Electric field-driven accumulation and separation of proteins at liquid-liquid interfaces in aqueous two-phase systems*

Liquid-liquid interfaces in aqueous two-phase systems (ATPS) represent a transport resistance to charged molecules driven by an electric field. As a result, depending on the applied electric field strength, the composition of the phases and the molecular properties, molecules accumulate at the interface. In this work, the interaction of two types of proteins with the interface of an ATPS is examined using a microfluidic device and confocal as well as epifluorescence microscopy. Both proteins are initially dissolved in the same phase of the ATPS and transported by an electric field normal to the interface. The temporal development of the fluorescence intensity at the interface and in the extraction phase indicates that one protein mainly accumulates at the interface while the other already crosses over, suggesting that the transport resistance differs for both types of proteins. A mathematical model based on the assumption that this transport resistance results from adsorption in a potential well at the interface supports the experimental results. Hence, the liquid-liquid interface of an ATPS can be utilized to separate different types of proteins.