The problem of genome packing in the viral capsid sits at the intersection of physics, chemistry, and virology. Neighboring works in the archive explore related aspects: how bacterial genes assemble into living droplets, how ionic transistors operate in nanopores, and how quantum effects manifest in biological systems. This paper stands out because it is the first to systematically include explicit ions, explicit water, discrete capsid charge sites, and full Coulombic electrostatics in molecular dynamics. Previous calculations either used implicit solvent, simplified polymer models, or fixed ionic conditions. Here, ionic screening emerges naturally from ion dynamics, allowing mean-field predictions to be tested at the molecular level.
The authors built a coarse-grained model in which RNA is represented by a linear chain of 1122 beads of three types (phosphate, sugar, sugar) based on the MARTINI-2 force field. The capsid is modeled as a double spherical shell with an inner radius of about 12 nm, on whose inner surface 180 positively charged sites are fixed, mimicking the N-terminal domains of the proteins. Water is described by a three-bead polarizable model, and sodium and chloride ions are standard MARTINI particles. Electrostatic interactions were calculated using the PPPM method with a dielectric constant of 2.5. Simulations were run in the LAMMPS package at 310 K with a Langevin thermostat. To validate the model, the persistence length of a free chain was measured at different salt concentrations (10–100 mM) and compared with experimental data for single-stranded RNA. Then the polymer was placed inside the capsid and radial and angular density profiles of monomers were analyzed after reaching equilibrium.
The model reproduced the experimental dependence of the persistence length of single-stranded RNA on salt concentration: as salt increases from 10 to 100 mM, the persistence length decreases from ~2.5 to ~1.0 nm, consistent with data from Chen et al. for poly(rU). Inside the capsid, increasing salt from 10 to 100 mM systematically shifts the radial distribution of the genome from the wall toward the center, as mean-field theories predict. However, the equilibration time increases sevenfold: from 64 ns at 10 mM to 470 ns at 100 mM. Independent variation of capsid charge and charge-site density showed that the total electrostatic coupling governs global organization, while geometric details modulate local genome–wall contact. Angular profiles revealed a double peak around each charge site at 180 sites, which disappears when the number of sites increases to 1470, as the distance between them becomes smaller than the persistence length of the chain. Representing the N-terminal domains as a single protruding bead splits the single maximum into two, indicating that the genome wraps around the protrusion rather than directly adhering.
This work stands apart from neighboring studies: unlike work on bacterial genes or ionic transistors, it directly tests mean-field predictions for polyelectrolyte packing in a charged spherical shell with explicit molecular details. Neighboring studies use either continuum models, implicit solvent, or fixed ionic conditions. Here, it is shown for the first time that discrete ions and protruding charge sites lead to quantitative deviations from mean-field behavior, especially in the angular distribution of the genome. This means that continuum approximations are correct for the coarse radial picture but miss the local organization important for understanding packaging specificity.
A natural next step is to include RNA secondary structure (base pairing) and branched topology, which may change the relative roles of total charge and charge distribution. It is also important to extend the model to multivalent ions (Mg2+ and others), which more strongly affect RNA folding. Long flexible N-terminal domains, beyond one or two beads, could add new conformational effects. Finally, moving from equilibrium organization inside a preformed capsid to assembly kinetics would allow direct comparison with virus self-assembly experiments.
The results are important for virology (understanding genome packaging), polyelectrolyte physics (behavior of charged chains in confined volumes), and nanotechnology (designing capsids for therapeutic nucleic acid delivery).
Immediate next steps are to include RNA secondary structure, model capsid assembly around the genome, and test the effects of multivalent ions.
In polyelectrolyte physics, the exact role of discrete ions and correlations in confined geometry remains an open question. This work shows that these effects are noticeable even with monovalent salt and moderate charges, posing the challenge of developing more accurate theories beyond mean field. For virology, the key unsolved problem is the link between RNA sequence, its secondary structure, and packaging efficiency in the capsid; this model provides a platform for studying that link.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors built a coarse-grained model of single-stranded RNA inside a viral capsid, explicitly accounting for ions, water, and electrostatics. The model reproduced the experimental dependence of RNA persistence length on salt concentration and showed that increasing salt shifts the genome toward the capsid interior, while the equilibration time increases sevenfold.
Where its strength is
The strength of the work lies in systematically incorporating molecular details (discrete charges, explicit ions, polarizable water) into an electrostatic model of genome packaging, which allows testing mean-field theory predictions at the molecular level.
Where it could go next
Worth trying
Why this may matter
Understanding how ionic conditions and charge discreteness govern genome packaging could aid in developing strategies to disrupt virus assembly or stabilize capsids for biotechnological applications.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.51, against a usual 0.64. It looks like few have drilled nearby.
The equilibration time in the model increases sevenfold when salt concentration rises from 10 to 100 mM, meaning that in a saltier environment the genome 'takes longer to find' its final fold. For comparison, at 10 mM equilibrium is reached in 64 nanoseconds, while at 100 mM it takes nearly half a microsecond.