Aqueous micellar systems in membrane protein crystallization: Partial miscibility of a nonionic surfactant in the presence of salt or polyethylene glycol

The effects of NaCl and polyethylene glycol (PEG) 4000 on the lower consolution boundary (LCB) of a nonionic surfactant (C 8E 5) were studied and compared. Micellar systems where NaCI and PEG 4000 are present are often used in membrane protein crystallization. While sodium chloride shifts the surfactant LCB to lower temperatures without a significant change in the shape of the boundary, PEG produces a large solubility change strongly depending on the surfactant concentration. The salt effect is explained by a reduced interaction of the micellar oligooxyethylene chains with the water and the PEG effect by an unfavourable configurational interaction between the C 8E 5 micelles and PEG molecules.