Amide III SFG Signals as a Sensitive Probe of Protein Folding at Cell Membrane Surface

A good understanding of membrane protein folding at the molecular level requires an effective means to determine the dynamical structural changes on coil-to-helix transition within the cell membrane and as yet remains challenging. Herein, we demonstrate that the amide III spectral signals of the protein backbone, generated in the sum frequency generation vibrational spectroscopy, are a powerful tool to probe the protein folding processes within the membrane in situ, in real time, and without exogenous labels. The amide III signals are capable of separating the spectral profiles of the random-coil and α-helical structures at the interface. The intensity ratio of coil and helix peaks becomes a prime indicator that allows one to directly capture the dynamical change of the coil–helix transition. With this approach, using pardaxin as a model, the influence of lipid charge on the peptide folding degree at the cell membrane surface has been nicely elucidated. It is evident that the negative charge of the lipid ...