On the temperature-induced coil to globule transition of poly-N-isopropylacrylamide in dilute aqueous solutions (original) (raw)
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Coil−Globule Transition of Poly(N-isopropylacrylamide): A Study of Polymer−Surfactant Association
Macromolecules, 1996
The phase behavior of a fractionated high molecular weight sample of poly(N-isopropylacrylamide) in dilute aqueous solution containing a surfactant, sodium dodecyl sulfate (SDS), is studied with temperature and surfactant Concentration as independent variables. Static and dynamic light scattering are used as methods of investigation. Without surfactant, the polymer exhibits a lower critical solution temperature (LCST) of 34 OC, above which it precipitates. At SDS concentrations of only 250 mg/L, aggregation is completely prevented (intermolecular solubilization) and the behavior of isolated polymer molecules can be studied in the whole temperature range: Upon heating, the polymer undergoes a coil-to-globule phase transition with a volume reduction by a factor of more than 300. The transition temperature depends on the surfactant concentration. It is first constant and equal to the LCST, but begins to increase above 300 mg/L surfactant. Therefore when increasing the surfactant concentration at constant temperatures above the LCST, we cross the phase boundary and observe intramolecular solubilization, Le., a surfactant-induced globule-to-coil transition. Below the LCST, the surfactant causes an expansion of the polymer coils also setting on at 300 mg/L.
Macromolecular Rapid Communications, 1995
The coil-globule transition of poly(Nisopropylacry1amide) (systematic name: poly-[ 1-(isopropylaminocarbonyl)ethylene]) has been viscometrically investigated in low-concentration aqueous sodium dodecyl sulfate solution. In this environment, even if the macromolecular coils collapse as the temperature increases above the lower critical solution temperature of the polymer, 3 4 T , the polymer does not precipitate. It seems that the hydrophobically collapsed macromolecular coils do not aggregate, but remain in solution with the aid of surface charges supplied by surfactant ions adsorbed through hydrophobic interactions.
Physical Chemistry Chemical Physics, 2012
We observed phase transition and phase relaxation processes of a poly(N-isopropylacrylamide) (PNIPAM) aqueous solution using the heterodyne transient grating (HD-TG) method combined with the laser temperature jump technique. The sample temperature was instantaneously raised by about 1.0 K after irradiation of a pump pulse to crystal violet (CV) molecules for heating, and the phase transition was induced for the sample with an initial temperature just below the lower critical solution temperature (LCST); the following phase relaxation dynamics was observed. Turbidity relaxation was observed in both the turbidity and HD-TG responses, while another relaxation process was observed only in the HD-TG response, namely via the refractive index change. It is suggested that this response is due to formation of globule molecules or their assemblies since they would have nothing to do with turbidity change but would affect the refractive index, which is dependent on the molar volume of a chemical species. Furthermore, the grating spacing dependence of the HD-TG responses suggests that the response was caused by the counter propagating diffusion of the coil molecules as a reactant species and the globule molecules as a product species and the lifetime of the globule molecules ranged from 1.5 to 5 seconds. Thus, we conclude that the turbidity reflects the dynamics of aggregate conditions, not molecular conditions. The coil and globule sizes were estimated from the obtained diffusion coefficient. The sizes of the coil molecules did not change at the initial temperatures below the LCST but increased sharply as it approaches LCST. We propose that the coil-state molecules associate due to hydrophobic interaction when the initial temperature was higher than LCST minus 0.5 K and that the globule-state molecules generated from the coil-state molecules showed a similar trend in temperature. The phase transition was also induced by heating under a microscope, and the relaxation process was followed using the fluorescence peak shift of a fluorescent molecule-labeled PNIPAM. The result also supports the existence of a globule molecule or its assembly remains for several seconds in the phase relaxation.
Impact of Hydrophobic Sequence Patterning on the Coil-to-Globule Transition of Protein-like Polymers
Macromolecules, 2012
Understanding the driving forces for the collapse of a polymer chain from a random coil to a globule would be invaluable in enabling scientists to predict the folding of polypeptide sequences into defined tertiary structures. The HP model considers hydrophobic collapse to be the major driving force for protein folding. However, due to the inherent presence of chirality and hydrogen bonding in polypeptides, it has been difficult to experimentally test the ability of hydrophobic forces to independently drive structural transitions. In this work, we use polypeptoids, which lack backbone hydrogen bonding and chirality, to probe the exclusive effect of hydrophobicity on the coil-to-globule collapse. Two sequences containing the same composition of only hydrophobic "H" N-methylglycine and polar "P" N-(2-carboxyethyl)glycine monomers are shown to have very different globule collapse behaviors due only to the difference in their monomer sequence. As compared to a repeating sequence with an even distribution of H and P monomers, a designed protein-like sequence collapses into a more compact globule in aqueous solution as evidenced by small-angle X-ray scattering, dynamic light scattering, and probing with environmentally sensitive fluorophores. The free energy change for the coil-to-globule transition was determined by equilibrium denaturant titration with acetonitrile. Using a two-state model, the protein-like sequence is shown to have a much greater driving force for globule formation, as well as a higher m value, indicating increased cooperativity for the collapse transition. This difference in globule collapse behavior validates the ability of the HP model to describe structural transitions based solely on hydrophobic forces.
Helix-Coil transition in polypeptides: A microscopical approach
Biopolymers, 1990
ABSTRACT In the framework of an earlier constructed model [N.S. Ananikyan et al. (1990) Biopolymers, Vol. 30, pp. 357-367], some analytical estimates for the correlation length and degree of helicity near the transition point were obtained in the case of an arbitrary topology of hydrogen bond closing (delta). It was shown that the Zimm-Bragg cooperativity parameter sigma is determined by the set of (delta-1) amino acid residues and so is nonlocal. An analytic expression for cooperativity parameters in a heteropolypeptide chain was obtained and numerical calculations showed that in case of heteropolypeptide with random primary structure the nonlocality of cooperativity parameter influenced the temperature dependence of helicity degree.
A Model for a Thermally Induced Polymer Coil-to-Globule Transition
Macromolecules, 2008
A semiquantitative mean-field model for the thermally induced (heating-induced) polymer coilto-globule transition (HCGT) is developed with no adjustable parameters. The transition temperature Θ is given for a long chain by the equation Θ) 2T p * [1-F B (Θ)], where T p * is the characteristic temperature of the polymer and F B (Θ) is the bulk solvent density at the transition temperature. The variables F B and T p * are obtained by invoking the Sanchez-Lacombe (S-L) equation of state. Calculated HCGT temperatures show good agreement with experimental lower critical solution temperatures (LCSTs). The predicted globular state is characterized by the dominance of attractive polymer self-interactions over excluded volume interactions. There is a critical value of the ratio of polymer to solvent S-L characteristic temperature below which no HCGT transition is predicted for an infinite chain. This model can be easily generalized to treat cross-linked gels and their contraction-expansion characteristics.
Coil–globule transition of a polymer involved in excluded-volume interactions with macromolecules
The Journal of Chemical Physics, 2015
Polymers adopt extended coil and compact globule states according to the balance between entropy and interaction energies. The transition of a polymer between an extended coil state and compact globule state can be induced by changing thermodynamic force such as temperature to alter the energy/entropy balance. Previously, this transition was theoretically studied by taking into account the excluded-volume interaction between monomers of a polymer chain using the partition function. For binary mixtures of a long polymer and short polymers, the coil-globule transition can be induced by changing the concentration of the shorter polymers. Here we investigate the transition caused by short polymers by generalizing the partition function of the long polymer to include the excluded-volume effect of short polymers. The coil-globule transition is studied as a function of the concentration of mixed polymers by systematically varying Flory's χ-parameters. We show that the transition is caused by the interplay between the excluded-volume interaction and the dispersion state of short polymers in the solvent. We also reveal that the same results can be obtained by combining the mixing entropy and elastic energy if the volume of a long polymer is properly defined.
Coil-globule transition in the denatured state of a small protein
Proceedings of the National Academy of Sciences, 2006
Upon transfer from strongly denaturing to native conditions, proteins undergo a collapse that either precedes folding or occurs simultaneously with it. This collapse is similar to the well known coil-globule transition of polymers. Here we employ singlemolecule fluorescence methods to fully characterize the equilibrium coil-globule transition in the denatured state of the IgGbinding domain of protein L. By using FRET measurements on freely diffusing individual molecules, we determine the radius of gyration of the protein, which shows a gradual expansion as the concentration of the denaturant, guanidinium hydrochloride, is increased all the way up to 7 M. This expansion is observed also in fluorescence correlation spectroscopy measurements of the hydrodynamic radius of the protein. We analyze the radius of gyration measurements using the theory of the coil-globule transition of Sanchez [Sanchez, I. C. (1979) Macromolecules 12, 980 -988], which balances the excluded volume entropy of the chain with the average interresidue interaction energy. In particular, we calculate the solvation energy of the denatured protein, a property that is not readily accessible in other experiments. The dependence of this energy on denaturant concentration is nonlinear, contrasting with the common linear extrapolation method used to describe denaturation energy. Interestingly, a fit to the binding model of chemical denaturation suggests a single denaturant binding site per protein residue. The size of the denatured protein under native conditions can be extrapolated from the data as well, showing that the fully collapsed state of protein is only Ϸ10% larger than the folded state.