“…As a powerful analytical technique suited for biointerface detection under nonvacuum conditions, sum frequency generation (SFG) vibrational spectroscopy has been widely employed to study interfacial proteins and lipid membranes, offering structural and dynamic information at the molecular level in situ in real time. − The SFG process is intrinsically surface/interface-selective, as it is typically permitted at surfaces/interfaces where inversion symmetry is broken, rendering SFG highly sensitive to the biointerfaces associated with structural asymmetry. ,− Lysozyme, as a positively charged precursor protein with four disulfide bonds, was selected as a model protein for their potential to undergo disulfide-based restructuring and amyloid-type aggregation at interfaces. ,, Tris(2-carboxyethyl)phosphine (TCEP) used in this system can reduce disulfide bonds to dithiols, thus inducing the transformation of lysozyme from an oxidized folding state to a reduced folding state . As demonstrated in the previous SFG studies, lysozyme aggregated at the lipid monolayer interface in the low-pH environment , or only structured at the air/water, solid/water, or nanodroplet interfaces. − However, lysozyme aggregation by the reductive pathway on lipid bilayer membranes, which is relatively efficient and physiological, , has not been fully appreciated and remains to be clarified, due to a lack of molecular-level understanding of the interfacial structures and the aggregation kinetics, along with the influence on transmembrane asymmetry.…”