Taming polymorphism of tubule self-assembly using templated growth

Self-closing assembly, in which the global finite-size is controlled by the geometry of the subunits, is one of the most promising strategies for making self-limiting assemblies. However, self-closing assembly is often plagued with polymorphism due to thermally-excited bending fluctuations. One way to overcome this challenge is by templating growth, a strategy commonly found in nature. By creating a precisely defined seed that can nucleate tubules, one can remove the dispersity of structures that results from spontaneous nucleation and tubule closure. We test this concept using a system of DNA-origami subunits, in which we prescribe the inter-subunit binding angles and specific interactions to assemble self-closing tubules of a user-specified diameter and helicity. In particular, we synthesize tubule templates with defined lengths and diameters using two strategies: 1) designing fully-addressable seeds; 2) purifying a specific seed from a polymorphic mixture. By tuning the seed and monomer concentrations and adjusting the assembly temperature, we show how we grow tubules from the seed and avoid spontaneous nucleation. We observe that tubules tend to follow the guidance of the seed and lower the formation of off-target states to produce a high selectivity of the target geometry. Our results demonstrate that employing a precisely defined seed to guide assembly can significantly decrease polymorphism in self-closing assembly in a controllable and economical way.