Packing and ejection of a semiflexible polymer in a capsid: Effect of helicity

The translocation of semiflexible polymers into confined geometries is central to many biological processes, including viral genome packaging. Understanding how helicity influences these processes offers new mechanistic insights into the role of torsional elasticity in confined semiflexible polymers. Here, we investigate how torsional rigidity influences the packing and ejection dynamics of a helical polymer confined within a spherical capsid. We find that torsional stiffness has a non-monotonic effect on the total packing time: a small but finite torsional rigidity minimizes the packing time, while further increases in stiffness first slow down and then accelerate the packing process. This behavior arises from a competition between the formation of spool-like configurations, which favor efficient packing, and the increase in persistence length, which hinders polymer folding under confinement. Notably, this non-monotonicity vanishes when confinement interactions are removed, underscoring the key role of spatial constraints. In addition, structural analyses reveal that torsional stiffness promotes coiled or spooled conformations, resembling those observed in bacteriophage DNA. Even in the absence of torsion, moderate bending stiffness combined with confinement can induce partial spooling, highlighting the importance of geometric and motor-driven effects. In contrast, ejection slows down monotonically with increasing torsional stiffness, as higher rigidity impedes uncoiling and exit through the pore. These results demonstrate that torsional elasticity critically shapes both packing and ejection, and they offer insights into viral DNA packaging, where polymer mechanics and confinement are intricately coupled.