Fig 2. The basic rules and the frustrated αβ-units pairing.

(A) The right-handed rule for crossover connections of βαβ-units is depicted. The left side shows schematics of left- and right-handed βαβ-units, while the right side presents their occurrence frequencies in the dataset. The arrows represent β-strands and the rectangles represent α-helices. (B) The αβ-rule is represented. The left side displays schematics of antiparallel and parallel αβ-units, and the right side shows their occurrence frequencies in the dataset. The square containing a red-filled circle symbolizes a single amino acid residue with a side chain located on the proximal side, and the red-colored filled square represents a residue with a side chain located on the opposite side. (C) The register shift rules for a β-strand of an αβ-unit and its neighboring β-strand are depicted. The left side provides a schematic representation of negative, zero, and positive register shifts, and the right side shows their occurrence frequencies in the dataset. (D) Schematics of structures with a topology that cannot simultaneously satisfy the three rules. The areas that violate the rules are highlighted in magenta. Beneath each schematic, a violated rule is documented. (E) A schematic of a frustrated αβ-units pairing is shown. (F) Schematics of structures with a topology that can simultaneously satisfy the three rules. (G) OFHTs of topology pairs are presented, where one is a topology that does not contain an FαβP (the horizontal axis) and the other is its reverse topology that does (the vertical axis).