  A molecular-level model is used to study the mechanical response of empty
cowpea chlorotic mottle virus (CCMV) and cowpea mosaic virus (CPMV) capsids.
The model is based on the native structure of the proteins that consitute the
capsids and is described in terms of the C-alpha atoms. Nanoindentation by a
large tip is modeled as compression between parallel plates. Plots of the
compressive force versus plate separation for CCMV are qualitatively consistent
with continuum models and experiments, showing an elastic region followed by an
irreversible drop in force. The mechanical response of CPMV has not been
studied, but the molecular model predicts an order of magnitude higher
stiffness and a much shorter elastic region than for CCMV. These large changes
result from small structural changes that increase the number of bonds by only
30% and would be difficult to capture in continuum models. Direct comparison of
local deformations in continuum and molecular models of CCMV shows that the
molecular model undergoes a gradual symmetry breaking rotation and accommodates
more strain near the walls than the continuum model. The irreversible drop in
force at small separations is associated with rupturing nearly all of the bonds
between capsid proteins in the molecular model while a buckling transition is
observed in continuum models.
