Counting of inter-protein 4-tuples in contact.

<p>Scatter plot for observed relative frequencies and their background relative frequencies for 4-tuples. The insert is an enlargement of a particular area of the graph, and the corresponding dots are marked with red circles. ‘AAAA’ is short for the aromatic-aromatic-aromatic-aromatic 4-tuple type. ‘SHNP’ stands for the small-hydrophobic-negative-polar 4-tuple type. ‘SHHH’ and ‘HHSA’ are the small-hydrophobic-hydrophobic-hydrophobic and hydrophobic-hydrophobic-small-aromatic 4-tuple types, respectively. The 4-tuples of ‘HHSA’ and ‘SHHH’ have the highest relative frequency to be contact 4-tuples, while ‘SHNP’ is expected to be the most frequent one at interface according to the background frequencies of S, H, N, and P on the surface. ‘AAAA’ is the type of contact 4-tuples with the largest divergence angle from the diagonal line.