Nonhelical conformations

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. 

A zinc(n) meso-meso linked porphyrin oligomer 57 exists in a nonhelical conformation in solution, but may adopt a dynamic helical conformation upon complexation with an achiral urea 58 through complementary hydrogen bonding interactions [115]. In the presence of the chiral diamine (S)-59, the 57-58 complex forms a predominantly one-handed helical conformation, thus showing a characteristic ICD in the absorption region of the porphyrin chromophore. This system may be used to sense the chirality of chiral diamines. 

N-3-H of uracil is hydrogen-bonded to N-7 of adenine. The Hoogsteen A—U pairs and the 9-methylacridine molecules form an infinite column of alternate, stacked rings. The adenine and uracil bases are in the anti disposition, and the conformation of the D-ribosyl group is C-2 endo in the adenosine moiety and C-3 endo in the uridine moiety. The orientation around both phosphoric diester P-O bonds is g+, resulting in a nonhelical conformation. 

The monomeric units of a macromolecule may lie in helical (h) or nonhelical (c) conformations. Such a polymer chain of helical and nonhelical sequences may, for example, have a time-average sequence of. . . hhchhhhhhcccchhccccchhhh. . . The proportions of helical and nonhelical conformations, here, will change with solvent, pressure, and temperature. These changes may be monitored by group-specific methods (ir, uv, nmr, ORD, CD) or by molecule specific methods (radii of gyration, viscosities). 

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... 

After the procollagen polypeptides are assembled into a triple helix, they are secreted by the classical route. They pass through the smooth endoplasmic reticulum and the Golgi complex, where they are packaged into membranous vesicles and secreted into the extracellular space by exo-cytosis. This process requires ATP and may involve microtubules and microfilaments. The conformation of procollagen markedly affects the rate of secretion. Prevention of the formation of a triple helix (e.g., lack of 4-hydroxylation due to vitamin C deficiency) leads to the accumulation of nonhelical propolypeptides within the cistemae of the rough endoplasmic reticulum and a delayed rate in its secretion. 

Figures 28-2 and 28-3 are schematic diagrams of the secondary structures of a and p chains, respectively. Each chain contains helical and nonhelical segments and surrounds a heme group. Almost 80% of the amino acids exist in an a-helical conformation.

The conformation of the helical polypeptides changes from helical to nonhelical with an increase in temperature, and the glycopolypeptides and polypeptides show similar thermal transition behavior (not shown) (23). CD spectra for all other random coil and a-helical glycopolypeptides were recorded and found to be consistent with our previously reported data (23, 24) with some variations in absolute intensities, regardless of the identity of the saccharides used in the modification (Gal or Cap). These observations confirmed that the glycosylation of polypeptides with different saccharides does not significantly alter the secondary structure of the polypeptides. 

Nonionic block copolypeptides made of PEGylated L-lysine and L-leucine residues, PELLys- -PLLeu (Fig. lOh) have also been described [52], The copolymers adopted a rod-like conformation, due to the strong tendency of both segments to form a-helices (CD spectroscopy), and produced a variety of self-assembled structures in aqueous solution. Micrometer vesicles and sheet-like membranes could be obtained for copolymers with fractions of the hydrophobic leucine ranging from 10 to 30mol%. Conventional uncharged block copolymers of this composition would be expected to form spherical or cylindrical micelles. The assembly into bilayers was related to a secondary structure effect, as illustrated in Fig. 12. Accordingly, samples with the same composition but nonhelical chain conformation (CD),... 

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