π-π interactions: the geometry and energetics of phenylalanine-phenylalanine interactions in proteins

The geometries of aromatic-aromatic interactions between phenylalanine residues in proteins are analysed in detail and correlated with energy calculations. A new definition of the interplanar angle is important for distinguishing favourable edge-to-face and unfavourable face-to-face orientations. The experimental observations are scattered over a wide range of conformational space, with no strongly preferred single orientation. However, Phe-Phe interactions occur almost exclusively in electrostatically attractive geometries: electrostatically unfavourable regions are only sparsely populated. Electrostatics dominate the geometry of interaction, while van der Waals' interactions are less significant, probably due to the hydrophobic environment of the protein core. The observations on proteins support the Hunter-Sanders rules for π-π interactions. In particular, offset stacked geometries, which theory predicts to be favourable, are observed experimentally. For monocyclic aromatics, use of a CH dipole, the approach used in molecular mechanics calculations, accounts well for these aromatic-aromatic interactions. Comparison with the results obtained from the small molecules database indicates that the protein and small molecule crystal environments are very different.