Size-exclusion partition coefficients

Stokes radius determination. The Stokes radii (Rs) of DnaK, RCMLA, NCA-SNase and their complexes were determined by SEC-HPLC using a series of standard globular proteins for which Stokes radii were available (15. and references therein). It is well established that size-exclusion partition coefficients can be correlated to the moleculzir mass, MM, of proteins by eq. 3 ... 

The elution volume, F/, and therefore the partition coefficient, is a function of the size of solute molecule, ie, hydrodynamic radius, and the porosity characteristics of the size-exclusion media. A protein of higher molecular weight is not necessarily larger than one of lower molecular weight. The hydrodynamic radii can be similar, as shown in Table 4 for ovalbumin and a-lactalbumin. The molecular weights of these proteins differ by 317% their radii differ by only 121% (53). 

When a dilute solution of a polymer (c << c ) is equilibrated with a porous medium, some polymer chains are partitioned to the pore channels. The partition coefficient K, defined as the ratio of the polymer concentration in the pore to the one in the exterior solution, decreases with increasing MW of the polymer (7). This size exclusion principle has been used successfully in SEC to characterize the MW distribution of polymer samples (8). 

Fig. 14. Prediction of solute permeability for solutes of different hydrodynamic radii in swollen gels. Permeability is calculated as the product of the partition coefficient, K, using the size exclusion theory of Schnitzer and the ratio of the solute diffusion coefficient in the gel, D, to its value in solution, D , using the theory of Yasuda et al. [123, 159, 160]...

The experiments which yielded the diffusion coefficients for acetaminophen in PNIPAAm gel in Fig. 16 also yielded the corresponding partition coefficients. While the diffusion coefficients fit theory, the partition coefficients as plotted in Fig. 18 do not at all. In fact, a trend opposite to theory is observed as the partition coefficients are seen to increase as the gel swelling decreases. In fact, above the transition temperature of the gel, at 35 °C, the partition coefficient is seven times the maximum possible size exclusion coefficient, 1. This implies the dominance of hydrophobic effects over steric effects, since acetaminophen is a relatively small, nonionic but hydrophobic solute, and while the gel mesh size shrinks with increasing temperature, its level of hydrophobicity increases with temperature. 

To test this, we determined the partition coefficients of two different solute extremes - vitamin B-12 and norethindrone. Vitamin B-12 is a relatively large, hydrophilic solute, while norethindrone is a relatively small, hydrophobic solute. Thus we would expect size exclusion to dominate partitioning with vitamin B-12, while hydrophobic interactions should dominate with norethindrone. This behavior is in fact observed in Figs. 19 and 20. The values of K... 

Fig. 18. The partition coefficients of acetaminophen in 10 x 4 PNIPAAm gels as a function of the swelling degree of the gel. The trend is opposite to that estimated by the ideal size exclusion theory of Schnitzer, shown as a solid line [17]. The dashed line is to guide the eye. Reprinted from the Journal of Controlled Release (1992) 18 1, by permission of the publishers, Elsevier Science Publishers...

The enthalpic term in eqn [8] can be considered as a partition coefficient Kp, whose value is unity AH° = 0) if the size exclusion is the only interaction of the solute molecules with the column packing. If the adsorption and/or partition of the solute takes place in the chromatographic system, Kp > 1 AH° is negative), or if an incompatibility between the solute and the column packing exists, Kp lies between zero and unity AH° is positive). Consequently, the en-tropic term in eqn [8] represents the distribution coefficient Ka of pure size exclusion. Ka acquires values between zero and unity and, in agreement with the experimental findings, is independent of the temperature. Hence, the distribution coefficients can be redefined as... 

Figure 7.1.26. Characteristics of size exclusion chrorrmtography/gel permeation chrorrmtography. (a) Partition coefficient afmacromol-ecular species i between solution and gel particle Mils die molecular weight of macromolecular species, (b) Elution volume of macromolecular species i. (c) Universal calibration plot of many polymers in tetrahydrofuran (THF) log [i ]i M[ vs. elution volume Vgi. (After Grubisic et al. (1967).)...

Size Exclusion Mechanism. Steric exclusion (or size exclusion) is the main process of polymer fractionation by GPC. This mechanism is based on a thermodynamic equilibrium between two phases the interstitial solvent in the dead volume V and the solvent filling the porous volume Vp. If is the partition coefficient, the elution volume of a macromolecule is defined by ... 

The only restriction on this technique is that there must be no interactions between the gel and solute beyond Donnan exclusion. Since the partition coefficient is often affected by several factors, including molecular size, electric potential, and hydrophobic interactions [52], this can be a serious problem. Since the lack of... 

Figure 6 shows the effects of a homologous series of alkyl sulfates and alkyl car-boxylates on the permeability properties of porcine skin to water. Consistent with the findings of others, maximal tissue damage was observed for the C12 species. This maximum reflects a delicate balance between several mutually exclusive forces. For example, lipo-philicity and the ability to partition into a oil phase increases with alkyl chain length. However, offsetting these effects are the limitations imposed by decreases in solubility and permeability coefficients due to increasing molecular size [130]. 

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