The State of Manganese in the Photosynthetic Apparatus. Ii. X-Ray Absorption Edge Studies on Manganese in Photosynthetic Membranes

X-ray absorption spectra at the Manganese K-edge are presented for spinach chloroplasts, and chloroplasts which have been Tris-treated and hence unable to evolve oxygen. A significant change in the electronic environment of manganese is observed and is attributed to the release of manganese from the thylakoid membranes with a concomitant change in oxidation state. A correlation of the K-edge energy, defined as the energy at the first inflection point, with coordination charge has been established for a number of manganese compounds of known structure and oxidation state. Comparison of the manganese K-edge energies of the chloroplast samples with the reference compounds places the average oxidation state of the chloroplasts between +2 and +3. Using the edge spectra for Tris-treated membranes which were osmotically shocked to remove the released manganese, difference edge spectra were synthesized to approximate the active pool of manganese. Coordination charge predictions for this fraction are consistent with an average resting oxidation state higher than +2. The shape at the edge is also indicative of heterogeneity of the manganese site, of low symmetry, or both

Is Mn-Bound Substrate Water Protonated in the S2 State of Photosystem II?

In spite of great progress in resolving the geometric structure of the water-splitting Mn4OxCa cluster in photosystem II, the binding sites and modes of the two substrate water molecules are still insufficiently characterized. While time-resolved membrane-inlet mass spectrometry measurements indicate that both substrate water molecules are bound to the oxygen-evolving complex (OEC) in the S2 and S3 states (Hendry and Wydrzynski in Biochemistry 41:13328–13334, 2002), it is not known (1) if they are both Mn-bound, (2) if they are terminal or bridging ligands, and (3) in what protonation state they are bound in the different oxidation states Si (i = 0, 1, 2, 3, 4) of the OEC. By employing 17O hyperfine sublevel correlation (HYSCORE) spectroscopy we recently demonstrated that in the S2 state there is only one (type of) Mn-bound oxygen that is water exchangeable. We therefore tentatively identified this oxygen as one substrate ‘water’ molecule, and on the basis of the finding that it has a hyperfine interaction of about 10 MHz with the electron spin of the Mn4OxCa cluster, we suggest that it is bound as a Mn–O–Mn bridge within a bis-μ2 oxo-bridged unit (Su et al. in J Am Chem Soc 130:786–787, 2008). Employing pulse electron paramagnetic resonance, 1H/2H Mims electron-nuclear double resonance and 2H-HYSCORE spectroscopies together with 1H/2H-exchange here, we test this hypothesis by probing the protonation state of this exchangeable oxygen. We conclude that this oxygen is fully deprotonated. This result is discussed in the light of earlier reports in the literature

The assignment of Qy(1,0) vibrational structure and Q x for chlorophyll a

International audienceWe used non-photochemical persistent spectral hole-burning at 1.4 K to investigate the Qy(1,0) vibrational structure of Chl a in a water-soluble chlorophyll-binding protein (WSCP) which exhibits resolved structure in its broadband optical spectra. Franck-Condon vibrational overlap factors were determined from the vibrational hole-burning data and used to simulate the Qy(1,0) spectra. The simulations were not able to accurately reproduce the details of the Qy(1,0) spectrum. This indicates a breakdown of the approximations used for the analysis and demonstrates that vibrationally induced mixing of electronic states (vibronic coupling) is active for Chl a. By considering the inhomogeneous broadening and vibrational hole-burning phenomena in the Qx and Qy(1,0) region of Chl-WSCP in addition to magnetic circular dichroism data, we favor the traditional placement of Qx at ?~570-590 nm rather than the alternate assignment underneath the Qy(1,0) absorption near ?~615-630 nm. © 2010 Elsevier B.V. All rights reserved