Figure 1.

Geometric criteria used to identify non-canonical interactions. (A) The aromatic ring system is represented as a hexagon. In the case of Trp, two ring systems, a five-member and a six-member ring system, are considered separately. The donor group is represented as X-H, where X can be main chain C^α, N atom or side chain N atom of Arg, Lys, S atom of Cys and O atom of Thr, Ser and Tyr. π_m represents the ring mid-point and the vector π_n represents the normal to the plane of the ring. P₁ and P₂ are distances from X and H to π_m respectively. P₃ is the angle between vectors X-H and H-π_m and P₄ is the angle between the vectors X-π_m and the ring normal, π_n. (B) Parameters used to identify C^α-H···O=C interactions. P₁ and P₂ are distances from C^α, H^α to the main chain carbonyl O. P₃ and P₄ are the angles C-H···O and H···O=C, respectively. Default values used to identify interactions based on these definitions are provided in Table 1.