Single random coil

In water, SPG exists as a thermodynamically stable triple-helix (Fig. 2) held together by hydrogen bonds. Under special conditions these hydrogen bonds can be broken for further discussion, it is important to know that, in DMSO, schizophyllan dissociates and exists as a single randomly-coiled chain, but it will re-associate into triple helices upon dilution with water. 

The intensity I(q) of scattering from a single random-coil polymer molecule in solution, obeying Gaussian statistics, can be written (see Section 5.2.3.1) as... 

An interesting outgrowth of these considerations is the idea that In r versus K or Vj should describe a universal calibration curve in a particular column for random coil polymers. This conclusion is justified by examining Eq. (9.55), in which the product [i ]M is seen to be proportional to (rg ), with r = a(rg 0 ) - This suggests that In rg in the theoretical calibration curve can be replaced by ln[r ]M. The product [r ]M is called the hydrodynamic volume, and Fig. 9.17 shows that the calibration curves for a variety of polymer types merge into a single curve when the product [r ]M, rather than M alone, is used as the basis for the cafibration. 

As for further analysis, curve fitting against the worm-like chain (WLC) model was conducted and indicated as a solid line in Figure 21.4. The model describes single polymer chain mechanics ranging from random-coil to fully extended forms, as follows ... 

Figure 2. Theoretically predicted size exclusion effects of different solute types (cylindrical pores of single pore size). (O) random coil (Rg) (%) hard sphere (R, = r VIJJ) CA rod (R, = L, VIT)...

Of the 20 reduced or disulfide bond-free proteins for which the Rg of the chemically or thermally unfolded state has ben reported, 17 fall on a single curve (Fig. 4). Fitting the Rg of these 17 proteins produces a strong, statistically significant correlation (r 2 = 0.96) and an exponent, v = 0.61 d= 0.03, startlingly close to the expected value for an excluded-volume random coil chain. 

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