Fig. 8.

(a) Centerlines of two α-helices making a classical coiled coil and defining the crossing angle φ of the supercoil. Here, the coiled coil has left-handed twist along its axis, and by convention, φ has a negative value. Not to scale: the angle φ shown here is some 50% larger than our largest computed values. See Table 1 for the method of calculation of crossing angle. (b) Schematic view of three α-helices, looking from left to right in Fig. 3 and showing the crossing angles between them. Nearest to the viewer, and shown thickest, is α-helix CD1 of subunit i + 5: its lower one-third, cd, forms a right-handed coiled coil (ψ) with the central portion ab of ND1a of subunit i. Here [cf. (a)], the α-helices are shown straight: it is the crossing angles that are of primary interest. The crossing angle between ND1a and CD1, both of subunit i, is designated φ. Values of φ and ψ are given in green for the straight L structure (Ref. 10) and in red for the straight R structure (Ref. 9). Of special interest is the twist angle (φ + ψ) (“in the glue” of Fig. 7) between CD1 (i + 5) and CD1(i) and, in particular, the change in its value in the L-to-R transition (see Table 1). (c) Close-up view of the switch connection ab/cd of (b). The portions cd shown in green and red correspond to the straight L and R structures, respectively: note that the switch connection is such that the change in angle ψ is directly related to the vertical shearing movement of 2.5 Å within the switch. Data from Refs. 9 and 10.