![]() For oriented single crystals or ordered membranes, the interatomic vector orientations can be deduced from dichroism measurements. However, ordered samples, such as membranes and single crystals, often increases the information obtained from XAS. The significant advantage of XAS over the X-ray crystallography is that the local structural information around the element of interest can be obtained even from disordered samples, such as powders and solution. Study of structurally well-characterized model complexes also provides a benchmark for understanding the EXAFS from metal systems of unknown structure. These complexes provide a basis for evaluating the influence of the coordination environment (coordination charge) on the absorption edge energy (Cinco et al. Metal complexes, as models with known structures, have been essential in order to understand the XAS of metallo-proteins. The development of intense third generation synchrotron radiation X-ray sources has also permitted the study of dilute samples. In such a case, the use of X-ray fluorescence for the detection of the absorption spectra, instead of using the transmission detection mode, has been the standard approach. In the PS II, for example, Mn may be at the level of 10 parts per million or less. Yet, X-ray spectroscopy of metallo-enzymes has been a challenge due to the small relative concentration of the element of interest in the sample. X-ray absorption spectroscopy (XAS) allows us to study the local structure of the element of interest without interference from absorption by the protein matrix, water or air. These two methods give complementary structural information, the XANES spectra reporting electronic structure and symmetry of the metal site, and the EXAFS reporting numbers, types, and distances to ligands and neighboring atoms from the absorbing element (Koningsberger and Prins 1988). X-ray absorption spectroscopy (XAS) is the measurement of transitions from core electronic states of the metal to the excited electronic states (LUMO) and the continuum the former is known as X-ray absorption near-edge structure (XANES), and the latter as extended X-ray absorption fine structure (EXAFS) which studies the fine structure in the absorption at energies greater than the threshold for electron release. The XES methods are discussed in the paper by Bergmann and Glatzel (this issue). In this article, we focus on XAS methods which have been used in the field of photosynthesis. The results from infrared and Raman spectroscopy can be related to specific elements through isotopic substitution, but the analysis of such spectra for metal clusters is complicated when the structure is not known. This is quite a contrast with other methods, such as optical or UV absorption, fluorescence, magnetic susceptibility, electrochemistry etc., which have been applied to study biological redox systems. The choice of the energy of the X-rays used, in most cases, determines the specific element being probed. Both methods characterize the chemical nature and environment of atoms in molecules, and synchrotron sources provide a range of X-ray energies that are applicable to most elements in the periodic table, in particular, those present in redox-active metallo-enzymes. In X-ray spectroscopy, transitions are involved in absorption (XAS, X-ray absorption spectroscopy) or emission (XES, X-ray emission spectroscopy) of X-rays, where the former probes the ground state to the excited state transitions, while the latter probes the decay process from the excited state. 1996 Penner-Hahn 1998 Yachandra 2005 Yano and Yachandra 2007 Sauer et al. The historical background of the XAS study on PS II, especially the early work, has been reviewed in some detail (Yachandra et al. 1971 Eisenberger and Kincaid 1978) overlaps the history of the structural research on the OEC in photosystem II (PS II). The history of the development of the experimental method of XAS (Sayers et al. It has been raising important questions of correlation between structure and function of the metal sites in metallo-proteins, including the photosynthetic oxygen-evolving complex (OEC Yano and Yachandra 2008). During the past 30 years, X-ray absorption spectroscopy (XAS) has made major contributions to a wide variety of biochemical research topics. ![]()
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