Coil-globule transition
Encyclopedia
In polymer physics
, the Coil-globule transition is the collapse of a macromolecule
from an expanded coil state through an ideal coil state
to a collapsed globule state, or vice-versa. The coil-globule transition is of importance in biology
due to the presence of coil-globule transitions in biological macromolecules such as protein
s and DNA
. It is also analogous with the swelling behavior of a crosslinked
polymer
gel
and is thus of interest in biomedical engineering
for controlled drug delivery. A particularly prominent example of a polymer possessing a coil-globule transition of interest in this area is that of Poly(N-isopropylacrylamide)
(PNIPAAm).
of the macromolecule scales as its chain length to the three fifths power. As it passes through the coil globule transition, it shifts to scaling as chain length to the half power (at the transition) and finally to the one third power in the collapsed state. The direction of the transition is often specified by the constructions 'coil-to-globule' or 'globule-to-coil' transition.
behavior to poor solvent behavior. The canonical coil-globule transition is associated with the Upper critical solution temperature
and the associated Flory theta point. In this case, collapse occurs with cooling and results from favorable attractive energy of the polymer to itself. A second type of coil globule transition is instead associated with the lower critical solution temperature
and its corresponding theta point. This collapse occurs with increasing temperature and is driven by an unfavorable entropy of mixing. An example of this type is embodied by the polymer PNIPAAM, mentioned above. Coil globule transitions may also be driven by charge effects, in the case of polyelectrolytes. In this case pH and ionic strength changes within the solution may trigger collapse, with increasing counterion concentration generally leading to collapse in a uniformly charged polyelectrolyte. In polyampholytes containing both positive and negative charges, the opposite may hold true.
Polymer physics
Polymer physics is the field of physics that studies polymers, their fluctuations, mechanical properties, as well as the kinetics of reactions involving degradation and polymerisation of polymers and monomers respectively....
, the Coil-globule transition is the collapse of a macromolecule
Macromolecule
A macromolecule is a very large molecule commonly created by some form of polymerization. In biochemistry, the term is applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...
from an expanded coil state through an ideal coil state
Ideal chain
An ideal chain is the simplest model to describe a polymer. It only assumes a polymer as a random walk and neglects any kind of interactions among monomers...
to a collapsed globule state, or vice-versa. The coil-globule transition is of importance in biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
due to the presence of coil-globule transitions in biological macromolecules such as protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s and DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. It is also analogous with the swelling behavior of a crosslinked
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
gel
Gel
A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
and is thus of interest in biomedical engineering
Biomedical engineering
Biomedical Engineering is the application of engineering principles and design concepts to medicine and biology. This field seeks to close the gap between engineering and medicine: It combines the design and problem solving skills of engineering with medical and biological sciences to improve...
for controlled drug delivery. A particularly prominent example of a polymer possessing a coil-globule transition of interest in this area is that of Poly(N-isopropylacrylamide)
Poly(N-isopropylacrylamide)
Poly is a temperature-responsive polymer that was first synthesized in the 1950s. It can be synthesized from NIPAM which is commercially available....
(PNIPAAm).
Description
In its coil state, the radius of gyrationRadius of gyration
Radius of gyration or gyradius is the name of several related measures of the size of an object, a surface, or an ensemble of points. It is calculated as the root mean square distance of the objects' parts from either its center of gravity or an axis....
of the macromolecule scales as its chain length to the three fifths power. As it passes through the coil globule transition, it shifts to scaling as chain length to the half power (at the transition) and finally to the one third power in the collapsed state. The direction of the transition is often specified by the constructions 'coil-to-globule' or 'globule-to-coil' transition.
Origin
This transition is associated with the transition of a polymer chain from good solvent behavior through ideal or Theta solventTheta solvent
In a polymer solution, a theta solvent is a solvent in which polymer coils act like ideal chains, assuming exactly their random walk coil dimensions...
behavior to poor solvent behavior. The canonical coil-globule transition is associated with the Upper critical solution temperature
Upper critical solution temperature
The upper critical solution temperature or upper consolute temperature is the critical temperature above which the components of a mixture are miscible in all proportions. The word upper indicates that the UCST is an upper bound to a temperature range of partial miscibility, or miscibility for...
and the associated Flory theta point. In this case, collapse occurs with cooling and results from favorable attractive energy of the polymer to itself. A second type of coil globule transition is instead associated with the lower critical solution temperature
Lower critical solution temperature
The lower critical solution temperature or lower consolute temperature is the critical temperature below which the components of a mixture are miscible for all compositions...
and its corresponding theta point. This collapse occurs with increasing temperature and is driven by an unfavorable entropy of mixing. An example of this type is embodied by the polymer PNIPAAM, mentioned above. Coil globule transitions may also be driven by charge effects, in the case of polyelectrolytes. In this case pH and ionic strength changes within the solution may trigger collapse, with increasing counterion concentration generally leading to collapse in a uniformly charged polyelectrolyte. In polyampholytes containing both positive and negative charges, the opposite may hold true.
See also
- Upper critical solution temperatureUpper critical solution temperatureThe upper critical solution temperature or upper consolute temperature is the critical temperature above which the components of a mixture are miscible in all proportions. The word upper indicates that the UCST is an upper bound to a temperature range of partial miscibility, or miscibility for...
- Lower critical solution temperatureLower critical solution temperatureThe lower critical solution temperature or lower consolute temperature is the critical temperature below which the components of a mixture are miscible for all compositions...
- Critical pointCritical pointCritical point may refer to:*Critical point *Critical point *Critical point *Construction point of a ski jumping hill-See also:*Brillouin zone*Percolation thresholds...
- Ideal solutionIdeal solutionIn chemistry, an ideal solution or ideal mixture is a solution with thermodynamic properties analogous to those of a mixture of ideal gases. The enthalpy of solution is zero as is the volume change on mixing; the closer to zero the enthalpy of solution is, the more "ideal" the behavior of the...