Rim fluctuations help virus assembly at weak binding. They hinder it at strong binding.
Classical nucleation theory treats a growing viral capsid as a disk with a rigid boundary. The line tension at the rim provides the energy cost of the open edge, opposing the area energy gained by adding subunits. Growth occurs when the disk is large enough that area wins.
Martín-González et al. (arXiv:2603.07549) extend this by letting the rim fluctuate thermally. A partially formed capsid doesn't have a rigid circular boundary — its edge undulates, exploring different geometric configurations. These fluctuations carry entropy.
At weak binding energies, rim fluctuations lower the nucleation barrier. The entropic contribution from edge flexibility effectively reduces the line tension, making it easier for small capsids to grow past the critical nucleus size. Fluctuations help.
But at strong binding energies, the effect reverses. The fluctuating rim stabilizes incomplete capsids by providing an entropy bonus for being open. The last few subunits that would close the shell eliminate the rim entirely — and with it, all the rim entropy. Closure imposes a finite-size entropy penalty. The capsid resists finishing because the complete shell has fewer microstates than the almost-complete shell.
The crossover is governed by a threshold in binding free energy. Below threshold, fluctuations promote assembly. Above threshold, they trap the system in partially assembled states. The same physics — thermal undulations of a boundary — switches from catalyst to inhibitor depending on a single parameter.
The structural lesson: when a boundary carries entropy, eliminating the boundary costs entropy. Any process that completes a structure by removing its edge pays a closing penalty — and that penalty grows with the freedom the edge had. The last step of assembly is the hardest precisely when the intermediate is most flexible.
Martín-González et al., "Effects of Rim Fluctuations in Classical Nucleation Theory of Virus Capsids," arXiv:2603.07549 (2026).