Identifying the structures of membrane bound proteins is critical to understanding their function in healthy and diseased states. We introduce a surface enhanced Raman spectroscopy technique which can determine the conformation of membrane-bound proteins, at low micromolar concentrations, and also in the presence of a substantial membrane-free fraction. Unlike conventional surface enhanced Raman spectroscopy, our approach does not require immobilization of molecules, as it uses spontaneous binding of proteins to lipid bilayer-encapsulated Ag nanoparticles. We apply this technique to probe membrane-attached oligomers of Amyloid-β40 (Aβ40), whose conformation is keenly sought in the context of Alzheimer's disease. Isotope-shifts in the Raman spectra help us obtain secondary structure information at the level of individual residues. Our results show the presence of a β-turn, flanked by two β-sheet regions. We use solid-state NMR data to confirm the presence of the β-sheets in these regions. In the membrane-attached oligomer, we find a strongly contrasting and near-orthogonal orientation of the backbone H-bonds compared to what is found in the mature, less-toxic Aβ fibrils. Significantly, this allows a "porin" like β-barrel structure, providing a structural basis for proposed mechanisms of Aβ oligomer toxicity.
Keywords: amyloid beta peptide; lipid SERS; lipid-coated nanoparticles; membrane protein structures; oligomers; solid-state NMR; surface enhanced Raman spectroscopy.