Backbone Rigidity Encodes Universal Viscoelastic Signatures in Biomolecular Condensates.

Biomolecular condensates exhibit a wide range of viscoelastic properties shaped by their molecular sequence and composition. Coarse-grained molecular models of intrinsically disordered proteins are widely used to complement experiments by revealing the structure and thermodynamics of condensates. However, fully flexible chain representations of inherently disordered proteins often fail to capture their complex viscoelastic behavior, instead predicting purely viscous responses. In this work, we demonstrate that introducing sequence-dependent chain rigidity enables the accurate reproduction of the elastic and viscous moduli for experimentally characterized condensates of A1-LCD and its numerous mutants. Furthermore, we show that the frequency-dependent loss factor can be described by a single parameter that universally correlates with viscosity across different sequences and variations of the coarse-grained molecular energy function. Our results also reveal that increased chain rigidity, indicated by a larger gyration radius, expands the condensates' elastic regime. Finally, we elucidate the microscopic origins of sequence-encoded viscoelasticity by showing how it can be tuned through sequence rearrangements that promote sticker cluster formation.