Low-angle diffraction pattern

A typical low-angle diffraction pattern from relaxed bony fish muscle is shown in Fig. 4B. Much of the intensity that is seen comes from the organization of the myosin heads on the myosin filaments in the resting state (probably mainly MADP.Pi). We know that the myosin heads lie approximately on three co-axial helices of subunit translation 143 A and repeat 429 A. This is most easily represented by the radial net shown in Fig. 16B-D. The radial net in D is like an opened-out surface view of the filament in B. Here the helical tracks become straight lines, and the black blobs represent the origins on the myosin filament surface of the pairs of myosin heads in each myosin molecule. From early studies it is known that the three crowns within the 429 A repeat are not exactly the same and that there is a perturbation. 

Myosin filament structure has been described by Squire et al. (2005). In vertebrate striated muscles the myosin filaments can be described approximately as three-stranded 9/1 helices. The helix pitch is 1287 A, but, because there are three strands and nine subunits in each strand, the structure repeats after C = 1287/3 = 429 A. Figure 12 shows the expected form of the low-angle diffraction pattern from such filaments. The modeling of this structure by X-ray diffraction was described by Squire et al. in terms of the three crowns of heads within each 429 A repeat. The crown repeat of 143 A gives rise to an m = +1 meridional reflection, which has been labeled as the M3 reflection in many muscle studies (as in Fig. 12). The myosin head array also gives rise to layer lines at orders of the repeat of 429 A. The first myosin layer line (ML1) is at 1/429 A-1, the second (ML2) at 2/429 = 1/ 214.5 A-1, and so on. The M3 reflection occurs on the third layer line at 3/429 = 1/143 A-1. 

Fig. 12. (A) The left half of a low-angle diffraction pattern from bony fish muscle...

Several groups have studied the effects on the muscle low-angle diffraction pattern of applying various mechanical perturbations to steady-state structures, either isometric contractions or rigor muscle at various strain levels. Huxley et al. (1981, 1983) used whole frog muscles and followed the effects of step changes of length of various amplitudes applied at the plateau of an otherwise isometric tetanus. They studied the effects on the M3 intensity as a whole. More recendy, with the two components of the active M3 resolved, Huxley et al. (2003) and Reconditi et al. (2003) have studied the separate behavior of these components. Huxley et al. (2003) found that the intensity ratio of the M30 to M3 varied from an initial value... 

FIGURE 1.8 Low-angle diffraction pattern of n-butylammonium vermiculite at T = 6°C and an external salt concentration of c = 0.1 M. 

Figure 4. Low angle diffraction patterns characteristic of (a) 1-D layer (smectic) phases, (b) 2-D hexagonal phases, and (c) 3-D cubic phases.

The schematic low-angle diffraction patterns from the various types of SmA phase are also shown in Fig. 16. The SmA,/2, SmAj, SmA(j and SmA2 phases are discriminated partly by the positions of the pseudo-Bragg peaks on the meridian, at Qq AkHq, Qq 1kHq, Qq 2tiIIq, and Qq kHq. In addition, the SmAj and SmA j phases may exhibit diffuse maxima at Qq-Ik/Iq and (2o 2 7t//o, corresponding to small domains of SmAj-like and SmAj-like ordering, respectively. 

Figure 18. Structures of modulated (a) SmA and (b) SmC antiphases. The schematic low angle diffraction patterns are shown to the right.

Figure 19. (a) Structure and (b) low angle diffraction pattern from the incommensurate SmA2.inc phase. 

Figure 21. Schematic low angle diffraction patterns from (a) the incommensurate TGBA phase, and (b) the commensurate TGBC phase with = 5. (Adapted from [14]).

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