RNA inside a virus is like a long thread with charges, and the virus shell is like a sphere with opposite charges. The thread is drawn to the wall because opposite charges attract. But the solution contains salt. Its ions are like a crowd of mediators: they stick to the charges and weaken their bond. The more salt, the weaker the thread holds to the wall.
Scientists built a computer model: a thread of 1122 beads, a sphere with 180 charged protrusions, surrounded by water and salt ions. Everything moved according to the laws of electricity. First they tested the model on a free thread—its flexibility matched experiments. Then they placed the thread inside the sphere and watched how it folded.
It turned out that salt pushes the thread away from the wall toward the center. This was known from theories that smooth out all charges. But the model revealed something new: if the protrusions on the wall are sparse, the thread wraps around each one individually. If there are many protrusions and they are tightly packed, the thread stops distinguishing them. A protruding protein tail makes the thread wrap around it rather than just sticking.
Previously, it was unclear how accurate the simple theories were. Now it is clear: they hold for the big picture but not for details. The charges on the shell create a map of attraction, and the thread reads it like a maze. Salt changes both the map and the speed of pathfinding. This knowledge will help build virus-like particles for drug delivery—containers that assemble themselves.
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.