Helix coil transformation

A. Wada, Helix-coil transformation and titration curve of poIy-L-glutamic acid. Molec. Phys. 3 409-416 (1960). 

The changes in IMM during the conformational helix-coil transformation in synthetic polypeptides have also been studied by PL The polymer used was poly(glutamic acid) (PGA) with anthracene-containing luminescent markers in the side chains (one marker per 1000 monomer units). The structure and the position of the marker are ... 

LCs were the earliest studied structures, in which polypeptide homopolymer rods pack in an ordered manner to form smectic, nematic, and cholesteric phases. The smectic LCs are mainly formed by polypeptide homopolymers with identical polymer length. The cholesteric phase can be prepared by synthetic polypeptides with polydisperse chain length. The nematic phase can be regarded as a special example of the cholesteric phase with an infinite cholesteric pitch. The cholesteric pitch and chirahty in the polypeptide LCs are dependent on many factors, such as temperature, polymer concentration, solvent nature, and polypeptide cOTiformation. Deep understanding of such phenomena is necessary for preparation of ordered polypeptide assembles with delicate stmctures. The addition of denaturing solvent to polypeptide solution can lead to an anisotropic-isotropic reentrant transition at low temperatures where the intramolecular helix-coil transformation occurs. However, the helical structure is more stable in LC phase than in dilute solution due to the conformational ordering effect. 

Figure 16 shows relationships between the number of introduced side chains and relaxation rigidity (G,) at 900 s for carboxymethylated wood binding various metal ions [341. Wood specimens were prepared from Japanese linden Tilia japonica Smik.). Carboxymethylation and the introduction of metal ions was the same procedure as mentioned in the previous section [32,33]. Stress relaxation measurements were carried out in an aqueous solution at 30°C. The relaxational property of carboxymethylated wood without metal ions is first discussed. For carboxymethylated wood (a broken line in Fig. 16), Gf (900) decreases with an increase in the number of introduced side chain. This rapid decrease appears to be caused by two factors. One is the effect of sodium hydroxide (NaOH). Young s modulus of wood treated with an aqueous solution of NaOH decreases remarkably under wet conditions, especially at concentrations above 10% NaOH [35]. The other factor is the electrostatic repulsion of ionized carboxymethyl groups in carboxymethylated wood, as mentioned in the above section [291. For example, conformation of polypeptide is influenced by the ionization of the side chains, and the structural change of the helix-coil transition has been interpreted as a reversible transformation. Theoretical treatment of the transformation has been reported to explain the mechanism [23-25, 36-43]. The conformation of component molecules in wood, however, cannot change markedly by ionization in comparison with soluble polyelectrolytes in water, because carboxymethylated wood is not dissolved in water. Only space among the main chains is expanded by the electrostatic repulsion due to negatively charged side chains. For these reasons, G (900) of carboxymethylated wood decreases with an increase in the number of introduced side chains.

The film prepared at 50 C differs in that it no longer contains the triple helix structure. If the helix-coil (or collagen gelatin) transformation is complete, the absorption should no longer be related to the different structural levels but instead controlled by the tortuosity of the diffusion paths. The rate of water absorption is considerably slower with a decrease in the weight absorbed at low humidities. But the energy profile remains similar to the sample prepared at 20 C. We believe that the disappearance of the triple helix structure is incomplete and that the water tends initially to be absorbed by the structures still present. 

Values for tte internal variabtes in thetmodynamic, internal equilibriwn are generally uniquely defined by the values for the external variables. For instance, in a simple, thermomechanical system (i.e. one that reacts mechanically solely volume-elastically) the equilibrium concentrations of the conformational isomers are uniquely described by temperature and pressure. In this case the conformational isomerism is not explicitly percqitible, but causes only overall effects, for example in the system s enthalpy or entropy. Elastic macroscopic effects may, however, occur when the relationship between internal and external variables is not single-valued. Then the response-functions of the system diverge or show discontinuities. The Systran undergoes a thermodynamic transformation. The best-known example of sudi a transformation based on conformational isomerism is the helix-coil transition displayed by sonte polymers in solution. An example in the scdid state is the crystal-to-condis crystal transition discussed in this paper. The conditions under which such transformations occur are dealt with in more detail in Sect 2.2. 

Amylose, amylopectin, and glycogen are decomposed into Schardinger dextrins by bacillus macerans. These dextrins are cyclic oligo[a-(1 4)-anhydroglucoses] from six (a-dextrin), seven (j8-dextrin), or eight (y-dextrin) glucose units. In the vacancies in the centers of these rings, iodine can be incorporated to form a blue iodine inclusion complex just as in the amylose helix. The transformation of the coils into helices, which takes... 

Hyperchromic effect increase in the absorbance of a solution at a particular wavelength due to structural changes in the solute molecules. The H.e. is a useful experimental index of DNA denaturation, since the A2J0 of a DNA solution increases when the double helix is transformed by heating into a disordered random coil (see Hybridization). 

Order-disorder transformation phenomena associated with macromolecules in solution are well-known examples include biopolymers such as polypeptides and DNA [96], 7i-conjugated systems such as the polydiacetylenes [88—93], and tr-bonded systems such as the poly silanes [97]. A detailed theoretical picture of the helix-coil transition in the biopolymers has been developed. Attempts toward a detailed theoretical understanding were made and possible schemes for the observed chromism in such systems were proposed as summarized in Figure 8.24. 

When individual, isolated molecules exist in helical, or other ordered forms, environmental changes, either in the temperature or solvent composition, can disrupt the ordered structure and transform the chain to a statistical coil. This conformational change takes place within a small range of an intensive thermodynamic variable and is indicative of a highly cooperative process. This reversible intramolecular order-disorder transformation is popularly called the helix-coil transition. It is an elementary, one-dimensional, manifestation of polymer melting and crystallization. 

This one-dimensional intramolecular structural transition, the helix-coil transition, has received extensive theoretical treatment by many investigators.(65-75) Although a variety of models and mathematical techniques have been brought to bear on this problem the basic conclusions have been essentially the same. The methods involved, and the results, have been eloquently summarized in the treatise by Poland and Scheraga.(76) As an example, we will outline the theoretical basis for the transformation in dilute solution of an isolated polypeptide chain from the alpha-helical to the coil form. 

Transforming a Helix-Coil into a Helix-Helix Transition and Eliminating... 

Fig. 10 The irreversible intramolecular electrocyclization of cw-transoidal polyphenylacetylene (a) taking place during the helix-coil transition of the polymer (b), its elimination by encapsulation of the polymer in a cylindrical supiamolecular polymer and the transformation of the helix-coil into a helix-helix transition (c)...

Some of the vacuum theories treated earlier in the book are repeated in Chapter 8. Examples are chemical equilibrium, allosteric phenomena, and helix-coil transition. The general procedure to transform a vacuum theory into a solution theory is developed. Then we emphasize possible large solvent effects that can significantly alter the vacuum theory, especially when the solvent is water. A detailed account of the thermodynamics of protein folding and protein association is also presented. 

The helix-coil transition has occupied polypeptide chemists as models for such structural transformations in proteins (Poland and Scheraga, 1970). Both theoretical and experimental approaches have been taken in such studies (Scheraga, 1978) in order to obtain quantitative measures of the tendency of each of the 20 naturally occurring amino acids to adopt the helix vs. the coil (i.e. all other, non-helical) conformations. These tendencies are expressed in terms of the Zimm-Bragg (1959) nucleation and growth parameters, O and s, respectively. The values of a and s may be used directly to predict the locations of a-helices in proteins. The values of a and s have been obtained from experimental studies of thermally-induced helix-coil transitions in host-guest random copolymers... 

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