Amphipols (APols) are short amphipathic polymers designed to adsorb onto the transmembrane surface of membrane proteins, keeping them water-soluble in the absence of detergent. Current APols carry charged groups, which is a limitation for certain types of applications. This has prompted the development of totally nonionic amphiphols (NAPols). In a previous work, glucose-based NAPols synthesized by free-radical cotelomerization of hydrophilic and amphiphilic monomers proved to be able to keep membrane proteins soluble (Sharma et al. Langmuir 2008, 24, 13581-13590). This provided a proof of principle, but the cumbersome synthesis prevented large-scale production and any detailed biochemical studies. In the present work, we describe a new synthesis route for NAPols based on grafting alkyl chains onto a glucosylated homotelomer. The NAPols thus prepared are highly water soluble. In aqueous solutions, they assemble into small, homogeneous particles similar to those formed by ionic APols. Two model membrane proteins, bacteriorhodopsin and the transmembrane domain of OmpA, form with NAPols small, well-defined water-soluble complexes whose size is comparable to that observed with ionic APols. Complexation by NAPols strongly stabilizes bacteriorhodopsin against denaturation. Glucosylated NAPols thus appear as a promising alternative to ionic APols for such applications as ion-exchange chromatography, isoelectrofocusing, and, possibly, structural approaches such as NMR and crystallography.
Bacterial Outer Membrane Proteins Bacteriorhodopsins