  Coupling between axial and torsional degrees of freedom often modifies the
conformation and expression of natural and synthetic filamentous aggregates.
Recent studies on chiral single-walled carbon nanotubes and B-DNA reveal a
reversal in the sign of the twist-stretch coupling at large strains. The
similarity in the response in these two distinct supramolecular assemblies and
at high strains suggests a fundamental, chirality dependent non-linear elastic
behaviour. Here, we seek the link between the microscopic origin of the
non-linearities and the effective twist-stretch coupling using energy based
theoretical frameworks and model simulations. Our analysis reveals a sensitive
interplay between the deformation energetics and the sign of the coupling,
highlighting robust design principles that determine both the sign and extent
of these couplings. These design principles have been already exploited by
Nature to dynamically engineer such couplings, and have broad implications in
mechanically coupled actuation, propulsion and transport in biology and
technology.
