Poly conformational transitions

Extensive data are given in the Uterature for the potentiometric titration of polymer acids which may be used to study the behaviour of polyelectrolyte systems under different conditions. For poly(a-D) galacturonic acid there are few data of this kind, especially in connection with the occurrence of a conformational transition induced by pH variations, or with the effect brought about by the addition or the exchange of counterions. Since for a polyacid not exhibiting a conformational transition in the course of titration, pK K denoting the apparent dissociation constant) increases monotonously with degree... 

In addition, data on the size, shape and solvation of the polymer particles in aqueous solutions at temperatures below and above the transition phenomena registered by HS-DSC have been obtained [42]. Table 2 shows the results of capillary viscometry and light scattering experiments for the fractions p and s of poly(NVCl-co-NVIAz) synthesized at 65 °C from the feed with the initial molar comonomer ratio equal to 85 15. Since fraction p precipitates from the aqueous solution at temperatures > 34 °C, its intrinsic viscosity can be determined only at 20 °C, whereas for the fraction s such measurements were possible above and below the temperatures of the HS-DSC-registered conformational transition. 

All the described properties of such a s-fraction of poly(NVCl-co-NVIAz) synthesized at the temperature above the PST of the reacting system allowed us to draw the conclusion that the chains of this type had the comonomer sequence, which at the temperatures above the conformation transition facilitated the formation of polymer particles, where H-blocks are in the interior shielded by the P-blocks against additional intermolecular association. Such a behaviour of this copolymer in aqueous media is close to that of oligomeric proteins similar to casein [46] possessing a rather hydrophobic core surrounded by the polar segments. 

H.-Z. Tang, M. Fujiki, and T. Sato, Thermodriven conformational transition of optically active poly[2,7- 9,9-bis[(5)-2-methyloctyl] fluorene] in solution, Macromolecules, 35 6439-6445, 2002. 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

The influence of the solvent on chiroptical properties of synthetic polymers is dramatically illustrated in the case of poly (propylene oxide). Price and Osgan had already shown, in their first article, that this polymer presents optical activity of opposite sign when dissolved in CHCI3 or in benzene (78). The hypothesis of a conformational transition similar to the helix-coil transition of polypeptides was rejected because the optical activity varies linearly with the content of the two components in the mixture of solvents. Chiellini observed that the ORD curves in several solvents show a maximum around 235 nm, which should not be attributed to a Cotton effect and which was interpreted by a two-term Drude equation. He emphasized the influence of solvation on the position of the conformational equilibrium (383). In turn, Furakawa, as the result of an investigation in 35 different solvents, focused on the polarizability change of methyl and methylene groups in the polymer due to the formation of a contact complex with aromatic solvents (384). 

The conformational behavior can be quite different in aqueous solution. Below pH 5, a sample of poly(L-glutamic acid) containing about 30 mol% azobenzene units adopts a P-structure that is not affected by light. Above pH 7, the polypeptide is random coil and the conformation is, once more, not affected by the photoisomerization of the azo side chains. However, at pH values in the range 5-7 (close to the pK of the conformational transition), irradiation causes a remarkable diminishing of the ordered structure, which is completely reversed in the dark.120,221... 

Sunlight-induced Conformational Transitions in Spiropyran-containing Polypeptides 13.2.2.1 Spiropyran-modified Poly(L-glutamate)s... 

Heijboer [28] has reported the dynamic mechanical properties of poly(nethacrylate)s with different size of the saturated ring as side chain. The y relaxation in these polymers is attributed to a conformational transition in the saturated ring. In the case of poly(cyclohexyl methacrylate), the transition is between the two chair conformations in the cyclohexyl ring. However, this type of internal motion in hindered by rather high intramolecular barriers, which can reach about 11 kcal mol-1. 

This is a typical relaxation for this kind of polymers [246,247] and it has been attributed to the chair-to-chair conformational transition of the cyclohexyl ring like in the cases of poly(cycloheyl methacrylate)s. 

The water-insoluble PMAA-poly(N-ethyl-4-vinylpyridinium bromide)polyelectro-lyte complex (5 l) formed at pH < 4 becomes soluble if the excess PMAA in the polyelectrolyte complex is ionized110. The solubility of the polyelectrolyte complex is apparently connected with the appearance of high negative charges of the complex particles and is accompanied by a conformational transition of PMAA in excess. It has been proposed that the water-soluble polyelectrolyte complex consists of the nucleus formed by bound base and acid units. This structure is retained in solution due to unbound parts (sequences) of ionized PMAA. 

The thermochromic effect in solutions of poly(3-alkylthiophene)s has been attributed to an intramolecular conformational transition to an extended chain conformation, principally on the basis of an observed isosbestic point <1987PSB1071, 1992MM2141>, but evidence for supramolecular aggregates was noted <1987PSB1071>. 

Most native DNAs and certain synthetic polynucleotides are able to undergo a B A conformational transition. In films and fibers, this transition may be induced by decreasing the relative humidity to which the sample is exposed. This is demonstrated by the Raman spectra of fibers (Erfurth et al., 1975 Martin and Wartell, 1982 Prescott et al., 1984) and by the IR spectra of films (Pilet and Brahms, 1973 Taillandier et al., 1985) made of native DNAs. In the case of polynucleotides, the decrease of relative humidity affords different results, depending on the sequence of the polymer. Thus, poly d(A-T) and poly d(A-C). poly d(G-T) - double stranded DNA with one strand adenine and cytosine and in the other guanine and thymine - show a classical B A transition (IR Brahms et al 1976 Taillandier et al., 1984a Adam et al., 1987. Raman Thomas and Benevides, 1985 Jenkins et al., 1986). 

The B —> A conformational transition can be induced in solution, for example by increasing the ionic strength (poly dG. poly dC exhibits the B form in 0.1 M NaCl and shows the A form in 4 M NaCl (Nishimura et al., 1986)). Another option is to work in aqueous ethanolic solution containing more than 80% alcohol (native DNA, Martin and Wartell, 1982). In all of these cases, the conformation is characterized by the previously discussed marker lines in the respective Raman spectra. 

An extreme case that has received attention is that of a cooperative conformational transition examplified by the coil-helix (c h) transition that occurs in poly-a-aminoacids. Whereas the polypeptide chain is quite flexible when it exists as a random coil, the rigid helical form may bring about formation of a liquid crystalline phase, as discussed above, if its concentration is sufficient. The conformational transition and the phase transition may therefore be coupled. The helix-coil transition may then acquire the character of a first-order phase transition, owing to generation of the liquid crystalline phase. 

Particular interest in poly(methacrylic acid), (PMA), has arisen because it represents the first clear example of a S5mthetic vinyl poly-electrol57te whose chain can undergo in aqueous solution a pH-induced conformational transition of distinctly cooperative character. Furthermore, to the author s knowledge, PMA is the only vinyl polyelectrol5de available in samples with a different degree and t5q)e of stereoregularity. Such samples have been prepared and examined in detail. 

In all cases, however, the potentiometric plots of PMA reveal an anomaly at around a 0.15, which is absent in the titration curve of poly(acrylic acid) or other vinyl polyelectrolytes so far studied. The anomalies have been attributed by Leyte and Mandel to a conformational transition of cooperative character taking place in the PMA chains over a narrow range of a-values (5). This anomalous behavior of PMA samples of different t5q)e and amount of chain stereoregularity over a given range of degree of neutralization has been confirmed using... 

External polymers can also be employed to control leakage of entrapped solutes. Adsorption of the hydrophobic polyelectrolyte poly(2-ethylacrylic acid) on phosphatidylcholine membranes causes the disruption of vesicle membranes (Figure 4.32) with small changes in pH (7.4—>6.5). The surface polyelectrolyte undergoes a well-defined conformational transition from an... 

Complexes between polyvinylpyrrolidone and poly(acrylic acid)s are also possible (IV). Such macromolecular reactions are highly selective and strongly dependent on molecular size and conformation. On mixing, some of the macromolecules might be involved in the complex while the rest will be free. The reason for compositional heterogeneity of the products could be the conformational transitions of macromolecules in the course of complex formation. 

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