Integral membrane protein/amphipol complexes and high-resolution solution-state NMR spectroscopy.

3_Development of pH-insensitive amphipols.

Comparative 2D [15N,1H]-TROSY spectra of bacteriorhodopsin in detergent solution, in nanodiscs, and in A8–35 have also been published (51). A drawback of A8-35 for NMR studies is the need to work at pH >7 in order to avoid aggregation. A high pH renders the observation of exchangeable protons more difficult, if not impossible in some cases, when the chemical exchange becomes too fast ( Zoonens et al. (2005), Catoire et al (2010b)). This has been one of the incentives leading to the development of pH-insensitive amphipols, such as SAPols and NAPols, both of which make it possible to record NMR spectra under acidic conditions (Dahmane et al. (2011),Bazzacco et al. (2012)). These new amphipols, however, suffer from other disadvantages: the purification of SAPols is demanding, which currently limits the amounts that can be conveniently produced, and NAPols, due to their complex chemical structure, cannot be easily perdeuterated.

The amphipol of choice, therefore, will depend on the nature of the experience to be performed (and on the availability of the respective amphipols; only A8-35 is commercially available yet).