Four-distance definition of inter-residue interactions for the Asn-Gln pair. (a) To define the interaction, one pair of side chain atom is chosen in the first residue (Asn) and another pair is chosen in the second residue (Gln). The four distances (OE1-OD1, OE1-ND2, NE2-OD1, NE2-ND2) define mutual positions of chosen side chain atoms. (b) A 2-dimensional projection (OD1-NE2 versus ND2-OE1 distance) of the 4-distance distributions for the Asn-Gln pair. Peaks in the histogram indicate preferred distance combinations. The peak indicated by the arrow corresponds to the arrangement of side chain atoms in the panel (a). The histogram was built with a bin size of 0.25 Å, without smoothing. The histogram is represented as a contour plot generated with MATLAB. All 4-distance distributions can be viewed in the supporting web site http://bioinfo.weizmann.ac.il/hunter/.