Sulfonated amphipols: synthesis, properties, and applications.

Amphipols (APols) are amphiphatic polymers that keep membrane proteins (MPs) water-soluble. The best characterized and most widely used APol to date, A8-35, comprises a polyacrylate backbone grafted with octyl- and isopropylamine side chains. The nature of its hydrophilic moieties prevents its use at the slightly acidic pH that is desirable to slow down the rate of amide proton exchange in solution NMR studies. We describe here the synthesis and properties of pH-insensitive APols obtained by replacing isopropyles with taurine. Sulfonated APols (SAPols) can be used to trap MPs in the form of small complexes, to stabilize them, and to keep them water-soluble even at low pH. [(15) N,(1) H]-transverse relaxation-optimized spectroscopy NMR spectra obtained at pH 6.8 of a bacterial outer MP folded in SAPols show that the protein is correctly folded. The spectra have a resolution similar to that achieved with A8-35 and reveal water-exposed amide and indole protons whose resonance peaks are absent at pH 8.0.