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Cation–π interactions in protein–protein interfaces

Arginine is an abundant residue in protein–protein interfaces. The importance of this residue relates to the versatility of its side chain in intermolecular interactions. Different classes of protein–protein interfaces were surveyed for cation–π interactions. Approximately half of the protein complexes and one‐third of the homodimers analyzed were found to contain at least one intermolecular cation–π pair. Interactions between arginine and tyrosine were found to be the most abundant. The electrostatic interaction energy was calculated to be ∼3 kcal/mol, on average. A distance‐based search of guanidinium:aromatic interactions was also performed using the Macromolecular Structure Database (MSD). This search revealed that half of the guanidinium:aromatic pairs pack in a coplanar manner. Furthermore, it was found that the cationic group of the cation–π pair is frequently involved in intermolecular hydrogen bonds. In this manner the arginine side chain can participate in multiple interactions, providing a mechanism for inter‐protein specificity. Thus, the cation–π interaction is established as an important contributor to protein–protein interfaces. Proteins 2005. © 2005 Wiley‐Liss, Inc.