Transition collapse

The basic features of folding can be understood in tenns of two fundamental equilibrium temperatures that detennine tire phases of tire system [7]. At sufficiently high temperatures (JcT greater tlian all tire attractive interactions) tire shape of tire polypeptide chain can be described as a random coil and hence its behaviour is tire same as a self-avoiding walk. As tire temperature is lowered one expects a transition at7 = Tq to a compact phase. This transition is very much in tire spirit of tire collapse transition familiar in tire theory of homopolymers [10]. The number of compact... 

In cases when the two surfaces are non-equivalent (e.g., an attractive substrate on one side, an air on the other side), similar to the problem of a semi-infinite system in contact with a wall, wetting can also occur (the term dewetting appHes if the homogeneous film breaks up upon cooHng into droplets). We consider adsorption of chains only in the case where all monomers experience the same interaction energy with the surface. An important alternative case occurs for chains that are end-grafted at the walls polymer brushes which may also undergo collapse transition when the solvent quality deteriorates. Simulation of polymer brushes has been reviewed recently [9,29] and will not be considered here. 

Fig. 18 The equilibrium temperatures (circles) and the heights of the free-energy barrier at these temperatures (triangles) for a single 512-mer as a function of B/Ep. The dashed line shows the demarcation for the occurrence of a prior collapse transition (Hu and Frenkel, unpublished results)...

Ziv, G., Thiramalai, D., Haran, G. Collapse transition in proteins. Phys. Chem. Chem. Phys. 2009, 11, 83-93. 

We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account Experimental results demonstrate a good correlation with theoretical analysis. 

In Sects. 2.1-2.3, we will deal with one-component solvent (apart from the presence of a low-molecular salt). At the same time, most of the experiments are performed for mixed solvents. In Sect. 2.4 we consider explicitly the collapse transition in various kinds of mixed solvents mixed low-molecular solvents, mixtures of low-molecular and polymer components (polymer solution). 

An analysis shows that the qualitative results on the character of the collapse transition do not depend essentially on the specific expression for elastic free energy therefore, the use of more modern modifications of Eq. (5) is not relevant. 

Ichiba, Y. and Yoshikawa, K. (1998) Single chain observation on collapse transition in giant DNA induced by negatively-charged polymer. Biochem. Biophys. Res. Commun., 242, 441 145. 

One of the most important phenomena in the polymer solvation is the change in the overall size of the polymer chain upon solvation. In fact at equilibrium the average size of isolated polymer molecules in solution is a function of solvent quality and varies from expanded conformations in good solvents to random walk conformations in poor solvents. This is referred to as collapse transition and was first predicted by Stockmayer [82] more than 45 years ago. The phenomenon was observed by Nishio et al. [83] and Swislow et al. [84] more than 25 years ago and is still a subject of much experimental, computational, and theoretical research today. So far many investigators have tried to study the chain size with solvation using a variety of methods. 

Equilibrium properties of the collapse transition and the relationship between polymer size and solvent quality are studied widely, and are addressed in the previous section, but up to now less is known about the collapse dynamics. There has been a... 

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