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
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
due to the presence of coil-globule transitions in biological macromolecules such as
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s
and
DNA. It is also analogous with the swelling behavior of a
crosslinked
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
gel
A gel is a semi-solid 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, although the liquid phase may still dif ...
and is thus of interest in
biomedical engineering
Biomedical engineering (BME) or medical engineering is the application of engineering principles and design concepts to medicine and biology for healthcare purposes (e.g., diagnostic or therapeutic). BME is also traditionally logical sciences ...
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).
Description
In its coil state, the
radius of gyration ''Radius of gyration'' or gyradius of a body about the axis of rotation is defined as the radial distance to a point which would have a moment of inertia the same as the body's actual distribution of mass, if the total mass of the body were concentr ...
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 solvent
In a polymer solution, a theta solvent (or θ solvent) is a solvent in which polymer coils act like ideal chains, assuming exactly their random walk coil dimensions. Therefore, the Mark–Houwink equation exponent is 1/2 in a theta solvent. Ther ...
behavior to poor solvent behavior. The canonical coil–globule transition is associated with the
Upper critical solution temperature
The upper critical solution temperature (UCST) 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 tem ...
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
The lower critical solution temperature (LCST) or lower consolute temperature is the critical temperature below which the components of a mixture are miscible in all proportions. The word ''lower'' indicates that the LCST is a lower bound to a t ...
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
polyelectrolyte
Polyelectrolytes are polymers whose repeating units bear an electrolyte group. Polycations and polyanions are polyelectrolytes. These groups dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are ...
s. 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 temperature
The upper critical solution temperature (UCST) 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 tem ...
*
Lower critical solution temperature
The lower critical solution temperature (LCST) or lower consolute temperature is the critical temperature below which the components of a mixture are miscible in all proportions. The word ''lower'' indicates that the LCST is a lower bound to a t ...
*
Critical point
*
Ideal solution
Citations
{{DEFAULTSORT:Coil-Globule Transition
Biochemistry
Thermodynamic processes
Polymer chemistry
Polymer physics