Molecular kinetics of protein unfolding at the air/water interface probed by vibrational sum-frequency spectroscopy

Protein behavior at aqueous interfaces is central to many soft-matter, biological, and technological systems, yet the molecular events underlying adsorption-induced structural change remain difficult to isolate. In particular, macroscopic measurements such as surface tension cannot directly reveal how proteins reorganize during interfacial adsorption. In this study, we report a time-resolved vibrational sum-frequency generation (VSFG) spectroscopy study of lysozyme adsorption and unfolding at the air/water interface, combined with simultaneous surface-tension measurements. By monitoring the interfacial CH-stretch response after subsurface protein injection, we directly correlate macroscopic changes in surface pressure with molecular restructuring of adsorbed lysozyme. During the first ∼10 min after injection, the surface tension remains essentially unchanged and no lysozyme CH resonance is detected. Together with the much shorter diffusion-only estimate, this argues against assigning the full induction period to bulk transport alone. The subsequent decrease in surface tension is accompanied by a rapid increase in the interfacial CH response, indicating adsorption-induced unfolding and exposure of hydrophobic moieties toward the vapor phase. After ∼30 min, the CH response saturates while the surface tension continues to evolve, revealing an additional kinetic regime associated with interfacial crowding, protein-protein interactions, and multilayer formation. Amide I VSFG spectra further show that interfacial lysozyme retains partial secondary structure, with enhanced beta-sheet contributions relative to the bulk solution. These results demonstrate how interface-selective nonlinear vibrational spectroscopy can resolve molecular kinetic steps that are hidden in surface-tension measurements alone, providing a detailed picture of protein structural dynamics at aqueous interfaces.