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Metalloproteins perform a wide variety of biological functions and, in doing so, in many cases exploit the redox properties and the coordination chemistry of the metal atom. The structural changes associated with the different coordination and redox changes can be quite small and can only be visualized at a very high resolution. The advent of synchrotron radiation has provided the possibility of studying these proteins by X-ray crystallography at `atomic' resolutions. Synchrotron radiation has also revolutionized another X-ray technique, XAFS (X-ray absorption fine structure), where modulations in atomic absorption take place due to the scattering of photo-excited electrons from the immediate surrounding of the photon-absorbing atom. The dependency of XAFS on synchrotron radiation is even more acute than protein crystallography, as a continuous X-ray spectrum of high intensity is required for the experiment. In fact, the source and monochomator requirements are very similar for XAFS and MAD (multiple-wavelength anamolous diffraction). The local nature of the XAFS has advantages in that no crystalline order is required and that the resolution is the same in the aqueous, amorphous or crystalline system, i.e. subatomic resolutions are intrinsically present in the data. This review, written in recognition of Sir John Walker's Nobel prize, for which synchrotron radiation played a key role, also provides some recent case studies to illustrate the advantages of this technique and its synergy with the synchrotron-based crystallography.
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