A molecular-level model is used to study the mechanical response of empty cowpea chlorotic mottlevirus (CCMV) and cowpea mosaic virus (CPMV) capsids. The model is based on the native structure of the proteins that constitute the capsids and is described in terms of the Ca 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 continuummodels and experiments, showing an elastic region followed by an irreversible drop in force. Themechanical response of CPMV has not been studied, but the molecular model predicts an order ofmagnitude 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 bedifficult 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 symmetrybreaking 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 bondsbetween capsid proteins in the molecular model, while a buckling transition is observed in continuum models.
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