Fig. 5.

Force-indentation (FZ) curves showing the force response of HSV1 capsids upon deformation. To calibrate the system, the reference curve FZ glass was recorded by bending the cantilever on the glass. The indentation curve on the capsid is shown by FZ forward, and the FZ backward curve depicts the retraction of the cantilever. The hysteresis between both curves shows the irreversibility of indentation after breaking. These large-scale disruptions appear permanent because waiting for several hours did not result in reversal to the initial mechanical properties of the capsids. The FZ glass and FZ virus curves were shifted along the x axis to have a coinciding contact point. The experiments were performed on B, A, and C capsids separately and at loading rates of ≈3 nN/sec. (A) Indentation of a penton-containing A capsid showing a clear drop of the force within the linear indentation part. FZ 2 and FZ 3 show the strongly increased flexibility of the particle for additional pushing cycles, indicating that the particle has been broken. (Inset) Indentation of a penton-containing A capsid showing the sometimes-occurring difference between the end of linearity and the breaking of the particle. Whereas the particle in the main panel shows 2 clear breaking events, the particle in the inset shows only one clear breaking event. (B) Indentation of a pentonless B capsid extracted with 2.0 M GuHCl. A qualitatively similar indentation behavior was observed for pentonless and intact capsids.