Polymers helical folding

RNA but of both handednesses. Upon appending a single homochiral residue such as L-lysine at the carboxy terminus of the helix, the peptide nucleic acid polymers predominantly fold into duplexes of a single handedness. This phenomenal positive cooperativity can be considered a form of molecular amplification. Additional examples on spontaneous and induced formations of helical morphologies were reviewed [114,182]. 

The helical nature of the amylose-iodlne-complex, with six D-glucose units in the C-1 configuration per turn of the helix and the iodine molecules packed inside the lumen, parallel to the axis of the helix has been well established by X-ray diffraction studies (1 ). Electron microscopic studies of amylose-iodlne-complex in the form of fibrils have revealed rod like structures with 4o nm in diameter and the length depending on the degree of polymerization of the polymer chain, and the helices folded parallel to the long axis (2). The recent studies of this complex by Kaman resonance and iodine-1 29 H6ssbauer spectroscopy have provided evidence for the presence of IE species within the amylose helix (I-I—I —I-I) (3). 

Further evidence for the existence of a helical conformation for meta-linked CPEs comes from circular dichroism (CD) studies of j-PPE-C02 (Figure 14.10). The basis for this experiment lies in the fact that a helix is an inherently chiral structure. Because the helix is chiral, one would expect that if an enantiomeric excess (ee) of the right- or left-handed forms (the P and M forms) exists in solution, then the CD spectrum would feature a bisignate signal due to exciton coupling of the backbone chromophores which are in a chiral environment [35]. However, when a polymer folds into a helical structure, because the polymer itself is achiral, the ensemble of helically folded polymers exists as a... 

Finally, one must observe that polymer helices may have any symmetry, but already of the shown helices in Fig. 5.11, the 5-, 7-, 8-, and 9-fold screw axes are not permitted as crystal symmetry, as are any not shown screw axes. Finally, the positions of the lattice points of a polymer helix can be computed from the definitions and equations in Fig. 5.11. 

Coordination catalysts based on aluminum alkyls and titanium halides yield isotactic polystyrene [171-174]. The polymer matches isotactic polystyrene formed with amylsodium. It is composed of head-to-tail sequences with the main chain fold being helical. There are three monomer units per each helical fold [171,172]. The catalyst composition, however, has a strong bearing on the microstructure of the resultant polymer [175, 176]. 

The conformation of the chains in space. The term conformation refers to the different arrangements of atoms and substituents of the polymer chain brought about by rotations about single bonds. Examples of different polymer conformations include the fully extended planar zigzag, helical, folded chain, and random coUs. Some conformations of a random coil might be... 

This polymer (15) folded into a helix in both organic solvents with different polarity and also in water, as proven by fluorescence spectroscopy. Monomer NI (l,8-naphthalimide-3,6-diamine) showed fluorescence emission at 420-450 nm and the emission of an excimer band at 540-560 nm. The excimer emission was stronger than the monomer emission in all solvents except trifluoroethanol, which means that the helix conformation was energetically favored. Trifluoroethanol interacted as a strong H-bond donor with the intramolecular H-bonding of the polymer, decreasing the stability of the helical conformation. 

Due to the presence of these different side chains, the solubility was increased. Both polymers were soluble in water and formed a stable helix. 24-L was also soluble and formed stable helical structures in different alcohols and in acetonitrile. It was found that a minimum number of seven monomers was needed to obtain a helical structure but, in order to obtain a very stable helix, almost 20 monomers were required. Finally, the methoxy group of 23 positively influenced the length of the polymer obtained but decreased the ability of the polymer to fold into a helical structure in different solvents (like acetonitrile). ... 

Host-guest complexes in foldamer structures are based on the combination of different non-convalent interactions, which force the polymer to fold into a helical structure around the guest. In the past years, many oligomers have been synthesized with the aim to complex with guests. Neutral (polar. 

Meudtner, R. M., Hecht, S. (2008), Responsive backbones based on alternating triazole-pyiidine/benzene copolymers From helically folding polymers to metaUosupramolec-ularly cross-linked gsls, Macromolecular Rapid Communications, 29,347-51. 

A change in the linkage from the para-position to the meta- or ortho-positions leads to the zig-zag systems of Grubbs and Kratz [52], or the helical folding polymers of Moore and coworkers (Scheme 10.4) [53]. It has been proposed that the formation of the helical structures of the latter is driven by solvophobic interactions that are sensitive to chain length, solvent quality, and temperature. 

Meudtner RM, Hecht S (2008) Responsive backbones based on alternating triazole-pyridine/ benzene copolymers from helically folding polymers to metallosupramolecularly crosslinked gels. Macromol Rapid Commun 29 347-351... 

Proteins are polymers made of amino acid units. The primary structure of a polypeptide is the sequence of amino acid residues secondary structure is the formation of helices and sheets tertiary structure is the folding into a compact unit quaternary structure is the packing of individual protein units together. 

This polymer crystallizes in three polymorphs. The threefold helical structure packs in a trigonal unit-cell with a = 14.3 A (1.43 nm) and c = 28.7 A (2.87 nm). The 8-fold helical structure occurs in a tetragonal unit-cell with a = 13.8 A (1.38 nm) and c = 78.2 A (7.82 nm). Axial periodicity in both cases is similar [h = 9.6 A (960 pm) and 9.8 A (980 pm), respectively], but the helix twist-angle is different (120 and 45°, respectively). Distribution of the charged side-groups in these helices was discussed. An orthorhombic form, with a twofold helical structure, has a repeat of 18.6 A (1.86 nm). 

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