GEOMETRY OF INTERPLANAR RESIDUE CONTACTS IN PROTEIN STRUCTURES

The relative spatial disposition of interacting side-chain planar grou ps (aromatic, guanidinium, amide, carboxyl, imidazole) is analyzed for 186 non-homologous well-resolved protein structures. The dihedral ang le of amide or carboxyl planar groups with other planar groups accords with a random distribution of planes. By contrast, the dihedral angle of the planes between close aromatic rings or of the histidine ring i nteracting with aromatic residues is significantly nonrandom, showing an approximately uniform distribution. Our results indicate that edge- to edge and edge-to-center spatial dispositions of residue planar sect ions are prevalent, while complete stacking configurations are uncommo n. The hypothesis that electrostatic forces are a major determinant of the geometry of interactions between side-chain planar groups is disc ussed.