Effect of Ca2+/Sr2+ substitution on the electronic structure of the oxygen-evolving complex of photosystem II: a combined multifrequency EPR, 55Mn-ENDOR, and DFT study of the S2 state

The electronic structures of the native Mn(4)O(x)Ca cluster and the biosynthetically substituted Mn(4)O(x)Sr cluster of the oxygen evolving complex (OEC) of photosystem II (PSII) core complexes isolated from Thermosynechococcus elongatus, poised in the S(2) state, were studied by X- and Q-band CW-EPR and by pulsed Q-band (55)Mn-ENDOR spectroscopy. Both wild type and tyrosine D less mutants grown photoautotrophically in either CaCl(2) or SrCl(2) containing media were measured. The obtained CW-EPR spectra of the S(2) state displayed the characteristic, clearly noticeable differences in the hyperfine pattern of the multiline EPR signal [Boussac et al. J. Biol. Chem.2004, 279, 22809-22819]. In sharp contrast, the manganese ((55)Mn) ENDOR spectra of the Ca and Sr forms of the OEC were remarkably similar. Multifrequency simulations of the X- and Q-band CW-EPR and (55)Mn-pulsed ENDOR spectra using the Spin Hamiltonian formalism were performed to investigate this surprising result. It is shown that (i) all four manganese ions contribute to the (55)Mn-ENDOR spectra; (ii) only small changes are seen in the fitted isotropic hyperfine values for the Ca(2+) and Sr(2+) containing OEC, suggesting that there is no change in the overall spin distribution (electronic coupling scheme) upon Ca(2+)/Sr(2+) substitution; (iii) the changes in the CW-EPR hyperfine pattern can be explained by a small decrease in the anisotropy of at least two hyperfine tensors. It is proposed that modifications at the Ca(2+) site may modulate the fine structure tensor of the Mn(III) ion. DFT calculations support the above conclusions. Our data analysis also provides strong support for the notion that in the S(2) state the coordination of the Mn(III) ion is square-pyramidal (5-coordinate) or octahedral (6-coordinate) with tetragonal elongation. In addition, it is shown that only one of the currently published OEC models, the Siegbahn structure [Siegbahn, P. E. M. Acc. Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al. Phys. Chem. Chem. Phys.2009, 11, 6788-6798], is consistent with all data presented here. These results provide important information for the structure of the OEC and the water-splitting mechanism. In particular, the 5-coordinate Mn(III) is a potential site for substrate 'water' (H(2)O, OH(-)) binding. Its location within the cuboidal structural unit, as opposed to the external 'dangler' position, may have important consequences for the mechanism of O-O bond formation.     

Investigation of the calcium-binding site of the oxygen evolving complex of photosystem II using 87Sr ESEEM spectroscopy

The proximity of the calcium/strontium binding site of the oxygen evolving complex (OEC) of photosystem II (PSII) to the paramagnetic Mn cluster is explored with (87)Sr three-pulse electron spin-echo envelope modulation (ESEEM) spectroscopy. CW-EPR spectra of Sr(2+)-substituted Ca(2+)-depleted PSII membranes show the modified g = 2 multiline EPR signal as previously reported. We performed three-pulse ESEEM on this modified multiline signal of the Mn cluster using natural abundance Sr and (87)Sr, respectively. Three-pulse ESEEM of the natural abundance Sr sample exhibits no detectable modulation by the 7% abundance (87)Sr. On the other hand, that of the (87)Sr enriched (93%) sample clearly reveals modulation arising from the I = (9)/(2) (87)Sr nucleus weakly magnetically coupled to the Mn cluster. Using a simple point dipole approximation for the electron spin, analysis of the (87)Sr ESEEM modulation depth via an analytic expression suggests a Mn-Ca (Sr) distance of 4.5 A. Simulation of three-pulse ESEEM with a numerical matrix diagonalization procedure gave good agreement with this analytical result. A more appropriate tetranuclear magnetic/structural model for the Mn cluster converts the 4.5 A point dipole distance to a 3.8-5.0 A range of distances. DFT calculations of (43)Ca and (87)Sr quadrupolar interactions on Ca (and Sr substituted) binding sites in various proteins suggest that the lack of the nuclear quadrupole induced splitting in the ESEEM spectrum of (87)Sr enriched PSII samples is related to a very high degree of symmetry of the ligands surrounding the Sr(2+) ion in the substituted Ca site. Numerical simulations show that moderate (87)Sr quadrupolar couplings decrease the envelope modulation relative to the zero quadrupole case, and therefore we consider that the 3.8-5.0 A range obtained without quadrupolar coupling included in the simulation represents an upper limit to the actual manganese-calcium distance. This (87)Sr pulsed EPR spectroscopy provides independent direct evidence that the calcium/strontium binding site is close to the Mn cluster in the OEC of PSII.     

Macrocyclic receptor showing extremely high Sr(II)/Ca(II) and Pb(II)/Ca(II) selectivities with potential application in chelation treatment of metal intoxication

Herein we report a detailed investigation of the complexation properties of the macrocyclic decadentate receptor N,N'-Bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6 (H(2)bp18c6) toward different divalent metal ions [Zn(II), Cd(II), Pb(II), Sr(II), and Ca(II)] in aqueous solution. We have found that this ligand is especially suited for the complexation of large metal ions such as Sr(II) and Pb(II), which results in very high Pb(II)/Ca(II) and Pb(II)/Zn(II) selectivities (in fact, higher than those found for ligands widely used for the treatment of lead poisoning such as ethylenediaminetetraacetic acid (edta)), as well as in the highest Sr(II)/Ca(II) selectivity reported so far. These results have been rationalized on the basis of the structure of the complexes. X-ray crystal diffraction, (1)H and (13)C NMR spectroscopy, as well as theoretical calculations at the density functional theory (B3LYP) level have been performed. Our results indicate that for large metal ions such as Pb(II) and Sr(II) the most stable conformation is Δ(δλδ)(δλδ), while for Ca(II) our calculations predict the Δ(λδλ)(λδλ) form being the most stable one. The selectivity that bp18c6(2-) shows for Sr(II) over Ca(II) can be attributed to a better fit between the large Sr(II) ions and the relatively large crown fragment of the ligand. The X-ray crystal structure of the Pb(II) complex shows that the Δ(δλδ)(δλδ) conformation observed in solution is also maintained in the solid state. The Pb(II) ion is endocyclically coordinated, being directly bound to the 10 donor atoms of the ligand. The bond distances to the donor atoms of the pendant arms (2.55-2.60 ?) are substantially shorter than those between the metal ion and the donor atoms of the crown moiety (2.92-3.04 ?). This is a typical situation observed for the so-called hemidirected compounds, in which the Pb(II) lone pair is stereochemically active. The X-ray structures of the Zn(II) and Cd(II) complexes show that these metal ions are exocyclically coordinated by the ligand, which explains the high Pb(II)/Cd(II) and Pb(II)/Zn(II) selectivities. Our receptor bp18c6(2-) shows promise for application in chelation treatment of metal intoxication by Pb(II) and (90)Sr(II).     

Ca(II), Sr(II) and Ba(II) ion interaction with the rheumatoid arthritis drug tenoxicam: Structural,thermal, and biological characterization

Recently, one of the most common conditions that manifests as joint and muscle inflammation is rheumatoid arthritis. One of the treatments for this arthritis includes non‐steroidal anti‐inflammatory drugs (NSAIDs) of the oxicam family, and the widest used drug in this family is tenoxicam (Tenox). In this study, the complexation properties of the drug Tenox with Ca(II), Sr(II) and Ba(II) ions in a (dichloromethane + water) binary solvent system are reported. The formed metal complexes were characterized structurally, thermally, and biologically. Tenox was found to act as a chelate monoanionic ligand towards all metal ions with complexation stoichiometry of 1:2 (Metal: Tenox) for Ca(II) and Sr(II) ions, and 1:1 for Ba(II) ions. The Tenox ligand behaves as a bidentate ligand when coordinated with Sr(II) or Ba(II) ions and as a tridentate ligand when coordinated with Ca(II) ions. The Sr(II) and Ba(II) complex of the Tenox ligand exhibited marked inhibitory effect on the cell growth of the C. albicans species.     

Novel structurally characterized hetero-bimetallic [Zn(II)2M(II)] (M = Ca and Sr) bis(salamo)-type complexes: DFT calculation,Hirshfeld analyses,antimicrobial and fluorescent properties

Two hetero-bimetallic [Zn(II)₂M(II)] (M = Ca and Sr) complexes [Zn₂Ca(L)(OAc)₂] ( 1 ) and [Zn₂Sr(L)(OAc)₂] ( 2 ) with a novel asymmetrical bis(salamo)-type tetraoxime ligand (H₅L) were designed and synthesized, and characterized by elemental analyses, FT-IR spectra, UV–vis absorption spectra, Density Functional Theory (DFT) calculation, Hirshfeld surface analyses and X-ray single crystal diffractions. Compared with the symmetric bis(salamo)-type ligands, the ligand H₅L and its [Zn(II)₂M(II)] (M = Ca and Sr) asymmetrical bis(salamo)-type complexes synthesized for the first time in this paper have more novel structures and better properties. The results of ultraviolet titration show that the coordination ratio of ligand, alkaline earth metal and transition metal is 1:1:2. Complexes 1 and 2 have good luminescent properties and obvious antimicrobial activities.     

Solid state and solution structures of alkaline-earth complexes with lariat ethers containing aniline and benzimidazole pendants

Herein we report the synthesis and structural characterization of Mg(II), Ca(II), Sr(II) and Ba(II) complexes with bibracchial lariat ethers derived from 1,7-diaza-15-crown-5 and 1,7-diaza-12-crown-4 containing aniline or benzimidazole pendant arms. The solid state structures of most of them have been determined by using single crystal X-ray crystallography. A coordination number of seven was observed for the Mg(II) complexes in the solid state, while the Ca(II), Sr(II) and Ba(II) complexes are 8-, 9- and 11-coordinate, respectively. The Ca(II), Sr(II) and Ba(II) complexes show a syn conformation, with the two pendant arms of the ligand disposed on the same side of the macrocyclic mean plane. However, the Mg(II) complex with the largest ligand derived from 1,7-diaza-15-crown-5 containing benzimidazole pendants presents an anti conformation in the solid state. ¹H and ¹³C NMR spectroscopy reveal that this conformation is maintained in acetonitrile solution.     

Chemischer Transport ternärer Oxide in den Systemen Ca/Mo/O und Sr/Mo/O

Chemical Vapour Transport of Ternary Oxides in the Systems Ca/Mo/O and Sr/Mo/O The chemical vapour transport behaviour of ternary phases in the Ca/Mo/O and Sr/Mo/O systems has been investigated using Cl₂ as transport agent in a temperature gradient 1423 to 1323 K. MMoO₄ (M= Ca, Sr) migrate in the above‐mentioned temperature gradient with rates of 0.1 to 0.2 mg/h. Starting from three phase mixtures crystals of the compounds MMo₅O₈ have been grown (migration rates: M = Ca 0.1 mg/h, M = Sr 0.01 mg/h). The observed transport behaviour is compared with predictions given by thermo dynamical model calculations and the influences of source composition and the moisture contents are described in detail.     

Spectroscopic, thermal and antitumor investigations of sulfasalazine drug in situ complexation with alkaline earth metal ions

The complexes of sulfasalazine (H₃Suz) with some of alkaline metals Mg(II), Ca(II), Sr(II) and Ba(II) have been investigated. Sulfasalazine complexes were synthesized and characterized by spectroscopic tools; infrared spectra, electronic and mass spectra. The IR spectra of the prepared complexes were suggested that the H₃Suz behaves as a bi-dentate ligand through the carboxylic and phenolic groups. The molar conductance measurements gave an idea about the non-electrolytic behavior of the H₃Suz complexes. The thermal decomposition processes for metal(II) complexes of H₃Suz viz: [M(HSuz)(H₂O)₄] (where M = Mg(II), Ca(II), Sr(II) or Ba(II)) have been accomplished on the basis of TG/DTG and DTA studies, and the formula conforms to the stoichiometry of the complexes based on elemental analysis. The kinetic analyses of the thermal decomposition were studied using the Coats–Redfern and Horowitz–Metzger equations. The antitumor and antimicrobial activities of the H₃Suz and their alkaline metal(II) complexes were evaluated.     

Spectrophotometric and potentiometric study of the complexes originated by the trisodium salt of 2-(p-sulfophenylazo)-1,8-dihydroxynaphthalene-3,6-disulfonic acid (SPADNS) and several metal ions

A study of the complexes originated by U(VI), Cu(II), Pb(II), Ni(II), Co(II), Zn(II), Cd(II), Mn(II), Ca(II), Mg(II), Ba(II), and Sr(II), and SPADNS (trisodium salt of 2-(p-sulfophenylazo)-1, 8-dihydroxynaphthalene-3,6-disulfonic acid) has been made by means of spectrophotometric and potentiometric methods. The dissociation constants of the ligand and the formation constants of the metal ion-SPADNS complexes have been determined at 25 ± 0.1 °C and ionic strength 0.1 (NaClO₄).     

Mixed Ca/Sr salt forms of salicylic acid: tuning structure and aqueous solubility

Ten isostructural single‐crystal diffraction studies of mixed cation Ca/Sr salt forms of the salicylate anion are presented, namely catena‐poly[[diaquacalcium(II)/strontium(II)]‐bis(μ₂‐2‐hydroxybenzoato)], [Ca_1–xSr_x(C₇H₅O₃)₂(H₂O)₂]_n, where x = 0, 0.041, 0.083, 0.165, 0.306, 0.529, 0.632, 0.789, 0.835 and 1. The structure of an isostructural Sr/Ba species, namely catena‐poly[[diaquastrontium(II)/barium(II)]‐bis(μ₂‐2‐hydroxybenzoato)], [Sr_0.729Ba_0.271(C₇H₅O₃)₂(H₂O)₂], is also described. The Ca/Sr structures form a series where, with increasing Sr content, the unit cell expands in both the crystallographic a and c directions (by 1.80 and 3.18%, respectively), but contracts slightly in the b direction (−0.31%). The largest percentage structural expansion lies parallel to the direction of propagation of the one‐dimensional coordination polymer that is the primary structural feature. This structural expansion is thus associated with increased M—O distances. Aqueous solubility measurements show that solubility generally increases with increasing Sr content. Thus, tuning the composition of these mixed counter‐ion salt forms leads to systematic structural changes and allows solubility to be tuned to values between those for the pure Ca and Sr species.     

Determination of the Sr/Ca ratio in bones by XRFA

Radionuclide X-ray fluorescence analysis was used for the determination of Sr/Ca ratio in bones to test the influence of a diet on this ratio. The significant differences were observed for Sr/Ca ratio in bones of various animals. Only small differences in the Ca/Sr ratio were observed for the samples of variaous prehistoric human bones.     

Developments in heterobimetallic s-block systems: synthesis and structural survey of molecular M/Ae (M=Li, Na, K, Cs; Ae=Ca, Sr) aryloxo complexes

A series of novel heterobimetallic group 1/strontium and group 1/calcium aryloxo complexes having the composition [MAe(Odpp)3] [Ae=Sr and M=Na (1), K (2, 3), Cs (4); Ae=Ca and M=Na (5), K (6), Cs (7)] or [M2Ae(Odpp)4] [M=Li and Ae=Sr (9), Ca (10)] have been prepared using 2,6-diphenylphenol (HOdpp) as the ligand. Through the use of solid-state direct metalation, these compounds were obtained either directly from the reaction vessel or after workup in toluene. The Lewis base adduct [KCa(Odpp)3(thf)] (8) was obtained by treatment of [KCa(Odpp)3] (6) with tetrahydrofuran (thf). All of the compounds displayed extensive metal-pi-arene interactions, which provide significant stabilization in these reactive species. The thermal stabilities and volatilities of representative heterobimetallic strontium and calcium complexes were investigated using thermogravimetric analysis.     

Energy transfer from Ca(3P) to Sr and quenching of Sr(3P) by the inert gases

A laser-induced fluorescence technique has been used to measure the rate of the energy-transfer reaction Ca(³P) + Sr(¹S) α Ca(¹S) + Sr(³P). Rates for quenching Sr(³P) by inert gases were also measured and correlated with an orbiting model.     

Recovery and purification of Sr(II) in perchloric acid medium

Perchloric acid was found to be a suitable medium for the quantitative leaching of Sr(II) from homogeneous and calcined (Th,Sr)O₂ particularly with respect to the contamination from Th(IV). ⁹⁰Sr is a cause of major concern to the environment due to its long half life (28.6 years), significant abundance in large inventory of spent nuclear fuels (~350 thousand tons) awaiting geological disposal and its chemical similarity to Ca(II), an essential element for the living beings. Application of ⁹⁰Sr as a parent radionuclide for ⁹⁰Y (used in therapy radiopharmaceuticals) is possible provided it can be made available at desired high purity. In this context, the distribution coefficients of Sr(II), Th(IV), Zr(IV), Y(III), ¹⁵²Eu(III) and ¹³⁷Cs(I) were determined using Sr selective crown ether 4,4′(5′)-di-tert-butyl-dicyclohexano-18-crown-6 by solid–liquid extraction in perchloric acid medium. Feasibility of employing extraction chromatography using Sr selective resin for the recovery and purification of Sr(II) from leached perchloric acid medium was explored. Perchloric acid medium is better than nitric acid medium for the uptake of Sr by Sr selective chromatographic resin under varying loading conditions of Sr(II). Similarly pH 2 solution appears better eluent of Sr(II) than distilled water. Present work offers a novel approach for setting up a ⁹⁰Sr–⁹⁰Y generator.     

Characterization of metal binding properties of rhamnogalacturonan II from plant cell walls by size-exclusion HPLC/ICP-MS.

The binding properties of metal ions to a pectic polysaccharide, rhamnogalacturonan II (RG-II), from plant cell walls were analyzed by size-exclusion HPLC/ICP-MS. The dimeric RG-II borate complex (dRG-II-B) from sugar beet and red wine contained 0.8 - 1.1 mol/mol of B, 0.8 - 1.1 mol/mol of Ca, 0.1 - 0.3 mol/mol of Sr, and 0.03 - 0.07 mol/mol of Ba. The treatment of dRG-II-B with Sr2+, Ba2+, Pb2+ or La3+ exchanged the originally present Ca, Sr and Ba. In contrast, monomeric RG-II (mRG-II), which contained approximately 0.1 mol/mol of Ca, formed complexes with La3+, Eu3+, and Lu3+ added to the solution, but did not do so with Ca2+, Sr2+, Ba2+ and Pb2+. The HPLC/ICP-MS and HPLC/RI (refractive index detector) analysis of the partially hydrolyzed mRG-II that was treated with La3+ indicated that the side chains and backbone of mRG-II together form a lanthanoid binding site.     

Constructing Phenoxo-Bridged Heterobimetallic [Zn(II)2M(II)] (M = Sr and Ba) Salamo-Based Complexes

Russian Journal of General Chemistry - Two new phenoxo-bridged heterobimetallic [Zn(II)2M(II)] (M = Sr and Ba) salamo-based complexes, [{Zn(L)(μ2-OAc)}2Sr]·0.33CH3OH·H2O (1) and...     

Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: isovalent substitution effect

In the Fe-Mo based B-site ordered double-perovskite, A₂FeMoO_6.0, with iron in the mixed-valence II/III state, the valence value of Fe is not precisely fixed at 2.5 but may be fine-tuned by means of applying chemical pressure at the A-cation site. This is shown through a systematic ⁵⁷Fe Mössbauer spectroscopy study using a series of A₂FeMoO_6.0 [A=(Ba,Sr) or (Sr,Ca)] samples with high degree of Fe/Mo order, the same stoichiometric oxygen content and also almost the same grain size. The isomer shift values and other hyperfine parameters obtained from the Mössbauer spectra confirm that Fe remains in the mixed-valence state within the whole range of A constituents. However, upon increasing the average cation size at the A site the precise valence of Fe is found to decrease such that within the A=(Ba,Sr) regime the valence of Fe is closer to II, while within the A=(Sr,Ca) regime it is closer to the actual mixed-valence II/III state. As the valence of Fe approaches II, the difference in charges between Fe and Mo increases, and parallel with this the degree of Fe/Mo order increases. Additionally, for the less-ordered samples an increased tendency of clustering of the antisite Fe atoms is deduced from the Mössbauer data.     

Organic acid solutions in the chromatography of inorganic ions. VII. Cation exchange of Mg(II), Ca(II), Sr(II), Ba(II), Mn(II), Cd(II), Co(II), Ni(II), Zn(II), Cu(II) and Fe(III) in succinate media

Summary The cation-exchange behaviour of Mg(II), Ca(II), Sr(II), Ba(II), Mn(II), Cd(II), Co(II), Ni(II), Zn(II), Cu(II) and Fe(III) in succinate media at various concentrations and pH, was studied with Dowex 50 WX8 resin (200–400 mesh) in the NH ₄ ⁺ form. As examples separations of Cd(II)/Co(II), Cd (II)/Ni(II), Fe(III)/Cu(II)/Ni(II) and Mg(II)/Ca(II)/Sr(II)/Ba(II) have been achieved.This work was supported by C.N.R. of Italy.     

Mononuclear transition and non-transition complexes of amlodipine besylate as antihypertensive agent: synthesis,spectral, thermal,and antimicrobial studies

Mononuclear Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Mg(II), Sr(II), Ba(II), Ca(II), Pt(IV), Au(III), and Pd(II) complexes of the drug amlodipine besylate (HL) have been synthesized and characterized by elemental analysis, spectroscopic technique (IR, UV–Vis, solid reflectance, scanning electron microscopy, X-ray powder diffraction, and ¹H-NMR) and magnetic measurements. The elemental analyses of the complexes are confirmed by the stoichiometry of the types [M(HL)(X)₂(H₂O)]·nH₂O [M = Mn(II), Co(II), Zn(II), Ni(II), Mg(II), Sr(II), Ba(II), and Ca(II); X = Cl^? or NO₃ ^?], [Cd(HL)(H₂O)]Cl₂, [Pd(HL)₂]Cl₂, [Pt(L)₂]Cl₂, and [Au(L)₂]Cl, respectively. Infrared data revealed that the amlodipine besylate drug ligand chelated as monobasic tridentate through NH₂, oxygen (ether), and OH of besylate groups in Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Mg(II), Sr(II), Ba(II), Ca(II), and Au(III) complexes, but in Pt(IV) and Pd(II) complexes, the amlodipine besylate coordinates via NH₂ and OH (besylate) groups. An octahedral geometry is proposed for all complexes except for the Cd(II), Pt(IV), and Pd(II) complexes. The amlodipine besylate free ligand and the transition and non-transition complexes showed antibacterial activity towards some Gram-positive and Gram-negative bacteria and the fungi (Aspergillus flavus and Candida albicans).