A new EPR signal attributed to the primary plastosemiquinone acceptor in Photosystem II

A study of signals, light-induced at 77 K in O₂-evolving Photosystem II (PS II) membranes showed that the EPR signal that has been attributed to the semiquinone-iron form of the primary quinone acceptor, Q^?_AFe, at g = 1.82 was usually accompanied by a broad signal at g = 1.90. In some preparations, the usual g = 1.82 signal was almost completely absent, while the intensity of the g = 1.90 signal was significantly increased. The g = 1.90 signal is attributed to a second EPR form of the primary semiquinone-iron acceptor of PS II on the basis of the following evidence. (1) The signal is chemically and photochemically induced under the same conditions as the usual g = 1.82 signal. (2) The extent of the signal induced by the addition of chemical reducing agents is the same as that photochemically induced by illumination at 77 K. (3) When the g = 1.82 signal is absent and instead the g = 1.90 signal is present, illumination at 200 K of a sample containing a reducing agent results in formation of the characteristic split pheophytin^? signal, which is thought to arise from an interaction between the photoreduced pheophytin acceptor and the semiquinone-iron complex. (4) Both the g = 1.82 and g = 1.90 signals disappear when illumination is given at room temperature in the presence of a reducing agent. This is thought to be due to a reduction of the semiquinone to the nonparamagnetic quinol form. (5) Both the g = 1.90 and g = 1.82 signals are affected by herbicides which block electron transfer between the primary and secondary quinone acceptors. It was found that increasing the pH results in an increase of the g = 1.90 form, while lowering the pH favours the g = 1.82 form. The change from the g = 1.82 form to the g = 1.90 form is accompanied by a splitting change in the split pheophytin^? signal from approx. 42 to approx. 50 G. Results using chloroplasts suggest that the g = 1.90 signal could represent the form present in vivo.     

Antiferromagnetic interactions through phenoxo bridges and lattice water: Synthesis, structure, and magnetic properties of new Mn(III) Schiff base complexes in combination with thiocyanate ligand

The Schiff base ligands, N,N′-bis(2-hydroxyacetophenone)-1,2-diaminoethane (acphenH₂) and N,N′-bis(2-hydroxyacetophenone)-1,3-diaminopropane (acphpnH₂), prepared in situ were used to synthesise two new Mn(III) complexes which were characterised by crystallography and variable temperature magnetic measurements. [Mn(acphen)NCS]₂ is a phenoxo-bridged dimeric compound with the thiocyanate coordinating in the usual bent mode (Mn-N-C angle, 152°) and is weakly antiferromagnetic. Since there are no significant inter-dimer contacts in the crystal, the low temperature magnetic behaviour is influenced by single ion zero-field splitting. Exact diagonalisation of the spin Hamiltonian was performed to derive the following parameters: J = −0.7 cm⁻¹, D = −0.6 cm⁻¹. Mn(acphpn)(H₂O)NCS is monomeric with an unusual linearly coordinated thiocyanate (Mn-N-C angle, 178°). Two lattice water molecules link the Mn(III) complex molecules through hydrogen bonds to form one-dimensional chains in the crystal. Magnetic exchange along the chain makes this compound also weakly antiferromagnetic with J ∼ -2 cm⁻¹.     

Absence of a Formate-Induced Release of Bicarbonate from Photosystem II

Formate has been proposed to inhibit electron flow in photosystem II by replacing endogenous bound bicarbonate on the reaction center complex. A mass spectrometer was used to measure directly the CO₂/HCO₃⁻ released when maize thylakoids, showing normal rates of electron flow, were treated with formate. Although the formate inhibited electron flow by 95%, no release (displacement) of CO₂/HCO₃⁻ was detected. This is consistent with the concept that membrane-bound HCO₃⁻ is not a requirement for normal rates of electron flow through photosystem II. Moreover, formate and other monovalent anions do not inhibit electron flow by removing bound HCO₃⁻ but by binding to empty sites. The “bicarbonate effect” is a reversal, by high concentrations of exogenous bicarbonate, of anion inhibition of photosystem II.     

Natural variation of carotenoids in the eggs and gonads of the echinoid genus, Strongylocentrotus: implications for their role in ultraviolet radiation photoprotection

We examined variability in carotenoid concentration in the gonads and eggs of four sea urchin species (Strongylocentrotus purpuratus, Strongylocentrotus franciscanus, Strongylocentrotus pallidus and Strongylocentrotus droebachiensis) to explore the possible role of carotenes as photoprotectants. Carotene concentrations were measured in gonads and gametes of each species, while in eggs the ultraviolet radiation (UV-R) sensitivity and self-shading capacity by carotenes were calculated. Mean concentrations of carotenes in gonads ranged from 0.13±0.017 mg g⁻¹ dw (S. purpuratus), 0.14±0.019 mg g⁻¹ dw (S. franciscanus), 0.29±0.079 mg g⁻¹ dw (S. pallidus) to 0.36±0.06 mg g⁻¹ dw (S. droebachiensis). In eggs, concentrations ranged from 0.026±0.003 to 0.09±0.034 mg g⁻¹ dw. UV-R sensitivity in eggs was quantified by measuring UV-R induced first-cleavage delay. Intra-specifically, cleavage delay varied significantly between individuals, and could be correlated with carotene concentration. Interspecific differences in cleavage delay and carotene concentrations were not correlated. Using the observed concentration of β, β-echinenone (which makes up between 82.4% and 94.9% of the total carotene concentration in the eggs) and a molar extinction coefficient of ε=13.7×10³ mol⁻¹ cm⁻¹ at 334 nm, we calculated self-shading efficiency in the eggs. Self-shading capacity (J₃₃₄) indicated that the eggs could only screen from 4.6% (J₃₃₄=0.046) down to 1.5% (J₃₃₄=0.015) of UV-R at 334 nm. While not sunscreens, we suggest that carotenes can photoprotective in echinoid eggs, probably by mitigating the effects of reactive oxygen species.     

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.     

Bicarbonate use in three aquatic plants

Our study aimed to test the ability of aquatic plants to use bicarbonate when acclimated to three different bicarbonate concentrations. To this end, we performed experiments with the three species Ceratophyllum demersum, Egeria densa, Lagarosiphon major to determine photosynthetic rates under varying bicarbonate concentrations. We measured bicarbonate use efficiency, photosynthetic performance and respiration. For all species, our results revealed that photosynthetic rates were highest in replicates grown at low alkalinity. Thus, E. densa had approx. five times higher rates at low (264 ± 15 μmol O₂ g⁻¹ DW h⁻¹) than at high alkalinity (50 ± 27 μmol O₂ g⁻¹ DW h⁻¹), C. demersum had three times higher rates (336 ± 95 and 120 ± 31 μmol O₂ g⁻¹ DW h⁻¹), and L. major doubled its rates at low alkalinity (634 ± 114 and 322 ± 119 μmol O₂ g⁻¹ DW h⁻¹). Similar results were obtained for bicarbonate use efficiency by E. densa (136 ± 44 and 43 ± 10 μmol O₂ mequiv. L⁻¹ g⁻¹ DW h⁻¹) and L. major (244 ± 29 and 82 ± 24 μmol O₂ mequiv. L⁻¹ g⁻¹ DW h⁻¹). As to C. demersum, efficiency was high but unaffected by alkalinity, indicating high adaptation ability to varied alkalinities. A pH drift experiment supported these results. Overall, our results suggest that the three globally widespread worldwide species of our study adapt to low inorganic carbon availability by increasing their efficiency of bicarbonate use.     

Synthesis, crystal structure, magnetic properties and EPR spectra of copper(II) acetate derivatives: tetra-(μ-acetato)bis(2-methylaminopyridine)copper(II) and catena-poly(aquadiacetato-μ-3-aminomethylpyridine)copper(II). Sheets formed by chains connected through hydrogen-bonds

The derivatives of Cu(OAc)₂ · H₂O with 2-methylaminopyridine and 3-aminomethylpyridine, [Cu₂(μ-OAc)₄(MeNHpy)₂] (1) and [Cu(OAc)₂(μ-NH₂CH₂py)(H₂O)]_n (2), respectively, have been synthesized and characterized. Compound 1 shows the dimer structure of [Cu₂(μ-OAc)₄(H₂O)₂], with four syn-syn bridging acetate groups and the MeNHpy ligand coordinated in the axial positions. It is antiferromagnetic (2J=−285 cm⁻¹). Signals of the triplet state are observed in its EPR spectrum and the zero-field splitting parameter has been calculated (D=0.36 cm⁻¹; g_∥=2.35; g_⊥=2.07). Otherwise, the ligand 3-aminomethylpyridine acts as bridging bidentate ligand in compound 2, forming infinite zig-zag chains. Each copper atom lies in a square-planar pyramidal coordination, determined by two nitrogen atoms of two bridge ligands, two oxygen atoms of two monodentate terminal acetate groups and a water molecule. The parallel chains form a sheet because of the hydrogen bonds between them. The shortest Cu-Cu distances are: 5.1270, 6.0952 and 6.2163 Å (inter-chains) and 7.875 Å (intra-chain). Compound 2 shows a slight antiferromagnetic effect below 30 K. The EPR spectra show an orthorhombic signal (g₁=2.26; g₂=2.08; g₃=2.06).     

EPR studies of the water oxidizing complex in the S1 and the higher S states: the manganese cluster and YZ radical

The parallel polarization electron paramagnetic resonance (EPR) method has been applied to investigate manganese EPR signals of native S₁ and S₃ states of the water oxidizing complex (WOC) in photosystem (PS) II. The EPR signals in both states were assigned to thermally excited states with S=1, from which zero-field interaction parameters D and E were derived. Three kinds of signals, the doublet signal, the singlet-like signal and g=11–15 signal, were detected in Ca²⁺-depleted PS II. The g=11–15 signal was observed by parallel and perpendicular modes and assigned to a higher oxidation state beyond S₂ in Ca²⁺-depleted PS II. The singlet-like signal was associated with the g=11–15 signal but not with the Y_Z (the tyrosine residue 161 of the D1 polypeptide in PS II) radical. The doublet signal was associated with the Y_Z radical as proved by pulsed electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. The electron transfer mechanism relevant to the role of Y_Z radical was discussed.     

Towards a spin coupling model for the Mn4 cluster in Photosystem II

The X-band EPR spectra of the IR sensitive untreated PSII and of MeOH- and NH₃-treated PSII from spinach in the S₂-state are simulated with collinear and rhombic g- and Mn-hyperfine tensors. The obtained principal values indicate a 1Mn(III)3Mn(IV) composition for the Mn₄ cluster. The four isotropic components of the Mn-hyperfine tensors are found in good agreement with the previously published values determined from EPR and ⁵⁵Mn-ENDOR data. Assuming intrinsic isotropic components of the Mn-hyperfine interactions identical to those of the Mn-catalase, spin density values are calculated. A Y-shape 4J-coupling scheme is explored to reproduce the spin densities for the untreated PSII. All the required criteria such as a S=1/2 ground state with a low lying excited spin state (30 cm⁻¹) and an easy conversion to a S=5/2 system responsible for the g=4.1 EPR signal are shown to be satisfied with four antiferromagnetic interactions lying between −290 and −130 cm⁻¹.     

Effects of formate binding on the quinone-iron electron acceptor complex of photosystem II

EPR was used to study the influence of formate on the electron acceptor side of photosystem II (PSII) from Thermosynechococcus elongatus. Two new EPR signals were found and characterized. The first is assigned to the semiquinone form of Q(B) interacting magnetically with a high spin, non-heme-iron (Fe2(+), S=2) when the native bicarbonate/carbonate ligand is replaced by formate. This assignment is based on several experimental observations, the most important of which were: (i) its presence in the dark in a significant fraction of centers, and (ii) the period-of-two variations in the concentration expected for Q(B)(?-) when PSII underwent a series of single-electron turnovers. This signal is similar but not identical to the well-know formate-modified EPR signal observed for the Q(A)(?-)Fe2(+) complex (W.F.J. Vermaas and A.W. Rutherford, FEBS Lett. 175 (1984) 243-248). The formate-modified signals from Q(A)(?-)Fe2(+) and Q(B)(?-)Fe2(+) are also similar to native semiquinone-iron signals (Q(A)(?-)Fe2(+)/Q(B)(?-)Fe2(+)) seen in purple bacterial reaction centers where a glutamate provides the carboxylate ligand to the iron. The second new signal was formed when Q(A)(?-) was generated in formate-inhibited PSII when the secondary acceptor was reduced by two electrons. While the signal is reminiscent of the formate-modified semiquinone-iron signals, it is broader and its main turning point has a major sub-peak at higher field. This new signal is attributed to the Q(A)(?-)Fe2(+) with formate bound but which is perturbed when Q(B) is fully reduced, most likely as Q(B)H? (or possibly Q(B)H(?-) or Q(B)(2?-)). Flash experiments on formate-inhibited PSII monitoring these new EPR signals indicate that the outcome of charge separation on the first two flashes is not greatly modified by formate. However on the third flash and subsequent flashes, the modified Q(A)(?-)Fe2(+)Q(B)H? signal is trapped in the EPR experiment and there is a marked decrease in the quantum yield of formation of stable charge pairs. The main effect of formate then appears to be on Q(B)H? exchange and this agrees with earlier studies using different methods.     

Synthesis and multinuclear magnetic resonance spectroscopy of the novel ionic Pt(II) mixed-ligands complexes cis- and trans-[Pt(pyrazine)2(Ypy)2](NO3)2 where Ypy = pyridine derivative

Novel ionic mixed-ligands complexes of the types cis- and trans-[Pt(pz)₂(Ypy)₂](NO₃)₂ (where Ypy is a pyridine derivative and pz = pyrazine) were synthesized and studied mainly in the solid state by IR spectroscopy and in aqueous solution by multinuclear (¹⁹⁵Pt, ¹H and ¹³C) magnetic resonance spectroscopy. The trans isomers with ligands containing a methyl group in ortho position on the pyridine ring could not be synthesized. The results of the solution NMR characterization have shown that the isolated compounds are pure. In ¹⁹⁵Pt NMR, the cis complexes containing a methyl group in ortho positions were observed at lower field (average −2337 ppm) than the other cis compounds (average −2427 ppm), which is explained by the solvent effect. The trans isomers were observed at very slightly lower fields (average −2422 ppm) than the equivalent cis complexes (average −2427 ppm). In ¹H NMR, the coupling constants ³J(¹⁹⁵Pt-¹H_Ypy) and ³J(¹⁹⁵Pt-¹H_pz) are larger in the cis compounds (∼40 Hz) than in the trans complexes (∼31 Hz). A few ⁴J(¹⁹⁵Pt-¹H_pz) were observed (∼16 Hz). In ¹³C NMR spectroscopy, the coupling constants ³J(¹⁹⁵Pt-¹³C_pz) and ³J(¹⁹⁵Pt-¹³C_Ypy) are also larger in the cis configuration (∼30 and ∼38 Hz, respectively) than in the trans isomers (∼20 Hz). One ⁴J(¹⁹⁵Pt-¹³C_pz) could be calculated (17 Hz). The presence of the syn and anti rotamers were observed in all the cis complexes containing a pyridine derivative with a -CH₃ group in ortho position. They were observed in ¹⁹⁵Pt, ¹H and ¹³C NMR spectroscopy. The proportion of the two rotamers is about 55% and 45%.     

Influence of counter anions on the structures and magnetic properties of trinuclear Cu(II) complexes containing a μ3-OH core and Schiff base ligands

Four trinuclear Cu(II) complexes, [(CuL¹)₃(μ₃-OH)](NO₃)₂ (1), [(CuL²)₃(μ₃-OH)](I)₂·H₂O (2), [(CuL³)₃(μ₃-OH)](I)₂ (3) and [(CuL¹)₃(μ₃-OH)][Cu^II₃] (4), where HL¹ (8-amino-4-methyl-5-azaoct-3-en-2-one), HL² [7-amino-4-methyl-5-azaoct-3-en-2-one] and HL³ [7-amino-4-methyl-5-azahept-3-en-2-one] are the three tridentate Schiff bases, have been synthesized and structurally characterized by X-ray crystallography. All four complexes contain a partial cubane core, [(CuL)₃(μ₃-OH)]²⁺ in which the three [CuL] subunits are interconnected through two types of oxygen bridges afforded by the oxygen atoms of the ligands and the central OH⁻ group. The copper(II) ions are in a distorted square-pyramidal environment. The equatorial plane consists of the bridging oxygen of the central OH⁻ group together with three atoms (N, N, O) from the Schiff base. The oxygen atom of the Schiff base also coordinates to the axial position of Cu(II) of another subunit to form the cyclic trimer. Magnetic susceptibilities have been determined for these complexes over the temperature range of 2-300 K. The isotropic Hamiltonian, H = −J₁₂S₁S₂ − J₁₃S₁S₃ − J₂₃S₂S₃ has been used to interpret the magnetic data. The best fit parameters obtained are: J = −54.98 cm⁻¹, g = 2.24 for 1; J = −56.66 cm⁻¹, g = 2.19 for 2;J = −44.39 cm⁻¹, g = 2.16 for 3; J = −89.92 cm⁻¹, g = 2.25 for 4. The EPR data at low temperature indicate that the phenomenon of spin frustration occurs for complexes 1-3.     

Magneto-structural relationships for a mononuclear Co(II) complex with large zero-field splitting

A mononuclear cobalt(II) complex, [Co(ac)₂(H₂O)₂(MeIm)₂], with heteroleptic coordination sphere possessing the {CoO₂O′₂N₂} chromophore has been prepared and structurally characterized. The magnetic data down to 2 K show an enhanced magnetic anisotropy manifesting itself in a large zero-field splitting (ZFS) parameter. As a consequence, the magnetization deviates substantially from the Brillouin-function behavior. A fit to the zero-field splitting model gave the following set of magnetic parameters: D/hc = +95 cm⁻¹, g_x = 2.530, zj/hc = −0.078, χ_TIP = 16.7 × 10⁻⁹ m³ mol⁻¹, (g_z = 2.0). The Griffith-Figgis model and the Generalized Crystal-Field model lie beyond the spin-Hamiltonian formalism; they gave analogous, although not identical ZFS parameters: D/hc = 109 cm⁻¹, and D/hc = 77 cm⁻¹, respectively. The absorption spectrum taken in the FAR-IR region exhibits manifold absorption peaks referring to the transitions among the crystal-field multiplets of the parent ⁴A_2g + ⁴E_g terms (D_4h), originating in a crystal-field splitting of the octahedral ⁴T_1g ground term.     

Structure and magnetic properties of a tetranuclear Cu4O4 open-cubane in [Cu(L)]4·4H2O with L = (E)-N′-(2-oxy-3-methoxybenzylidene)benzohydrazide

The in-situ formed hydrazone Schiff base ligand (E)-N′-(2-oxy-3-methoxybenzylidene)benzohydrazide (L²⁻) reacts with copper(II) acetate to a tetranuclear open cubane [Cu(L)]₄ complex which crystallizes as two symmetry-independent (Z′ = 2) S₄-symmetrical molecules in different twofold special positions with a homodromic water tetramer. The two independent (A and B) open- or pseudo-cubanes with Cu₄O₄ cores of 4 + 2 class (Ruiz classification) each have three different magnetic exchange pathways leading to an overall antiferromagnetic coupling with J_1B = J_2B = −17.2 cm⁻¹, J_1A = −36.7 cm⁻¹, J_2A = −159 cm⁻¹, J_3A = J_3B = 33.5 cm⁻¹, g = 2.40 and ρ = 0.0687. The magnetic properties have been analysed using the H = −Σ_i,jJ_ij(S_iS_j) spin Hamiltonian.     

EPR characterisation of the triplet state in photosystem II reaction centers with singly reduced primary acceptor QA

The triplet states of photosystem II core particles from spinach were studied using time-resolved cw EPR technique at different reduction states of the iron-quinone complex of the reaction center primary electron acceptor. With doubly reduced primary acceptor, the well-known photosystem II triplet state characterised by zero-field splitting parameters |D| = 0.0286 cm⁻¹, |E| = 0.0044 cm⁻¹ was detected. When the primary acceptor was singly reduced either chemically or photochemically, a triplet state of a different spectral shape was observed, bearing the same D and E values and characteristic spin polarization pattern arising from RC radical pair recombination. The latter triplet state was strongly temperature dependent disappearing at T = 100 K, and had a much faster decay than the former one. Based on its properties, this triplet state was also ascribed to the photosystem II reaction center. A sequence of electron-transfer events in the reaction centers is proposed that explains the dependence of the triplet state properties on the reduction state of the iron-quinone primary acceptor complex.     

Hydrogencarbonate is not a tightly bound constituent of the water-oxidizing complex in photosystem II

Since the end of the 1950s hydrogencarbonate (‘bicarbonate’) is discussed as a possible cofactor of photosynthetic water-splitting, and in a recent X-ray crystallography model of photosystem II (PSII) it was displayed as a ligand of the Mn₄O_xCa cluster. Employing membrane-inlet mass spectrometry (MIMS) and isotope labelling we confirm the release of less than one (≈ 0.3) HCO₃⁻ per PSII upon addition of formate. The same amount of HCO₃⁻ release is observed upon formate addition to Mn-depleted PSII samples. This suggests that formate does not replace HCO₃⁻ from the donor side, but only from the non-heme iron at the acceptor side of PSII. The absence of a firmly bound HCO₃⁻ is corroborated by showing that a reductive destruction of the Mn₄O_xCa cluster inside the MIMS cell by NH₂OH addition does not lead to any CO₂/HCO₃⁻ release. We note that even after an essentially complete HCO₃⁻/CO₂ removal from the sample medium by extensive degassing in the MIMS cell the PSII samples retain ≥ 75% of their initial flash-induced O₂-evolving capacity. We therefore conclude that HCO₃⁻ has only ‘indirect’ effects on water-splitting in PSII, possibly by being part of a proton relay network and/or by participating in assembly and stabilization of the water-oxidizing complex.     

Primary production,respiration and calcification of the temperate free-living coralline alga Lithothamnion corallioides

Calcification and primary production responses to irradiance in the temperate coralline alga Lithothamnion corallioides were measured in summer 2004 and winter 2005 in the Bay of Brest. Coralline algae were incubated in dark and clear bottles exposed to different irradiances. Net primary production reached 1.5 μmol C g⁻¹ dry wt h⁻¹ in August and was twice as high as in January–February. Dark respiration showed significant seasonal variations, being three-fold higher in summer. Maximum calcification varied from 0.6 μmol g⁻¹ dry wt h⁻¹ in summer 2004 to 0.4 μmol g⁻¹ dry wt h⁻¹ in winter 2005. According to P–E curves and the daily course of irradiance, estimated daily net production and calcification reached 131 μg C g⁻¹ dry wt and 970 μg CaCO₃ g⁻¹ dry wt in summer 2004, and 36 μg C g⁻¹ dry wt and 336 μg CaCO₃ g⁻¹ dry wt in winter 2005. The net primary production of natural L. corallioides populations in shallow waters was estimated at 10–600 g C m⁻² y⁻¹, depending on depth and algal biomass. The mean annual calcification of L. corallioides populations varied from 300 to 3000 g CaCO₃ m⁻². These results are similar to those reported for tropical coralline algae in terms of carbon and carbonate productivity. Therefore, L. corallioides can be considered as a key element of carbon and carbonate cycles in the shallow coastal waters where they live.     

A new (μ3-carbonato)tricopper(II) complex with symmetry related equilateral triangular array of metal centers; structure and magnetism

Using an unsymmetrical N-tridentate (2-pyridyl)alkylamine ligand methyl[2-(2-pyridyl)ethyl](2-pyridylmethyl)amine (mpepma), a new tricopper(II) complex [{(mpepma)Cu(OClO₃)}₃(μ₃-OCO₂)][ClO₄] (1), having triply bridging μ₃-carbonate group, with three symmetry related triangular array of copper(II) centers has been synthesized. In 1, each copper ion is five-coordinate (almost perfect square pyramidal) by three nitrogens from the ligand (two pyridyl and an amine) and an oxygen atom of the carbonate group in the equatorial plane and by an oxygen atom of the perchlorate ion in the apical position. Magnetic susceptibility data (2-300 K) were fitted using the Hamiltonian: , which corresponds to a triangular array of spins, corrected with a zJ′ intertrimer interaction or zero-field splitting within the ground state, to fit low temperature data. It is concluded that the copper(II) ions in 1 are ferromagnetically coupled. The best parameters obtained are g = 2.11, J = 18 ± 1 cm⁻¹, zJ′ = −0.36 ± 0.02 cm⁻¹ or D = 0.50 ± 0.05 cm⁻¹.     

Structural effects on electrochemical properties of the carboxamide complexes [Ni(Mebpb)] and [Ni(Mebqb)]

Two Ni(II) complexes of the dianionic ligands, Mebpb²⁻, [H₂Mebpb = N,N′-bis(pyridine-2-carboxamido)-4-methylbenzene] and Mebqb²⁻, [H₂Mebqb = N,N′-bis(quinoline-2-carboxamido)-4-methylbenzene] have been synthesized and characterized by elemental analyses, IR, and UV-Vis spectroscopy. The crystal and molecular structures of [Ni(Mebpb)], (1), and [Ni(Mebqb)], (2), were determined by X-ray crystallography. Both complexes exhibit distorted square-planar NiN₄ coordination figures with two short and two long Ni-N bonds (Ni-N ∼1.84 and ∼1.95 Å, respectively). The electrochemical behavior of these complexes with the goal of evaluating the structural effects on the redox properties has been studied.     

Kinetic and thermodynamic properties of the folding and assembly of formate dehydrogenase

The folding mechanism and stability of dimeric formate dehydrogenase from Candida methylica was analysed by exposure to denaturing agents and to heat. Equilibrium denaturation data yielded a dissociation constant of about 10⁻¹³ M for assembly of the protein from unfolded chains and the kinetics of refolding and unfolding revealed that the overall process comprises two steps. In the first step a marginally stable folded monomeric state is formed at a rate (k₁) of about 2 × 10⁻³ s⁻¹ (by deduction k₋₁ is about10⁻⁴ s⁻¹) and assembles into the active dimeric state with a bimolecular rate constant (k₂) of about 2 × 10⁴ M⁻¹ s⁻¹. The rate of dissociation of the dimeric state in physiological conditions is extremely slow (k₋₂ ∼ 3 × 10⁻⁷ s⁻¹).