Table 2. The most common binary anion–π interactions in proteins. Pairs of interacting residues and their occurrences in number (amount), percentage (%), and residues' representativities for each distinct anion (%A^–) and π-system (%π). The expected amount of each interaction pair, according to its relative abundance, and the statistical significance are shown (Methods). Statistical significance is denoted with ** for p-value < 0.01, and *** for p-value < 0.001.

Interaction	Amount (expected)	%	%A–	%π
Glu–His	13 763 (13 805)	16.7	29.8	55.8
Glu–Tyr	13 592 (13 428)	16.5	29.5	56.6
Glu–Phe	13 060 (13 446)**	15.8	28.3	54.3
Asp–Phe	10 477 (9972)***	12.7	30.6	43.6
Asp–His	10 180 (10 239)	12.3	29.8	41.3
Asp–Tyr	9797 (9959)	11.9	28.6	40.6
Glu–Trp	5717 (5450)**	6.9	12.4	58.7
Asp–Trp	3760 (4042)**	4.6	11.0	38.6
SO4–His	405 (316)**	0.5	38.4	1.6
SO4–Tyr	274 (307)	0.3	26.0	1.1