to this work is more evidence that the a-helix exists at the air-water interface. While some have appeared reluctant to accept this view, no good theoretical reason exists why it should not be stable. Where the nature of the side chain might provoke other conformations [as in poly(l-valine)] or the molecular weight is low or monolayers are spread from poor solvents miscible with water, other conformations are detectable, and the monolayer properties are significantly different (5). 

Molecular dynamics calculations have been carried out to simulate a conformational model and vibrational spectra of poly(dichlorophospha-zenes). Molecular dynamics similations for [NP(OCH2CF3)2] and the isomers [NP(OBu")2] , [NP(OBu )2] and [NP(OBu )2] show a reasonable agreement between the calculated and experimental values of density and glass transition temperature as well as for gas transport parameters in these polymers.Small molecule models have been used for a theoretical approach of poly(thionylphosphazenes). ... 

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