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Figure 4
Left: X-ray diffraction pattern of collagen fibrils in rat-tail tendon. Reproduced with permission by Elsevier Press from Orgel et al. (2000BB24). The X-ray diffraction pattern of rat-tail tendon contains a series of intense meridional reflections corresponding to axial molecular organization. Three-dimensional microcrystalline domains of collagen molecules produce discrete Bragg peaks in the equatorial (m = 0, n = 0) helix layer plane and parallel to the meridian of the diffraction pattern. The resolution of the diffraction data extends to approximately 1.0 nm along the equator and 0.54 nm parallel to the meridian. Elements of diffraction can also be seen corresponding to the (m = 0, n = −1) helical layer line. Such diffraction images were used to produce the electron density maps of collagen structure shown on the right. This is reproduced with permission by Elsevier Press from Orgel et al. (2001BB25). Here the higher-density overlap region (bottom) can be seen to contain five molecular segments whereas the less easily visible gap region contains more diffuse density although some evidence of molecular paths can be seen. The gap region is likely to contribute to the liquid-like order in the collagen fibril. The vertical aspect represents the 67 nm periodicity and this has been compressed by a factor of eight so that the molecular paths are exaggerated. The horizontal scale is 5 nm.

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