The reduced S<Subscript>−1</Subscript> and S<Subscript>−2</Subscript> oxidation states of the O<Superscript>2</Superscript>-evolving complex of photosystem II: An EPR microwave power saturation study

Progressive microwave power saturation (P_1/2) measurements have been performed on the tyrosine D radical (Y_D ^•) of photosystem II (PSII) in order to examine its relaxation enhancement by the oxygen-evolving complex (OEC) poised to the reduced S₋₁ and S₋₂ oxidation states by NO treatment. Analysis of the power saturation curves showed that the S₋₁ oxidation state of the OEC does not enhance the relaxation of Y_D ^•: it therefore possesses a diamagnetic ground state. In contrast, the Mn(II)-Mn(III) multiline electron paramagnetic resonance (EPR) signal characteristic of the S₋₂ oxidation state of the OEC was shown to provide a relaxation enhancement pathway for Y_D ^•, however less efficient relative to the one provided by the S₂-state multiline EPR signal. We also examined the Y_D ^• relaxation enhancement characteristics of the EPR-silent oxidation state produced after brief (1–5 min) dark incubation at 0°C of a PSII sample poised to the EPRactive S₋₂ state. This EPR-silent oxidation state denoted as “0°C incubation” state was shown to possess remarkably similar P_1/2 values with the EPR-active S₋₂ state in the overall examined temperature range (6–20 K). In addition, these values remained unchanged after successive cycles of the OEC between the EPR-active S₋₂ state and the “0°C incubation” state. The data presented in this work point to the conclusion that the “0°C incubation” state is indeed an S₋₂ oxidation state with half-integer spin.     

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.Abbreviations Chl chlorophyll - D tyrosine 160 on the D2 protein, an accessory electron donor in PS II - D⁺ the oxidized form of D - EDTA ethylene-diaminetetraacetic acid - EPR electron paramagnetic resonance - EXAFS extended X-ray absorption fine structure - HL py-2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol - Mes 2-(N-morpholino)ethanesulfonic acid - N4 py-tris(2-pyridylmethyl)amine - OEC oxygen evolving complex - P680 primary electron donor of PS II - PS II Photosystem II - Q₄₀₀ a high spin Fe³⁺ of the iron-quinone acceptor complex in PS II - SSD solid state detector - XAFS X-ray absorption fine structure - XANES X-ray absorption near edge structure     

Involvement of manganese peroxidase in the transformation of macromolecules from low-rank coal by Phanerochaete chrysosporium

Manganese peroxidase (Mn peroxidase) catalyses the oxidation of Mn(II) to Mn(III), a diffusible non-specific oxidant likely to be involved in the transformation of polyphenolic macromolecules from brown coal by the white-rot fungus Phanerochaete chrysosporium. We report here that solubilised macromolecules from Morwell brown coal were depolymerised by Mn(III) ions when incubated under hyperbaric O₂. However, under N₂ or air they were polymerised, suggesting that net depolymerisation by Mn(III) requires molecular oxygen to inhibit coupling of coal radicals. Coal macromolecules were also polymerised when separated by a semipermeable membrane from a culture of P. chrysosporium or from a solution of Mn peroxidase, Mn(II) and H₂O₂, probably by Mn(III) crossing the membrane. In oxygenated cultures in which Mn peroxidase␣was up-regulated by Mn(II), the extent of depolymerisation correlated with cumulative Mn peroxidase activity suggesting that Mn-peroxidase-generated Mn(III) has a central role in initial depolymerisation of coal molecules in vivo. However, mutant ME446-B17-1, which produces Mn peroxidase but not lignin peroxidase, polymerised coal macromolecules in oxygenated cultures. In sum, it appears Mn peroxidase can both polymerise and depolymerise brown coal macromolecules and that, in vivo, both hyperbaric O₂ and lignin peroxidase are also required to force net depolymerisation to products assimilable by cells. Received: 4 September 1997 / Received revision: 29 January 1998 / Accepted: 30 January 1998     

D1-S169A substitution of photosystem II reveals a novel S2-state structure

In photosystem II (PSII), photosynthetic water oxidation occurs at the O₂-evolving complex (OEC), a tetramanganese-calcium cluster that cycles through light-induced redox intermediates (S₀–S₄) to produce oxygen from two substrate water molecules. The OEC is surrounded by a hydrogen-bonded network of amino-acid residues that plays a crucial role in proton transfer and substrate water delivery. Previously, we found that D1-S169 was crucial for water oxidation and its mutation to alanine perturbed the hydrogen-bonding network. In this study, we demonstrate that the activation energy for the S₂ to S₁ transition of D1-S169A PSII is higher than wild-type PSII with a ~1.7–2.7× slower rate of charge recombination with Q_A⁻ relative to wild-type PSII. Arrhenius analysis of the decay kinetics shows an E_a of 5.87 ± 1.15 kcal mol⁻¹ for decay back to the S₁ state, compared to 0.80 ± 0.13 kcal mol⁻¹ for the wild-type S₂ state. In addition, we find that ammonia does not affect the S₂-state EPR signal, indicating that ammonia does not bind to the Mn cluster in D1-S169A PSII. Finally, a QM/MM analysis indicates that an additional water molecule binds to the Mn4 ion in place of an oxo ligand O5 in the S₂ state of D1-S169A PSII. The altered S₂ state of D1-S169A PSII provides insight into the S₂➔S₃ state transition.     

Ca2+ effects on Fe(II) interactions with Mn-binding sites in Mn-depleted oxygen-evolving complexes of photosystem II and on Fe replacement of Mn in Mn-containing,Ca-depleted complexes

Fe(II) cations bind with high efficiency and specificity at the high-affinity (HA), Mn-binding site (termed the “blocking effect” since Fe blocks further electron donation to the site) of the oxygen-evolving complex (OEC) in Mn-depleted, photosystem II (PSII) membrane fragments (Semin et al. in Biochemistry 41:5854, 2002). Furthermore, Fe(II) cations can substitute for 1 or 2Mn cations (pH dependent) in Ca-depleted PSII membranes (Semin et al. in Journal of Bioenergetics and Biomembranes 48:227, 2016; Semin et al. in Journal of Photochemistry and Photobiology B 178:192, 2018). In the current study, we examined the effect of Ca²⁺ cations on the interaction of Fe(II) ions with Mn-depleted [PSII(-Mn)] and Ca-depleted [PSII(-Ca)] photosystem II membranes. We found that Ca²⁺ cations (about 50 mM) inhibit the light-dependent oxidation of Fe(II) (5 µM) by about 25% in PSII(-Mn) membranes, whereas inhibition of the blocking process is greater at about 40%. Blocking of the HA site by Fe cations also decreases the rate of charge recombination between Q_A^? and Y_Z^?+ from t_1/2?=?30 ms to 46 ms. However, Ca²⁺ does not affect the rate during the blocking process. An Fe(II) cation (20 µM) replaces 1Mn cation in the Mn₄CaO₅ catalytic cluster of PSII(-Ca) membranes at pH 5.7 but 2 Mn cations at pH 6.5. In the presence of Ca²⁺ (10 mM) during the substitution process, Fe(II) is not able to extract Mn at pH 5.7 and extracts only 1Mn at pH 6.5 (instead of two without Ca²⁺). Measurements of fluorescence induction kinetics support these observations. Inhibition of Mn substitution with Fe(II) cations in the OEC only occurs with Ca²⁺ and Sr²⁺ cations, which are also able to restore oxygen evolution in PSII(-Ca) samples. Nonactive cations like La³⁺, Ni²⁺, Cd²⁺, and Mg²⁺ have no influence on the replacement of Mn with Fe. These results show that the location and/or ligand composition of one Mn cation in the Mn₄CaO₅ cluster is strongly affected by calcium depletion or rebinding and that bound calcium affects the redox potential of the extractable Mn4 cation in the OEC, making it resistant to reduction.

     

Influence of Selenium on Hepatic Mitochondrial Antioxidant Capacity in Ducklings Intoxicated with Aflatoxin B<Subscript>1</Subscript>

The aim of the study was to investigate the effect of selenium on hepatic mitochondrial antioxidant capacity in ducklings administrated with aflatoxin B₁ (AFB₁). Ninety 7-day-old ducklings were randomly divided into three groups (groups I–III). Group I was used as a blank control. Group II was administered with AFB₁ (0.1 mg/kg body weight). Group III was administered with AFB₁ (0.1 mg/kg body weight) plus selenium (sodium selenite, 1 mg/kg body weight). All treatments were given once daily for 21 days. The results showed that the activities of mitochondrial superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and glutathione reductase (GR) in group II ducklings significantly decreased when compared with group I (P < 0.01). Furthermore, the content of hepatic mitochondrial malondialdehyde (MDA) significantly increased (P < 0.01). However, the activities of hepatic mitochondrial SOD, CAT, GSH-Px, and GR in group III ducklings significantly increased when compared with group II (P < 0.05). In addition, the content of hepatic mitochondrial MDA significantly decreased (P < 0.01). These results revealed that AFB₁ significantly induced hepatic mitochondrial antioxidant dysfunction. However, sodium selenite could significantly ameliorate the negative effect induced by AFB₁.     

Electron transfer in the water-oxidizing complex of Photosystem II

An overview is presented of secondary electron transfer at the electron donor side of Photosystem II, at which ultimately two water molecules are oxidized to molecular oxygen, and the central role of manganese in catalyzing this process is discussed. A powerful technique for the analysis of manganese redox changes in the water-oxidizing mechanism is the measurement of ultraviolet absorbance changes, induced by single-turnover light flashes on dark-adapted PS II preparations. Various interpretations of these ultraviolet absorbance changes have been proposed. Here it is shown that these changes are due to a single spectral component, which presumably is caused by the oxidation of Mn(III) to Mn(IV), and which oscillates with a sequence +1, +1, +1, –3 during the so-called S₀ S₁ S₂ S₃ S₀ redox transitions of the oxygen-evolving complex. This interpretation seems to be consistent with the results obtained with other techniques, such as those on the multiline EPR signal, the intervalence Mn(III)-Mn(IV) transition in the infrared, and EXAFS studies. The dark distribution of the S states and its modification by high pH and by the addition of low concentrations of certain water analogues are discussed. Finally, the patterns of proton release and of electrochromic absorbance changes, possibly reflecting the change of charge in the oxygen-evolving system, are discussed. It is concluded that nonstoichiometric patterns must be considered, and that the net electrical charge of the system probably is the highest in state S₂ and the lowest in state S₁.     

Electron spin-lattice relaxation studies of different forms of the S2 state multiline EPR signal of the Photosystem II oxygen-evolving complex

The pulsed EPR inversion recovery sequence has been utilized to monitor the temperature dependence of the electron spin-lattice relaxation rate of the Mn cluster of the Photosystem II oxygen evolving complex poised in a variety of S ₂ state forms giving rise to g = 2 multiline EPR signals. A previous study (Lorigan and Britt (1994) Biochemistry 33: 12072–12076) showed that for PS II membranes treated with 5% ethanol, the S ₂ state Mn cluster relaxes via the Orbach spin-lattice relaxation mechanism, where the relaxation is enhanced via phonon scattering off an excited state spin manifold, in this case at an energy of Δ = 36.5 cm⁻¹ above the S = 1/2 ground state giving rise to the multiline EPR signal. Parallel experiments are reported for PS II membranes with 5% methanol, treated with ammonia, and following short and long term dark adaptation. In each case, the temperature dependence of the electron spin-lattice relaxation rate is consistent with Orbach relaxation, and the range of excited state energies is relatively narrow (33.8 cm⁻¹ ≤ Δ ≤ 39.7 cm⁻¹). In addition, short term dark adapted (6 min, ‘active state’) PS II membranes show biphasic recovery traces which indicate that a minority fraction of the oxygen evolving complexes are trapped in a form with greatly slowed spin-lattice relaxation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Low-frequency fourier transform infrared spectroscopy of the oxygen-evolving complex in Photosystem II*

In this communication, we report our progress on the development of low-frequency Fourier transform infrared (FTIR) spectroscopic techniques to study metal-substrate and metal-ligand vibrational modes in the Photosystem II/oxygen-evolving complex (PS II/OEC). This information will provide important structural and mechanistic insight into the OEC. Strong water absorption in the low-frequency region (below 1000 cm⁻¹), a lack of suitable materials, and temperature control problems have limited previous FTIR spectroscopic studies of the OEC to higher frequencies (>1000 cm⁻¹). We have overcome these technical difficulties that have blocked access to the low-frequency region and have developed successive instruments that allow us to move deeper into the low-frequency region (down to 350 cm⁻¹), while increasing both data accumulation efficiency and S/N ratio. We have detected several low-frequency modes in the S₂/S₁spectrum that are specifically associated with these two states. Our results demonstrate the utility of FTIR techniques in accessing low-frequency modes in Photosystem II and in proteins generally. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cardiovascular control via angiotensin II and circulating catecholamines in the spiny dogfish, Squalus acanthias

The contributions of circulating angiotensin II (Ang II) and catecholamines to cardiovascular control in the spiny dogfish were investigated by monitoring the effects of exogenous and endogenous dogfish [Asn¹, Pro³, Ile⁵]-Ang II (dfAng II) on plasma catecholamine levels and blood pressure regulation. Bolus intravenous injections of dfAng II (30–1200 pmol kg⁻¹) elicited dose-dependent increases in plasma adrenaline and noradrenaline concentrations, caudal artery pressure (P _CA), and systemic vascular resistance (R _S), and a decrease in cardiac output (Q). Similar injections of Ang II in dogfish pre-treated with the α-adrenoceptor antagonist yohimbine (4 mg kg⁻¹) also elicited dose-dependent increases in plasma catecholamine levels yet the cardiovascular effects were abolished. Dogfish treated with yohimbine were hypotensive and had elevated levels of plasma Ang II and catecholamines. Intravenous injection of the smooth muscle relaxant papaverine (10 mg kg⁻¹) elicited a transient decrease in P _CA and R _S, and increases in plasma Ang II and catecholamine levels. In dogfish first treated with lisinopril (10⁻⁴ mol kg⁻¹), an angiotensin converting enzyme inhibitor, papaverine treatment caused a more prolonged and greater decrease in P _CA and R _S, an attenuated increase in plasma catecholamines, and no change in plasma Ang II. By itself, lisinopril treatment had little effect on P _CA, and no effect on R _S, plasma Ang II or catecholamines. In yohimbine-treated dogfish, papaverine treatment elicited marked decreases in P _CA, R _S, and Q, and increases in plasma Ang II and catecholamines. Among the three papaverine treatments, there was a positive linear relationship between plasma Ang II and catecholamine concentrations, and the cardiovascular and hormonal changes were most pronounced in the yohimbine + papaverine treatment. Therefore, under resting normotensive conditions, while Ang II does not appear to be involved in cardiovascular control, catecholamines play an important role. However, during a hypotensive stress elicited by vascular smooth muscle relaxation, Ang II indirectly contributes to cardiovascular control by dose-dependently stimulating catecholamine release. Accepted: 24 February 1999     

Bridging-type changes facilitate successive oxidation steps at about 1 V in two binuclear manganese complexes--implications for photosynthetic water-oxidation

The redox behavior of two synthetic manganese complexes illustrates a mechanistic aspect of importance for light-driven water oxidation in Photosystem II (PSII) and design of biomimetic systems (artificial photosynthesis). The coupling between changes in oxidation state and structural changes was investigated for two binuclear manganese complexes (1 and 2), which differ in the set of first sphere ligands to Mn (N(3)O(3) in 1, N(2)O(4) in 2). Both complexes were studied by electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) in three oxidation states which had been previously prepared either electro- or photochemically. The following bridging-type changes are suggested. In 1: Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(II)<-->Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(III)-->Mn(III)-(mu-OR)(mu-OCO)(mu-O)-Mn(III). In 2: Mn(II)-(mu-OR)(mu-OCO)(2)-Mn(III)<-->Mn(III)-(mu-OR)(mu-OCO)(2)-Mn(III)-->Mn(III)-(mu-OR)(mu-OCO)(mu-O)-Mn(IV). In both complexes, the first one-electron oxidation proceeds without bridging-type change, but involves a redox-potential increase by 0.5-1V. The second one-electron oxidation likely is coupled to mu-oxo-bridge (or mu-OH) formation which seems to counteract a further potential increase. In both complexes, mu-O(H) bridge formation is associated with a redox transition proceeding at approximately 1V, but the mu-O(H) bridge is observed at the Mn(2)(III,III) level in 1 and at the Mn(III,IV) level in 2, demonstrating modulation of the redox behavior by the terminal ligands. It is proposed that also in PSII bridging-type changes facilitate successive oxidation steps at approximately the same potential.     

Protection of Selenium on Hepatic Mitochondrial Respiratory Control Ratio and Respiratory Chain Complex Activities in Ducklings Intoxicated with Aflatoxin B<Subscript>1</Subscript>

To investigate the protection of selenium on hepatic mitochondrial functions, 90 7-day-old ducklings were randomly divided into three groups (groups I–III). Group I was used as a blank control. Group II was administered with aflatoxin B₁ (0.1 mg/kg body weight). Group III was administered with aflatoxin B₁ (0.1 mg/kg body weight) plus selenium (sodium selenite, 1 mg/kg body weight). All treatments were given once daily for 21 days. The results showed that the activities of hepatic mitochondrial complexes I–IV in group II ducklings significantly decreased when compared with group I (P < 0.01). Furthermore, the activities of hepatic mitochondrial complexes I–IV in group III significantly increased when compared with group II (P < 0.05). The hepatic mitochondrial respiratory control ratio (RCR) in group II ducklings significantly decreased when compared with group I (P < 0.01). In addition, the hepatic mitochondrial RCR in group III significantly increased when compared with group II (P < 0.05). These results revealed that the aflatoxin B₁ significantly induced hepatic mitochondrial dysfunction in the activities of hepatic mitochondrial respiratory chain complexes I–IV and the RCR in ducklings. However, sodium selenite could significantly ameliorate the negative effect induced by aflatoxin B₁.     

EPR properties of mixed-valent μ-oxo and μ-hydroxo dinuclear iron complexes produced by radiolytic reduction at 77 K

 Radiolytic reduction at 77 K of oxo-/hydroxo-bridged dinuclear iron(III) complexes in frozen solutions forms kinetically stabilized, mixed-valent species in high yields that model the mixed-valent sites of non-heme, diiron proteins. The mixed-valent species trapped at 77 K retain ligation geometry similar to the initial diferric clusters. The shapes of the mixed-valent EPR signals depend strongly on the bridging ligands. Spectra of the Fe(II)OFe(III) species reveal an S=1/2 ground state with small g-anisotropy as characterized by the uniaxial component (g _z –g _av /2<0.03) observable at temperatures as high as ∼100 K. In contrast, hydroxo-bridged mixed-valent species are characterized by large g-anisotropy (g _z –g _av /2>0.03) and are observable only below 30 K. Annealing at higher temperatures causes structural relaxation and changes in the EPR characteristics. EPR spectral properties allow the oxo- and hydroxo-bridged, mixed-valent diiron centers to be distinguished from each other and can help characterize the structure of mixed-valent centers in proteins. Received: 27 June 1998 / Accepted: 25 February 1999     

Influence of Selenium on Body Weights and Immune Organ Indexes in Ducklings Intoxicated with Aflatoxin B<Subscript>1</Subscript>

To investigate the influence of selenium on body weights and the immune organ indexes in ducklings administrated with aflatoxin B₁ (AFB₁), 90 7-day-old ducklings were randomly divided into three groups (groups I–III). Group I was used as a blank control. Group II was administered with AFB₁ (0.1 mg/kg body weight). Group III was administered with AFB₁ (0.1 mg/kg body weight) plus sodium selenite (1 mg/kg body weight). All treatments were given once daily for 21 days. It showed that the ducklings’ bursa of fabricius, thymus indexes, and body weights in group II significantly decreased when compared with group I (P < 0.01). Furthermore, the spleen indexes significantly decreased (P < 0.01). However, the ducklings’ bursa of fabricius and thymus indexes, body weights in group III ducklings significantly increased when compared with group II (P < 0.01). In addition, the spleen indexes significantly decreased (P < 0.01). These results revealed that AFB₁ significantly affect ducklings’ growth and immune organs development. However, selenium significantly ameliorated the negative effects induced by AFB₁.     

Oxygen-Evolving System of Inactive Photosystem II Complexes in Chlorella

Incubation of green alga Chlorella pyrenoidosa Chick in darkness at 37–38°C for 10–30 h resulted in inactivation of the oxygen-evolving complex (OEC): the maximum yield of oxygen evolution during a series of short light flashes shifted from the third to the fifth flash; the transition of S₂- and S₃-states of OEC to a stable S₁-state was markedly accelerated. This inactivation of OEC was accompanied by the accumulation of inactive complexes of photosystem II (PSII), in which the reduction of primary quinone acceptor and the conversion into the closed state occurred with a low efficiency, even in the presence of 5 M DCMU. The treatment of light-grown algal cells with hydroxylamine impaired OEC functioning, in similarity to the effect of dark incubation, but caused no accumulation of inactive PSII complexes. We conclude that the inactivation of OEC is not the cause of the inactivation of PSII complex. The decline in the efficiency of electron-transport reactions, both on the donor and acceptor sides of PSII may be related to modification of major proteins in the PS II reaction center.     

Acetyl-<Emphasis Type="SmallCaps">l</Emphasis>-carnitine prevents carbon tetrachloride-induced oxidative stress in various tissues of Wistar rats

Acetyl-l-carnitine (ALCAR) has been shown to prevent experimental selenite cataractogenesis, a manifestation of oxidative stress, but little is known about its potential in other settings of oxidative stress. The present study was based on the hypothesis that ALCAR prevents carbon tetrachloride (CCl₄)-induced oxidative stress in vital tissues. Male albino Wistar rats were divided into three groups, each of six rats. Group I (control) rats received only vehicle (1 ml/kg b.w.) for 4 days; Group II (CCl₄-exposed, untreated) rats received CCl₄ (2 ml/kg b.w.) on the second and third days and vehicle on the first and fourth days; Group III (CCl₄-exposed, ALCAR-treated) rats received ALCAR (200 mg/kg b.w.) for 4 days and CCl₄ on the second and third days. All administrations were made intraperitoneally. After the experimental period, significantly (P < 0.05) elevated mean serum levels of aspartate transaminase, alanine transaminase, alkaline phosphatase, and lactate dehydrogenase were observed in Group II rats when compared to Group I and Group III rats. The mean levels of vitamin C, vitamin E, and reduced glutathione and the mean activities of superoxide dismutase, catalase, and glutathione peroxidase were significantly (P < 0.05) lower in samples of hemolysate and of liver, kidney, and brain tissues of Group II rats than those in Group I and Group III rats. The mean level of lipid peroxidation was significantly (P < 0.05) higher in Group II rats than that in Group I and Group III rats. Moreover, the CCl₄-induced upregulation of inducible nitric oxide synthase expression was prevented by ALCAR in the liver and brain tissues. These results suggest that ALCAR is able to prevent the CCl₄-induced oxidative stress.     

Orientation of the Mn(II)-Mn(III) dimer which results from the reduction of the oxygen-evolving complex of photosystem II by NO: an electron paramagnetic resonance study

The central part of the oxygen-evolving complex of photosystem II is a cluster of four manganese atoms. The known EPR spectra in the various oxidation states of the cluster are complicated by the magnetic interactions of the four Mn ions and accordingly are difficult to analyze. It has been shown recently that NO at -30 degrees C slowly reduces the cluster to a Mn(II)-Mn(III) state [Sarrou, J., Ioannidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3581-3587). We study herein the orientation dependence of the Mn(II)-Mn(III) EPR spectrum with respect to the thylakoid membrane plane. Both the powder and the oriented spectra are satisfactorily simulated with the same set of fine and hyperfine parameters assuming axial symmetry and collinear g and A tensors. The axial component of the tensors is found to be oriented at an angle of 20 degrees +/- 10 degrees to the membrane plane normal (mosaic spread Omega = 40 degrees ). We make the reasonable assumption that the Mn(II)-Mn(III) dimer is one of the di-mu-oxo units that has been suggested to comprise the Mn tetramer. On the basis of the sign of the hyperfine tensor anisotropy, the axial direction is assigned to the d(z(2)) orbital of Mn(III), which by comparison with synthetic model complexes is assumed to be oriented perpendicular to the Mn-(mu-oxo)-Mn plane. The present results complement earlier orientation studies by EXAFS and suggest that the Mn-(mu-oxo)-Mn plane makes a small angle (approximately 20 degrees) with the membrane plane and the axis connecting the bridging oxygens is approximately parallel to the plane.     

Hemoprotein models based on a covalent helix-heme-helix sandwich. 3. Coordination properties, reactivity and catalytic application of Fe(III)- and Fe(II)-mimochrome I

 The coordination state of Fe(III)- and Fe(II)-mimochrome I, a covalent peptide-deuteroheme sandwich involving two nonapeptides bearing a histidine residue in a central position, was studied by UV-visible, EPR, and resonance Raman spectroscopy. The ferric and ferrous states of this new iron species mainly exist, at pH 7, in a low-spin hexacoordinate form with two axial histidine ligands coming from the peptide chains. A minor amount of high-spin form for the ferric state is also present at pH 7. However, it is mainly high-spin at pH 2 or in DMSO. Fe(II)-mimochrome I binds CO with an affinity comparable to that of myoglobin and hemoglobin. Fe(III)-mimochrome I reacts with alkylhydroxylamine and arylhydrazines, leading to the corresponding Fe(II)-nitrosoalkyl and Fe(III)-σ-aryl complexes, respectively. These reactions were greatly dependent on the solvent used and on the pH, and were much slower than the corresponding reactions performed by deuterohemin in the presence of excess imidazole. All these results indicate that the reactivity of iron-mimochrome I is controlled by the binding of the peptide chains to the iron. The reactivity shown by this complex at neutral pH is intermediate between that observed for iron porphyrins in the presence of excess imidazole and that of hemoproteins characterized by a strong bis-histidine axial coordination, such as cytochrome b ₅. Fe(III)-mimochrome I is able to catalyze styrene epoxidation by using a [Fe(III)-mimochrome I]/[H₂O₂]/[stryrene] ratio of 1 : 10 : 2000 in phosphate buffer solution (pH 7.2) containing 2% CTAB both under strictly anaerobic conditions and in the presence of oxygen, at 0  °C. Received: 26 May 1998 / Accepted: 20 August 1998     

Structure and characterization of a μ-phenoxo-bis μ-acetato dinuclear Mn(II,II) complex [Mn2(LO)(μ-OAc)2](ClO4) (LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl}-4-methylphenol): Substituent effects

The synthesis, structure and characterization of the dinuclear Mn(II) complex [Mn₂(LO)(μ-OAc)₂](ClO₄) (1) where LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl)}-4-methylphenol are reported. The reaction of Mn(ClO₄)₂ · 6H₂O with the dinucleating ligand LOH and H₃CCOONa in the presence of NEt₃ in dry, degassed methanol and under an argon atmosphere, yields 1 as a colorless powder. The crystal structure of 1, determined by X-ray diffraction methods, shows a dinuclear Mn(II) complex in which two Mn(II) ions, each in six-coordinate approximate octahedral coordination, are bridged by the phenolate oxygen of LO⁻ and by two acetate ions in a syn,syn-1,3-bridging mode. The Mn-Mn distance is 3.557(1) Å and Mn-O_phenolate-Mn angle is 112.50(9)°. Cyclic voltammetry of 1 in acetonitrile solution shows a quasi-reversible wave at E_1/2 = 0.65 V, for the Mn₂(II,II)/Mn₂(II,III) redox process, and an irreversible oxidation peak at E_p,c = 1.22 V versus Ag/AgCl for the Mn₂(II,III) to Mn₂(III,III) oxidation process. Controlled potential electrolysis of 1 in acetonitrile solution at 0.85 V (versus Ag/AgCl) takes up 1 F of charge per mole of 1 to yield a brown solution of the Mn₂(II,III) state of the complex, which, however, is unstable and reverts back to the Mn₂(II,II) state in solution at room temperature. Least square fitting of the variable temperature magnetic susceptibility measurements on powdered sample of 1 is obtained with g = 1.888, J = −2.75 cm⁻¹, Par = 0.008, TIP = 0. The low −J value and the room temperature calculated magnetic moment of the complex (5.30 BM per Mn(II)), which is less than the spin-only moment of Mn(II), show that the two Mn(II) ions are weakly antiferromagnetically coupled.     

Electrochemistry of the CuA domain of Thermus thermophilus cytochrome ba 3

 The electrochemistry of a water-soluble fragment from the Cu_A domain of Thermus thermophilus cytochrome ba ₃ has been investigated. At 25  °C, Cu_A exhibits a reversible reduction at a pyridine-4-aldehydesemicarbazone-modified gold electrode (0.1 M Tris, pH 8) with E° = 0.24 V vs NHE. Thermodynamic parameters for the [Cu(Cys)₂Cu]^+/0 electrode reaction were determined by variable-temperature electrochemistry (ΔS°_rc = –5.4(12) eu, ΔS° = –21.0(12) eu, ΔH° = –11.9(4) kcal/mol;ΔG° = –5.6 (11) kcal/mol). The relatively small reaction entropy is consistent with a low reorganization energy for [Cu(Cys)₂Cu]^+/0 electron transfer. An irreversible oxidation of [Cu(Cys)₂Cu]⁺ at 1 V vs NHE confirms that the Cu^II:Cu^II state of Cu_A is significantly destabilized relative to the Cu^II state of analogous blue-copper proteins. Received: 3 June 1996 / Accepted: 26 August 1996