Solvent-Isotope Effects in Biomolecular Phase Separation and Fibrillation of Disordered Proteins and Peptides

Heavy water (D2O) is widely used in biomolecular spectroscopy and imaging, often under the assumption that it is an inert replacement for H2O. However, D2O differs subtly in hydrogen-bonding, viscosity, and dielectric properties, which can alter biomolecular interactions and self-assembly. Here, we test how solvent isotope substitution modulates protein/peptide phase separation and amyloid formation in multiple intrinsically disordered systems. Using turbidity-based phase diagrams and microscopy, we quantify how D2O shifts protein-RNA complex coacervation boundaries and alters condensate morphology. Droplet recovery measurements indicate significant solvent-dependent changes in condensate material properties. We further evaluate amyloid formation kinetics, in the presence or absence of a cofactor, supported by orthogonal structural characterization, and assess the functional consequences of tau fibrils using a tau biosensor seeding assay with explicitly defined seed delivery conditions. Together, these results show that D2O can systematically bias liquid-liquid phase separation and aggregation readouts and should be treated as an active experimental variable rather than a neutral solvent substitute.