Response of Viral Shells under Nano-Indentation

Abstract

Viral capsids are self assembled nano-containers with remarkable material properties. They combine extreme simplicity of construction with both, toughness and resilience protecting the viral genome, and with complex functionality that the virus needs for targeting and infecting new host cells. We have experimentally, with atomic force microscopy, and numerically, with finite element analysis, studied viral shells under external mechanical stress. While gently probing bacteriophage Φ29 shells with small forces, we could measure linear response properties and estimate a Young's modulus for the shell proteins. In images we observed patterns following symmetry elements. When we irreversibly destroyed the shells in a controlled fashion with higher applied forces, we found that the capsids fractured along well-defined lines revealing trimers as stable building blocks. Similar experiments on capsids of the cowpea chlorotic mottle virus (CCMV) at pH 4.8 revealed an initial reversible linear regime up to indentations of ∼ 20% of the diameter followed by irreversible deformation. At a pH 6.0, the response of the shell changes dramatically and becomes soft. Modeling predicts that the nature of structural failure is determined by a simple and universal physical characteristic, namely, the Foppl-von Karman (FvK) number, a dimensionless control parameter that emerges from the continuum theory of thin shells.