Poly conformational transition induced

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... 

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

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. 

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. 

Photophysical studies on a conformational transition of PMA induced by cationic surfactants have been reported (7). The stretched PMA chain at pH 8 collapses on addition of cationic surfactants that is, the hydrophobic interactions between the cationic surfactants that are bonded to the PMA chain lead to refolding of the polymer chain, and thus provide a hydrophobic site for fluorescence probes at pH 8. The cationic polyelectrolyte poly(4-vinylpyridine) quatemized with n-dodecyl bromide (8 i0) or hexadecyl bromide (11) are also examples of hydrophobically modified polyelectrolytes. 

Fluorescence studies 14, 15) using pyrene, pyrene derivatives, and cationic probes in poly(methacrylic acid) have shown that a conformational transition from a closed compact coil to extended form induced by pH is a progressive process over several pH units (pH 4-6). The emission spectrum of 4 X 10 M R6G and 4 X 10 M RB excited at 480 nm in water is not dependent on pH. However, in aqueous solutions of PM A, the spectra are significantly dependent on pH (shown in Figure 6). At pH 4-5, the spectra are similar to the typical emission of RB at pH 2-3 and 6-7, the spectra in PMA display stronger emission at 550 nm and at pH 8, the spectra are identical to those in water. 

The modified titration curves of the fractionated (MA-St)n aqueous NaCl at various ionic strengths from 0.009 to 0.27 resembled the potentiometric behaviors of the maleic acid copolymer with n-butyl vinyl ether and poly(methacrylic acid) in aqueous salts. There was no precipitation or turbidity in the deionized polymer solution, except in aqueous NaCl of 0.27 M. The conformational transition from the compact to extended coil form was seen in diagp ams of the intrinsic viscosity vs. qp and the reduced viscosity in the range 0.1 < aj < 0.4 was independent of polymer concentration. Both the potentiometric and viscometric titration data showed that the pH-induced conformational transition of (MA-St)n in aqueous NaCl is due to intramolecular interaction. The unperturbed molecular dimensions were calculated from the viscosity data and the acid compact form of this copolymer was found to be not so compact as the conformations of globular proteins. Dilatometry did also indicate existence of the transition,and the transition was observed in aqueous solutions of various salts chlorides, bromides, perchlorates and thiocyanates of Na" ", K" ", Rb" ", Cs" " and... 

Chee CK, Riuuner S, Soutar I, Swanson L (1997) Time-resolved fluorescence anisotropy studies of the temperature-induced intramolecular conformational transition of poly (N-isopropylacrylamide) in dilute aqueous solution. Polymer 38(2) 483 86. doi 10.1016/ s0032-3861(96)00636-2... 

Chee CK, Rimmer S, Soutar I, Swanson L (2001) Fluorescence investigations of the thermally induced conformational transition of poly(N-isopropylacrylamide). Polymer 42 (12) 5079-5087. doi 10.1016/s0032-3861(00)00821-l... 

In addition to the commonly used synthetic polymers such as pNIPAM and poly(N-vinylcaprolactam) (PVCL), elastin-like polymers were introduced as thermoresponsive polymers for affinity precipitation (Shimazu et al., 2003). An elastin-like polymer consists of the repeating penta-peptide, Val-Pro-Gly-Xaa-Gly (Xaa being any amino acid except proline). It undergoes reversible phase transition under temperature or salt concentration changes similar to pNIPAM (Shimazu et a/., 2003). An elastin-like polymer was used as a terminal tag for the facilitation of recombinant protein purification as attaching it to the hydrolase enzyme improves its purification. Purification, in excess of 1300-fold was achieved after two cycles of reversible changes of elastin-like polypeptide conformation precipitation induced by environmental changes (Shimazu et al., 2003). 

Strong optical changes in polythiophene solutions can also be induced by varying the solvent composition [17,18]. For instance, the addition of methanol (poor solvent) to a solution of poly(3-hexylthiophene) in chloroform (good solvent) results in a yellow to red-violet color change. This phenomenon is again related to a cooperative planar/nonplanar conformational transition of the backbone induced by a variation of the solvent quality. 

Since the shear induces an unusual conformational transition in the PAA to form a self-complexation, it is worthwhile to investigate how the complexation is affected by the interaction with competitive species such as poly(ethylene oxide) [15] and cationic surfactants. With the latter, it is particularly interesting to see how the PAA interchain complexation is interfered with by the presence of a hydrophobic environment due to the bound surfactant molecules, and how their microdomain formation is altered by the presence or absence of shear stress. To monitor the conformational state changes of the PAA, we chose two fluorescence probes, pyrene and AuO the former as a probe for microenvironmental polarity in the absence of shear and the latter as a probe for local rigidity along the PAA chain under shear. 

Poly (methacrylic acid). pH-induced conformational transition thermodynamic data. Summary of microcalorimetric and potentiometric data for the conformational transition of poly (methacrylic acid) in aqueous solution... 

Mitsui et al. (1970) reported an unusual X-ray diffraction pattern of ethanol-precipitated fiber of poly(dIdC) poly(dIdC) and proposed a left-handed helical structure for this copolymer. Pohl and Jovin (1972) found that poly(dGdC)-poly(dGdC) underwent a salt-induced conformational transition in solution to a new form with an inverted CD spectrum and suggested that a left-handed helix could be a possible explanation. Subsequently, Pohl (1976) also observed the same CD spectral change for... 

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