Polymers helix

Their studies involved the partial polymerization of NCAs of mixtures of specific amino adds having known e.e.s, followed by determination of the e.e.s of the amino adds in both the resulting polypeptides and in the residual unreacted NCA monomers. [94] In a typical experiment it was found that when an optically impure leucine NCA monomer having an l > d e.e. of 31.2% was polymerized to the extent of 52 % to the helical polyleucine peptide, the e.e. of the polymer was enhanced to 45.4 %, an increase of 14.2 %. In the same experiment the e.e. of the unreacted leucine NCA monomer was depleted to a similar extent. Analogous experiments with valine NCAs of known e.e.s, however, led to a reverse effect, namely, the preferential incorporation of the racemate rather than one enantiomer into the growing polyvaline peptide. This finding was interpreted to be the result of the fact that polyvaline consists of (3-sheets rather than a-helices, emphasizing that the Wald mechanism applies only to a-helix polymers. At about the same time Brach and Spach [95] showed that, under proper conditions, (3-sheet polymers could also be implicated in the amplification of amino add e.e.s. 

Eg 3.5 eV, of the (C2S) helix polymer and indicated that the electron localization occurs already for n < 7 [64]. These results are in contrast to electron delocalization in [n]heliphenes, [njhelicenes and [njthiahelicenes, which possess much lower Eg values of 2.1-2.5 eV. These results are consistent with cyclic voltam-metric data. [nJHelicenes 40, 44 and 45 showed reversible cyclic voltammetric waves (1.2-1.3 V vs. SCE) the second oxidation wave for 40 was found at 1.8 V, i.e. at a significantly more positive potential [64, 85]. 

D-polymers (Sects. 9.1-9.2), the monomers (R,R)-65 in the helix polymer (P)-66 are not positioned perpendicular to each other. 

OTHER POLYMERS SHOWING THIS SPEriAL PROPERTY Poly(n-hexyl isocyanate) rigid polymer backbone with bulky side chain. Triarylmethyl methacrylates high value of optical activity with helix polymer structure. 

Doubly stranded DNA is densely packed in cell nuclei, sperm heads, bacteria, and viruses, with an estimated concentration of up to 800 mg/ml. At lower concentrations in saline solutions, a cholesteric (chiral nematic) phase forms, but at hundreds of mg/ml, a typical columnar texture can be observed (Figure 11.16). PBLG (poly-y-benzyl-L-glutamate), also a double-helix polymer, exhibits similar textures. X-ray diffraction and freeze-fracture investigations prove that the high concentration phase is indeed columnar, not smectic, with the molecular double helices packed into a hexagonal pattern (Figure 11.17). When the concentration is increased from the cholesteric to... 

The above discussion points out the difficulty associated with using the linear dimensions of a molecule as a measure of its size It is not the molecule alone that determines its dimensions, but also the shape in which it exists. Linear arrangements of the sort described above exist in polymer crystals, at least for some distance, although not over the full length of the chain. We shall take up the structure of polymer crystals in Chap. 4. In the solution and bulk states, many polymers exist in the coiled form we have also described. Still other structures are important, notably the helix, which we shall discuss in Sec. 1.11. The overall shape assumed by a polymer molecule is greatly affected... 

This particular sequence of conformations-trans bonds that advance the helix along the axis, alternating with gauche bonds which provide the twist-takes the chain through a series of relatively low potential energy states and generates a structure with minimum steric hindrance between substituents. If the polymer series is extended to include bulkier substituents, for example. 

Figure 1.10 Helical conformations in polymer molecules, (a) A vinyl polymer with R substituents has three repeat units per turn, (b) The a helix of the protein molecule is stabilized by hydrogen bonding. [From R. B. Corey and L. Pauling,/ end. Inst. Lombardo Sci. 89 10 (1955).]...

The special stability of the helix with three repeat units per turn for isotactic vinyl polymers in which the substituent is not too large was discussed in connection with Fig. 1.10a. Rodriguez has suggested a simple paper model which is also helpful in illustrating this structure. The following steps summarize this method additional details are available in this readily accessible reference. 

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 ... 

For example, a polypeptide is synthesized as a linear polymer derived from the 20 natural amino acids by translation of a nucleotide sequence present in a messenger RNA (mRNA). The mature protein exists as a weU-defined three-dimensional stmcture. The information necessary to specify the final (tertiary) stmcture of the protein is present in the molecule itself, in the form of the specific sequence of amino acids that form the protein (57). This information is used in the form of myriad noncovalent interactions (such as those in Table 1) that first form relatively simple local stmctural motifs (helix... 

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. 

Physical Properties. Table 3 Hsts physical properties of stereoregular polymers of several higher a-olefins. Crystal ceU parameters of these polymers ate available (34—36). AU. stereoregular polyolefins have helix conformations ia the crystalline state. Their densities usually range from 0.90 to 0.95 g/cm. Crystalline PMP, however, represents an exception its density is only 0.812—0.815 g/cm, lower even than that of amorphous PMP (0.835—0.840 g/cm ), thus making it one of the lowest densities among plastics. 

Polymer Melting poiut, °C Crystal type Helix type Crystalline density, g/cm ... 

Fibrous proteins can serve as structural materials for the same reason that other polymers do they are long-chain molecules. By cross-linking, interleaving and intertwining the proper combination of individual long-chain molecules, bulk properties are obtained that can serve many different functions. Fibrous proteins are usually divided in three different groups dependent on the secondary structure of the individual molecules coiled-coil a helices present in keratin and myosin, the triple helix in collagen, and P sheets in amyloid fibers and silks. 

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... 

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