Plot of coordination number versus cation radius for commercially available divalent metal cations. Those cations that support Csk activity are shown in blue. Zn2+, which also binds to Csk but does not support Csk activity, is shown in red. Fe2+ and Cr2+ are also labelled. Their ability to support Csk activity is discussed in the text. The coordination numbers and the cation radii are taken from reference  and the inhibition data is taken from reference .