Phase behavior and protein partitioning in aqueous two-phase systems of cationic-anionic surfactant mixtures

Cationic-anionic surfactant mixtures can form aqueous two-phase systems. Su ch aqueous surfactant two-phase systems (ASTP systems) can be used for sepa ration and purification of biomaterials. In this work we investigated the p hase behavior and the partitioning of BSA and lysozyme in the ASTP system f ormed by mixtures of dodecyltriethylammonium bromide and sodium dodecylsulf ate (SDS). The pseudo ternary phase diagram of these mixtures at low total surfactant concentrations contains two narrow two-phase regions, which repr esent two kinds of different ASTP systems formed when cationic and anionic surfactants are in excess, respectively (called ASTP-C and ASTP-A). The pha se separation is associative, one phase is surfactant-rich, and the other p hase is surfactant-depleted. Mechanisms behind the phase behavior are discu ssed. The phase behavior, especially phase separation time and phase volume ratio, is strongly influenced by total concentration and molar ratio of mi xed surfactants. The effect of molar ratio is strong, which enables one to get desired phase systems also at very low total concentration by tuning th e molar ratio of the surfactants. It was shown that the marked differences of surfactant concentration between the phases makes proteins distribute wi th different partitioning coefficients. The charges on the micellar surface , which can be adjusted by tuning the molar ratio of cationic surfactants t o anionic surfactants, enhance the selectivity of protein partitioning by e lectrostatic effects. At pH 7.1, in the ASTP-C systems, negatively charged BSA is concentrated in the surfactant-rich phase and positively charged lys ozyme in the surfactant-depleted phase, while in ASTP-A systems, a totally opposite partitioning was observed. It was shown that lysozyme could retain activity in ASTP systems. (C) 2000 Elsevier Science B.V. All rights reserv ed.