A globule is a state (set of conformations) of a polymer chain in which fluctuations in the concentration of units are small: their correlation radius is much smaller than the size of a macromolecule. The concentration of units in the globular state is much higher than in the polymer coil , and this concentration is constant throughout the volume of the globule, except for a thin layer on the surface (for example, the polymer / solvent interface), called the edge of the globule.
Content
Tangle-Globule Transition
As a rule, in a homopolymer solution, when the quality of the solvent deteriorates (usually with decreasing temperature), the polymer chain experiences a coil- globule transition. Physically, this is because a poor solvent corresponds to the attractive bulk interaction of the units . First, just below the θ temperature (by an order of magnitude where - the degree of polymerization of the chain, that is, the number of links in it) the coil collapses into a loose globule close in size to the coil; with further deterioration in the quality of the solvent, the globule becomes dense. The tangle-globule phase transition was phenomenologically studied by P.J. Flory in 1949 [1] ; modifications of the Flory approach proposed by Ptitsyn and Eisner, Birshtein and Pryamitsyn (1986) are often used. A much more rigorous, but complex method was proposed in 1979 by I. M. Lifshitz , A. Yu. Grosberg, and A. R. Khokhlov based on an approach representing the conformational entropy of a chain in the form of a density functional ( Lifshitz formula ) [2] .
Proteins can serve as an example of a globule. However, they exhibit complex behavior during denaturation-renaturation: although some of the above methods were developed precisely in view of the transition from native to denatured conformation in proteins, they turned out to be inapplicable to such a complex object.
Soluble Globules
As a rule, the surface tension of the globule is positive, that is, the globules in solution aggregate and precipitate. This is easy to explain: if the interaction of the links with each other and with the solvent molecules causes them to attract and form a globule within the same chain, then the links of different chains will also be attracted.
However, some substances - for example, proteins - are soluble globules. However, proteins have a rather complex structure, which is difficult for chemical synthesis and is difficult to theoretically analyze. The preparation of soluble globules can be promising both for explaining the properties of soluble globular biopolymers and in practical applications: for targeted drug delivery , molecular scaffolds, catalysis, and fuel cells . Experimentally soluble globules formed by amphiphilic homopolymers were obtained (1990s) and are studied in the group of F. Vinnik (Françoise Winnik) [3] . In computer experiments, the existence of soluble globules was discovered in the early 2000s in the group of V. Vasilevskaya. Theoretically, this behavior was explained somewhat later by A. Semenov et al.
Literature
- A. Yu. Grosberg, A.R. Khokhlov, Statistical Physics of Macromolecules . - M .: Nauka, 1989. ISBN 5-02-014055-4
- G. M. Bartenev, S. Ya. Frenkel, Polymer Physics . - L .: Chemistry, 1990. ISBN 5-7245-0554-1
Notes
- ↑ Paul J. Flory, The Configuration of Real Polymer Chains , J. Chem. Phys. 17 , 303 (1949)
- ↑ I. M. Lifshits, A. Yu. Grosberg, A. P. Khokhlov, Volume interactions in the statistical physics of a polymer macromolecule , Phys. Uspekhi 127 (3) (1979)
- ↑ See, for example, A. Laukkanen, L. Valtola, FM Winnik, and H. Tenhu, Formation of Colloidally Stable Phase Separated Poly (N-vinylcaprolactam) in Water , Macromolecules 2004, 37 , 2268–2274