The structure of the manganese complex of Photosystem II in its dark-stable S1-state—EXAFS results in relation to recent crystallographic data

Abstract

Determination of the nuclear geometry of the Mn4Ca complex of photosystem II (PSII) may facilitate an in-depth discussion of the mechanism of photosynthetic water oxidation. The first step of structural analysis by EXAFS (extended X-ray absorption fine-structure) spectroscopy and protein crystallography is the determination of the structure of the dark-stable S1-state of the Mn complex. Approaches for normalization and simulation of EXAFS spectra are critically evaluated, in particular with respect to the accuracy of the determined coordination numbers. It is shown that the number of Mn–Mn interactions with about 2.7 A length is likely to be two in the S1-state. The presence and orientation of the 2.7 A Mn–Mn vectors as well as of the Mn–Ca distances of ∼3.3 A length is predicted by EXAFS analysis and seems to be compatible with the available crystallographic data (Zouni, Witt, J. Kern, P. Fromme, N. Kraus, W. Saenger and P. Orth, Nature, 2001, 409, Kamiya and Shen, Proc. Natl. Acad. Sci. USA, 2003, 100, 98; Ferreira, Iverson, Maghlaoui, Barber and Iwata, Science, 2004, 303, 1831). In the light of XAS results on radiation-induced modifications of the Mn4Ca complex, however, we conclude that in the course of the crystallographic data collection the Mn4Ca complex may be significantly affected by X-ray photoreduction so that the obtained electron density maps do not represent the intact complex in its S1-state. Thus, any detailed structural modelling by combination of EXAFS results and crystallographic data seems to be premature. Two motifs for the connection of Mn ions in the complex, namely either by (μ2-O)2 or (μ2-O, μ3-O) bridges, may account for Mn–Mn distances close to 2.7 A. Three basic possibilities to arrange the Mn ions of the Mn4Ca complex in its S1-state are discussed.