Pulse EPR Spectroscopic Characterization of the S₃ State of the Oxygen-Evolving Complex of Photosystem II Isolated from Synechocystis

Abstract

The S 3 state is the last semi-stable state in the water splitting reaction that is catalyzed by the Mn 4 O 5 Ca cluster that makes up the oxygen-evolving complex (OEC) of photosystem II (PSII). Recent high-field/frequency (95 GHz) electron paramagnetic resonance (EPR) studies of PSII isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus have found broadened signals induced by chemical modification of the S 3 state. These signals are ascribed to an S 3 form that contains a fivecoordinate Mn IV center bridged to a cuboidal Mn IV 3 O 4 Ca unit. High-resolution X-ray free-electron laser studies of the S 3 state have observed the OEC with all-octahedrally coordinated Mn IV in what is described as an open cuboid-like cluster. No five-coordinate Mn IV centers have been reported in these S 3 state structures. Here, we report high-field/frequency (130 GHz) pulse EPR of the S 3 state in Synechocystis sp. PCC 6803 PSII as isolated in the presence of glycerol. The S 3 state of PSII from Synechocystis exhibits multiple broadened forms (≈69% of the total signal) similar to those seen in the chemically modified S 3 centers from T. elongatus. Field-dependent ELDOR-detected nuclear magnetic resonance resolves two classes of 55 Mn nuclear spin transitions: one class with small hyperfine couplings (|A| ≈ 1−7 MHz) and another with larger hyperfine couplings (|A| ≈ 100 MHz). These results are consistent with an all-Mn IV 4 open cubane structure of the S 3 state and suggest that the broadened S 3 signals arise from a perturbation of Mn4A and/or Mn3B, possibly induced by the presence of glycerol in the as-isolated Synechocystis PSII.

Redox Isomerism in the S3 State of the Oxygen‐Evolving Complex Resolved by Coupled Cluster Theory

“…[ 14 , 20 ] For this process to be feasible, the two forms must be close in energy. X‐ray absorption spectroscopy (XAS) [51] and X‐ray emission spectroscopy (XES) experiments [9] as well as EPR studies[ 3a , 5a , 5d ] are in favor of a IV–IV–IV–IV valence distribution. In addition, Corry and O'Malley reported [20] that the calculated 55 Mn hyperfine coupling constants for a peroxo model do not agree with experiment, in contrast to the oxo‐peroxo form.…”

CaMn3IVO4 Cubane Models of the Oxygen‐Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

“…[15,40] Similar to the S 2 state,astructural interpretation for the observed spectral heterogeneity in the S 3 state is the subject of ongoing investigation. [34] An alternative interpretation for the spin state interconversion in the S 2 state is based on the observation that this process is pHd ependent, with ah igher pHf avoring the high-spin ground state. [24,41] Notably,o ther effects such as low temperature irradiation of the S 1 state and the introduction of F À favor formation of the high-spin state.…”

“…[20][21][22][23][24] Connected to Mn oxidation state and structural changes, each of the S n state intermediates adopts a unique electronic structure with a characteristic spin ground state S G : [25] S G (S 0 ) = 1/2, [26] S G (S 1 ) = 0 [27] or higher [28][29] , S G (S 2 ) = 1/2 or 5/2, [30][31][32] S G (S 3 ) = 3. [15,[33][34][35] The interconversion between the S G = 1/2 and S G = 5/2 forms of the S 2 state is notable. [30] One interpretation invokes structural changes in the CaMn 4 core centered around the location of a particular O(5) oxo moiety, with concomitant changes in the magnetic coupling interactions resulting in a different S G .…”

“…Connected to Mn oxidation state and structural changes, each of the S n state intermediates adopts a unique electronic structure with a characteristic spin ground state S G : [25] S G (S 0 )=1/2, [26] S G (S 1 )=0 [27] or higher, [28, 29] S G (S 2 )=1/2 or 5/2, [30–32] S G (S 3 )=3 [15, 33–35] . The interconversion between the S G =1/2 and S G =5/2 forms of the S 2 state is notable [30] .…”

CaMn3IVO4 Cubane Models of the Oxygen‐Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

“…[20][21][22][23][24] Connected to Mn oxidation state and structural changes, each of the S n state intermediates adopts au nique electronic structure with ac haracteristic spin ground state S G : [25] S G -(S 0 ) = 1/2, [26] S G (S 1 ) = 0 [27] or higher, [28,29] S G (S 2 ) = 1/2 or 5/ 2, [30][31][32] S G (S 3 ) = 3. [15,[33][34][35] Thei nterconversion between the S G = 1/2 and S G = 5/2 forms of the S 2 state is notable. [30] One interpretation invokes structural changes in the CaMn 4 core centered around the location of aparticular O(5) oxo moiety, with concomitant changes in the magnetic coupling interactions resulting in ad ifferent S G .…”