Tunable lipid-coated nanoporous silver sheet for characterization of protein-membrane interactions by surface-enhanced Raman scattering (SERS)

Membrane environments affect protein structures and functions through protein-membrane interactions in a wide range of important biological processes. To better study the effects from the lipid's hydrophilic and hydrophobic interaction with protein on different membrane regions, we developed the lipid-coated nanoporous silver sheets to provide tunable supported lipid monolayer/bilayer environments for in situ surface-enhanced Raman vibrational spectroscopy ( SERS) characterizations. Under the controllable surface pressure, lipid monolayer/bilayer was coated along the microscopic curved surface of nanoporous silver sheets to serve as a cell membrane mimic as well as a barrier to avoid protein denaturation while empowering the high SERS enhancements from the underlying metallic bases allowing detection sensitivity at low physiological concentrations. Moreover, we fine-tuned the lipid packing density and controlled the orientation of the deposited lipid bilayers and monolayers to directly monitor the protein structures upon interactions with various membrane parts/positions. Our results indicate that lysozyme adopted the a-helical structure in both hydrophilic and hydrophobic interaction with lipid membrane. Interestingly, alpha-synuclein folded into the a-helical structure on the negatively charged lipid heads, whereas the hydrophobic lipid tails induced the ss-sheet structural conversion of alpha-synuclein originated from its unstructured monomers. These direct observations on protein hydrophilic and hydrophobic interaction with lipid membrane might provide profound insights into the formation of the ss-sheet-containing alpha-synuclein oligomers for further membrane disruptions and amyloid genesis associated with Parkinson's disease. Hence, with the controllability and tunability of lipid environments, our platform holds great promise for more general applications in investigating the influences from membranes and the correlative structures of proteins under both hydrophilic and hydrophobic effects.

Automated summary

Membrane environments play a vital role in affecting protein structures and functions through protein-membrane interactions in important biological processes. To investigate these effects, we developed lipid-coated nanoporous silver sheets that provide tunable supported lipid monolayer/bilayer environments for in situ surface-enhanced Raman vibrational spectroscopy (SERS) characterizations. By controlling the surface pressure, we can coat lipid monolayer/bilayer on the microscopic curved surface of the silver sheets, mimicking cell membranes and preventing protein denaturation. Our results indicate that lysozyme adopts an α-helical structure in both hydrophilic and hydrophobic interactions with the lipid membrane, while alpha-synuclein folds into an α-helical structure on the negatively charged lipid heads and β-sheet structure on the hydrophobic lipid tails. These observations provide insights into the formation of alpha-synuclein oligomers and its associated amyloid genesis in Parkinson's disease. Therefore, our controllable and tunable lipid platform holds great promise for studying the influence of membranes on protein structures under hydrophilic and hydrophobic effects.