Ballistic diffusion fronts in biomolecular condensates

Biomolecular condensates in cells compartmentalize vital processes by enriching molecules through molecular recognition. However, it remains elusive how transport occurs in biomolecular condensates and how it relates to their dynamic and/or viscoelastic state. We show that the transport of molecules in DNA model condensates does not follow classical Fickian diffusion, which has a blurry front with a square root of time dependence. By contrast, we identify a new type of transport with an ultrasharp front that propagates linearly with time. Our data reveal that this ultrasharp ballistic diffusion front originates from molecular recognition and an arrested-to-dynamic transition in the condensate properties. This diffusion mechanism is the result of intertwining chemical kinetics and condensate dynamics on transport in biomolecular condensates. We believe that our understanding will help to better explain and tune the dynamics and properties in synthetic condensate systems and for biological functions. Adding short, complementary oligonucleotides to single-stranded DNA condensates creates a concentrated, linearly propagating, sharp diffusion front that contradicts the fuzzy concentration gradients and nonlinear kinetics typical of Fickian diffusion.