Intermolecular Interactions at the Interface Quantified by Surface-Sensitive Second-Order Fermi Resonant Signals

Accurate determination of intermolecular interaction forces at the surface and the interface is essential to identify the nature of interfacial phenomena such as absorption, interfacial assembly, and specific ion effect, but it still represents a major technical challenge. In this study, we proposed a novel method to deduce the interfacial interaction forces by using surface-sensitive second-order Fermi resonant signals, generated in sum frequency generation vibrational spectroscopy (SFG-VS). By investigating the influence of lipid chain length and intermolecular distance on the Fermi resonant signals of phospholipid monolayer at the air/CaF₂ surface and the air/water interface, a linear correlation between the Fermi resonant intensity ratio and the dominated interactions in the lipid monolayer has been observed. It implies that the amplitude of the intensity ratio can be used as an effective <i>in situ</i> vibrational optical ruler to characterize the total intermolecular interaction forces at the surface and the interface. Such a relationship further enables us to elucidate the specific ion effects on the interfacial interactions, allowing us to identify different contributions from van der Waals, electrostatic, and hydration interactions. This study clearly demonstrates the power of the second-order Fermi resonant signals for evaluating the interfacial interaction forces <i>in</i> <i>situ</i> and in real time

Amide I SFG Spectral Line Width Probes the Lipid–Peptide and Peptide–Peptide Interactions at Cell Membrane In Situ and in Real Time

The balance of lipid–peptide and peptide–peptide interactions at cell membrane is essential to a large variety of cellular processes. In this study, we have experimentally demonstrated for the first time that sum frequency generation vibrational spectroscopy can be used to probe the peptide–peptide and lipid–peptide interactions in cell membrane in situ and in real time by determination of the line width of amide I band of protein backbone. Using a “benchmark” model of α-helical WALP23, it is found that the dominated lipid–peptide interaction causes a narrow line width of the amide I band, whereas the peptide–peptide interaction can markedly broaden the line width. When WALP23 molecules insert into the lipid bilayer, a quite narrow line width of the amide I band is observed because of the lipid–peptide interaction. In contrast, when the peptide lies down on the bilayer surface, the line width of amide I band becomes very broad owing to the peptide–peptide interaction. In terms of the real-time change in the line width, the transition from peptide–peptide interaction to lipid–peptide interaction is monitored during the insertion of WALP23 into 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phospho-(1′-<i>rac</i>-glycerol) (DPPG) lipid bilayer. The dephasing time of a pure α-helical WALP23 in 1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phospho-(1′-<i>rac</i>-glycerol) and DPPG bilayer is determined to be 2.2 and 0.64 ps, respectively. The peptide–peptide interaction can largely accelerate the dephasing time