Conformation helix-coil transition

The conformational transitions in the presented model take place accord-itig to the all-or-nothing law, i.e. they occur at the certain r.h. value. The same behaviour has been observed, for example, for the helix-coil transition of the model double-stranded structure A(pA)i7-U(pU)i7 [24]. It is worth noting that this structure is homogeneous, the same is supposed in our model. 

Teramoto, A. and Fujita, H. Conformation-dependet Properties of Synthetic Polypeptides in the Helix-Coil Transition Region. Vol. 18, pp. 65— 149. 

Typical examples are the conversion of the neutral form of an amino acid into its zwitterionic form, the helix-coil transitions in polypeptides and polynucleotides, and other conformational changes in biopolymers. Reactions of higher molecularity in which reactants and products have different dipole moments are subject to the same effect (association of the carboxylic acids to form hydrogen-bonded dimers). Equilibrium involving ions are often more sensitive to the application of an electric field ... 

Both ORD and CD are important tools in the study of molecular conformation in solution. These techniques are of special importance in the study of biomolecules, their helical content, and helix-coil transitions. 

When the collagen rod can be extracted in the native form it is soluble in acidic solutions, at room temperature. If the solutions are heated, the collagen is denaturated the chains lose their helical conformation. The characteristic temperature of this helix —> coil transition is around 36 C. The solution then contains principally single chains, but also some double and triple chains which were initially covalently bound and some sub-units of the single chains. This product is gelatin. 

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). 

Conformation-Dependent Properties of Synthetic Polypeptides in the Helix-Coil Transition Region... 

Current investigations on dilute polymer solutions are still largely limited to the class of macromolecular solutes that assume randomly coiled conformation. It is therefore natural that there should be a growing interest in expanding the scope of polymer solution study to macromolecular solutes whose conformations cannot be described by the conventional random-coil model. The present paper aims at describing one of the recent studies made under such impetus. It deals with a nonrandom-coil conformation usually referred to as interrupted helix or partial helix. This conformation is a hybrid of random-coil and helix precisely, a linear alternation of randomly coiled and helical sequences of repeat units. It has become available for experimental studies through the discovery of helix-coil transition phenomena in synthetic polypeptides. 

Chapter B outlines a typical statistical-mechanical formulation of polypeptide conformations in terms of these three parameters and describes its use for the evaluation of s and tr from observed helix-coil transition curves. Then the reported values of AH and a for selected polypeptide-solvent pairs are given and their implications are briefly discussed from a molecular standpoint. Here AH denotes the transition enthalpy derived from s by a thermodynamic relation. 

Although the statistical mechanical theories such as those described above yield exact analytic expressions for various quantities characterizing the conformation of an interrupted helix, those expressions are so complicated that it is of both theoretical and practical value to simplify them, with the imposition of suitable restrictions on parameters, to forms that are amenable to straightforward computations and also, hopefully, to direct comparisons with observed data. Various attempts have been made, and they are summarized in Poland-Scheraga s book (10). Though not available at the time this book was published, the approximations worked out by Okita et al. (13) are of great practical use for their wide applicability and simplicity. Their method is described below in some detail, because it has been consistently used in our statistical-thermodynamic analyses of helix-coil transition phenomena. 

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