Fig. 4.

Pushing the limits of the capsids. (A) Three consecutive FZ curves taken in rapid succession (8 msec) on a prohead. This shell responded linearly to applied forces up to ≈2.5 nN. At higher forces, a softer reaction to applied force was observed. The elastic constant was calculated from the linear region of each curve shown. (B). The change of the measured spring constant with repeated indentations on three different proheads (red, blue, and magenta). The red curve (related to the curves in A) shows a gradual change of the elasticity after ≈30 indentations, whereas the blue curve shows a sudden decrease. Both show that the rigidity of the object is lowered permanently after the decrease (marked by the vertical lines). This decrease was an indication of the destruction of the viral shell. The magenta curve shows how repeated indentation of a prohead can eventually disintegrate it completely. This prohead appeared to be broken at the first scan, and its rigidity decreased further with each consecutive indentation. (C) The magenta and blue curves relate to the magenta and blue curves shown in B. The blue curves display a discrete breakage event in the prohead, which resulted in a permanently lower spring constant of the object. The magenta FZ curves represent the first and last curves of a shell, which disintegrated with every consecutive push.