Illustrated are the principles for isolating proteins from solution by sorption into a polymer gel phase, driven by the addition of a water-soluble polymer to the protein solution. The separation is shown to be analogous to conventional two-phase aqueous extraction. However, the use of a gel phase rather than a solution for absorbing the protein makes separation of the protein from the polymer and the recycling of the gel phase much simpler. The model system used was linear poly(ethylene glycol) (PEG) and dextran gel. Increasing the molecular weight and concentration of the PEG favored sorption by the gel of ovalbumin, bovine serum albumin, cytochrome c, and hemoglobin. The proteins could be quantitatively recovered by immersing the gel in PEG-free solution.

The state of aggregation of cholesterol at various temperatures in CHCl, and in CCl, solutions has been determined by vapor pressure osmometry (VPO). There is hydrogen bonding by the CHC1, to the cholesterol, so the self-association of cholesterol is weaker in CHC1, than in CCl,, where no solvent hydrogen bonding occurs. With both solvents the self-association increased as the temperature decreased. In CHC1, the self-association is described by two models-an indefinite self-association with all odd species beyond monomer absent, or a monomer-dimer association. In CC14 the self-association was best described by a monomer-trimer-hexamer model. Values of the thermodynamic functions for these self-associations are reported. Proton nuclear magnetic resonance studies on these solutions at 33.0 "C parallel the VPO studies. The chemical shift of the hydroxyl proton showed linear concentration dependence in CHCl,, whereas a parabolic concentration dependence was observed in CC4. These downfield chemical shifts indicated that hydrogen bonding was involved in the self-association.

Vapor pressure osmometry (VPO) and nuclear magnetic resonance (NMR) studies have been carried out on solutions of methyl lithocholate, methyl deoxycholate, and methyl cholate in nonaqueous solvents (CC14, CHC13, CH2C12, and CS2). At 37 O C the VPO experiments indicated that the self-association of each bile-acid ester was greater in CC14 than in CHC13 solutions and that an increase in the number of hydroxyl groups promoted self-association in both solvents. The concentration dependence of the proton chemical shift of the hydroxyl group(s) tended to parallel the VPO experiments. As the number of hydroxyl groups increases, the concentration dependence of the chemical shift becomes more pronounced. The type of self-association present, the values of the equilibrium constant(s), and the values of the nonideal term were calculated from the VPO experiments. The NMR experiments could not provide this information, but they did give insight into the nature of the bonding involved in the self-association, the sites of interaction, and the internal mobility in the aggregates.

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