Globules, collapsed

When polymer solutions encounter situations for which the thermodynamic quality of the solvent becomes poor, individual chains undergo a coil-to-globule collapse, the globules associate immediately, and macroscopic phase separation seems unavoidable. However, it has been reported that a number of polymers in water or in organic solvents form equilibrium globules, i.e. single-chain globules that remain in solution without immediate association and precipitation. 

In Fig. 12.1(a), the specific-heat curve for a system of two identical semiflexible polymers (2 x i3) is compared with the energetic fluctuations of a single chain (1 ycA ). The single chain exhibits a very weak coil globule collapse transition (shoulder near T 0.88), whereas the crystallization near T 0.24 is a pronounced, separate process. The thermodynamic phase behavior of single semiflexible polymers in solvent has already been discussed in Chapter 7. The first result for the semiflexible multiple-cham system obtained from... 

The parameter /r tunes the stiffness of the potential. It is chosen such that the repulsive part of the Leimard-Jones potential makes a crossing of bonds highly improbable (e.g., k= 30). This off-lattice model has a rather realistic equation of state and reproduces many experimental features of polymer solutions. Due to the attractive interactions the model exhibits a liquid-vapour coexistence, and an isolated chain undergoes a transition from a self-avoiding walk at high temperatures to a collapsed globule at low temperatures. Since all interactions are continuous, the model is tractable by Monte Carlo simulations as well as by molecular dynamics. Generalizations of the Leimard-Jones potential to anisotropic pair interactions are available e.g., the Gay-Beme potential [29]. This latter potential has been employed to study non-spherical particles that possibly fomi liquid crystalline phases. 

The collapse of the unfolded state to generate the molten globule embodies the main mystery of protein folding. What is the driving force behind the choice of native tertiary fold from a randomly oriented polypeptide chain ... 

For (Ar) interactions the collapsed state of the polymer is a tight globule from which solvent is excluded. Figure 15 shows the polymer bead and solvent radial distribution functions relative to the center of mass of the globule,... 

An excluded-volume random-coil conformation will be achieved when the solvent quality exceeds the theta point, the temperature or denatu-rant concentration at which the solvent-monomer interactions exactly balance the monomer—monomer interactions that cause the polymer to collapse into a globule under more benign solvent conditions. A number of lines of small-angle scattering—based evidence are consistent with the suggestion that typical chemical or thermal denaturation conditions are good solvents (i.e., are beyond the theta point) and thus that chemically or thermally unfolded proteins adopt a near random-coil conformation. 

These spectra are similar to that of native RNase A at 95°C (not shown). The far-UV spectrum at 95°C indicates a retention of substantial p-sheet secondary structure, but a significant loss of the a-helix conformation as indicated by the decrease of intensity at 222 nm.48 The near-UV spectrum at 95°C indicates a complete collapse of tertiary structure as is seen in molten globule proteins.49 Trace 3 is the sample from trace 2 after cooling the protein to 23°C. Both spectra reveal little recovery of either secondary or tertiary protein structure. 

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