Self-Limited Self-Assembly of Chiral Subunits

A simple computational model demonstrates the assembly of self-limited filamentous bundles. The images are taken from dynamic Monte Carlo simulations in which "chiral" subunits ( with a preference to form twisted arrays of one "handedness") spontaneously assemble under different interaction strengths and degrees of chirality.

(a) Moderate interactions and moderate chirality reproducibly lead to bundles of complex internal structure, with self-limited diameters, whose size depends on the degree of chirality. Weaker chirality allows larger bundles. (b) Lengthwise view of a self-limited two-layer bundle. (c) With strong interactions, frustration is relieved by defects, which enable the formation of branched networks and irregular bundles.

The assembly of filamentous bundles with controlled diameters is essential for structure and regulation in biological systems and desirable for the development of nanomaterials. The objective of this project is to determine, without assumptions about assembly pathways or assemblage geometries, if chirality can result in stable bundles with finite diameters. To answer this general question, the MRSEC investigators constructed a minimal model for pairwise chiral interactions that drive assembly into filamentous bundles. They then combined free energy calculations and recently developed path sampling techniques explore the structures that spontaneously assemble for varying degrees of chirality. The simulations have demonstrated that frustration due to chirality can result in regular self-limited bundles for a range of interaction strengths.  With stronger interactions, however, the frustration is relieved by defects, which give rise to branched networks or irregular bundles. While some regular self-limited structures that emerge from model trajectories can be modeled as twisted filaments arranged with local hexagonal symmetry (as done in several prior equilibrium calculations), other structures are surprising in their complexity and could not have been predicted with geometrical equilibrium arguments.