Cation-π interactions in structural biology

Cation-π interactions in protein structures are identified and evaluated by using an energy-based criterion for selecting significant sidechain pairs. Cation-π interactions are found to be common among structures in the Protein Data Bank, and it is clearly demonstrated that, when a cationic sidechain (Lys or Arg) is near an aromatic sidechain (Phe, Tyr, or Trp), the geometry is biased toward one that would experience a favorable cation-π interaction. The sidechain of Arg is more likely than that of Lys to be in a cation-π interaction. Among the aromatics, a strong bias toward Trp is clear, such that over one-fourth of all tryptophans in the data bank experience an energetically significant cation-π interaction.