Surface Charge and Solvent Exposure Dictate the pH-Dependent Viscosity of Therapeutic Antibodies

Understanding the molecular basis that influences the viscosity behavior of antibodies is essential for the development of high-concentration antibody therapeutics. In this study, we investigated the pH-dependent viscosity behavior of four IgG₄ antibodies and dissected the structural and physicochemical factors contributing to their viscosity profiles. Two antibodies (mAb2 and mAb3) showed marked viscosity increases with rising pH, whereas the viscosity of mAb1 and mAb4 remained largely unchanged. To explore the mechanisms behind this divergence, we performed molecular dynamics simulations, surface charge mapping, and measurements of protein-protein interaction coefficients (k_D). Antibodies with increasing viscosity over pH displayed significantly larger solvent-accessible surface areas (SASA), prominent negative surface charge patches in the Fv region, and stronger intermolecular attraction at elevated pH. These trends were further validated by viscosity reduction under high-salt conditions, suggesting a key role of electrostatic interactions. These findings highlight that solvent exposure and localized charge distributions act synergistically to promote self-association, offering a framework to better understand and mitigate antibody viscosity in molecular design and formulation.