Nutrient import across Gram-negative bacteria's outer membrane is powered by the proton-motive force, delivered by the cytoplasmic membrane protein complex ExbB-ExbD-TonB. Having purified the ExbB-ExbD complex in the detergent dodecyl maltoside, we substituted amphipol A8-35 for detergent, forming a water-soluble membrane protein/amphipol complex. Properties of the ExbB-ExbD complex in detergent or in amphipols were compared by gel electrophoresis, size exclusion chromatography, asymmetric flow field-flow fractionation, thermal stability assays, and electron microscopy. Bound detergent and fluorescently labeled amphipol were assayed quantitatively by 1D NMR and analytical ultracentrifugation, respectively. The structural arrangement of ExbB-ExbD was examined by EM, small-angle X-ray scattering, and small-angle neutron scattering using a deuterated amphipol. The amphipol-trapped ExbB-ExbD complex is slightly larger than its detergent-solubilized counterpart. We also investigated a different oligomeric form of the two proteins, ExbB-ExbD, and propose a structural arrangement of its transmembrane α-helical domains. [ABSTRACT FROM AUTHOR]