Figure 3.

Subunit illustrated as an equilateral triangle (a). The subunits bond together edge to edge, and the growth of the shell proceeds by adding the subunit to the location with the smallest opening angle, α (b). As the shell grows, the two unbound edges can either bind to form a pentamer (c), or a new subunit can be added and bound along both edges to form a hexamer (d). The choice between forming a pentamer and a hexamer is based on what leads to a lower energy per subunit in the growing shell.