Coiled coil structures discontinuities

M molar mass), where I and III are the tricritical or -regions. Here, the chain molecules exhibit an unperturbed random coil confirmation. In contrast, I and II are the critical or good solvent regimes, which are characterized by structural fluctuations in direction of an expanded coil conformation. According to the underlying concept of critical phenomena, the phase boundaries have to be considered as a continuous crossover and not as discontinuous transitions. 

A discontinuous coil to stretch transition is evident at sc = 0.000725. The transition point sc was found by using two different initial conformations as described above. For values lower than ec> the random chain will eventually coil, form a folded chain crystalline structure and stay in that conformation until the end of the run for relatively long run times. On the other hand, a prestretched chain would fluctuate and eventually form a crystallized folded chain that is stable. Similarly, for flow rates higher than sCy a pre-stretched chain will never coil and a random chain will eventually stretch. 

Figure 25-16 A model of myoglobin to show the way in which the polypeptide chain is coiled and folded. The shaded sections correspond to regions in which the chain is coiled into an a helix. Each fold, and the regions near the C-terminus and the /V-terminus, represent discontinuities in the helical structure. The position of the heme group is represented by the disoiike shape.

The thermodynamic affinity of cyclohexane to polystyrene is known to increase with temperature and, naturally, increasing the temperature must further raise the volume of the polystyrene networks in cyclohexane. There is, however, an additional point we should consider. The plot of Q vs. temperature exhibits a steplike discontinuity at around 30°C (Fig. 1.14). This discontinuity, resemhling very much a -transition, is located 3-5°C below the -temperature for linear polystyrene in cyclohexane and about 8°C above the -point for star-shaped polystyrene macromolecules. This phenomenon is outside the scope of the questions discussed here, but, naturally, the first assumption of the authors [143] seems to be very logical, according to which the discontinuity reflects a transition from Gaussian coil to a supercoiled compact structure on cooling the swollen gel below that temperature zone. 

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