Innovative separation methods in bioprocessing 185 their partition behaviour in two phase systems(Hachem, 1992, Asenjo et al., 1994 Schmidt, 1994). It has been possible to correlate the partition coefficient of a representative number of proteins to their hydrophobicity measured by precipitation. This correlation was not as ood if the hydrophobicity was evaluated by hydrophobic interaction chromatography HIC)or also by reverse phase -HPLC (RP-HPLC)(Hachem, 1992; Asenjo et al., 1994 Hachem et al., 1994). In a typical protein precipitation graph as that shown in Fig. 7.4 (S=protein in solution), the proteins solubility (and thus 'hydrophilicity) can be expressed by point m, which is oint at which the protein starts precipitating, Thus the hydrophobicity, P, was evaluated as 1/m. The correlation found between hydrophobicity and partition in PEG /salt systems with a high concentration of NaCl is shown in Fig. 7.5. This can be represented by the equati gK=D△ w log P-D△ w log Po co AW corresponds to the tie- line length of a system and is evaluated by the difference in oncentration of one component(e. g. PEG or salt) between the phases which is constan for one particular system. D is the'discrimination factor'and thus D Aw is the slope in Fig.7.5 which corresponds to the resolution of a particular system to exploit differences in hydrophobicity between proteins. Table 7. 2 gives values of resolution (D Aw) and trinsic hydrophobicity'(Po) found for PEG/phosphate systems with different concen trations of NaCl. Clearly the systems with higher concentrations of Nacl give a higher resolution to exploit the protein s hydrophobicity in partitioning 7.3.2 Reverse micelle extraction Water-in-oil microemulsions, or reverse micelles, are stable, monodisperse aggregates of surface-active molecules (1-10 nm diameter) in an organic solvent. Typically, the =-012330+0.8431x-8.83470e-2x22=0.972 y=87919-24846XP=0.985 08L Concentration of ammonium sulphate(M) Fig. 7. 4. The fitted curves o oulin a at 25c in a solution containing added ammonium the first represents the represe salting-in region and the equati0.4 0.2 0.0 E . 9 v) -0.2 0, - -0.4 -0.6 -0.8 - - I - - - - - I I