title: Visible light induction of an electron paramagnetic resonance split signal in Photosystem II in the S(2) state reveals the importance of charges in the oxygen-evolving center during catalysis: a unifying model.
abstract: Cryogenic illumination of Photosystem II (PSII) can lead to the trapping of the metastable radical Y(Z)(•), the radical form of the redox-active tyrosine residue D1-Tyr161 (known as Y(Z)). Magnetic interaction between this radical and the CaMn(4) cluster of PSII gives rise to so-called split electron paramagnetic resonance (EPR) signals with characteristics that are dependent on the S state. We report here the observation and characterization of a split EPR signal that can be directly induced from PSII centers in the S(2) state through visible light illumination at 10 K. We further show that the induction of this split signal takes place via a Mn-centered mechanism, in the same way as when using near-infrared light illumination [Koulougliotis, D., et al. (2003) Biochemistry 42, 3045-3053]. On the basis of interpretations of these results, and in combination with literature data for other split signals induced under a variety of conditions (temperature and light quality), we propose a unified model for the mechanisms of split signal induction across the four S states (S(0), S(1), S(2), and S(3)). At the heart of this model is the stability or instability of the Y(Z)(•)(D1-His190)(+) pair that would be formed during cryogenic oxidation of Y(Z). Furthermore, the model is closely related to the sequence of transfers of protons and electrons from the CaMn(4) cluster during the S cycle and further demonstrates the utility of the split signals in probing the immediate environment of the oxygen-evolving center in PSII.
authors.affiliation: Photochemistry and Molecular Science, Department of Chemistry, Ångström Laboratory, Uppsala University, SE-751 20 Uppsala, Sweden.
authors.forename: Johannes
authors.initials: J
authors.lastname: Sjöholm
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
authors.forename: Kajsa G V
authors.initials: KG
authors.lastname: Havelius
authors.forename: Felix M
authors.initials: FM
authors.lastname: Ho
mesh_terms: D002384:Catalysis; D004578:Electron Spin Resonance Spectroscopy; D008027:Light; D008345:Manganese / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry; D055668:Photochemical Processes; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism / Q000528:radiation effects; D018724:Spinacia oleracea / Q000737:chemistry; D055672:Static Electricity
chemical_list: D045332:Photosystem II Protein Complex; D008345:Manganese; D010100:Oxygen