Zigzag-helix conformation

Crystal structure of P(3HB) (a) and two types of molecular conformations (b) the 2/1 helix conformation (a-form) and the planar zigzag conformation (p-form)... 

Figure 15(b) shows the wide-angle X-ray diffraction (WAXD) profiles of as-spun P(3HB) nanofiber mats spun from 1 wt.% solution. In the profile before enzymatic degradation, diffraction peaks assigned to both a-form crystal with 2/1 helix conformation and p-form with planar zigzag conformation were observed at 2 = 19.6°. 

Tanaka et al. and Iwata et performed enzymatic degradation of highly oriented P(3HB) monofilaments, in which both the 2/1 helix conformation (a-form) and planar zigzag conformation (p-form) existed. They demonstrated that the enzymatic erosion rate of the p-form is faster than the a-form for P(3HB) fibers. This result can also be explained in terms of conformational matching of the polymer chain with the active site of the enzyme. 

The X-ray diffraction of a bundle of ten pieces of P(3HB) fibers (beam size 300 pm) as shown in Fig. 2c includes reflections simultaneously from both the a-structure (2/1 helix conformation Okamura and Marchessault 1967 Yokouchi et al. 1973) and the p-structure (planar zigzag conformation Orts et al. 1990) of P(3HB) that are drawn in Fig. 3. It is well known that P(3HB) crystallizes as an orthorhombic crystal system with unit cell parameters of a = 0.576 nm, ft = 1.320 nm, and c(fiber axis) = 0.598 nm and space group (a-structure ... 

PHB can take up the helix conformation (called the a-form) or a planar zigzag conformation (called the p-form). The a-conformation results in lamellar crystals while the less common p-form is usually formed by extending the uniaxially oriented films of the PHB further, and occupies the amorphous regions between the lamellar crystals of the a-form. ... 

Fig. 6.14 Pseudo phase diagram of syndiotactic poly(styrene) in trans decalin. O crystallization of helix conformation crystallization of zigzag conformation melting of helix conformation melting of zigzag conformation. (From Deberdt and Berghmans (292))...

The DNA helix has major and minor grooves Z-DNA forms a zigzag pattern B-DNA is the preferred conformation in vivo Specific base sequences can be recognized in B-DNA Conclusion Selected readings... 

A better knowledge of force constants, the use of more detailed physical models, and the availability of large computers and of new methods of calculation has permitted the prediction, within close approximation, of the frequencies and, to a lesser degree, the intensities of absorptions of stereoregular polymers in ordered conformations (helix, zigzag, etc.). In this way data on molec-... 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

Z-form DNA is a more radical departure from the B structure the most obvious distinction is the left-handed helical rotation. There are 12 base pairs per helical turn, and the structure appears more slender and elongated. The DNA backbone takes on a zigzag appearance. Certain nucleotide sequences fold into left-handed Z helices much more readily than others. Prominent examples are sequences in which pyrimidines alternate with purines, especially alternating C and G or 5-methyl-C and G residues. To form the left-handed helix in Z-DNA, the purine residues flip to the syn conformation, alternating with pyrimidines in the anti conformation. The major groove is barely apparent in Z-DNA, and the minor groove is narrow and deep. 

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