Random coil—globule transition

The pair interaction potential models the interatetion among the molecules in any substance and is a superposition of van der Waals (intcrmolecular) forces of repulsion and attraction of different nature, namely, dipole, inductive, and disperse ones. 

The peculiarity of a macromolecule as a system containing a huge number of elements N (with a rather adequate extrapolation /V oo) is in the structural elements being connected to a chain, which leads to characteristic features of the subsystem (and the system as a whole) but cannot suppress the basic properties due to the attraction and repulsion forces. 

For the macromolecule s segments in LM WL medium, the interaction potential properly changes (renormalizes) to consider the influence of LMWL molecules. 

It follows from the aforesaid that, when T 0 (for definiteness, take systems with an UCST), the repulsion forces between chain-distant segments predominate over the attraction ones. 

For a system of a huge number of LMW component (disconnected segments), such a situation means the existence of vapour with a lower density in comparison with the liquid density. 

---

Random distribution of a significant number of hydrophilic NVIAz units along the polymer chain could result in uniform hydrophilization. This, in turn, could lead to a loss of ability for the coil-globule transition, which is caused by the hydrophobic interactions. As a result, such copolymers should be water-soluble over a wide temperature range. 

P.-G. de Gennes later also considered the multisegment attraction regime. He suggested the so-called p-cluster model [11] in order to explain certain anomalies in behavior observed in many polymer species such as polyethyle-neoxide (PEO) see also [12]. The scenario of coil-globule transition with dominating multisegment interaction first considered by I.M. Lifshitz has been recently studied in [13]. The authors used a computer simulation of chains in a cubic spatial lattice to show that collapse of the polymer can be due to crystallization within the random coil. 

As the solvent quality turns poorer to the polymer from the theta condition, polymer-solvent contacts become more unfavorable, and the chain contracts even more. Eventually, the random-coil conformation changes to a globular shape to minimize the polymer-solvent contacts and maximize the contacts between monomers The chain dimension should be now proportional to A /, as expected for a packed sphere. When N is sufficiently large, the change from R = to hfV / is rather abrupt therefore, it is called coil-globule transition. Figure 2.27 summarizes how... 

Helix-Random coil] transition vs. [Coil-Globule] transition... 

Fig. 8 Conformational transition of single chain molecule in dilute solution (HELIX-RANDOM COILJ vs. [COIL-GLOBULE] transition...

Mel nikov, S.M., Sergeyev, V.G. and Yoshikawa, K. (1995b) Transition of double-stranded DNA chains between random coil and compact globule states induced by cooperative binding of cationic surfactants. J. Am. Chem. Soc., 117,9951-9956. 

The effect of copolymer sequence on coil-to-globule transition was also studied using Langevin molecular dynamics [103]. The method for estimation of the quality of reconstruction of core-shell globular structure after chain collapse was proposed. It was found that protein-like sequences exhibit better reconstruction of initial globular structure after the cooling procedure, as compared to purely random sequences. 

Fig. 2 Schematic of four thermodynamically stable states (random coil, crumpet coil, molten globule and collapsed globule) of a homopolymer chain in the coil-to-globule and the globule-to-coil transitions. There exists a hysteresis between the two transitions around the 0-temperature ( 30.6 °C) of the PNIPAM solution [37]...

---