Noncovalent interactions have a constitutive role in the science of intermolecular relationships, particularly those involving aromatic rings such as π-π and cation-π. In recent years, anion-π contact has also been recognized as a noncovalent bonding interaction with important implications in chemical processes. Yet, its involvement in biological processes has been scarcely reported. Herein we present a large-scale PDB analysis of the occurrence of anion-π interactions in proteins and nucleic acids. In addition we have gone a step further by considering the existence of cooperativity effects through the inclusion of a second noncovalent interaction, i.e. π-stacking, T-shaped, or cation-π interactions to form anion-π-π and anion-π-cation triads. The statistical analysis of the thousands of identified interactions reveals striking selectivities and subtle cooperativity effects among the anions, π-systems, and cations in a biological context. The reported results stress the importance of anion-π interactions and the cooperativity that arises from ternary contacts in key biological processes, such as protein folding and function and nucleic acids-protein and protein-protein recognition. We include examples of anion-π interactions and triads putatively involved in enzymatic catalysis, epigenetic gene regulation, antigen-antibody recognition, and protein dimerization.
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