3i Helices

Fig. 5. Comparison of ab initio, DFT/BPW91/6-31G -computed IR and VCD spectra over the amide I, II, and III regions for model peptides (of the generic sequence Ac-Alaw-NHCH3). These are designed to reproduce the major structural features of an o -helix (top left, n— 6, in which the center residue is fully H-bonded), a 3i helix (PLP Il-like, top right, n— 4), and an antiparallel /1-sheet (n= 2, 3 strands, central residue fully H-bonded) in planar (bottom left) and twisted (bottom right) conformations. The computations also encompass all the other vibrations in these molecules, but those from the CH3 side chains were shifted by H/D exchange (CH3) to reduce interference with the amide modes.

The case of isotactic polypropylene is somewhat different issues related to the crystallization of the chiral /1-form [42,43] of this polymer have been discussed by various authors and probably the model presented by Lotz is the most articulated and detailed [14]. It is known that iPP, unlike sPP, adopts in all its crystalline modifications its stable conformation, namely the 3i helix with a 6.50 A periodicity [36], which may be right-handed or... 

Fig. 8 The changes occurring to [Ni(CH2NH2NH2CH2)3][N03]2 from ambient conditions left) to low temperature right). The molecular geometry remains almost unchanged, but the molecule moves away from a crystallographic threefold axis producing a 3i helix. The phase change occurs abruptly and at (Tj., the two structures are different and in equilibrium...

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. 

As an NMR methodology for elucidating conformational stability in the polypeptide blends, the conformation-dependent 13C NMR chemical shift for polypeptides in the solid state has been reported. It has been elucidated that the 13C NMR chemical shifts of a number of polypeptides in the solid state, as determined by the 13C CP/MAS method, are significantly displaced, depending on their particular conformations such as a-helix, 3i-helix or p-sheet form.11,17 31... 

The observed 13C CP/MAS NMR spectra of PG, PLV and the PG/PLV (20/80, 50/50 and 80/20 wt/wt%) blend samples as prepared by adding a TFA solution with a 2.0 wt/wt% amount of H2S04 to alkaline water (Method 5) are shown in Fig. 10. In the 13C CP/MAS NMR spectra, homopolypeptides of PG (3i-helix) and PLV ((5-sheet) are prepared using the same conditions as for PG/PLV blend samples. The assignments of these spectra were made by the above-mentioned method. The 13C chemical shift values of these blend samples are listed together with those for PG in the 3i-helix form (PG-II) and the p-sheet form (PG-I), and PLV in the p-sheet form in Table 7.21,22,26 The two intense peaks at 172.9 and 42.9 ppm, which appear in the spectrum of PG (Fig. 10 a) can be assigned to the C=0 and Ca carbons. From these 13C chemical shift values, it is found that the PG used in this work takes the 3 -helix form. There are no peaks which come from the p-sheet form. On the other hand, in the spectrum of PLV (Fig. 10 e), the four intense peaks appear at 172.0, 58.6, 32.8 and 19.0 ppm and can be assigned to the C=0, Ca, Cp and Cy carbons, respectively. From these 13C chemical shift values, it is found that PLV takes the p-sheet form. 

Another new peak of the Cot carbon of PG appears at about 44.3 ppm in addition to a small peak assigned to the 3 -helix form (42.6 ppm), and can be assigned to the p-sheet form (44.3 ppm). These results show that the 3i-helix form of PG in the PG/PLV blends is almost transformed to the p-sheet form. 

Homopolypeptides of PG (3i-helix) and PLV (p-sheet) are prepared using same condition as PG/PLV (20/80, 50/50, 80/20) blend samples. c Not determined because of the overlap of the minor peak with the major peak. 

For this isotactic polypropylene, different structures based on a 3i helix chain structure have been proposed. Natta and Corradini (16) reported that the space group is Cc or C2/c,... 

Figure 4. Folded and twisted conformations of (/i)3-hydroxybutanoate oligolides (left) containing one (heptamer, octamer) or two (hexamer) single turns of a right-handed 3i-helix, a model of which is shown on the right side in views from the side and along the helix axis. The helix is covered with methyl groups and has a dipole moment resulting from the unidirectional arrangement of the C=0 bonds parallel to the helix axis [4].

Nakano et al) reported a C CP/MAS NMR study on poly(L-Alanine) (PLA)/Polyglycine (PC) blends. The comparison between the C CP/MAS spectra of the blends and those of the parent polymers discloses that upon blending, new conformations of PLA and PC are fonned, which is closely related to the presence of intermolecular hydrogen-bonding interactions. Tip measurements demonstrate that the major parts of PC and PLA in the blends, which are in 3i-helix and a-helix conformation forms respectively, are phase separated. On the other hand, the p-sheet forms of PC and PLA, which are newly formed upon blending, exhibit similar values, demonstrating that these two parts are miscible at the scale of 3-4 nm. 

Isotactic polypropylene is semicrystalline. In the crystalline regions the molecules form a 3i helix, that is three monomer units form one turn of a helix. This helix performs rotational Jumps around its own axis. After one jump, each monomer occupies the place of its next neighbour before the jump. Molecular reorientation by discrete jumps... 

Problem 2.4 What is the repeat distance between pendant methyl groups that form a row (a) in the 3i helix of isotactic polypropylene and (b) in the 2i helix of syndiotactic polypropylene. Assume each carbon-carbon bond length is 1.54 Aand each bond angle is 109.5°. 

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