Osmotically-induced rupture of viral capsids

A simple model is proposed aimed to investigate how the amount of dissociated ions influences the mechanical stability of viral capsids. After an osmotic and mechanical equilibrium is established with the outer solution, a non-adiabatic change in salt concentration at the external environment is considered, which results in a significant solvent inflow across the capsid surface, eventually leading to its rupture. The key assumption behind such an osmotic shock mechanism is that solvent flow takes place at timescales much shorter than the ones typical of ionic diffusion. In order to theoretically describe this effect, we herein propose a thermodynamic model based on the traditional Flory theory. The proposed approach is further combined with a continuum Hookian elastic model of surface stretching and pore-opening along the lines of a classical nucleation theory, allowing us to establish the conditions under which capsid mechanical instability takes place. Despite its non-local character, the proposed model is able to capture most of the relevant physical mechanisms controlling capsid stability, namely the volume exclusion and entropy of mixing effects among the densely-packed components, the elastic cost for capsid stretching and further pore opening, the Donnan equilibrium across the interface, as well as the large entropy loss resulting from folding the viral genome into close-packed configurations inside the capsid. It is shown that, depending on the particular combination of initial condition and capsid surface strength, the capsid can either become unstable after removal of a prescribed amount of external salt, or be fully stable against osmotic shock, regardless of the amount of ionic dilution.