The S₀ State EPR Signal from the Mn Cluster in Photosystem II Arises from an Isolated S = ¹/₂ Ground State

Abstract

During oxygen evolution, the Mn cluster in Photosystem II cycles through five oxidation states, S0-S4. S0 and S2 are paramagnetic, and can be monitored by electron paramagnetic resonance (EPR). Recently a new EPR signal from the S0 state was discovered [Ahrling et al. (1997) Biochemistry 36, 13148-13152, Messinger et al. (1997) J. Am. Chem. Soc. 119, 11349-11350]. Here, we present a well-resolved S0 spectrum, taken at high power and low temperature. The spectrum is wider and more resolved than previously thought, with structure over more than 2500 G, and appears to have at least 20 reproducible peaks on each side of g = 2. We also present the temperature dependence of the unsaturated S0 signal amplitude. A linear relationship was found between signal intensity and reciprocal temperature (1/T) in the region 5-25 K, clearly extrapolating to 0. This obeys the Curie law, indicating that the S0 state is a ground S = 1/2 state with no thermally accessible excited state. The data are consistent with a minimum energy gap of 30 cm-1 between the ground and first excited states.

Interaction of methanol with the oxygen-evolving complex: atomistic models, channel identification, species dependence, and mechanistic implications

“…62 Older estimates in the literature place the native Δ E of spinach to ca. 6 cm –1 , 47 while for methanol-treated samples previous estimates included 12 cm –1 , 50 30 cm –1 , 47 and 36.5 cm –1 . 46 Therefore, our principal target here is to identify methanol interaction sites that correlate with an increase of Δ E .…”

“…The phenomenology of split EPR signals of both the S 1 Y Z ˙ and the S 0 Y Z ˙ states has been rationalized in terms of an increase in Δ E upon addition of methanol, 60 so a similar mode of interaction as for the S 1 and S 2 states (operating on the Mn3–Mn4 antiferromagnetic coupling) would be valid for the S 0 state as well. We prefer not to propose specific S 0 models at this point because of the more complex EPR phenomenology of this state 50,77,116,117 and remaining uncertainties in the protonation states of terminal water-derived ligands and oxo bridges. 35,72,118–120 EPR studies of the S 3 state reveal a rich and complex phenomenology.…”

X-ray absorption spectroscopy to analyze nuclear geometry and electronic structure of biological metal centers?potential and questions examined with special focus on the tetra-nuclear manganese complex of oxygenic photosynthesis

“…10). This means that an unbiased inspection of the inflectionpoint shift for the S 0 →S 1 transition is suggestive of a twoelectron oxidation, a presumption that is clearly unreasonable in the light of EPR results [66,73,74,75,76,77]. For the S 2 →S 3 transition, it has been reported that the edge-shift is clearly smaller than the S 1 →S 2 shift [70,71] whereas others found it to be of comparable magnitude [31,34,35].…”

Detection of an Electron Paramagnetic Resonance Signal in the S0 State of the Manganese Complex of Photosystem II from Synechococcus elongatus

“…The S 0 multiline signal, centered at g ) 2, was spread over about 2000 G and has been proposed to originate from a Mn 4 cluster containing a di-µ-oxo(Mn II Mn III ) dimer (26)(27)(28). A temperature dependence study of the S 0 multiline signal has shown that the S ) 1 / 2 state responsible for the signal is an isolated ground state (29).…”