The aromatic/arginine stack motif. a) Stick representation of the residues involved in W/R stacks; arginines and lysines are coloured blue, aromatic residues are coloured green, and one cysteine residue in brown. Three forms of stack formation: b) parallel (W1012/R1038/W965), c) perpendicular (R1046/W992), and d) aliphatic stacking (R1040/W1012). Maps are 2mFo-DFc from Phenix.refine contoured at 1.03 Sigma. Figure drawn with CCP4mg .