ε 2 provides a gauge of whether the addition of another shell to the fit is justified. A detailed description of error analysis is presented by Lytle et al. (1989). The importance of the EXAFS technique to the Stattic mw biochemist or structural biologist depends directly on the fact that the EXAFS modulations contain information about the distance between
the absorbing and backscattering atoms within a distance of about 5 Å, as well as the identity and number of the backscattering atoms. Essentially, EXAFS analysis is used to determine the radial distribution of atoms around a particular selleck products absorbing atom, thus providing a probe for the local structure in the vicinity of the absorbing atom; for example, the metal in the active site of an enzyme. These vector lengths (distances) can be determined to a precision of 0.02 Å and much more precisely than by conventional X-ray crystallography. Advantages and limitations of XAS We summarized the advantages and the limitations (Eisenberger and Brown 1979)
of the XAS method as follows. Advantages (1) X-ray absorption spectroscopy (XAS) is element specific, so one can focus on one element without interference from other elements present in the sample. In a protein, which has more than one metal like cytochrome oxidase (Cu and Fe), or nitrogenase (Fe and Mo), it is possible to study the structural environment of each metal atom selectively. The element Cell Cycle inhibitor specificity and the fact that it is always next possible to obtain an X-ray spectrum of an element also means that one ‘sees’ all of the metal of interest, which is present in the sample. This makes it imperative that one is sure of the biochemical homogeneity of the sample and, if there is more
than one site for the same metal, to resolve the structural parameters of the different sites. (2) Another important advantage of XAS is that the metal of interest is never ‘silent’ with respect to X-ray absorption spectra. The system could be ‘silent’ with respect to EPR, optical, or other spectroscopic methods, but one can always probe the metal site structure by XAS. (3) X-ray absorption spectroscopy (XAS) is not limited by the state of the sample, because it is sensitive only to the local metal site structure. The sample can be prepared as a powder, a solution or, as is done most often, as a frozen solution for biological samples. It is not necessary to obtain single crystals of the material to examine the local structure of the metal. However, having oriented crystals such as membranes and single crystals significantly increases the structural information obtained from the XAS method. This will be discussed in a latter section.