Interactions between coils

This proportion yields a number that characterizes the degree to which two coils in the 0 state see each other. The effect is overlayed by any specific interactions between coils. In the simplest case it describes the opportunity for topological entanglements. Thus, entanglement effects that show up on a macroscopic time scale are characterized by a particular degree of interaction pi2 (e.g., neutron scattering (37) and viscometry). At short times the... 

In an improved approximation, we have to take into account the interactions between coils in a good solvent two coils tend to repel each other. Flory has shown that in this regime the coils behave essentially like hard spheres of radius This implies an equation of state of the form... 

Many of the mesoscale techniques have grown out of the polymer SCF mean field computation of microphase diagrams. Mesoscale calculations are able to predict microscopic features such as the formation of capsules, rods, droplets, mazes, cells, coils, shells, rod clusters, and droplet clusters. With enough work, an entire phase diagram can be mapped out. In order to predict these features, the simulation must incorporate shape, dynamics, shear, and interactions between beads. 

We saw in Sec. 1.11 that coil dimensions are affected by interactions between chain segments and solvent. Both the coil expansion factor a defined by Eq. (1.63) and the interaction parameter x are pertinent to describing this situation. 

In addition to an array of experimental methods, we also consider a more diverse assortment of polymeric systems than has been true in other chapters. Besides synthetic polymer solutions, we also consider aqueous protein solutions. The former polymers are well represented by the random coil model the latter are approximated by rigid ellipsoids or spheres. For random coils changes in the goodness of the solvent affects coil dimensions. For aqueous proteins the solvent-solute interaction results in various degrees of hydration, which also changes the size of the molecules. Hence the methods we discuss are all potential sources of information about these interactions between polymers and their solvent environments. 

Metal contained in the channel is subjected to forces that result from the interaction between the electromagnetic field and the electric current in the channel. These inward forces produce a circulation that is generally perpendicular to the length of the channel. It has been found that shaping the channels of a twin coil inductor shown in Figure 10 produces a longitudinal flow within the channel and significantly reduces the temperature difference between the channel and the hearth (12). 

The length of the zone and the diameter of the tod are chosen in such a way that surface tension and interactions between circulating electric currents in the molten zone and the radio-frequency (r-f) field from the surrounding induction coil keep the molten zone in place. As of this writing (ca 1996), the maximum sihcon rod diameter that can be purified in this manner is ca 125 mm. Initially, additional purification can be obtained by making mote sweeps of the zone. Eventually, however, more sweeps do not remove any additional impurities. The limiting profile is given by equation 4 ... 

Figure 10.18 Side-chain interactions in the leucine zipper structure, (a) The hydrophobic side chains in spikes a and d (see Figure 10.17) form a hydrophobic core between the two coiled a helices, (b) Charged side chains in spikes and g can promote dimer formation by forming complementary charge interactions between the two a helices.

The structure of the chain, i.e., whether it is a helix or a random coil, might influence not only the rate but also the stereospecificity of the growing polymer. For example, it is plausible to expect that in normal vinyl polymerization helix formation might favor specific placement, say isotactic, while either placement would be approximately equally probable in a growing random coil. Formation of a helix requires interaction between polymer segments, and this intramolecular interaction is enhanced by bad solvents particularly those which precipitate the polymer. 

The Zimm model predicts correctly the experimental scaling exponent xx ss M3/2 determined in dilute solutions under 0-conditions. In concentrated solution and melts, the hydrodynamic interaction between the polymer segments of the same chain is screened by the host molecules (Eq. 28) and a flexible polymer coil behaves much like a free-draining chain with a Rouse spectrum in the relaxation times. 

Amino acid substitutions on the native y52 8sKIpeptide, coiled-coil domain of human fibrin were able to stabilize the coiled-coil formation. These substitutions were targeted to the positions that compose the interface between coiled-coil strands while the solvent-exposed residues were left unperturbed. This strategy aimed at reducing the likelihood of immunogenicity for future in vivo apphcafion of these materials. In contrast to PEG block copolymers with end blocks that are not used for directed assembly, PEG copolymers with coiled-coil protein motives aim to enhance intermolecular interactions and control over the assembly conditions [85, 173]. 

The CD spectra of nine proteins in 6 M Gdm-HCl were studied by Cortijo etal. (1973). Those proteins with disulfide bridges were reduced and carboxymethylated. The spectra of individual proteins were not reported, but the range of values at wavelengths from 240 to 210 nm was given. The [0]222 values ranged from —800 to —2400 deg cm2/dmol. From this substantial variation, Cortijo etal. (1973) concluded that the proteins studied are not true random coils in 6 M Gdm-HCl, because random coils should have CD spectra essentially independent of amino acid composition and sequence. The observed variation was attributed to differences in the conformational distribution between allowed regions of the Ramachandran map or to residual interactions between different parts of the chain that are resistant to Gdm-HCl denaturation. 

Protein-protein interactions between the histone subunits are undoubtedly important in promoting formation of a nucleosome in which 146 base pairs of DNA are coiled around the outside of the histone core. One molecule of histone HI binds to an exterior region of each nucleosome, but histone HI is not needed to determine nucleo-some structure. The distance between nucleosomes is approximately 200 base pairs consequently, in electron micrographs, nucleosomes resemble evenly spaced beads on a string of DNA. Neutron and x-ray diffraction data are also consistent with this structure. 

The expansion of a polymer coil is determined by its interaction with the solvent. The more favorable the interaction between the polymer segments and the solvent molecules (good solvent), the better the polymer dissolves and the more the coil expands. 

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