PHASE-SEPARATION PROPERTIES OF SEVERAL AQUEOUS ALKYLDIMETHYLPHOSPHINE OXIDE/PHOSPHOLIPID MIXTURES AND THEIR POTENTIAL USE FOR PROTEIN-PURIFICATION

The phase separation properties of several aqueous alkyldimethylphosphine oxide/phospholipid mixtures were investigated. The addition of synthetic phospholipids with acyl residues ranging from 12 to 18 carbon atoms, a soybean phospholipid extract, and a red blood cell ghost extract lowered the temperature of phase separation below that of the surfactant alone. The high relative viscosity of the top layer suggests that asymmetric mixed micelles exist, whereas the relative viscosity of the bottom layer approached the value of 2.5 expected for spherical particles. The relative concentration of lipid in the top layer decreased with increasing molecular weight for the synthetic lecithins. The addition of 0.1 M univalent salts was investigated and no dependence of the cloud point on cation size was observed. However, a set of potassium halides exhibited a trend which followed the Hofmeister series. Finally, the composition of the top and bottom layers was investigated at 38, 42, 48 and 55 degrees C for one DodDPO/DMPC mixture, and the solute was observed to be progressively concentrated in a smaller volume as the temperature was increased for each of five initial concentrations ranging from 12 to 53.6 mg/ml.