Surfactant structure and interactions guide membrane protein crystallization

In spite of the recent progress, crystallization of integral membrane proteins still poses enormous challenges. These amphipatic proteins require surfactants in order to be extracted from biological membranes, and the surfactant complicates the interactions between and among the solubilized membrane proteins. Because of the complicate interplay of surfactant and protein interactions, most of the known protocols for growing membrane protein crystals have been developed slowly by trial and error. A rational approach based on knowledge of surfactant properties in solution would be an extremely helpful guide and accelerate the screening of crystallization conditions. Towards this goal, we are investigating the properties of nonionic surfactant mixtures, specifically octylglucoside (C8βG1) and triethylene glycol monooctylether (C8E3), at solution conditions suitable for crystallization of the membrane protein OmpF. Small-angle neutron scattering was used to investigate structure and interactions of C8βG1-C8E3 mixed micelles near the cloud point. Results confirm previous views of the role of the phase boundary on tuning attractive micellar interactions. Furthermore, the effect of such surfactant solutions on colloidal interparticle interactions has been explored by examining the behavior of colloidal silica particles in mixtures near the cloud point. Particle attractions and aggregation depend on the proximity of the cloud curve, and show how such phase behavior features can influence protein interactions.