Poly helix

It is not the purpose of this book to discuss in detail the contributions of NMR spectroscopy to the determination of molecular structure. This is a specialized field in itself and a great deal has been written on the subject. In this section we shall consider only the application of NMR to the elucidation of stereoregularity in polymers. Numerous other applications of this powerful technique have also been made in polymer chemistry, including the study of positional and geometrical isomerism (Sec. 1.6), copolymers (Sec. 7.7), and helix-coil transitions (Sec. 1.11). We shall also make no attempt to compare the NMR spectra of various different polymers instead, we shall examine only the NMR spectra of different poly (methyl methacrylate) preparations to illustrate the capabilities of the method, using the first system that was investigated by this technique as the example. 

The intrinsic viscosity of poly(7-benzyl-L-glutamate) (Mq = 219) shows such a strong molecular weight dependence in dimethyl formamide that the polymer was suspected to exist as a helix which approximates a prolate ellipsoid of revolution in its hydrodynamic behaviorf ... 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Short segments of poly(dG—dC) incorporated within plasmids, or citcular DNA, adopt the Z-conformation under negative superhehcal stress. This left-handed DNA may be important in genetic control. On the other hand, the stmctural alteration of the helix requited in a B-to-Z transition within a plasmid is radical, and would involve either a multistep mechanism or the complete melting and reformation of helix. The improbability of such transitions has led to questions concerning the feasibility of a biological role for Z-DNA. 

Licjuid Crystals. Ferroelectric Hquid crystals have been appHed to LCD (Uquid crystal display) because of their quick response (239). Ferroelectric Hquid crystals have chiral components in their molecules, some of which are derived from amino acids (240). Concentrated solutions (10—30%) of a-helix poly(amino acid)s show a lyotropic cholesteric Hquid crystalline phase, and poly(glutamic acid ester) films display a thermotropic phase (241). Their practical appHcations have not been deterrnined. 

The commercial poly-(4-methypent-1-ene) (P4MP1) is an essentially isotactic material which shows 65% crystallinity when annealed but under more normal conditions about 40%. For reasons given later the material is believed to be a copolymer. In the crystalline state P4MP1 molecules take up a helical disposition and in order to accommodate the side chains require seven monomer units per two turns of the helix (c.f. three monomers per turn with polypropylene and polybut-I-ene). Because of the space required for this arrangement the density of the crystalline zone is slightly less than that of the amorphous zone at room temperature. 

FIGURE 6.18 Poly(Gly-Pro-Pro), a collagen-like right-handed triple helix composed of three left-handed helical chains. (Adaptedfrom Miller Scheraga, H. A., 1976, Calculation of the... 

Several years ago Makino et al. 86) studied the influence of anions on the conformation of poly-[L-methionine-S-methylsulfonium] salts in solution. They found that especially perchlorate will induce a-helix formation whereas Cl- and Br do not. Since then several authors 87 92) have found a similar a-helix inducing effect in the case of poly-L-lysine (Lys) and other BPAA at low pH-values where the polymer molecules usually attain an extended conformation due to the electrostatic repulsion of the ammonium groups. Therefore, the a-helix inducing effect is obviously an... 

Fig. 8. Space Ailing model of a poly-L-lysine a-helix with CIO4 anions inserted between the NH3 -side groups forming a left-handed superhelix 911...

Fig. 9. CD for poly(L-proIine) II helix in water. Data from Carver et al.18)...

Fig. 18.—Antiparallel packing arrangement of 2-fold poly(ManA) (15) helices, (a) Stereo view of two unit cells roughly normal to the hoplane. The helix at the center (filled bonds) is antiparallel to the two in the back (open bonds). Intrachain hydrogen bonds stabilize each helix. Association of helices through direct hydrogen bonds involve the carboxylate groups for parallel chains, but involve the axial hydroxyl groups for antiparallel chains, (b) A view of the unit-cell contents down the t-axis highlights the interactions between the helices.

Fig. 2.2 Most favored helical structures proposed for two crystal forms of poly(a-n-butyl-/ -L-aspartate) (7, R=Bu) [28]. (A) Model of a (P)-3.25i4-helix. (B) Model of a (P)-4is-helix...

Blout ER, Idelson M (1956) Polypeptides VI. Poly-alpha-glutamic acid preparation and helix-coil conversions. J Am Chem Soc 78 497-498... 

Figure 11a shows a force-distance profile measnred for poly(L-glutamic acid) brushes (2C18PLGA(44)) in water (pH = 3.0, 10 M HNO3) deposited at 40 mN/m from the water subphase at pH = 3.0. The majority of peptides are in the forms of an a-helix (38% determined from the amide I band) and a random coil. Two major regions are clearly seen in... 

Most DNA, natural and synthetic, can adopt the B-form as defined by its characteristic X-ray pattern [3]. There are few a species of synthetic DNA, namely poly(dA) poly(dT), poly(dI) poly(dC), poly(dA-dI) poly(dC-dT) and poly(dI-dC)-poly(dI-dC), which have a B-form that significantly differs from that of the other complementary deoxy polymers due to their different intramolecular packing arrangements and slightly changed value for the rise per residue. The helix parameters are presented in Table 1. The linear sequen-... 

The head-to-tail-coupling reactions described above are potentially useful in the design of dynamic combinatorial libraries. Features of these reactions include the rapid and reversible formation of carbon-carbon bonds, multifunctional ene-imine building blocks, and formation of stereo centers upon ene-imine linkage. Support for template-directed synthesis utilizing ene-imine building blocks is the formation of a poly ene-imine species that could recognize 3 -GGA-5 sequences of DNA.48 It is noteworthy that some polyene-imines are helical and could form a triple helix with DNA. 

Iso tactic poly(methyl methacrylate) (it-PMMA) can form a stereocomplex with st-PMMA. Recent X-ray studies 179) of this material indicate that the two polymer chains probably interact to form a double helical structure. The it-PMMA chain forms the inner helix and is surrounded by the st-PMMA helical chain which winds around it. If subsequent work confirms this model, this material would constitute a most unusual inclusion compound involving only one monomeric substance. 

E Pefferkorn, A Schmitt, R Varoqui. Helix-coil transition of poly(a,L-glutamic acid) at an interface Correlation with static and dynamic membrane properties. Biopolymers 21 1451-1463, 1982. 

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