3i-Helical conformation

Fig. 8. Theoretical simulation of VCD (top) and IR absorption (bottom) spectra of alanine dodecapeptides for the amide V bands for a fully a-helical conformation (left) and a fully left-handed 3i-helical conformation (right). The simulations are for the same three isotopically labeled (13C on the amide C=0 for four Ala residues selected in sequence) peptides as in Figure 7 N-terminal tetrad (4AL1), middle (4AL2), and C-terminal (4AL4). The 13C feature is the same for all sequences, confirming the experimentally found unfolding of the C-terminus. The agreement with the shapes in Figure 7 is near quantitative. Reprinted from Silva, R. A. G. D., Kubelka, J., Decatur, S. M., Bour, R, and Keiderling, T. A. (2000a). Proc. Natl. Acad. Sci. USA 97, 8318-8323. 2000 National Academy of Science, U.S.A.

A certain isotactic vinyl polymer crystallises with the chains in a simple 3i helical conformation, with only one chain passing through each unit cell. At approximately what angle will the second layer line be observed in an X-ray fibre photograph obtained using radiation of wavelength 0.194 nm ... 

The team in Milan had to face two major scientific problems assign the structure of the new polymer and explain its genesis. The first part could be accomplished with a clever analysis of the X-ray diffraction patterns, recognized by Corradini [20 and references therein] as compatible with the 3i helical conformation anticipated by Bunn [21] for a hypothetical stereoregular hydrocarbon with the configuration that we now name isotactic after the suggestion by Natta s wife [11] (Fig. 3). 1 am not a believer in Lukacs reflection theory [22], but 1 find it a fascinating coincidence that Watson and Crick published their fundamental work on the a-helix of DNA at about the same time [23]. 

Figure 2.15 Model of packing of a form of iPP with chains in 3i helical conformation packed in a monoclinic unit cell according to the space group C2lc or P2ilc [29,101,102]. The heights of the carbon atoms of the methyl groups are indicated in c/12 units. R and L indicate right-handed and left-handed helices, respectively.

Isotactic polypropylene (iPP) exhibits a stable crystalline a form consisting of chains in the 3i helical conformation (tgtgtg), packed in a monoclinic unit cell. Left- and right-handed helices face each other in the a form. The metastable -form crystals contain hexagonally packed 3i helical chains arranged in groups of the same helical handedness (left or right). There is also a smectic, or partially ordered, form... 

The conformational analysis was developped recently by different authors first on the disaccharide units to investigate the role of the charge on C-6 position on the conformation [29-32]. From this study, the 2i or right-handed 3i helices were described as most problable conformations [30,31]. These conformations were also demonstrated from x-ray diffraction [33] or on the basis of circular dichroTsm [34], From these data, the repeat unit in the axial-axial conformation has a 4.3 A° length which will be used to characterize the electrostatic properties. 

From such a background, some kinds of polypeptide blend samples have been studied by solid state NMR.27,72 74 Especially, detailed information for four kinds of blend samples such as poly(L-alanine) (PLA)/poly(L-valine) (PLV), PLA/poly(L-isoleucine) (PLIL), poly(D-alanine) (PDA)/PLV and polyglycine (PG)/PLV blends, have been reported. Here, let us describe some reasons why PLA/PLV, PDA/PLV, PLA/PLIL and PG/PLV blends are interesting systems. PLA and PDA in the solid-state can take the a-helix and (3-sheet forms due to intra- and intermolecular HBs, respectively. PG in the solid-state can take the 3j-helix (PG-II) and (3-sheet (PG-I) forms due to intra- and intermolecular HBs, respectively. However, PLIL and PLV in the solid state can predominantly take the (3-sheet form as the stable conformation. For this reason, it is interesting to know whether an isolated a-helix or 3i-helix form polypeptide surrounded by a major polypeptide in the (3-sheet form can take the helical conformation, or not, due to the balance between intramolecular and intermolecular hydrogen bonds. In addition, we would like to know whether a polypeptide in the (3-sheet form surrounded by a major polypeptide in the a-helix or 3 -helix form can take the (3-sheet form. 

Figure 4 Right-handed 3i g-helical conformation formed by a doubie repetition of the -(A Phe-A Phe-L-Aia)- modei peptide views orthogonai to (a) and down (b) the heiix axis.

Poly(3-hydroxybutyrate) is a linear polyester with helical macromolecules. The secondary stracture of PHB is specified as left-hand 2j helix in a g g tt conformation, while the structure of oligolides consists of right-hand 3i helices [99]. The surface of the 3i(-l-) helix is covered by methyl groups, leading to the lipophilic nature of the macromolecule. The carbonyl bonds in the 2i(—) helix are placed perpendicularly, while in the 3i(-l-) counterpart, they are parallel to the helix axis. The latter is the reason for ability to form ionic complexes. 

Fig. 2. (a). A 3i helical structure simplified using mathematical expression, (b). Projection of 3i helical at right angle to the helical axis for iPP. The filled circle represents CH3. Therefore, four possible conformations exist with identical free energy A. Right-handed d helix B. Left-handed helical I helix C. Left-handed d helix and D. Right-handed I helix (6). 

Structural conformations that can develop under different processing or treatment conditions [42]. In particular, melt or cold crystallization and solution spinning processes at low drawing temperatures and/or low hot draw ratios induce the formation of a-structures characterized by a left-handed IO3 helix that packs in an orthorhombic unit cell with parameters a = 1.06 nm, b = 1.737 nm, and c = 2.88 nm [46]. Stretching, solution spinning, or high hot draw ratios induce the formation of 3-crystal structures, a unit cell with a = 1.031 nm, Z = 1.821 nm, and c = 0.900 nm, and a chain conformation with left-handed 3i helices [47]. More recently, an additional crystalline form named y that develops under epitaxial crystallization has been reported [48]. 

When polymers possess helical symmetry, this symmetry changes the types of vibrational modes that can be observed in the IR and Raman spectra in a specific manner that can be used to determine the chain conformation (Fig. 6.9). Thus, for the planar 2i and 3i helices, differences in selection rules for Raman and IR spectra allow a direct determination of the conformation. For helical conformations with pitches greater than that of a 3i hehx, the selection rules do not change but the frequencies shift. 

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