Masami Kusunoki

X-ray Detection of the Period-Four Cycling of the Manganese Cluster in Photosynthetic Water Oxidizing Enzyme.

X-ray absorption near-edge structure spectra of the manganese (Mn) cluster in physiologically native intermediate states of photosynthetic water oxidation induced by short laser flash were measured with a compact heat-insulated chamber equipped with an x-ray detector near the sample surface. The half-height energy of the Mn Kedge showed a period-four oscillation dependent on cycling of the Joliot-Koks oxygen clock. The flash number-dependent shift in the Mn K-edge suggests that the Mn cluster is oxidized by one electron upon the S0-to-S1, S1-to-S2, and S2-to-S3 transitions and then reduced upon the S3-to-S0 transition that releases molecular oxygen.

Spin-exchange interactions in the S2-state manganese tetramer in photosynthetic oxygen-evolving complex deduced from g=2 multiline EPR signal

Abstract Possible spin-exchange structures of the Mn(III,IV,IV,IV) cluster in an S 2 state of plant photosystem II were computer-searched, within the range compatible with X-ray absorption data, by diagonalizing each Heisenberg spin-exchange Hamiltonian and then by checking whether it can take the S =1/2 ground state capable of explaining the effective 55 Mn hyperfine constants determined from oriented multiline spectra and the first excited state with excitation energy around 20–50 cm −1 , or not. The possible spin-exchange structures were found to be distributed in those that contain only one strong-antiferromagnetic coupling and at most two intermediate coupling(s). The most probable structures are discussed in detail.

A new paramagnetic hyperfine structure effect in manganese tetramers. The origin of “multiline” EPR signals from an S2 state of a photosynthetic water-splitting enzyme

Abstract A new theory of EPR spectra for singlet states of Mn tetramers, allowing spin-hybridizations for component composite spins, is developed to elucidate the origin of the “multiline” EPR signals from the Mn clusters in an S 2 state of photosystem II. By computer simulation it is shown that the Mn cluster most probably exists in the Mn(III, IV, IV, IV) oxidation state with a strong-antiferromagnetic exchange interaction ( J ab S a · S b ) between a pair of Mn(III) and Mn(IV) ions and the other weak or intermediate ones satisfying the relations, (3/2) p 01 (2 J ac −2 J ad + J bd − J bc ) cos 2θ=[ J cd − J ac − J ad + 1 2 ( J bc + J bd )] sin 2θ⪢0, where p 01 =1.291 and θ, a polar angle of the spin—state vector in the x – y plane of S cd =0 and S cd =1 spin configurations, is ≈ 38°–39° in control samples.

Manganese K-edge X-ray absorption spectra of the cyclic S-states in the photosynthetic oxygen-evolving system

A set of Mn K-edge XANES spectra due to the redox states S0−S3 of the OEC were determined by constructing a highly-sensitive X-ray detection system for use with physiologically native PS II membranes capable of cycling under a series of saturating laser-flashes. The spectra showed almost parallel upshifts with relatively high K-edge half-height energies given by 6550.9±0.2 eV, 6551.7±0.2 eV, 6552.5±0.2 eV and 6553.6±0.2 eV for the S0, S1, S2 and S3 states, respectively. The successive difference spectra between S0 and S1, S1 and S2, and S2 and S3 states were found to exhibit a similar peak around 6552–6553 eV, indicating that one Mn(III) ion or its direct ligand is univalently oxidized upon each individual S-state transition from S0 to S3. The present data, together with other observations of EPR and pre-edge XANES spectroscopy, suggest that the oxidation state of the Mn cluster undergoes a periodic change; S0: Mn(III,III,III,IV) → S1: Mn(III,IV,III,IV) → S2: Mn(III,IV,IV,IV) → S3: Mn(IV,IV,IV,IV) or Mn(III,IV,IV,IV)·L+ with L being a direct ligand of a Mn(III) ion.

EPR evidence for the primary water oxidation step upon the S2 → S3 transition in the Joliot-Kok cycle of plant photosystem II

Abstract A new type of 90–240 G wide EPR signal from the modified S3 state of Ca2+-depleted photosystem II (PSII) is concluded to arise from a partially oxidized water radical with spin S = 1 2 interacting with the S = 1 2 S2-state manganese tetramer (‘Mn4’). This is based exclusively on the fact that the average g value of the radical is ≈ 2.010–2.012, a value close to that of OH radical (2.011) and significantly larger than either one of an oxidized imidazole (2.00226) or an oxidized tyrosine (2.0046), indicating that the radical may be (HOOH)−, the most probable intermediate produced by abstracting two protons and one electron from a bound water dimer. The effective interactions between the Mn4 and radical spins (S1 and S2, respectively) of the form H int = J 12 S 1 · S 2 + S 1 · D 12 · S 12 have been thoroughly investigated to find which Mn4-radical complex can reasonably make both J 12 and D 12 as small as ≈ 100 G in magnitude and can, simultaneously, yield an X-ray absorption Mn K-edge energy 0.7 ± 0.3 eV higher than that in the modified S2 state. As the most probable model, we propose that the radical must form the third bridging ligand between di-μ-oxo or μ2-oxo-(μ3-oxo) bridged Mna(III) and Mnb(IV) ions on the opposite side of mono-μ2-oxo bridged Mnc and Mnd ions.

Flash induced XANES spectroscopy for the Ca-depleted Mn-cluster in the photosynthetic O2-evolving enzyme

Flash‐induced changes of the Mn K‐edge absorption spectra have been studied in the oxygen‐evolving complex depleted of Ca. The Mn K‐edge energy for the Ca‐depleted S1‐state was lower by 1.5 eV than that for the normal S1‐state. The K‐edge energy upshifted by 1 eV after one flash, indicative of an oxidation of Mn. After two flashes, the K‐edge was elevated as well by 0.4 eV, and then reached a steady‐state high level after continuous illumination where the K‐edge energy was higher by 0.9 eV than that after one flash. The results indicate that the Mn‐cluster and/or its direct ligand could be oxidized up to two electrons but further events are blocked.

Mn K-edge XANES Spectroscopy of Photosynthetic Water Oxidation Enzyme in the S0-, S1-, S2- and S3-States Induced by Flash Excitation

Electronic and structural rearrangement of the Mn-cluster during the four step oxidation of water in photosynthetic oxygen evolution was studied by XANES spectroscopy. The Mn K-edge energy of the spectrum was changed with flash number to show a clear quadruple oscillation, indicating a periodic change in oxidation and electronic state of the Mn-cluster depending on Joliot and Koks oxygen clock.

MOLECULAR ORBITAL STUDY ON THE ROLE OF MANGANESE IN PHOTOSYNTHETIC WATER OXIDATION: A MODEL FOR THE MOLECULAR MECHANISM

Publisher Summary This chapter presents a model for the molecular mechanism of the role of manganese in photosynthetic water oxidation. The major information about the mechanism of water oxidation has come mainly from kinetic studies under flashing light conditions with the general acceptance of the S-state model. Upon flash excitation, a number of oscillatory phenomena in photosystem II (PSII) display a basic periodicity of four suggesting that the water-splitting enzyme–substrate complex can exist in a cycle of four stable or long-lived oxidation S-state. This implies that M1 (special Mn clusters) can bind only two water molecules at a time for oxidation. Therefore, the number of catalytic Mn atoms in M1, which binds a water molecule on each Mn atom, must be two. The other Mn atoms would be noncatalytic and are either imbedded within a protein involved in the main path of electron transfer or separate from the electron transport components. In a study described in the chapter, an essential role of the Mn catalyst was investigated by comparing the calculated proton transfer energies for the without-Mn-catalyst, BII, BIII, and BIV systems.

High-Resolution Xanes Spectra from Manganese Ions in Spinach Photosystem II and a Proposal for the Protein Binding Sites

Photosynthetic water oxidation to dioxygen takes place in a Mn-containing protein in PSII via Kok’s intermediates, labeled S0–S4[1]. These S-states probably reflect different redox states of some or all of four Mn ions involved in the enzyme-substrate(H2O) complex. The molecular structure and oxidation states of this Mn-cluster in each S-state has been the subject of considerable recent spectroscopic studies involving EPR[2–5] and X-ray absorption[6,7]. In this paper, we present high-quality Mn K-edge XANES spectra measured for spinach PSII membranes in S1 and S2 states, which exhibit informative pre-edge feature due to 1s to 3d transition as well as four fine structures superimposed on the principal absorption band. These features are semi-empirically analized in comparison with those of authentic Mn complexes. Based on the results obtained, we propose a hypothetical model for the Mn-cluster which is compatible with EPR data[2–5] in S2-state.

Catalytic Cooperativity of Mono-Manganese and Tri-Manganese Clusters for Water-Splitting and Oxygen-Evolving Reaction in Photosystem II: Chemical Mechanistic Insight

Applying the UDFT/B3LYP/(lacvp**, lacv3p**) geometry optimization method together with a Poisson-Boltsman equation solver in the e = 4 dielectric medium to a version-upped “truncated-OEC-cluster” model of MT-type, we found that (1) Upon the Si-state transitions (I = 0 — 4) in a cyclic change of the most-stable tautomer(s), a proton release pattern of 1:0:1:2 has been derived with use of calculated exothermic vs endothermic energies, to yield the oxidation states: S0Mna III; Mnb III, Mnc III, Mnd IV, S1Mna III; Mnb IV, Mnc III, Mnd IV, S2 +Mna IV; Mnb IV, Mnc III, Mnd IV, S3 +Mna IV; Mnb IV, Mnc IV, Mnd IV and S4aMna IV; Mnb IV, Mnc IV, Mnd IV, (2) The redox potential for the last S3/S4a + oxidation step has been evaluated to be ca.1.07 V, a significantly-reduced value due to the H-bonding network between YZ, H190 and Ca 2+-binding site in the Mn4Ca cluster, (3) The S4a-intermediate contains the catalytic Mna IV ion binding two adjoining substrate derivatives, a hydroxyl anion (W1 = HO−) and an oxo radical (W2 = O−·), and (4) The O-O bond formation is thermally induceable by a proton-coupled electron transfer (PCET) via a transition state with an activation energy of ca. 11.2 kcal/mol and a small exothermicity of ca. −4.5 kcal/mol, to yield a side-on superoxo anion radial bound to the Mna III ion in the second intermediate, formulated as S4bMna III:O2−·(W1 = W2); Mnb IV, Mnc III, Mnd IV, where the third Mnc III ion is in a low-spin state of S c=1.