The Use of Amphipols in Mass Spectrometry

Abstract

Mass spectrometry (MS) analyses of membrane proteins (MPs) have progressed spectacularly over the past decade. Yet, the presence of detergents remains a source of problems. In matrix-assisted laser desorption ionization (MALDI), only certain detergents are acceptable, which are not necessarily those best tolerated by the proteins under study. In electrospray ionization-mass spectrometry (ESI-MS) studies, where MPs can be brought to the gas phase under their native oligomeric state and conformation, a fine line has to be navigated between the Charybdis of retaining too many detergent molecules, yielding unexploitable spectra, and the Scylla of using too much collisional energy to strip away the detergent and denaturing the protein. The use of amphipols (APols) as alternatives to detergents has been validated in these two experimental configurations. MALDI-time-of-flight (TOF) experiments have been carried out using either A8-35 or non-ionic APols and have yielded mass determinations, identification of posttranslational modifications and associated lipids, and sequence data. In ESI-MS experiments, often combined with ion mobility (IM) measurements of the collisional cross section of the protein, which yield information about its conformation, A8-35 has proven particularly convenient, protecting MPs against unfolding better than the much used detergent dodecylmaltoside. Fast photochemical oxidation has revealed details of the way the two surfactants interact with MPs that tally well with the conclusions from molecular dynamics and NMR investigations. Whether APols can be used to preserve the oligomeric state of MPs in the gas phase, however, is uncertain. Finally, A8-35 has been used as a powerful tool to simplify whole proteome investigations.