Revisiting the Explanations of the Beta-Sheet Twist and Its Handedness.

The β-sheet, consisting of several β-strands, is one of the most important secondary structures of proteins. Most β-sheets differ greatly from the fully extended, all-trans form due to twisting and/or bending. When looked at in the direction of the β-strands rather than along the hydrogen bonds, the twist is usually right-handed. Although numerous studies have investigated the origin of the right-handed twist of β-sheets or β-strands in proteins, there is no common agreement about its causes. The twist can be seen from the dihedral angles in the Ramachandran plot. Here, we discuss the opposing roles of the dihedral angles ϕ and ψ. The key role is played by the angle ϕ, which is controlling the distance between the carbonyl group of the backbone and the side chain of the next amino acid. There are two antisymmetric effects: the change in ϕ in the clockwise direction is initiated by a Cβ… O clash and delimited by a subsequent Cβ… NH clash, while the opposite relationship holds for the counter-clockwise change in ψ. The impact of the twist on tertiary structures is examined. The understanding of the molecular effects within a strand is deepened by 3D computer images and ball-and-stick models. The use of (tangible) physical models is highlighted in view of teaching structural biology to undergraduate students.