XAS of Mn in the Photosynthetic Oxygen, Evolving Complex

Nearly all of the dioxygen which animals and aerobic bacteria use during respiration originates from the photosynthetic oxygen evolving complex (OEC) which is found in the chloroplast thylakoid membranes of higher plants and algae. Four specifically bound Mn are required for OEC activity [l]. These manganese do not exhibit the six line epr spectrum which is characteristic of Mn(IIj in solution, but instead, a multiline epr signal [2], indicating that at least two, and as many as four, manganese ions form a magnetically coupled cluster in the OEC. A second epr spectral feature from S2 becomes apparent at geR cu.. 4.1 if preparation conditions are slightly altered [ 31. In a series of studies [4]! Klein, Sauer and coworkers found that the average Mn coordiantion enviroment includes an Mn-(N,O) distance of 1.751.8 b, a disordered shell of oxygen or nitrogen at an average distance of 2.15 A from the Mn, and a Mn-Mn distance of 2.7A. The shorter Mn-(N,O) distance was assigned to a bridging ligand and the longer distance to terminal ligands. The observed Mn-Mn distance of 2.7 W provides additional evidence that at least two manganese ions are in close proximity. The recently reported procedure for isolating PSI1 reaction centers using the detergent octylglucopyranoside in the presence of high ionic strength has permitted the preparation of highly purified, highly concentrated samples of the OEC [53. This procedure has allowed us to prepare samples containing ca. 1.5 mM Mn, thus permitting us to obtain data having a significantly enhanced signal to 9 oise ratio. EXAFS. The k -weighted EXAFS data for two different OEC samples (both in the Sl form) are shown in Figure 1. The high signal/noise ratio and good reproducibility of these data is apparent. The higher quality of these data in comparison with earlier XAS studies of the OEC is attributable to the higher concentrations of Mn in our samples and the use of a solid-state detector array for monitoring x-ray fluorescence. The Fourier transforms (FTs) of the data in Figure 1 are shown in Figure 2. The major features of the FTs are similar to those reported by Klein, Sauer and co-workers. We see three, and possibly four, peaks which are reproducibly present above the noise level of the data. The first two peaks, corresponding to shells of atoms at ca. 1.9b and 2.7A from the Mn, are identical to those reported in the most recent work by Yachandra et al. In addition to these two main peaks, we also see smaller peaks corresponding to atoms at distances of ca. 3.3A and 4.5.4 from the Mn. The first shell peak can be fitted with ca. four oxygens at 1.91A. The low first shell coordination number likely reflects the presence of several ligands distributed over a range of longer distances, however we see no