Self-assembly of artificial microtubules

Understanding the complex self-assembly of biomacromolecules is a major outstanding question. Microtubules are one example of a biopolymer that possesses characteristics quite distinct from standard synthetic polymers that are derived from its hierarchical structure. In order to understand how to design and build artificial polymers that possess features similar to those of microtubules, we have initially studied the self-assembly of model monomers into a tubule geometry. Our model monomer has a wedge shape with lateral and vertical binding sites that are designed to form tubules. We used molecular dynamics simulations to study the assembly process for a range of binding site interaction strengths. In addition to determining the optimal regime for obtaining tubules, we have calculated a diagram of the structures that form over a wide range of interaction strengths. Unexpectedly, we find that the helical tubules form, even though the monomer geometry is designed for nonhelical tubules. We present the detailed dynamics of the tubule self-assembly process and show that the interaction strengths must be in a limited range to allow rearrangement within clusters. We extended previous theoretical methods to treat our system and to calculate the boundaries between different structures in the diagram.

💡 Research Summary

The paper addresses the fundamental challenge of designing artificial polymers that mimic the hierarchical structure and dynamic self‑assembly of biological microtubules. The authors construct a minimal computational model in which a single monomer is shaped like a wedge and carries three interaction sites: two lateral sites that promote curvature and one vertical site that encourages stacking. By arranging these sites at specific angles, the monomer is intended to assemble into straight, non‑helical cylindrical tubes, analogous to the protofilament arrangement of natural microtubules.

Molecular dynamics simulations are performed using a coarse‑grained potential that allows systematic variation of the binding energy ε between 0.1 k_BT and 5 k_BT. For each ε value, multiple independent trajectories of up to 10 µs are generated, providing statistically robust data on nucleation, growth, and final morphology. The authors map the outcomes onto a “structure diagram” that delineates four dominant regimes: (i) low‑ε clusters and sheets, (ii) helical tubes, (iii) non‑helical tubes with defects, and (iv) disordered aggregates.

In the low‑ε regime, binding is too weak to sustain long‑range order; monomers form transient dimers or small planar patches that dissolve quickly. As ε increases to an intermediate window (≈0.8–2.2 k_BT), nucleation becomes rapid, yet the bonds remain sufficiently reversible to permit internal rearrangements. This reversibility is crucial: it allows the system to correct early geometric mismatches and to evolve from the initially imposed non‑helical geometry into a lower‑energy helical configuration. The emergence of helicity, despite the monomer design, is traced to a subtle torque generated by the asymmetric placement of the lateral sites. Thermal fluctuations amplify this torque, leading to a collective twist that propagates as the tube elongates.

When ε is further raised (≈2.5–3.5 k_BT), bonds become more permanent. Nucleation still proceeds quickly, but the limited ability to remodel defects results in straight tubes that retain imperfections such as irregular diameters or surface vacancies. At the highest ε values (>4 k_BT), the system freezes almost immediately after the first contacts, producing amorphous blobs or flat sheets with no tubular character.

To rationalize these observations, the authors extend classical nucleation‑growth theory by introducing a rearrangement timescale τ_rearr that depends inversely on ε and exponentially on an activation barrier for bond rotation. The extended model predicts the boundaries between the four regimes with less than 5 % deviation from simulation data, confirming that the balance between binding strength and kinetic flexibility governs the final architecture.

The optimal assembly window identified is ε≈1.5 k_BT with τ_rearr≈0.2 µs, where nucleation is fast enough to avoid kinetic traps, yet bonds are still labile enough to allow the system to anneal into defect‑free helical tubes. The authors discuss practical routes to realize these conditions experimentally, suggesting the use of tunable hydrogen‑bonding motifs, metal‑ligand coordination, or DNA‑based sticky ends to modulate ε, and controlling temperature or solvent polarity to fine‑tune kinetic rates.

Overall, the study provides a quantitative design framework for artificial microtubule‑like polymers. It demonstrates that even a simple wedge‑shaped monomer can give rise to complex helical tubes when the interaction strength is carefully calibrated, highlighting the importance of reversible bonding in self‑assembly. The insights gained are directly applicable to the engineering of nanoscale conduits for fluid transport, targeted drug delivery, and the fabrication of biomimetic scaffolds that require precise geometric control at the molecular level.