Extending the methodology of X-ray crystallography to non-crystalline specimens
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Since the emergence of powerful X-ray sources such as synchrotron radiation, the area of X-ray structure determination gradually evolved into two fields. The relatively small one is X-ray Microscopy which employs X-ray lenses such as zone plates, compound refractive lenses, curved mirrors and glass capillaries to focus X-rays to determine the structures of non-crystalli ne specimens. The resolution of X-ray Microscopy is limited by the resolution of the X-ray lenses and, for biological specimens, radiation damage. While the radiation damage problem can be alleviated somewhat by cooling the specimens down to liquid nitrogen temperature, the resolution of X-ray lenses is limited by fabrication difficulties. At present, the highest resolution achievable is around SO-SOnm1'2. The other far more successful field is Xray crystallography. By employing crystalline specimens, where constructive interference among the large number of identical unit cells generates strong Bragg peaks, X-ray crystallography can achieve atomic resolution without serious radiation damage to the specimens. The major difficulty of X-ray crystallography, however, is that the specimens should be crystalline, while most complex biological specimens, for example, can not be or are too big to be crystallized.