Meridional Bragg reflection

The model for the a helix received strong support when Max Perutz, working with hemoglobin and tilted fibers of keratin, observed a prominent meridional reflection at 1.5 A. This had been previously overlooked, and corresponds to the translation per amino acid residue along the polypeptide chain. The observation of such a meridional Bragg reflection is a good indication of the presence of an a helix. The existence of a helices was finally verified in the crystal structure of myoglobin. ... 

The X-ray diffraction work of Gutierrez et al (1983) showed the presence of aperiodic meridional Bragg reflections in poly(hydroxynaphthoic acid-co-... 

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. 

A fiber diffraction pattern of the potassium salt is shown in Figure 2 ( 8). Sharp Bragg reflections extend to approximately 3.0 A resolution with meridional intensities on the 6th and 9th layer lines. The diffraction pattern can be indexed on the basis of a... 

HBA, 2,6-dihydroxynaphthalene and terephthalic acid can be improved by refining the average residue orientations with respect to the fiber axis, and it is likely that this procedure will also improve the agreement for copoly(HBA/HNA). In addition, the meridional intensities may be affected by interchain interferences as a result of a preferred axial stagger in parts of the specimens, which is indicated by the presence of sharp equatorial and off-equatorial Bragg reflections. 

Equatorial and meridional scans of the wide-angle X-ray scattering patterns of the as-spun CPE-1 fibers are shown in Figure 6. This X-ray pattern shows a weU-pronounced broad halo at diffraction angles of 10°-35° and two weak diffuse maxima at 2 =5°-9° and 12°-16° (Figure 6a). On the meridian, one can distinguish three narrow and well-defined Bragg reflections (20=14.20°, 28.67°, and 43.63°) that are associated with different orders of one reflection and which are located on the second. 

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