It is useful to compare the spectra from
the unknown complex to some known model complexes (assuming that there is evidence that the structure resembles that of the model complex) and then use Debye–Waller parameters obtained from the model complexes in the fits. This method works reasonably well, when the structure of the system being studied is well-modeled by inorganic complexes. X-ray absorption spectroscopy studies of photosystem II One of the advantages of XAS is that one can potentially study the chemical events from each element which is involved in the reaction. In the OEC, Mn, Ca, and possibly Cl are the key elements we can focus on, in order to obtain Sirolimus the mechanistic information during the catalytic cycle.
The XAS results, with emphasis on results from our laboratory, will be used to highlight the utility of the technique for the study of the Mn4Ca cluster in PS II. Mn XAS The geometric and electronic structural changes of the OEC have been studied intensively using Mn XAS. Figure 3 shows the Mn K-edge spectrum of each S-state of spinach PS II after deconvolution of the spectra obtained from consecutive flash illumination into pure S-state spectra, and their second derivative spectra (Messinger et al. 2001). Traditionally, the inflection point MK-8669 cell line of the rising Mn K main edge (electron 1s to 4p transition) has been used as an indicator of the oxidation states in the field of XAS. The edge positions for each of the S-states have been quantitated by measuring the inflection
point energy (IPE), given by the zero-crossing of the second derivative. Extensive model compound studies have shown that, when Mn is oxidized by one electron in a set of Mn model compounds with similar ligands, the IPE shifts 1–2 eV to higher energy (Visser et Montelukast Sodium al. 2001). Clear differences in absorption edge energy attributed to Mn oxidation were seen in the S0 → S1 and S1 → S2 transitions in the OEC, but the absorption edges for S2 and S3 did not show a significant difference. These results were taken to indicate the absence of Mn oxidation during the S2 → S3 transition, although different interpretation exists. However, one has to be aware that the edge position cannot be simply an indicator of only the oxidation state and it is problematic to conclude oxidation state changes based only on the XANES inflection point. Due to the size of the metal 4p orbital, this orbital overlaps with p orbitals of the ligands, either through σ- or π-bonding. Consequently, XANES is sensitive not only to the oxidation state but also to the ligand environment of the metal. Additionally, no definite theory is available for calculating main K-edge spectra for transition-metal complexes, owing to several factors that affect the metal p-density.