The charge contacts frequency
differences between superfamily heterodimeric and homodimeric protein interfaces, respectively:

(filled bars), and

(open bars). Here

and

are the numbers of (+−) atomic contacts (normalized by the total number of interface atoms) across superfamily hetero- and homodimeric interfaces, respectively.

and

are the analogous numbers for (++) contacts (see Methods). The results are highly statistically significant, as the computed Kolmogorov-Smirnov
p-values demonstrate (control data not shown). Inset: The analogous data for the statistics of charge contacts across
model protein interfaces. Model contact numbers count the residue contacts normalized by the total number of residues at the interface.