Elucidation of the Effects of Electrostatic Interactions on Protein Structures at Buried Solid/Liquid Interfaces: A Combined Study with Experiment, Simulation, Isotope Labeling, and Spectra Calculation

Protein structures and interactions at buried solid/liquid interfaces are critical for applications in biosensors, biomedical devices, and antifouling coatings. Here, we investigated the conformations and orientations of wild-type (WT) and mutant (MT) protein GB1s adsorbed onto a (3-aminopropyl)triethoxysilane (APTES) self-assembled monolayer (SAM) surface using a combined approach of sum frequency generation (SFG) vibrational spectroscopy, isotope labeling, atomistic molecular dynamics (MD) simulations, and Hamiltonian spectral calculations. Our results revealed that WT GB1 and MT GB1 adopt similar structures on the APTES surface due to dominant electrostatic interactions, in contrast to their distinct structures at graphene and polystyrene interfaces where π-π interactions are significant. Here, the C-terminal β-sheet of protein GB1 predominantly interacts with the positively charged APTES surface, as evidenced by MD simulations and SFG spectra matching. Isotope labeling enhances structural elucidation by providing additional independently measured parameters without altering protein conformation. The study demonstrates the power of this integrative method for resolving protein structures at interfaces in situ, contributing to our understanding of interfacial protein behavior and function.