Meridional reflections

The occurrence of the mesophase in the fiber is confirmed by x-ray diffraction examination. The occurrence of three equatorial reflections 010, 110, and 100, the absence of layer and meridional reflections, and the manifestation of the intensity maximum of diffusively scattered radiation at 20 = 19 in the fiber diffraction pattern are the criterion for the presence of the mesophase. The... 

Fig. 2. (continued)—(d) an aggregate of microcrystallites whose long axes are parallel, but randomly oriented (left), diffracts to produce a series of layer lines (right) and (e) a polycrystalline and preferentially oriented specimen (left) diffracts to give Bragg reflections on layer lines (right). The meridional reflection on the fourth layer line indicates 4-fold helix symmetry. 

Figure 8. (Continued). As described above, the packing of myosin molecules into the thick filament is such that a layer of heads is seen every 14.3 nm, and this reflection is thought to derive from this packing. Off the meridian the 42.9 nm myosin based layer line is shown. This arises from the helical pitch of the thick filament, due to the way in which the myosin molecules pack into the filament. The helical pitch is 42.9 nm. c) Meridional reflections from actin. Actin based layer lines can be seen at 35.5 nm, 5.9 nm and 5.1 nm (1st, 6th, and 7th layer lines)and they all arise from the various helical repeats along the thin filament. Only the 35.5 nm layer line is shown here.The 5.9 nm and 5.1 nm layer lines arise from the monomeric repeat. The 35.5 nm layer line arises from the long pitch helical repeat and is roughly equivalent to seven actin monomers. A meridional spot at 2.8 nm can also be seen, d) The equatorial reflections, 1,0 and 1,1 which arise from the spacings between crystal planes seen in cross section of muscle.

The use of unimodal meridional reflections for determination of the orientation distribution and desmearing (Sect. 9.6). 

In Fig. 9.5 two sides of the spherical triangle merge (

directly reflects the shape of the orientation distribution. 

The fiber repeat is 15.24 A (1.524 nm). The threefold screw symmetry of the chain was confirmed by the observation of a third layerline, meridional reflection. 

The repeat distance is 18.6 A (1.86 nm), with a meridional reflection on the first layer-line. A tetrasaccharide repeating-unit was favored. It was suggested that the linkage between the 2-acetamido-2-deoxy-D-glucose and D-glucuronic acid residues is alternately a-D-(l — 4) and /3-d-(1 — 4). 

Fig. 1. Cross-/] structure of amyloid fibrils. (A) Cartoon representation of a cross-/] X-ray diffraction pattern. The defining features are a meridional reflection at 4.7 A and an equatorial reflection on the order of 10 A. The 4.7-A reflection is generally much brighter and sharper than the reflection at 10 A. (B) The cross-/] core structure of amyloid fibrils. Parallel /(-sheets are depicted, but the structure could equivalendy be composed of antiparallel /(-sheets or a mix of parallel and antiparallel. The 4.7-A spacing of /(-strands within each /(-sheet is parallel to the long fibril axis. The depicted 10-A sheet-to-sheet spacing actually ranges from about 5 to 14 A (Fandrich and Dobson, 2002), depending on the size and packing of amino acid side chains. Amyloid fibrils have diameters on the order of 100 A.

Fig. 13a and b. Intensity contour maps around the 5.9-nm and 5.1-nm actin layer lines (indicated by arrows) a resting state b contracting state. Z is the reciprocal-space axial coordinate from the equator. M5 to M9 are myosin meridional reflections indexed to the fifth to ninth orders of a 42.9-nm repeat, (c) intensity profiles (in arbitrary units) of the 5.9- and 5.1-nm actin reflections. Dashed curves, resting state solid curves, contracting state. Intensity distributions were measured by scanning the intensity data perpendicular to the layer lines at intervals of 0.4 mm. The area of the peak above the background was adopted as an integrated intensity and plotted as a function of the reciprocal coordinate (R) from the meridian... 

Electron diffraction patterns from the primary wall of 15-day-old cotton fiber showed sharp, meridional reflections, with d spacings of 0.517 nin and 0.258 nm. Broad maxima on the equator, with spacings of 0.416 nm and 0.570 nm were also observed. On this basis, it was suggested that the primary wall of cotton contains the cellulose IVi polymorph, which is simply a laterally disordered structure of cellulose I. A discussion of the morphology of cellulose in the primary wall was given. 

A layer line-spacing of 4.13 nm was observed. Occurrence of meridional reflections only on layer line s with 1 = 3n, and stereochemical analysis, led to a 3(— 1.377) conformation. The intrachain hydrogen-bonds are OH-4—0-5 in the Rhap-(1— 3)-Rhap segment, OH-4—0-5 in the Rhap-(1—>3)-Galp segment, and OH-2—0-2 in the Galp-(1—>3)-Rhap segment. The influence of Rhap on the conformation of the chain was discussed. 

X-Ray diffraction showed meridional reflections on the third and the sixth layer-lines. Model-building calculations led to a 3(— 1.291) helical conformation, with the following, intrachain hydrogen-bonds (i) 0H-3-(Fuep)—0-5-(GlcpA), (n) OH-2-(Fucp)—0-2-(Glcp), and (iii) 0H-.3-(Glqi)—0-5-(Galp ) ... 

In addition, for the simplest type of discrete helix, in which there is an integral number of residues per turn of the helix, this integer Pip is the same as number / of the layer line on which the first meridional intensity peak occurs. Note that helix d contains exactly six residues per turn, and that the first meridional intensity peak above or below the center of the pattern occurs on layer line l = 6. To review, the layer-line spacing Z is proportional to l/P, and the distance from the origin to the first meridional reflection is proportional to 1 Ip. 

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