Introduction to Fiber Diffraction

Some Macromolecules form Fibers rather than Crystals

Many biological macromolecules will not or cannot crystallize. However, an important group of fibrous macromolecules such as DNA or many of the components of the cytoskeleton form orientated fibers in which the axes of the long polymeric structures are parallel to each other. Often, as in the case of muscle fibers, the orientation is intrinsic; sometimes the long molecules can be induced to form orientated fibers by pulling them from a gel with tweezers, sometimes by flowing a gel through a capillary tube, or even by subjecting them to intense magnetic fields. The experimental set-up is rather simple: the orientated fiber is placed in a collimated x-ray beam at right angles to the beam and the "fiber diffraction pattern" is recorded on a film placed a few cm away from the fibre.

Fibers show helical symmetry rather than the three-dimensional symmetry taken on by crystals. By analysing the diffraction from orientated fibers one can deduce the helical symmetry of the molecule and in favourable cases one can deduce the structure. In general this is done by constructing a model of the fiber (as in DNA) and then calculating the expected diffraction pattern. By comparing the calculated and observed diffraction patterns one eventually arrives at a better model.

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