THE EFFECTS OF PH AND IONIC-STRENGTH ON THE PARTITIONING OF 4 PROTEINS IN REVERSE MICELLE SYSTEMS

Four proteins with different physicochemical properties have been part itioned in reversed micelle systems: thaumatin, ribonuclease A, soybea n trypsin inhibitor, and alpha-lactalbumin. The organic phase was form ed by sodium salt (AOT) in isooctane, and the aqueous phase contained KCl, KBr, MgCl2, or NaCl. Aqueous phase pH was varied between 2 and 13 and ionic strength from 0.1 to 1.0 M. Small changes in pH around the isoelectric point (pl) were found to influence the solubilization of ribonuclease A and trypsin inhibitor, but for thaumatin the pH change necessary to affect partition was much greater as a consequence of th e difference in net charge (titration curves) of these protein molecul es as pH changes. The type of ions present in the system was also a de termining factor for partition; the larger ions (K+) produced more ele ctrostatic screening and hence less protein solubilization than the sm aller ions (Na+). With changes in ionic strength surface hydrophobicit y was a dominant factor affecting solubilization of thaumatin in NaCl- containing systems at high pH. Charge distribution and hydrophobicity are thought to be important parameters when partitioning the protein a lpha-lactalbumin. (C) 1994 John Wiley & Sons, Inc.