Coil state

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

How does the polymer chain sneak out from the entangled random coil state and how does it order into a crystalline state ... 

Fig. 8. CD spectra of collagen in 0.01 m acetic add. (A) At 24 °C in the native state, (B) at 44 °C in a randomly coiled state. Data from Piez et al.la)...

To answer the question whether the ds-transisomerization of the bridged polypeptides with a Ala-Gly-Pro sequence represents the rate-determining step, the following experiment was carried out The polypeptide with a chain length n = 8 was denaturated in a rapid reaction with a temperature jump from 9.2 to 30 °C and subjected to renatura-tion at 9.2 °C after an incubation time of 25 s. In a second and a third experiment, the incubation in the coiled state was prolonged respectively to 75 and 125 s. It could be observed that the amplitude of the rapid phase depends on the time that lapses between the denaturation and renaturation (Fig. 32). 

At higher temperatures (T > Tm), model II predicts larger values of 0, because this increases the number of possibilities of forming a helix, in contrast to the AON model (I). On the other hand, at lower temperatures, model I gives more possibilities of the realization of the coiled state and thus the transition curve near = 1 is flat in comparison with the AON case. 

The interpretation of these results is, however, problematic since no data on the absolute enthalpy and entropy of the respective triple helix and coiled state are available. Though it may be taken as an established fact that the entropy of conformation of a (Pro-Pro-Gly) coil is lower than in the case of a (Pro-Ala-Gly)n coil, we are not sure whether the entropy of the triple helix depends on the imino acid content. 

Plotting U as a function of L (or equivalently, to the end-to-end distance r of the modeled coil) permits us to predict the coil stretching behavior at different values of the parameter et, where t is the relaxation time of the dumbbell (Fig. 10). When et < 0.15, the only minimum in the potential curve is at r = 0 and all the dumbbell configurations are in the coil state. As et increases (to 0.20 in the Fig. 10), a second minimum appears which corresponds to a stretched state. Since the potential barrier (AU) between the two minima can be large compared to kBT, coiled molecules require a very long time, to the order of t exp (AU/kBT), to diffuse by Brownian motion over the barrier to the stretched state at any stage, there will be a distribution of long-lived metastable states with different chain conformations. With further increases in et, the second minimum deepens. The barrier decreases then disappears at et = 0.5. At this critical strain rate denoted by ecs, the transition from the coiled to the stretched state should occur instantaneously. 

If e is now decreased, with the chain in the extended state, the dumbbell nevertheless stays in the stretched state where the potential is the lowest. The transition back to the coiled state occurs only when there is a single minimum on the potential energy curve, i.e. at et = 0.15. Since the critical strain-rate for the stretch-to-coil transition (esc) is much below the corresponding value for the coil-to-stretch transition (eca), the chain stretching phenomenon shows hysteresis (Fig. 11). 

First, proteins refold from the denatured state, not from the hypothetical random coil state. It is the starting point of all refolding reactions, whether in a cell or in a test tube. To understand any chemical reaction, structural features of the reactant and the product must be compared to quantify the changes that occur, for these changes define the reaction. 

To further our understanding of the behavior of unfolded proteins, it is necessary to employ experimental techniques able to discriminate between the dynamic true random coil state and more static types of disorder, including situations in which some ordered secondary structure may also be present. One such technique is a novel chiroptical... 

Equation 20 predicts a free-energy barrier for primary crystal nucleation (i.e., the free-energy difference between the top of the barrier and the initial coil state) as... 

Fig. 10 Equilibrium radius of gyration of a molecule plotted as a function of temperature the molecule is composed of 1000 beads. The radius of gyration shows a steep increase and a large fluctuation above 700 K. The insets show typical chain conformations at indicated temperatures. Note that the ideal random coil state of this fully flexible chain should have the mean-square radius of gyration R2 = 1000 x (1.54/3.92)2/6 = 25.7, the value is around 800 K...

Fig.1 A space-fiUing model showing the conformational equUihrium between hehcal and random coil states for a mPE oligomer (n=18). Side chains have been omitted for clarity...

Changes in reactivity and reaction kinetics in comparison to conversions of low-molecular-weight substances can also be attributed to the fact that macromolecules in solution are in a more or less coiled state. During the course of a reaction this state changes through alteration of the solubility parameters, thus facilitating or aggravating conversion. Finally one observes in-... 

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