Chain conformation energy calculations

According to conformational energy calculations, the POx chain can assume conformations other than planar zigzag, such as T3G and T2G2, and, according X-ray and IR studies, these two conformations are stable. The POx assuming them is thicker than the planar zigzag one, so that it may be included in / -CD as well as a-CD. 

A RIS model with neighbor dependence is used to calculate mean-square dipole moments and their temperature coefficients for PDMS chains over a wide range of molecular weight. Chain conformational energies required in the calculations are obtained from a previous analysis of the random-coil dimensions of PDMS chains in the limit of large x (S 116). 

Dielectric constants are determined for pure liquid dimethylsiloxane oligomers. Mean-square dipole moments, calculated from the Onsager equation, are in good agreement with predicted values based on the RIS model (S 117) with neighbor dependence and chain conformational energies obtained in an independent analysis of the random-coil dimensions of such chains. In addition, the observed temperature coefficients of are in qualitative agreement with calculated results. 

Conformational energy calculations are coupled with dipole moment measurements to derive a conformational description of P2VP. When an RIS model is used to calculate the dipole moments of P2VP chains with different stereosequences, it is found that the calculated dipole moments are nearly independent of the P2VP stereosequence. 

Conformational energy calculations are carried out for monomeric and trimeric oligomers of PMMA and for four-bond segments embedded in stereoregular PMMA chains. All incident interactions are taken into account. 

The helical parameters corresponding to the various skeletal conformations of the blsphenol A polycarbonate chain are calculated. Combining these results with the conformational energy calculations shows that flat-helical and extended conformations are of equal energy for this chain. In addition, cyclic structures are also found to be stereochemically possible. The small values of the characteristic ratio of the unperturbed end-to-end distance and its temperature coefficient are attributed to the equal energy of the flat-helical and extended-helical, as well as the nonhelical, conformers. 

Conformational energy calculations and estimation of the static chain stiffness are carried out for a number of polyfalkyl methacrylates), The intramolecular energies of the diad model compounds of MMA, EMA, and /-PMA are calculated. 

The conformational energy calculated for a frans-L-prolyl residue when Isolated from other prolyl residues in a polypeptide chain is characterized by two minima of comparable energy occurring near vj/ - 125° (A) and ly = 325° (C), where v is the angle of rotation about the C —C bond the angle of rotation about the N —C bond is fixed at approximatly 122° by the proline ring. 

A mathematical method is developed to provide a solution to two problems hitherto arising in conformational energy calculations of oligomers and polymers, when bond length and bond angles are maintained fixed. The two problems are the calculation of the sets of dihedral angles which lead to (a) exact ring closure in cyclic molecules and Ibl local conformational deformations of linear or cyclic molecules. Most of the emphasis is placed on polypeptide chain molecules. 

Yokouchi et al (2) used conformational energy calculations, revealing six low-energy conformations. Each of these was placed in the unit cell with the chain axis coincident with the c-axis and rotated about this axis to minimise the R-factor. One of them gave a much lower R-factor than the others, and this was chosen as the starting model. 

Besides this statistical mechanical approach to the question of helix stability, the problem has also been addressed by conformational energy calculations. First, the helix-breaking tendencies of such residues as serine and aspartic acid can be accounted for by the tendency toward formation of side chain-backbone hydrogen bonds in nonhelical conformations163 (Figures 20 and 21). Second, the free energies of the helical and statistical coil forms in water have... 

Solution and Solid-State Structures of PDBS and PDFS. A well-developed picture of the structure of PDHS has been obtained from the experimental work just discussed. A less complete picture is available for the structures of other polysilylenes. The structure of the dipentyl polymer, PDFS, was determined by Miller et al. (11) by using Raman and X-ray techniques. PDFS does not have an sl -trans conformation, unlike phase I of PDHS it is a 7/3 helix see previous discussion of conformational energy calculations and Figure 7). The same chain conformation has been found by Schilling et al. (10) for the dibutyl polymer, PDBS. 

An empirical conformation energy calculation based on the ECEPP functions indicated two stable side-chain orientations (A and B) are possible for a-helical main chain. A theoretical CD computation, however, suggested that orientation B = 290°, X2 285°) is more populated than A. 

Excimer Emission and CPF Spectra Fluorescence spectra of the two polypeptides in TMP solution are shown in Figure 7 (lower curves). Small excimer emissions are observed in the two polymers. The monomer/excimer intensity ratio was independent of the polymer concentration at least down to [pyr] = 1x10 mol L, suggesting an intramolecular character of the excimer. Since the interchromo-phore distances in the most probable conformations predicted from the conformational energy calculation are much longer than the exci-mer-forming distance, the excimers should be formed at the point where conformations of the main chain and/or the side chain are largely distorted. 

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