Helical conformation, origin

By contrast, in the CD spectrum of the p derivative (39), no bands were observed even at —80°C, as shown in Figure 4.19b. This is because the Cotton band of the p derivative is canceled by an oppositely signed CD band originating in an equivalent proportion of opposite screw sense helical segments (which may be referred to as masked helicity or masked circular dichro-ism ). The difference between the helical conformations of m and p derivatives may be due to the degree of steric hindrance of (S>2-methylbutoxyphenyl rings, since the p derivative may have enough space to freely rotate the... 

Incorporation of the (.S )-2-mcthyloctoxy group afforded optically active polymers with preferential helical screw sense (see Section 3.11.6.1). The observed helicity was corroborated by force field calculations, which indicated similar helical conformations for both dialkoxy- and dialkyl-substituted polymers. Based on their similar conformational properties, it was suggested that the origin of the spectral red shift was electronic, due to a a-n mixing interaction, as for polymers 76 above, rather than conformational. 

In an attempt to investigate the phase structure of this sample, the line shape analysis of the CH2 resonance line in the DD/MAS spectrum at 87 °C that is shown in Fig. 25 was examined. The result is shown in Fig. 26-(a). The elementary line shape of the crystalline phase was obtained as the line shape of the longest Tic component by Torchia s pulse sequence [53]. It was a doublet and was represented approximately by two down- and upheld Lorentzians with an intensity ration of 2 1 (Spectrum A shown by dotted line in Fig. 26). Since all methylene carbons in the a-crystalline form of this polymer are equivalent in the intramolecular helical conformation, the origin of the doublet could be attrib-... 

In order to show that HIT-1 was a-helix compatible and induces a-helical conformations in short peptides from the N- and C-terminus as well as from internal positions, template-linked peptides have been compared to nonconstrained reference peptides. In the N- and C-cap projects a hydrophobic 12-mer peptide was used as a standard reference. The 12-mer reference peptide 104 was originally developed for host guest experiments to evaluate the a-helix propensities of unnatural amino acids (see Section II.A) and had later been used to confirm N-cap induced helix formation.141,202,214 In the C-cap series and for position-independent templates the same reference peptide could be used after minor modification of the terminal protection groups. The suitably protected reference peptide and the corresponding template-linked model peptides used for the evaluation of N-terminal, C-terminal, and internal helix induction are depicted in Figure 39. 

The existence of helical conformations in polyolefin melts was originally suggested by Garth Wilkes et al in the early 1970s [35]. These authors observed... 

Table XI also compares the amino acid analyses of Merino 64 s wool with that of cuticle obtained by subjecting the wool to ultrasonic irradiation in 98 % formic acid (Bradbury and Chapman, 1964). The cuticle contained less arginine, aspartic acid, glutamic acid, leucine, and phenylalanine than whole wool, but more cystine, proline, serine, and valine. Cys-teic acid is probably formed during the ultrasonic treatment. These analyses cannot be accounted for quantitatively on the basis of a simple combination of low-sulfur and high-sulfur protein fractions, but the data suggest that the cuticle would not contain as much material in the a-helical conformation as the original fiber. The values obtained by Bradbury (1960) in an earlier examination of cuticle-rich material obtained by a mechanical descaling technique are in reasonable agreement with the values in Table XI.

Although originally defined for the conformation adopted by polymers of proline, the PPII helical conformation can be adopted by amino acid sequences other than those based on proline. This was noticed during several statistical surveys of structures in the Protein Data Base (PDB), which estimated that up to 10% of residues that are not assigned to a regular secondary structure have PPII dihedral angles but contain little proline (Sreerama and Woody, 1994). 

---