Highly efficient photoactivation of Mn-depleted photosystem II by imidazole-liganded manganese complexes

The oxygen-evolving complex (OEC) of Mn-depleted photosystem II (PSII) can be reconstituted in the presence of exogenous Mn or a Mn complex under weak illumination, a process called photoactivation. Synthetic Mn complexes could provide a powerful system to analyze the assembly of the OEC. In this work, four mononuclear Mn complexes, [(terpy)₂Mn^II(OOCH₃)]·2H₂O (where terpy is 2,2′:6′,2″-terpyridine), Mn^II(bzimpy)₂, Mn^II(bp)₂(CH₃CH₂OH)₂ [where bzimpy is 2,6-bis(2-benzimidazol-2-yl)pyridine] and [Mn^III(HL)(L)(py)(CH₃OH)]CH₃OH (where py is pyridine) were used in photoactivation experiments. Measurements of the photoreduction of 2,6-dichorophenolindophenol and oxygen evolution demonstrate that photoactivation is more efficient when Mn complexes are used instead of MnCl₂ in reconstructed PSII preparations. The most efficient recoveries of oxygen evolution and electron transport activities are obtained from a complex, [Mn^III(HL)(L)(py)(CH₃OH)]CH₃OH, that contains both imidazole and phenol groups. Its recovery of the rate of oxygen evolution is as high as 79% even in the absence of the 33-kDa peptide. The imidazole ligands of the Mn complex probably accelerate P ₆₈₀ ^•+ reduction and consequently facilitate the process of photoactivation. Also, the strong intermolecular hydrogen bond probably facilitates interaction with the Mn-depleted PSII via reorganization of the hydrogen-bonding network, and therefore promotes the recovery of oxygen evolution and electron transport activities.     

Rapid degradation of the tetrameric Mn cluster in illuminated, PsbO-depleted photosystem II preparations

A “decoupling effect” (light-induced electron transport without O₂ evolution) was observed in Ca-depleted photosystem II (PSII(-Ca)) membranes, which lack PsbP and PsbQ (Semin et al. (2008) Photosynth. Res., 98, 235–249). Here PsbO-depleted PSII (PSII(-PsbO)) membranes (which also lack PsbP and PsbQ) were used to examine effects of PsbO on the decoupling. PSII(-PsbO) membranes do not reduce the acceptor 2,6-dichlorophenolindophenol (DCIP), in contrast to PSII(-Ca) membranes. To understand why DCIP reduction is lost, we studied light effects on the Mn content of PSII(-PsbO) samples and found that when they are first illuminated, Mn cations are rapidly released from the Mn cluster. Addition of an electron acceptor to PSII(-PsbO) samples accelerates the process. No effect of light was found on the Mn cluster in PSII(-Ca) membranes. Our results demonstrate that: (a) the oxidant, which directly oxidizes an as yet undefined substrate in PSII(-Ca) membranes, is the Mn cluster (not the Y_Z radical or P680⁺); (b) light causes rapid release of Mn cations from the Mn cluster in PSII(-PsbO) membranes, and the mechanism is discussed; and (c) rapid degradation of the Mn cluster under illumination is significant for understanding the lack of functional activity in some PSII(-PsbO) samples reported by others.     

Catalysis on dinuclear Mn(II) centers: hydrolytic and redox activities of rat liver arginase

 Rat liver arginase contains a dinuclear Mn₂(II,II) center in each subunit having EPR properties similar to those observed in Mn-catalases. The principal physiologic role of arginase is catalyzing the hydrolytic cleavage of l-arginine to produce l-ornithine and urea. Here we demonstrate that arginase catalyzes the disproportionation of hydrogen peroxide by a redox mechanism analogous to Mn-catalases, but at rates that are 10^–5 to 10^–6 of k _cat for the Mn-catalases, and also exhibits peroxidase activity. The dinuclear Mn₂(II,II) center is essential for maximal catalase activity, since both the H101N and H126N mutant arginases containing only one Mn(II)/subunit have catalase activities that are <3% of that for the wild-type enzyme. Like the Mn-catalases, the catalase activity of arginase is not inhibited by millimolar concentrations of CN^–, the most potent inhibitor of heme catalases, or by EDTA, a chelator of free metal ions. The catalase activity of arginase is not significantly inhibited by Cl^– or F^–, in contrast to Mn-catalases, while potent inhibitors of the hydrolytic activity are also effective inhibitors of the catalase activity. These results suggest that lower affinity of hydrogen peroxide to the active site of arginase contributes to the lower catalase activity. EPR spectroscopy reveals that potent inhibitors of the hydrolytic reaction, including N ^ω-hydroxy-l-arginine, l-lysine, and l-valine, decouple the electronic interaction between the Mn²⁺ ions, most probably by removing a μ-bridging ligand or by increasing the intermanganese separation. The capacity for arginase to deliver a hydroxide ion to hydrolyze the l-arginine substrate is suggested to arise from a "dinuclear effect", wherein the two metal ions contribute more or less equivalently in deprotonation of metal-bound water molecule. Structure-reactivity analyses of these reactions will provide insights into the factors that control redox versus hydrolytic function in dimanganese clusters. Received: 18 November 1996 / Accepted: 7 April 1997     

Fluorescence Induction Kinetics in Membrane Preparations of Photosystem II with Heterogeneous Metal Clusters (Mn/Fe) in the Oxygen-Evolving Complex

Transport of electrons in spinach photosystem II (PSII) whose oxygen-evolving complex (OEC) contains heterogeneous metal clusters 2Mn2Fe and 3Mn1Fe was studied by measuring the fluorescence induction kinetics (FIK). PSII(2Mn,2Fe) and PSII(3Mn,1Fe) preparations were produced using Cadepleted PSII membranes (PSII(–Ca)). It was found that FIK in PSII(2Mn,2Fe) membranes is similar in form to FIK in PSII(–Ca) samples, but the fluorescence yield is lower in PSII(2Mn,2Fe). The results demonstrate that, just as in PSII(–Ca) preparations, there is electron transfer from the metal cluster in the OEC to the primary plastoquinone electron acceptor Q_A. They also show that partial substitution of Mn cations with Fe has no effect on the electron transport on the acceptor side of PSII. Thus, these data demonstrate the possibility of water oxidation either by the heterogeneous metal cluster or just by the manganese dimer. We established that FIK in PSII(3Mn,1Fe) preparations are similar in form to FIK in PSII(2Mn,2Fe) membranes but PSII(3Mn,1Fe) is characterized by a slightly higher maximal fluorescence yield, F_max. The electron transfer rate in PSII(3Mn,1Fe) preparations significantly (by a factor of two) increases in the presence of Ca²⁺, whereas Ca²⁺ has hardly any effect on the electron transport in PSII(2Mn,2Fe) membranes. In Mndepleted PSII membranes, FIK reaches its maximum (the so-called peak K), after which the fluorescence yield starts to decrease as the result of two factors: the oxidation of reduced primary plastoquinone Q _A ^? and the absence of electron influx from the donor side of PSII. The replacement of Mn cations by Fe in PSII(?Mn) preparations leads to fluorescence saturation and disappearance of the K peak. This is possibly due to the deceleration of the charge recombination process that takes place between reduced primary electron acceptor Q _A ^? and oxidized tyrosine Y _Z ^+. which is an electron carrier between the OEC and the primary electron donor P680.     

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.

     

Kinetics and DFT studies on the reaction of copper(II) complexes and H2O2

Copper(II) complexes supported by bulky tridentate ligands L1^H (N,N-bis(2-quinolylmethyl)-2-phenylethylamine) and L1^Ph (N,N-bis(2-quinolylmethyl)-2,2-diphenylethylamine) have been prepared and their crystal structures as well as some physicochemical properties have been explored. Each complex exhibits a square pyramidal structure containing a coordinated solvent molecule at an equatorial position and a weakly coordinated counter anion (or water) at an axial position. The copper(II) complexes reacted readily with H₂O₂ at a low temperature to give mononuclear hydroperoxo copper(II) complexes. Kinetics and DFT studies have suggested that, in the initial stage of the reaction, deprotonated hydrogen peroxide attacks the cupric ion, presumably at the axial position, to give a hydroperoxo copper(II) complex retaining the coordinated solvent molecule (H ^R ·S). H ^R ·S then loses the solvent to give a tetragonal copper(II)-hydroperoxo complex (H ^R ), in which the –OOH group may occupy an equatorial position. The copper(II)–hydroperoxo complex H ^R exhibits a relatively high O–O bond stretching vibration at 900 cm⁻¹ compared to other previously reported examples.Electronic Supplementary Material Supplementary material is available for this article at     

Bicarbonate protects the water-oxidizing complex of photosystem II against thermoinactivation in intact <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis> cells

Currently available data about bicarbonate (BC) action on the Mn-containing water-oxidizing complex (WOC) of the photosystem II (PSII) were obtained almost solely in vitro, e.g. on subchloroplast membrane fragments enriched with PSII. To investigate the in vivo BC effect on the PSII donor side, we used the method of dark thermoinactivation of intact Chlamydomonas reinhardtii cells. Photosynthetic activity of PSII was measured as photoinduced changes in the PSII chlorophyll fluorescence yield and as the rate of photosynthetic oxygen evolution. To exclude a “direct” effect of the absence of BC on the PSII activity, before measurements of the photosynthetic activity, the concentration of BC in all samples was equalized by addition of NaHCO₃ to each of them (except for those that contained 5 mM of NaHCO₃ during thermoinactivation) to reach the final concentration of 5 mM. This allowed registering only so-called “irreversible” (i.e., not reversible by subsequent addition of BC) effect of the absence of BC during thermoinactivation. It was shown that, if 5 mM NaHCO₃ was added to the medium before thermoinactivation, the rate of inactivation of the PSII donor side was lower than in BC-depleted medium 1.5-to 2-fold. The obtained results are interpreted as an indication that BC protects the donor side of PSII against thermoinactivation in vivo, in intact C. reinhardtii cells. This proves the correctness of the earlier proposition that BC is an integral constituent of the Mn-containing water-oxidizing complex of PSII. Published in Russian in Fiziologiya Rastenii, 2007, Vol. 54, No. 3, pp. 342–349. The article was translated by the authors.     

Characteristic features of the interaction between Fe(II) cations and the donor side of the manganese-depleted photosystem II

Ferrous iron cations Fe(II) can effectively bind to the donor side of the manganese-depleted photosystem II (PSII(-Mn)) and in this way block electron transfer from diphenylcarbazide (DPC) to the major donor for P680, Y_Z. The present study was focused on the characteristic features of this process. The oxidation and subsequent binding of Fe(II) cations to PSII(-Mn) may proceed in the absence of an artificial electron acceptor, and therefore we investigated the role of O₂ as a putative endogenous acceptor. Oxygen was shown to participate in the blockade of Y_Z by Fe cations, apparently as a structural element of Fe cluster formed at the donor side of PSII(-Mn). The kinetic study of blocking Y_Z by Fe(II) as dependent on light intensity demonstrated that the quantum efficiency of Fe cations binding to the donor side of PSII(-Mn) considerably exceeded that of Mn cations. We also compared the possibilities of extracting the native Mn cluster and reconstructed Fe cations from PSII and an alternative electron transport from DPC to P680⁺ under the conditions of the Y_Z blockade by Fe cations. Neither an alternative donor for P680, Y_D , nor cytochrome b ₅₅₉ participated in the latter process. As a whole, our evidence shows that many features of binding Fe cation to the donor side of PSII(-Mn) are in common with photoassembling the Mn cluster.Translated from Fiziologiya Rastenii, Vol. 52, No. 1, 2005, pp. 12–20.Original Russian Text Copyright © 2005 by Lovyagina, Davletshina, Kultysheva, Timofeev, Ivanov, Semin.     

Synthesis, crystal structure and photoelectric property of Mn(II/IV) coordination complexes

Three novel coordination complexes [Mn(tpha)(phen)]_n (1); [Mn(na)₂(H₂O)₂]_n (2); {[Mn(phen)₂(OH)Cl] · Cl · (OH) · (C₉H₁₁NO₂) · 2H₂O} (3) have been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction (H₂tpha = terephthalic acid, Hna = nicotinic acid, phen = 1,10-phenanthroline). The tpha groups in complex 1 bridge the Mn(II) ions to an infinite 3D framework. Complex 2 exhibits a 2D network structure in which the Mn(II) ions are linked by nicotinic groups. Complex 3 is connected to a 2D coordination supramolecule by hydrogen bonds. The results of surface photovoltage spectra (SPS) of complexes 1-3 indicate that they all exhibit positive surface photovoltage (SPV) responses in the range of 300-800 nm. However, the intensity, position and numbers of SPV responses are obviously different. The distinctions can be mainly attributed to their structures, valences and coordination environments of the manganese ions in the three complexes. Moreover the external field induced surface photovoltage spectra (FISPS) of the three complexes have been measured.     

Effect of monogalactosyldiacylglycerol on the interaction between photosystem II core complex and its antenna complexes in liposomes of thylakoid lipids

The non-bilayer lipid monogalactosyldiacylglycerol (MGDG) is the most abundant type of lipid in the thylakoid membrane and plays an important role in regulating the structure and function of photosynthetic membrane proteins. In this study, we have reconstituted the isolated major light-harvesting complexes of photosystem II (PSII) (LHCIIb) and a preparation consisting of PSII core complexes and minor LHCII of PSII (PSIICC) into liposomes that consisted of digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG), with or without MGDG. Transmission electron microscopy and freeze-fracture studies showed unilamellar proteoliposomes, and demonstrated that most of the MGDG is incorporated into bilayer structures. The impact of MGDG on the functional interaction between LHCIIb and PSIICC was investigated by low temperature (77 K) fluorescence emission spectra and the photochemical activity of PSII. The additional incorporation of LHCIIb into liposomes containing PSIICC markedly increased oxygen evolution of PSIICC. Excitation at 480 nm of chlorophyll (Chl) b in LHCIIb stimulated a characteristic fluorescence emission of the Chl a in PSII (684.2 nm), rather than that of the Chl a in LHCIIb (680 nm) in the LHCIIb–PSIICC proteoliposomes, which indicated that the energy was transferred from LHCIIb to PSIICC in liposome membranes. Increasing the percentage of MGDG in the PSIICC–LHCIIb proteoliposomes enhanced the photochemical activity of PSII, due to a more efficient energy transfer from LHCIIb to PSIICC and, thus, an enlarged antenna cross section of PSII.     

Effect of Oxygen Tension, Mn(II) Concentration, and Temperature on the Microbially Catalyzed Mn(II) Oxidation Rate in a Marine Fjord

We present evidence that the oxidation of Mn(II) in a zone above the O₂/H₂S interface in the water column of Saanich Inlet, British Columbia, Canada, is microbially catalyzed. We measured the uptake of ⁵⁴Mn(II) in water samples under in situ conditions of pH and temperature and in the presence and absence of oxygen. Experiments in the absence of oxygen provided a measure of the exchange of the tracer between the dissolved and solid pools of Mn(II); we interpret the difference between experiments in the presence and absence of oxygen to be a measure of Mn(II) oxidation. Using this method we examined the effect of oxygen tension, Mn(II) concentration, and temperature on the initial in situ Mn(II) oxidation rate (V₀). Mn(II) oxidation was almost twice as fast under conditions of 67% air saturation (V₀=5.5 nM h⁻¹) as with the in situ concentration of 15 μM (5% air saturation; V₀=3.1 nM h⁻¹). Additions of ca. 18 μM Mn(II) completely inhibited all Mn(II) oxidation at three different depths in the oxidizing zone, and there was a temperature optimum for Mn(II) oxidation of around 20°C. These results are consistent with biologically mediated Mn(II) oxidation and indicate that the rate is limited by both oxygen and the concentration of microbial binding sites in this environment.     

Properties of Chlamydomonas Photosystem II Core Complex with a His-Tag at the C-Terminus of the D2 Protein

A His-tagged PSII core complex was purified from recombinantChlamydomonas reinhardtii D2-H thylakoids by single-step Ni²⁺-affinitycolumn chromatography and its properties were partially characterizedin terms of their PSII functions and chemical compositions.The PSII core complex that has a His-tag extension at the C-terminusof the D2 protein evolved oxygen at a high rate of 2,400 µmol(mg Chl)^–1h^–1 at the optimum pH of 6.5 with ferricyanideand 2,6-dichlorobenzoquinone as electron acceptors in the presenceof Ca²⁺ as an essential cofactor, and approximately 90% of theactivity was blocked by 10 µM DCMU. The core complex exhibitedthe thermoluminescence Q-band but not the B-band regardlessof the presence or absence of DCMU, although both bands wereobserved in the His-tagged thylakoids. The core complex wasfree from PSI and contained one Y_D, Tyr 160 of the D2 protein,four Mn atoms, two cytochrome b-559, about 46 Chl a molecules,and probably one Q_A, the primary acceptor quinone of PSII. Itwas inferred from these results that His-tagging at the C-terminusof the D2 protein does not affect the functional and structuralintegrity of the PSII core complex, and that the ‘His-tagstrategy’ is highly useful for biochemical, physicochemical,and structural studies of Chlamydomonas PSII. (Received October 22, 1998; Accepted December 25, 1998)     

Structure and O2 uptake properties of a novel nickel(II) complex of pyridyl-pendant dioxocyclam [1-(2-pyridyl)methyl-5,7-dioxo-1,4,8,11-tetraazacyclotetradecane]

 Novel potentially five-coordinate pyridyl–pendant dioxocyclam [1-(2-pyridyl)methyl-5,7-dioxo-1,4,8,11-tetraazacyclotetradecane (H₂L) and its homologs (6-methyl and 6,6-dimethyl derivatives)] have been synthesized to study nickel(II) complexation. A purple nickel(II) complex with a deprotonated amide (NiHL) was isolated from aqueous equimolar solution of H₂L and Ni(ClO₄)₂. A yellow nickel(II) complex with two deprotonated amides (NiL) was crystallized from an H₂O/CH₃CN solution of H₂L and Ni(OH)₂. The X-ray crystal study of NiL showed a square-planar nickel(II) complex with the pyridyl–pendant remaining uncoordinated. It is concluded from the visible absorption and NMR study of NiL in aqueous solution that the four-coordinate NiL is in equilibrium with a five-coordinate square-pyramidal nickel(II) complex with the apical coordination of the pyridyl–pendant. A voltammetric study disclosed a low nickel(II/III) redox potential of +0.29 V vs SCE for NiL at pH 9.5 and 25  °C with 0.10 M Na₂SO₄. The nickel(II) complex NiL absorbed an equimolar amount of O₂ at pH 9.5 and 25  °C, and the O₂ was activated to cleave plasmid DNA. Received: 5 August 1996 / Accepted: 24 October 1996     

Probing tyrosine Z oxidation in photosystem II core complex isolated from spinach by EPR at liquid helium temperatures

Tyrosine Z (Tyr_Z) oxidation observed at liquid helium temperatures provides new insights into the structure and function of Tyr_Z in active Photosystem II (PSII). However, it has not been reported in PSII core complex from higher plants. Here, we report Tyr_Z oxidation in the S₁ and S₂ states in PSII core complex from spinach for the first time. Moreover, we identified a 500 G-wide symmetric EPR signal (peak position g = 2.18, trough position g = 1.85) together with the g = 2.03 signal induced by visible light at 10 K in the S₁ state in the PSII core complex. These two signals decay with a similar rate in the dark and both disappear in the presence of 6% methanol. We tentatively assign this new feature to the hyperfine structure of the S₁Tyr_Z ^• EPR signal. Furthermore, EPR signals of the S₂ state of the Mn-cluster, the oxidation of the non-heme iron, and the S₁Tyr_Z ^• in PSII core complexes and PSII-enriched membranes from spinach are compared, which clearly indicate that both the donor and acceptor sides of the reaction center are undisturbed after the removal of LHCII. These results suggest that the new spinach PSII core complex is suitable for the electron transfer study of PSII at cryogenic temperatures.     

Photoconsumption of molecular oxygen on both donor and acceptor sides of photosystem II in Mn-depleted subchloroplast membrane fragments

Oxygen consumption in Mn-depleted photosystem II (PSII) preparations under continuous and pulsed illumination is investigated. It is shown that removal of manganese from the water-oxidizing complex (WOC) by high pH treatment leads to a 6-fold increase in the rate of O₂ photoconsumption. The use of exogenous electron acceptors and donors to PSII shows that in Mn-depleted PSII preparations along with the well-known effect of O₂ photoreduction on the acceptor side of PSII, there is light-induced O₂ consumption on the donor side of PSII (nearly 30% and 70%, respectively). It is suggested that the light-induced O₂ uptake on the donor side of PSII is related to interaction of O₂ with radicals produced by photooxidation of organic molecules. The study of flash-induced O₂ uptake finds that removal of Mn from the WOC leads to O₂ photoconsumption with maximum in the first flash, and its yield is comparable with the yield of O₂ evolution on the third flash measured in the PSII samples before Mn removal. The flash-induced O₂ uptake is drastically (by a factor of 1.8) activated by catalytic concentration (5-10 μM, corresponding to 2-4 Mn per RC) of Mn²⁺, while at higher concentrations (> 100 μM) Mn²⁺ inhibits the O₂ photoconsumption (like other electron donors: ferrocyanide and diphenylcarbazide). Inhibitory pre-illumination of the Mn-depleted PSII preparations (resulting in the loss of electron donation from Mn²⁺) leads to both suppression of flash-induced O₂ uptake and disappearance of the Mn-induced activation of the O₂ photoconsumption. We assume that the light-induced O₂ uptake in Mn-depleted PSII preparations may reflect not only the negative processes leading to photoinhibition but also possible participation of O₂ or its reactive forms in the formation of the inorganic core of the WOC.     

Three Mn(II) supramolecular coordination complexes containing carboxylate-tetrazolate ligands

Three novel complexes [Mn(atza)₂(H₂O)₄] (1), [Mn(nptza)₂(CH₃OH)₄] (2), and [Mn(a4-ptz)₂(H₂O)₂]_n · 2nH₂O] (3) [atza = 5-aminotetrazole-1-acetato, nptza = 5-[(4-nitryl)phenyl] tetrazole-1-acetato, a4-ptz = 5-[N-acetato(4-pyridyl)] tetrazole] containing carboxylate-tetrazolate ligands have been synthesized and characterized by element analysis. X-ray crystallography shows that complexes 1 and 2 both contain mononuclear structure. The complex 3 is a 1D polymeric chain structure. Compounds 1-3 are self-assembled to form supramolecular structures through hydrogen bonds interactions.     

Trinuclear-based coordination compounds of Mn(II) and Co(II) with 4-amino-3,5-dimethyl-1,2,4-triazole and azide and thiocyanate anions: Synthesis, structure and magnetic properties

A 2D layer complex 1 and a linear trinuclear complex 2 with mixed ligands have been synthesized and characterized by elemental analyses, IR and single-crystal X-ray diffraction. In 1, the Mn(II) ions are six-coordinated and lie in distorted octahedron coordination environments. Complex 1 is connected into a 2D layer structure based on a linear trinuclear Mn₃(admtrz)₄(N₃)₆ (admtrz = 4-amino-3,5-dimethyl-1,2,4-triazole) building unit with either (6,3) topology when Mn1 cations as three-connected nodes or (4,4) network when the coordination trinuclear units being regarded as four connected nodes. In 2, the Co(II) ions are in slightly distorted octahedron coordination geometries. The magnetic behaviors are investigated in the temperature range 1.8-300 K. The magnetic susceptibility measurements show that the Mn(II) ions of complex 1 are weakly antiferromagnetically coupled with g = 1.98(1), J1 = −6.31(5) cm⁻¹ and J2 = −1.88(1) cm⁻¹. There is dominant zero field splitting (ZFS) effects with g values, g_// = 2.38(2) and g_⊥ = 4.96(4), indicated a significant presence of the spin-orbit coupling and magnetization experiment reveals large, uniaxial zero-field splitting parameters of D = −29.55 cm⁻¹ for 2.     

Encapsulation of [Mn(bpy)3] cations in 2D [Mn(dca)3] sheet: Synthesis, X-ray structure and EPR study

The reaction of Mn(NO₃)₂ · 4H₂O, 2,2′-bipyridine (bpy) and sodium dicyanamide (dca) in aqueous medium yielded the {[Mn(bpy)₃][Mn(dca)₃]₂}_n (1). The single-crystal X-ray analysis of 1 revealed that the anionic part of the complex, [Mn(dca)₃]⁻, features infinite 2D sheets with a honeycomb-like porous structure having a void space of ca. 12 Å in which [Mn(bpy)₃]²⁺ cations are encapsulated to yield a fascinating molecular assembly. Mn^II ions possess an octahedral geometry both in the anionic and cationic components of complex 1. In the anionic component, each Mn^II ion is bridged by three pairs of dicyanamide anions in an end-to-end fashion with two other Mn^II ions from adjacent [Mn(dca)₃]⁻ moieties. This type of linking propagates parallel to the bc crystallographic plane to form 2D sheets. [Mn(bpy)₃]²⁺ is found to have somewhat “squeezed” upon encapsulation. No measurable magnetic interaction was evidenced through variable temperature magnetic susceptibility measurements. However, in addition to the broad g ≈ 2 resonance typical of magnetically diluted [Mn(bpy)₃]²⁺ cations, EPR spectroscopy evidenced exchange narrowing of the [Mn(dca)₃]⁻ resonance at g ≈ 2 thus indicating operation of weak magnetic interactions extended over the whole 2D network through the dca⁻ bridges.     

Synthesis, characterization, X-ray molecular structure and catalase-like activity of a non-heme iron complex: Dichloro[N-propanoate-N,N-bis-(2-pyridylmethyl)amine]iron(III)

In this work we present the synthesis and characterization of the complex dichloro[N-propanoate-N,N-bis-(2-pyridylmethyl)amine]iron(III) [Fe^III(PBMPA)Cl₂]. The ligand LiPBMPA was synthesized through the Michael reaction of BMPA with methylacrylate, followed by alkaline hydrolysis. The complex [Fe^III(PBMPA)Cl₂] has been synthesized by the reaction of the ligand with FeCl₃ · H₂O and was mainly characterized by cyclic voltammetry, conductivimetry, and electronic, infrared and Mössbauer spectroscopies, and by X-ray structural analysis, which showed an iron center coordinated by one carboxylate oxygen in a monodentate way, one tertiary amine, two pyridine groups and two chloride ions. It has been proposed that in water the chloride ligands are shifted by the solvent molecules and the species [Fe^III(PBMPA)(H₂O)₂]Cl₂ is predominant. The catalase-like activity of the complex was tested in water, and it proved to be active in the hydrogen peroxide dismutation. Kinetics studies were conducted following the initial rates method. The reaction is first order in relation to both the complex and the hydrogen peroxide. Based on the presence of a lag phase that depends on the initial complex concentration, we propose that the active species that shows in situ catalase-like activity, is a binuclear complex.