Poly helix-coil transitions

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

Helix formation of poly(L-glutamic acid) has been studied by ORD234), by potentiometric titration235) and by viscosity methods236). Thermodynamic parameters for the coil-helix transition of the uncharged polymer have been determined and it has been found235) that the helix is an order of magnitude more stable in NMA than in water. 

In contrast to the spiropyran-modified polypeptides derived from poly(L-glutamic acid), the poly(L-lysine) analogs have been reported to show no photomodulation of the side-chain conformation in pure HFP, although they exhibited similar photochromic behavior (Figure 2).32,33 However, when appropriate amounts of triethylamine were added to the HFP solution, exposure to visible light resulted in the reversible formation of a helix by the poly(L-lysine) chains.35 Thus, addition of triethylamine to the HFP solution induced the coil - helix transition, but the amount of base necessary was different for the dark-adapted sample as compared with the irradiated one. The authors claimed... 

The spiropyran-modified poly(L-glutamic acid) PGA-2 undergoes a coil helix transition upon exposure to visible light in hexafluoro-2-propanol solution. In the dark, the polypeptide, containing 30-80 mol% chromophore units in the open charged form, adopts a random coil conformation. Irradiation causes isomerization in the side chains, as indicated by complete bleaching of the colored solution (see Scheme 5.4). The formation of the colorless and uncharged spiropyran form induces spiralization of the polypeptide chain. The coil helix transition can be followed with the aid of CD spectra, as shown in Fig. 5.4. 

Fig. 5.4 Coil helix transition of poly(glutamic acid) PGA-2 containing 80 mol% spiropyran units in the side chains. CD spectra recorded in hexafluoro-2-propanol solution in the dark (1) and after exposure to sunlight (2). Adapted from Pieroni et al. [14] with permission from Elsevier.

Lin J, Abe A, Furuya H, Okamoto S (1996) Liquid crystal formation coupled with the coil-helix transition in the ternary system poly(y-henzyl L-glutamate)/dichloroacetic acid/dichlo-... 

A similar two-step mechanism of gelation through coil-helix transition was confirmed for synthetic polymers with stereo-regularity. For instance, in solutions of syndiotactic poly(methyl methacrylate) (PMMA) in toluene, a fast intramolecular conformational change is followed by the intermolecular association leading eventually to gelation [49,50]. 

These theoretical expectations have been satisfied by experimental observations when proper theoretical account is taken of finite chain length. Another example of the coil- helix transition is given in Fig. 3.19 for poly-y-benzyl glutamate in dichloroacetic acid-ethylene dichloride mixtures.(79) The transition in this case is... 

Experiments reveal that the formation of the elements of various internal structures in macromolecules changes the IMM of the latter The appearance of local regions with internal structures in the molecules of PMAA the coil-helix transition in synthetic polypeptides and, finally, the formation of a globular structure in poly(l,2-dimethoxyethylene) molecules " lead to a several fold increase in the relaxation times characterizing IMM an, when the globular structure appears, the relaxation times increase by an order of magnitude (Table 11). 

KA1 Kagemoto, A. and Fujishiro, R., The heat of the coil-helix transition of poly(y-benzyl-L-glutamate) in the solution, Makromol. Chem., 114, 139, 1968. 

Very recently, however, two papers were published by the group of Fujiki which report successful solid-state CD studies of chiral polysilanes. In the first, a helix-coil transition was described for film samples of poly[(A)-3,7-dimethyloctyl- -propylsilylene)], 113.327 This polymer has a relatively low glass transition temperature, T, which was considered critical for the observation of a helix-helix transition in the solid state, since helical inversion would be precluded if the inversion temperature, Tc, were below Ts as the segmental motion of the chain,... 

The soluble polythiophenes are the first conducting polymers that can be taken above their glass transition without decomposition and it will be interesting to study morphology-property relationships. Heeger et al.262) have recently described conformational changes in solutions of poly-3-hexylthiophene which seem to involve a coil-helix transformation as the temperature is decreased or a poor solvent is added. 

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. 

Studies with other poly(/3-i -aspartate)s demonstrate that these polymers not only adopt conformational patterns that are similar to poly(a-amino acids), but that they exhibit greater conformational versatility. The range of conformations now include extended chain structures, arranged as antiparallel packings that come about by stretching poly(a-methyl-/3-L-aspartate) films in boiling water.209 In solution, the helix—coil conformational transition is a phenomenon common to the whole family of poly-(a-alkyl-/3-L-aspartates).210 The ordered conformation is responsive to environmental factors such as temperature and solvent in much the same way as for poly(a-peptides). 

If the above conclusions are applied to address the abrupt thermochromism of poly(dihexylsilane) not only in the solid state but also in solution, the conclusion is that the increase in UV absorption intensity does indeed result from a coil-to-rod transition but that the red shift arises from a helix-to-helix transition of the main chain. Recent calculations have indicated that the absorption maximum gradually red shifts as the helical dihedral angle changes from 60° to... 

On the basis of CD data one can safely exclude the righthanded a-helix from the possible ordered forms. Myer and Barnard [10] interpreted the CD spectra of the poly (His) ordered conformations in terms of a j -structure, this assignment however being complicated by the influence of side-chain chromophores [11]. In our study we have collected more evidences in favour of Myer and Bernard s proposal and investigated the thermodynamics of the P coil conformational transition [5]. Among the different types of data and informations derived from this study, I shall here collect and illustrate to you a few results more directly connected with the energetics of the transition of poly (His) and which, in my opinion, afford a further example of the useful joint applications microcalorimetry and potentiometry may find in the field of our concern. 

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. 

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. 

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). 

Figure 5 illustrates, with the data for poly(/J-benzyl L-aspartate) (PBLA) (22), that there are two types of thermal helix-coil transition, normal and inverse. It should be noted that for a given polypeptide the type of transition depends on the solvent in which the polymer is studied. This suggests that polymer-solvent interactions play a decisive role in the helix-coil transition phenomena of polypeptides. 

N 025 "Solution Properties of Synthetic Polypeptides. V. Helix-Coil Transition in Poly(p-benzyl-L-asparate)"... 

Water-soluble random copolymers containing L-serine and A/5-(4-hydroxybutyl)-L-glutamine are prepared, and the thermally induced helix-coil transition of these copolymers in water is studied. The Z/mm-Bragg parameters cr and s for the (hypothetical) helix-coil transition in poly(l-serine) in water are deduced. The values of s, computed from both the Lifson and Allegra theories are (.Llfson, o-1 x 10-4 / Allegra, o = 7.5 x 10 5 s - 0.667/0.726 (273 K), 0.757/0.784 (293 K), 0.777/0.792 (313 K), 0.731/0.744 (333 K),... 

The synthesis and characterization of water-soluble "random" copolymers containing t-valine with either A -O-hydroxypropyD-L-glutamine or / -(A-hydroxybutyD-L-glutamine are described, and the thermally induced helix-coil transitions of these copolymers in water are studied. The incorporation of /.-valine is found to decrease the helix content of the polymer at low temperatures and increase it at high temperatures. The Zimm-Bragg parameters o and s for the helix-coil transition in poly(t-valine) in water are deduced from an analysis of the melting curves of the copolymers. The values of s, computed for o = 1 x 1CI-4, are s 0.85 (273 K), 0.93 (293 K), 1.00 (313 K), 1.06 (333 Kl. 

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