Double Helical Conformation and Extreme Rigidity in a Rodlike Polyelectrolyte

TL;DR

This paper reports the discovery of a double helical conformation in a synthetic polyelectrolyte, PBDT, which exhibits extreme rigidity and high axial persistence length, providing insights into molecular self-assembly and composite formation.

Contribution

It is the first demonstration of a double helical structure in a synthetic non-chiral macromolecule with unprecedented rigidity and high charge density.

Findings

Double helix confirmed by X-ray, NMR, and MD simulations.

PBDT exhibits an axial persistence length of ~1 micrometer.

High mechanical modulus (~1 GPa) with liquid-like ion motions inside.

Abstract

The ubiquitous biomacromolecule DNA has an axial rigidity persistence length of ~50 nm, driven by its elegant double helical structure. While double and multiple helix structures appear widely in nature, only rarely are these found in synthetic non-chiral macromolecules. Here we describe a double helical conformation in the densely charged aromatic polyamide poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) or PBDT. This double helix macromolecule represents one of the most rigid simple molecular structures known, exhibiting an extremely high axial persistence length (~1 micrometer). We present X-ray diffraction, NMR spectroscopy, and molecular dynamics (MD) simulations that reveal and confirm the double helical conformation. The discovery of this extreme rigidity in combination with high charge density gives insight into the self-assembly of molecular ionic composites with high mechanical modulus (~1 GPa) yet with liquid-like ion motions inside, and provides fodder for formation of new 1D-reinforced composites.