Figure 5.
a) Sr K-edge XANES spectra of Sr-PS II from Thermosynechococcus elongatus (top), and the corresponding second derivatives of the XANES spectra (bottom) in the S1, S2, S3 and S0 states and an inactive control sample (HYD). The inflection point of the edges and the shape of the spectra are clearly different between the control and the intermediate S-states. There are small but distinct differences in the S-state spectra which are easier to see in the second derivatives (bottom). The two vertical dashed lines indicate the clear systematic differences in the Sr K-edge spectra between the S-states; the feature labeled a shifts to higher energy and feature b shifts to lower energy as we advance from S0 through S3 states. Other differences are also seen in the second derivatives between the S states, the most significant being that between the S2 and S3 and the S3 to S0 states. b) Fourier transforms of Sr EXAFS of Sr-PS II in the different S-states (S1, S2, S3, S0), and an inactive control sample prepared by hydroxylamine treatment of Sr-PS II. The dominant Fourier peak I is due to ligating oxygens in the first coordination sphere to Sr. Peak II is best fit by four Sr-Mn interactions; short distances at ~3.5 and longer Sr-Mn distances at ~4.0 Å. FT peak II from Sr-Mn is dependent on the particular S-state and shows that structure of cluster changes as we advance through the S-states, with a significant change occurring between the critical S2 to S3 transition suggesting that the cluster is flexible. The control sample shows only the FT peak from Sr-O backscattering, because the cluster is disrupted and the Sr-Mn interactions are lost.