INDEX random coil

It is essential that the solution be sufficiently dilute to behave ideally, a condition which is difficult to meet in practice. Ordinarily the dilutions required are beyond those at which the concentration gradient measurement by the refractive index method may be applied with accuracy. Corrections for nonideality are particularly difficult to introduce in a satisfactory manner owing to the fact that nonideality terms depend on the molecular weight distribution, and the molecular weight distribution (as well as the concentration) varies over the length of the cell. Largely as a consequence of this circumstance, the sedimentation equilibrium method has been far less successful in application to random-coil polymers than to the comparatively compact proteins, for which deviations from ideality are much less severe. 

Fig. 3 Dissolving kinetics (in terms of average hydrodynamic radius Rh) of collapsed single-chain PNIPAM globules, where t is the standing time after the solution temperature was quenched from 33.02 to 30.02 °C and the dashed line represents a stable average value of Rh of individual PNIPAM random coils at 30.02 °C. The weight-average molar mass (Mw) of the PNIPAM sample used is 1.08 x 107g/mol with a polydispersity index (Mw/Mn) less than 1.1 [32]...

Figure 3. Values ofr as a function ofV( 15)fP(90) for solvent refractive index 1.405. Random coils (---------)y spheres (-----).

Figure 3. Chemical shift deviation of the assigned residues compared to the random coil values for the first twelve amino terminal residues. Bottom panel shows the consensus chemical shift index for the complete protein.

Conformations of polymer chains in dilute solutions under theta conditions are essentially identical to the random coil conformations of chains in amorphous polymers [26], where the interactions of polymer chains with solvent molecules are replaced by interactions between polymer chains. The density and the refractive index in the amorphous limit of a polymer are therefore the appropriate values of pP and nP to use in calculating the specific refractive index increment via Equation 8.16. The correlation developed for V(298K) in Section 3.C was hence used to calculate pP, and the correlation developed in Section 8.C was used to calculate nP. 

Scattering Factor The scattering function P 6) depends on polymer dimension, wavelength of the light, and the refractive index of the solvent. The following function has been derived for random coil polymer (Doty and Edsall, 1951) ... 

Figure 9-8. Dependence of the dissymmetry coefficient z of scattered light at angles of 45° and 135° on the ratio of >/X for spheres (sph), unimolecular random coils cm)y polymolecular (Mn) = 2) random coils Cp), and rods (r). Here Xo is the wavelength of light in the medium of refractive index n, and D corresponds to the diameter of the spheres, the length of the rods, and the chain end-to-end distance of coiled macromolecules.

Hyperchromic effect increase in the absorbance of a solution at a particular wavelength due to structural changes in the solute molecules. The H.e. is a useful experimental index of DNA denaturation, since the A2J0 of a DNA solution increases when the double helix is transformed by heating into a disordered random coil (see Hybridization). 

The problem of the sense of the helix has also been attacked with the aid of optical rotation measurements in the following manner. Downie et al. (1957) investigated the optical rotation of copolymers of l- and D-leucine in benzene and in trifluoroacetic acid. In the latter solvent the copolymers exist as random coils, independent of the fraction of the L-form [designated as l/(d -f- l)]. Since the residue rotation (corrected for the dispersion of the refractive index) is a linear function of l/(d - - l), as shown in Fig. 96b, and is zero when l/(d + l) = 0.5, the observed rotation may be attributed to the excess of l- over n-residues. This conclusion is based on the assumption that the optical rotation of independent groups is additive, and is supported by the straight-line graphs obtained for a variety of solvents and randomly coiled polypeptides. The existence of the polypeptide in the randomly coiled form in trifluoroacetic acid is consistent with the absence of a contribution from any helical configuration. 

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