Reducing polymorphism in the self-assembly of tubules through soft-modes

Biological systems can create complex self-limited structures, such as viral capsids and microtubules, by controlling the valence and binding angles of biomolecular interactions. Inspired by this strategy, we create DNA origami subunits with specific interactions and prescribed binding angles that assemble into tubules whose self-limited width is much larger than the subunit size. Although we target a single tubule geometry, we find that thermal fluctuations can produce a variety of assemblies close to the target structure. This challenge is compounded by the fact that specific, valence-limited interactions localize subunit binding and limit their mobility within an assembly. Therefore, the tubule structures are unable to relax to equilibrium even as they continue to grow. To circumvent this challenge, we tweak the interactions to have softer localization that enables the subunits to slide along one another, creating soft modes within the assemblies. These soft modes allow the tubules to anneal towards their equilibrium structure, reducing the overall polymorphism we observe in our assemblies. These results show how including degrees of freedom for assemblies to anneal out of kinetics traps can help reduce polymorphism as we strive to make more complex structures.