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

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.