Immobilization of catalase via adsorption on poly(styrene-d-glycidylmethacrylate) grafted and tetraethyldiethylenetriamine ligand attached microbeads

Fibrous poly(styrene-d-glycidylmethacrylate) (P(S-GMA)) brushes were grafted on poly(styrene-divinylbenzene) (P(S-DVB)) beads using surface initiated-atom transfer radical polymerization (SI-ATRP). Tetraethyldiethylenetriamine (TEDETA) ligand was incorporated on P(GMA) block. The multi-modal ligand attached beads were used for reversible immobilization of catalase. The influences of pH, ionic strength and initial catalase concentration on the immobilization capacities of the P(S-DVB)-g-P(S-GMA)-TEDETA beads have been investigated. Catalase adsorption capacity of P(S-DVB-g-P(S-GMA)-TEDETA beads was found to be 40.8 ± 1.7 mg/g beads at pH 6.5 (with an initial catalase concentration 1.0 mg/mL). The K_m value for immobilized catalase on the P(S-DVB-g-P(S-GMA)-TEDETA beads (0.43 ± 0.02 mM) was found about 1.7-fold higher than that of free enzyme (0.25 ± 0.03 mM). Optimum operational temperature and pH was increased upon immobilization. The same support was repeatedly used five times for immobilization of catalase after regeneration without significant loss in adsorption capacity or enzyme activity.     

Kinetic and equilibrium studies of acrylonitrile binding to cytochrome c peroxidase and oxidation of acrylonitrile by cytochrome c peroxidase compound I

Ferric heme proteins bind weakly basic ligands and the binding affinity is often pH dependent due to protonation of the ligand as well as the protein. In an effort to find a small, neutral ligand without significant acid/base properties to probe ligand binding reactions in ferric heme proteins we were led to consider the organonitriles. Although organonitriles are known to bind to transition metals, we have been unable to find any prior studies of nitrile binding to heme proteins. In this communication we report on the equilibrium and kinetic properties of acrylonitrile binding to cytochrome c peroxidase (CcP) as well as the oxidation of acrylonitrile by CcP compound I. Acrylonitrile binding to CcP is independent of pH between pH 4 and 8. The association and dissociation rate constants are 0.32 ± 0.16 M⁻¹ s⁻¹ and 0.34 ± 0.15 s⁻¹, respectively, and the independently measured equilibrium dissociation constant for the complex is 1.1 ± 0.2 M. We have demonstrated for the first time that acrylonitrile can bind to a ferric heme protein. The binding mechanism appears to be a simple, one-step association of the ligand with the heme iron. We have also demonstrated that CcP can catalyze the oxidation of acrylonitrile, most likely to 2-cyanoethylene oxide in a “peroxygenase”-type reaction, with rates that are similar to rat liver microsomal cytochrome P450-catalyzed oxidation of acrylonitrile in the monooxygenase reaction. CcP compound I oxidizes acrylonitrile with a maximum turnover number of 0.61 min⁻¹ at pH 6.0.     

Spectroscopic speciation and structural characterisation of uranyl(VI) interaction with pyridine carboxylic acid N-oxide derivatives

Spectroscopic speciation of U(VI) solutions holding pyridine carboxylic acid N-oxides in a range pH 2.3-4.5 results in the single component spectrum of the U(VI) isonicotinic acid N-oxide complex. The molar absorption is 14 ± 2 L mol⁻¹ cm⁻¹ at 415.4 nm. The formation constant lg K_UL = 2.1 ± 0.2 (k = 2) is derived from solution modelling and by multivariate chemometric analysis. The first crystal structure analysis of a U(VI) pyridine carboxylic acid N-oxide revealed a sheet-like structure where the isonicotinic acid N-oxide binds to the uranyl(VI) both bidentately by the carboxylate group and monodentately by the N-O group. The single component spectrum of the [UO₂L]⁺ (where L⁻ is isonicotinate N-oxide) is compared to the small number of other U(VI) single ligand species. The comparison revealed the possible pitfalls of U(VI) spectroscopic speciation close to the pH region where U(VI) hydrolysis starts to interfere. On basis of the results for U(VI)-L coordination and physicochemical properties of the pyridine carboxylic acid N-oxides some conclusions could be drawn on the likely behaviour of nicotinic acid N-oxide and picolinic acid N-oxide. For the former, complex formation in a narrow range of pH and U(VI) concentrations close to the hydrolysis range of U(VI) might reveal thermodynamic data. In the case of picolinic acid N-oxide, additional experimental evidence is required to characterize suitable conditions.     

Design of a flexible ligand for the construction of a one-dimensional metal-organic coordination polymer

A new ligand LH (where LH = N-(picolinoyl)-biurate) has been prepared and characterized. The presence of three amide linkages make this ligand sufficiently flexible to act as N,N,O donor tridentate blocking ligand in the formation of a one dimensional metal-ligand layer like structure. Reaction of LH and dicyanamide (dca) with Co(NO₃)₂ · 6H₂O gives [CoL(dca)]_n (1). In this compound picolinamide modulated ligand L coordinated the central Co(II) ion in a meridonal-fashion. The single crystal X-ray crystallography revealed that in 1, dca acts as μ_1,5− singly bridging ligand whereas μ_1,5− doubly bridging is the more common type. This gives rise to the 1D undulated waves like structure. The Co(II) centre is surrounded in a distorted square pyramidal coordination geometry. The variable temperature magnetic (VTM) susceptibility measurements show that the global feature of the χ_MT versus T curve for 1 is characteristic of very weak antiferromagnetic interactions through the dicyanamide ligand and between 300 and 5 K the best fit parameter was determined as J = −3.52 cm⁻¹. The X-ray structure, VTM study and UV-Vis spectrum of the compound show that 1 is a low-spin square-pyramidal compound whereas high-spin compounds are more common for the five coordinated cobalt (II) compounds. The X-band EPR spectrum of 1 at room temperature shows only one isotropic band centred at g = 2.08.     

Synthesis, crystallography, phosphorescence of platinum complexes coordinated with 2-phenylpyridine and a series of β-diketones

Metallocyclic platinum(II) complexes coordinating with 2-phenylpyridine (ppy) and a series of β-diketone ancillary O^∧O ligands, (ppy)Pt(acac), (ppy)Pt(ba), (ppy)Pt(dbm), and (ppy)Pt(tta) (acac = acetylacetone, ba = benzoylacetone, dbm = dibenzoylmethane, tta = thenoyltrifluoroacetone) were synthesized. The crystal structure, absorption, emission, quantum yield and phosphorescence life time were characterized. As the conjugative π system of the O^∧O ligand increases in the order acac < ba < dbm, or there is a group -CF₃to attract the electron density of the tta ligand, the quantum yield decreases in the order (ppy)Pt(acac) > (ppy)Pt(ba) > (ppy)Pt(dbm) > (ppy)Pt(tta) due to an energy back-flow from ppy to the O^∧O ligand, a trend also in contrast to the phosphorescence emission spectra and time decay (biexponentially, ∼0.7-13 μs).     

Recovery of active N-acetyl-D-glucosamine 2-epimerase from inclusion bodies by solubilization with non-denaturing buffers

Overexpression of recombinant N-acetyl-d-glucosamine 2-epimerase, one of the key enzymes for the synthesis of N-acetylneuraminic acid, in E. coli led to the formation of protein inclusion bodies. In this study we report the recovery of active epimerase from inclusion bodies by direct solubilization with Tris buffer. At pH 7.0, 25% of the inclusion bodies were solubilized with Tris buffer. The specific activity of the solubilized proteins, 2.08 ± 0.02 U/mg, was similar to that of the native protein, 2.13 ± 0.01 U/mg. The result of circular dichroism spectroscopy analysis indicated that the structure of the solubilized epimerase obtained with pH 7.0 Tris buffer was similar to that of the native epimerase purified from the clarified cell lysate. As expected, the extent of deviation in CD spectra increased with buffer pH. The total enzyme activity recovered by solubilization from inclusion bodies, 170.41 ± 10.06 U/l, was more than 2.5 times higher than that from the clarified cell lysate, 67.32 ± 5.53 U/l. The results reported in this study confirm the hypothesis that the aggregation of proteins into inclusion bodies is reversible and suggest that direct solubilization with non-denaturing buffers is a promising approach for the recovery of active proteins from inclusion bodies, especially for aggregation-prone multisubunit proteins.     

Reactivity of [Ru(hedtra)(H2O)] with thio-amino acids and protease inhibition

Reaction of [Ru^III(hedtra)(H₂O)] (hedtra = N-hydroxyethylethylenediaminetriacetate) with thio-amino acids, L (L = cysteine, N-acetylcysteine, glutathione and penicilamine), was studied kinetically. Kinetic studies were performed at different concentrations of reactants, pH and temperature. Based on the kinetic results, it is suggested that the formation of S-bound substituted product takes place in a rapid ligand dependent rate determining step. Kinetic data and activation parameters are accounted for operation of an associative mechanism and discussed in reference to the data reported earlier for edta⁴⁻ (ethylenediaminetetraacetate) complex of ruthenium(III). Results of cysteine protease inhibition studies revealed that inhibition activities of Ru-pac complexes are enzyme specific.     

The mechanism of inactivation of glucose oxidase from Penicillium amagasakiense under ambient storage conditions

Glucose oxidase (GOx) from Penicillium amagasakiense has a higher specific activity than the more commonly studied Aspergillus niger enzyme, and may therefore be preferred in many medical and industrial applications. The enzyme rapidly inactivates on storage at pH 7.0-7.6 at temperatures between 30 and 40 °C. Results of fluorimetry and circular dichroism spectroscopy indicate that GOx inactivation under these conditions is associated with release of the cofactor FAD and molten globule formation, indicated by major loss of tertiary structure but almost complete retention of secondary structure. Inactivation of GOx at pH < 7 leads to precipitation, but at pH ≥ 7 it leads to non-specific formation of small soluble aggregates detectable by PAGE and size-exclusion chromatography (SEC). Inactivation of P. amagasakiense GOx differs from that of A. niger GOx in displaying complete rather than partial retention of secondary structure and in being promoted rather than prevented by NaCl. The contrasting salt effects may reflect differences in the nature of the interface between subunits in the native dimers and/or the quantity of secondary structure loss upon inactivation.     

Synthesis and crystal structures of five-coordinated β-pyrrole tetra(phenyl/cyano) substituted zinc(II)-tetraphenylporphyrins

Crystal structures of a series of 2,3,5,10,12,13,15,20-octaphenylporphinato zinc(II) complexes with varying axial ligands have been examined to elucidate the role of fifth ligand on the stereochemistry of the porphyrin macrocycle. The nonplanarity of the macrocycle varies in the order, ZnOPP < ZnOPP(pyridine) < ZnOPP(H₂O). The electron deficient porphyrin complex of five-coordinated Zn(II), ZnTPP(CN)₄(CH₃OH) showed enhanced nonplanarity of the macrocycle and it is less than that of ZnOPP(H₂O)·TCE complex. Normal coordinate structure decomposition analysis of the out-of-plane displacement of the porphyrin ring in these structures revealed negligible wave distortion in planar four-coordinated ZnOPP and saddle, ruffled and domed distortions in other five-coordinated Zn(II)-porphyrins. The pronounced distortion of the macrocyclic ring in these structures is possibly due to the axial ligand, solvate and/or intermolecular interactions compared to steric crowding of the peripheral substituents.     

Impact of hapten presentation on antibody binding at lipid membrane interfaces

We report the effects of ligand presentation on the binding of aqueous proteins to solid supported lipid bilayers. Specifically, we show that the equilibrium dissociation constant can be strongly affected by ligand lipophilicity and linker length/structure. The apparent equilibrium dissociation constants (K_D) were compared for two model systems, biotin/anti-biotin and 2,4-dinitrophenyl (DNP)/anti-DNP, in bulk solution and at model membrane surfaces. The binding constants in solution were obtained from fluorescence anisotropy measurements. The surface binding constants were determined by microfluidic techniques in conjunction with total internal reflection fluorescence microscopy. The results showed that the bulk solution equilibrium dissociation constants for anti-biotin and anti-DNP were almost identical, K_D(bulk) = 1.7 ± 0.2 nM vs. 2.9 ± 0.1 nM. By contrast, the dissociation constant for anti-biotin antibody was three orders of magnitude tighter than for anti-DNP at a lipid membrane interface, K_D = 3.6 ± 1.1 nM vs. 2.0 ± 0.2 μM. We postulate that the pronounced difference in surface binding constants for these two similar antibodies is due to differences in the ligands’ relative lipophilicity, i.e., the more hydrophobic DNP molecules had a stronger interaction with the lipid bilayers, rendering them less available to incoming anti-DNP antibodies compared with the biotin/anti-biotin system. However, when membrane-bound biotin ligands were well screened by a poly(ethylene glycol) (PEG) polymer brush, the K_D value for the anti-biotin antibody could also be weakened by three orders of magnitude, 2.4 ± 1.1 μM. On the other hand, the dissociation constant for anti-DNP antibodies at a lipid interface could be significantly enhanced when DNP haptens were tethered to the end of very long hydrophilic PEG lipopolymers (K_D = 21 ± 10 nM) rather than presented on short lipid-conjugated tethers. These results demonstrate that ligand presentation strongly influences protein interactions with membrane-bound ligands.     

Frequent mutations in NOTCH1 ligand-binding regions in Japanese oral squamous cell carcinoma

Recent studies showed that head and neck squamous cell carcinoma (HNSCC) including oral squamous cell carcinoma (OSCC) of Caucasian, Chinese and Indian patients frequently have NOTCH1 mutations. We found eight of 84 OSCC in Japanese patients have point mutations (9.5%) correspond to the ligand binding region of NOTCH1 protein. Two set of them are the same mutations and all mutations are non-synonymous G > A transitions. In addition, median disease-free survival is significantly longer in patients with NOTCH1-mutated tumors as compared to those without the mutation (P < 0.05). The protein structure simulation based on X-ray crystallography indicated that new p.A465T mutation leads to a conformational change of NOTCH1 ligand binding domain as well as the p.G481S mutant NOTCH1 with a loss of flexibility around this residue. These results suggest that NOTCH1 mutation occurs frequently in Japanese OSCC in the vicinity of the ligand binding region and, these mutations cause downregulation of the NOTCH1 function.     

Complexes of the highly preorganized ligand PDALC (2,9-bis(hydroxymethyl)-1,10-phenanthroline) with trivalent lanthanides. A thermodynamic and crystallographic study

Metal ion complexing properties of the highly preorganized tetradentate ligand PDALC (2,9-bis(hydroxymethyl)-1,10-phenanthroline) are presented. The structure of [Gd(PDALC)(NO₃)₃]·H₂O (1) is reported: triclinic, , a = 7.545(12), b = 10.811(17), c = 11.909(18) Å, α = 97.71(2)°, β = 91.56(2)°, γ = 109.06(2)°, V = 907(2) Å³, Z = 2, R = 0.0354. The Gd is 10-coordinate, with the coordination sphere comprising the four donor atoms of the PDALC plus the six O-donors of three chelated nitrates. Comparison with structures in the literature suggests that the Gd-L (L = ligand) bond lengths, particularly those to the alcoholic O-donors of PDALC, are a little short. It was suggested that the short Gd-L bond lengths in 1 were due to the efficiency of packing of the nitrates around the Gd, with the short ‘bite’ distances of the nitrate ligand. Formation constants (log K₁) were measured spectroscopically in 0.1 M NaClO₄ at 25 °C by monitoring the variation of the π-π∗ transitions of 2 × 10⁻⁵ M PDALC in the range 200-350 nm as a function of pH, in the presence of 1:1 concentrations of the lanthanide(III) (Ln(III)) metal ion. The measured log K₁ values varied from 5.34 (La(III)) to 6.40 (Lu(III), which is an unusually small variation across the series of Ln(III) ions. Values of log K₁ with PDALC were also measured for Y(III) (5.85) and Sc(III) (6.02). The small amount of variation in log K₁ for PDALC across the series of Ln(III) ions was rationalised in terms of the effect of neutral oxygen donors on complex stability, which promotes selectivity for larger metal ions such as La(III). It was discussed how the small amount of variation in log K₁ across the Ln(III) series might lead to optimal selectivity for the Am(III) ion relative to the Ln(III) ions as a group.     

Acidic pH triggers conformational changes at the NH2-terminal propeptide of the precursor of pulmonary surfactant protein B to form a coiled coil structure

Pulmonary surfactant protein SP-B is synthesized as a larger precursor, proSP-B. We report that a recombinant form of human SP-B_N forms a coiled coil structure at acidic pH. The protonation of a residue with pK = 4.8 ± 0.06 is the responsible of conformational changes detected by circular dichroism and intrinsic fluorescence emission. Sedimentation velocity analysis showed protein oligomerisation at any pH condition, with an enrichment of the species compatible with a tetramer at acidic pH. Low 2,2,2,-trifluoroethanol concentration promoted β-sheet structures in SP-B_N, which bind Thioflavin T, at acidic pH, whereas it promoted coiled coil structures at neutral pH. The amino acid stretch predicted to form β-sheet parallel association in SP-B_N overlaps with the sequence predicted by several programs to form coiled coil structure. A synthetic peptide (⁶⁰W-E⁸⁵) designed from the sequence of the amino acid stretch of SP-B_N predicted to form coiled coil structure showed random coil conformation at neutral pH but concentration-dependent helical structure at acidic pH. Sedimentation velocity analysis of the peptide indicated monomeric state at neutral pH (s_{20, w} = 0.55 S; Mr ~ 3 kDa) and peptide association (s_{20, w} = 1.735 S; Mr = ~ 14 kDa) at acidic pH, with sedimentation equilibrium fitting to a Monomer-Nmer-Mmer model with N = 6 and M = 4 (Mr = 14692 Da). We propose that protein oligomerisation through coiled-coil motifs could then be a general feature in the assembly of functional units in saposin-like proteins in general and in the organization of SP-B in a functional surfactant, in particular.     

A dinuclear silver compound with 5,6,7-trimethyl-[1,2,4]triazolo[1,5-a]pyrimidine with a short Ag-Ag bond. Synthesis, characterization, single-crystal structure analysis and cytostatic activity

A new dinuclear coordination compound with the ligand 5,6,7-trimethyl[1,2,4]triazolo[1,5-a]pyrimidine (abbreviated as tmtp) is described, together with its 3D crystal structures and spectroscopic properties. The compound shows a high cytostatic activity in ovarian cancer cell lines.The solid state structure of the compound is dinuclear based, in which the ligand tmtp uses two nitrogens (N3 and N4) to chelate to two different Ag(I) ions. The Ag-Ag distance is relatively short (3.109 Å), but is considered as too long for M-M bonding. The Ag(I) coordination is completed by weakly coordinated, asymmetric bidentate nitrate anions (Ag-O = 2.481-2.819 Å).     

Synthesis of the donor-acceptor ligand 2-(4-dimethylaminobenzylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd) and X-ray diffraction structure of the platinum(II) compound PtCl2(dbpcd)·1.5CH2Cl2

Knoevenagel condensation of 4-(dimethylamino)benzaldehyde with 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) gives the donor-acceptor ligand 2-(4-dimethylaminobenzylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd). The reaction of dbpcd with PtCl₂(cod) affords the platinum(II) complex PtCl₂(dbpcd) in high yield. The free dbpcd ligand and PtCl₂(dbpcd) have been isolated and fully characterized in solution by IR and NMR spectroscopies, and the solid-state structure of PtCl₂(dbpcd) determined by X-ray diffraction analysis. PtCl₂(dbpcd), as the 1.5CH₂Cl₂ solvate, crystallizes in the triclinic space group , a = 11.7412(7) Å, b = 12.0486(7) Å, c = 14.4781(9) Å, α = 82.866(1), β = 75.049(1), γ = 83.905(1), V = 1957.6(2) Å³, Z = 2, and D_calc = 1.678 mg/m³, R = 0.0291 and wR₂ = 0.0723 for 8315 reflections with I > 2σ(I). The molecular structure of PtCl₂(dbpcd)·1.5CH₂Cl₂ consists of a square-planar platinum architecture containing two chlorines and the ancillary dbpcd diphosphine ligand. The redox properties of the dbpcd ligand and PtCl₂(dbpcd) have been explored by cyclic voltammetry, and these data are discussed with respect to extended Hückel MO calculations.     

Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)

The stability constants of Am⁺³, Cm³⁺ and Eu³⁺ with ortho silicate, were measured at pH 3.50 and in ionic strengths of 0.20-1.00 M (NaClO₄) by the solvent extraction method. The Am⁺³, Cm³⁺ and Eu³⁺ forms 1:1 complex with ortho silicate ion at pH 3.60 with the stability constant (log β₁) value of 8.02 ± 0.10, 7.78 ± 0.08 and 7.81 ± 0.11, respectively. The stability of these metal ions decrease with increased ionic strength from 0.20 to 1.00 M (NaClO₄) for silicic acid concentrations of 0.002-0.020 M. Increasing silicic acid concentration above 0.02 M increased the amount of M³⁺ extracted into the organic phase, contrary to the trend usually observed for increased ligand concentration in solvent extraction. This reversed trend is likely due to the extraction of cationic species of silicic acid by HDEHP. Aging time (60-300 min) had no effect on the stability constant of these metal ions for 0.002-0.020 M silicic acid at pH 3.50 and I = 0.20 M (NaClO₄).The fraction of polymeric silicic acid present in solutions of 0.20-4.50 M NaClO₄ solutions at pH 3.0-10.0, T = 0-60 °C and aging time = 5-300 min was measured for determination of the silicomolybdate reaction to ascertain the proper conditions to study metal-silicate complexation.     

Synthesis, characterization and bioactivity of a novel 18-metallacrown-6: [Mn(pcshz)(CH3OH)]6 · 4CH3OH · 4H2O

The reaction of N-propionyl-5-chlorosalicylhydrazide (H₃pcshz) with Mn(OAc)₂ · 4H₂O in methanol solution gives a novel 18-metallacrown-6 [Mn(pcshz)(CH₃OH)]₆ · 4CH₃OH · 4H₂O (1). The structure of 1 has been determined by X-ray diffraction and it crystallizes in the monoclinic system and space group P2(1)/n. The ring of the metallacrown is consisted of six interlink [Mn-N-N] repeated units through hydrazide N-N group bridging the ring manganese ions. And the ligand enforces the metal ions to form the stereochemistry as a propeller configuration with alternation Λ/Δ form. The largest diameters of the disc-shaped hexanuclear ring are about 8.82 Å at entrance, 9.83 Å at the central of the cavity, respectively. The solution integrity and stability of the metallacrown was studied by electrospray ionization (ESI)-MS and UV-Vis spectroscopy. The results show it is soluble and stable in methanol. Antibacterial screening data indicate the forming of the complex weakens dramatically the antibacterial activity of the ligand H₃pcshz except for Eschericha coli.     

Aqueous acid-base chemistry involving dioxovanadium(V) complexes of 2,6-pyridinedimethanol, and the X-ray structures of Na[VO2{2,6-(OCH2)2NC5H3}] · 4H2O and [1-H-2,6-(HOCH2)2NC5H3]Cl

Reaction of NH₄VO₃ with 2,6-pyridinedimethanol in water at 85 °C followed by the room temperature addition of HCl (aq) yields [HVO₂(pydim)]_x (pydim = 2,6-pyridinedimethanolato dianion), as a sparingly soluble off-white solid. This acid may be deprotonated by titration with NaOH (aq), yielding Na[VO₂(pydim)] · 4H₂O, which has been structurally characterized by single-crystal X-ray diffraction. Treating Na[VO₂(pydim)] · 4H₂O with HCl (aq) regenerates [HVO₂(pydim)]_x, but reaction with additional NaOH (aq) displaces the pyridinedimethanolato ligand from the vanadium center. Similarly, treating [HVO₂(pydim)]_x with excess HCl (aq) strips the pyridinedimethanolato ligand from the vanadium center and yields the adduct [H₃(pydim)]⁺Cl⁻ as one component in a mixture of products. This adduct has been structurally characterized by single-crystal X-ray diffraction. The optimum pH range for stable dioxovanadium(V) complexes stabilized by the 2,6-pyridinedimethanolato ligand is at least 1.5-9.4.     

From triplesalen to triplesalacen: Synthesis spectroscopic, redox, and magnetic properties of the trinuclear Cu3 triplesalacen complex

The tris(tetradentate) triplesalacen ligand H₆talacen and its trinuclear copper complex, namely [(talacen)Cu^II₃], were synthesized and its molecular and electronic structure determined. The ligand is prepared by the triple condensation of 2,4,6-trisacetyl-1,3,5-trihydroxybenzene (1) and excess acacen half-unit (2). The molecular structure of [(talacen)Cu^II₃] exhibits a strong ligand folding around the central Cu^II-phenolate bonds. This ligand folding is stronger than that observed in the trinuclear copper triplesalen complexes. The electronic absorption spectrum exhibits a prominent intense band at 32 400 cm⁻¹ and d-d transitions around 19 000 cm⁻¹. These spectral features are related to the ligand folding, which opens an O^Ph(p_z)-Cu^II(d_x²-y²) bonding pathway. Electrochemical studies provide an irreversible oxidation at a relatively low potential of 0.17 V versus Fc⁺/Fc, which is assigned to a ligand-centered oxidation of the central phloroglucinol backbone. A ferromagnetic coupling of J = 1.20 cm⁻¹ (H=∑_i<j-2J_ijS_iS_j) is established in agreement with the spin-polarization mechanism. A critical analysis of the bond distances of the central phloroglucinol unit provides evidence that not only the usual phenolate-imine but also the keto-enamide mesomeric form contribute to the resonance hybrid. This might explain the relatively weak ferromagnetic coupling.     

Spin crossover in a mononuclear compound [Fe(EPPA)(bpym)](ClO4)2 (EPPA = N-(2-aminoethyl)-N-(3-aminopropyl)-2-(aminomethyl)pyridine, bpym = 2,2′-bipyrimidine): Synthesis, structure, and magnetic properties

The synthesis, magnetic properties and single crystal study of a new spin crossover compound [Fe(EPPA)(bpym)](ClO₄)₂ with EPPA = N-(2-aminoethyl)-N-(3-aminopropyl)-2-(aminomethyl)pyridine, bpym = 2,2′-bipyrimidine are reported. Variable-temperature magnetic susceptibility data collected in the temperature range 10-294 K reveal the occurrence of a relatively cooperative spin transition with T_1/2 = 108 K. The crystal structure of [Fe(EPPA)(bpym)](ClO₄)₂ was determined by single-crystal X-ray diffraction method. The structure of the complex consists of mononuclear [Fe(EPPA)(bpym)](ClO₄)₂ units. The potentially bis-bidentate bpym ligand acting as a bidentate one, is coordinated to iron(II) in cis-position by two nitrogen atoms. The four remaining positions in the pseudooctahedral [FeN₆] core are occupied by one pyridinic and three aliphatic nitrogens of the EPPA ligand. The network of cooperative links in the crystal lattice is represented by H-bonding and π stacking interactions.