Twist-bend instability for toroidal DNA condensates

TL;DR

This paper introduces a two-stage collapse process for semiflexible polymers in poor solvents, where a toroidal structure transitions into a twisted state to minimize bending energy, explaining various experimental DNA behaviors.

Contribution

It presents the concept of topological ripening in DNA toroids, linking structural transitions to experimental observations and potential gene delivery improvements.

Findings

DNA toroids undergo a second transition to a twisted state in poor solvents.

Topological ripening explains DNA entanglement in viral capsids.

The twisted state may enhance gene delivery efficiency.

Abstract

We propose that semiflexible polymers in poor solvent collapse in two stages. The first stage is the well known formation of a dense toroidal aggregate. However, if the solvent is sufficiently poor, the condensate will undergo a second structural transition to a twisted entangled state, in which individual filaments lower their bending energy by additionally orbiting around the mean path along which they wind. This ``topological ripening'' is consistent with known simulations and experimental results. It connects and rationalizes various experimental observations ranging from strong DNA entanglement in viral capsids to the unusually short pitch of the cholesteric phase of DNA in sperm-heads. We propose that topological ripening of DNA toroids could improve the efficiency and stability of gene delivery.