Domains Dictate Antibody Adsorption Architectures at Biocompatible Interfaces: Elucidation by QCM‐D and Neutron Reflectometry

ABSTRACT The interfacial stability of therapeutic monoclonal antibodies (mAbs) remains a critical challenge in pharmaceutical development, particularly in the context of biocompatible silicone oil‐coated delivery devices. Here, we present a molecular‐level investigation of antibody conformational dynamics at model polydimethylsiloxane (PDMS) interfaces under varying pH conditions. Using a multi‐technique approach combining neutron reflection (NR), spectroscopic ellipsometry (SE), and quartz crystal microbalance with dissipation (QCM‐D), the pH‐dependent structural adaptations of two engineered antibodies, COE‐3 and COE‐7, and their constituent Fab and Fc fragments were revealed. Both COE‐3 and COE‐7 underwent distinct conformational transitions between pH 5.5 and 8.0, with a remarkable shift from monolayer to bilayer architectures. At pH 5.5, both antibodies formed compressed monolayers, indicating substantial molecular deformation. As pH approached the isoelectric point (pH 8.0), a unique bilayer architecture emerged, with a densely packed inner layer supporting a more diffuse and non‐deformed outer layer. Notably, this pH‐induced structural reorganization was primarily driven by the Fc region, while the Fab fragments maintained consistent monolayer conformations regardless of pH conditions. Our work has established a quantitative framework for illustrating antibody‐surface interactions and the contribution of individual fragments in the adsorption process. This mechanistic understanding opens new avenues for enhancing the stability of antibody‐based pharmaceuticals on silicone oil‐coated delivery devices.