Mixed-valency with cyanides as terminal ligands: Diruthenium(III,II) complexes with the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine bridge and variable co-ligands (CN vs. bpy or NH3)

New diruthenium complexes (PPN)₄[(NC)₄Ru(μ-bptz)Ru(CN)₄], (PPN)₄1, and [(bpy)₂Ru(μ-bptz)Ru(CN)₄], 2, (PPN⁺ = bis(triphenylphospine)iminium; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; bpy = 2,2′-bipyridine), were synthesised and characterised by spectroscopic and electrochemical techniques. The comproportionation constant K_c = 10^7.0 of the mixed-valent species [(NC)₄Ru(μ-bptz)Ru(CN)₄]³⁻ as obtained by oxidation of 1⁴⁻ in CH₃CN is much lower than the K_c = 10^15.0 previously detected for [(H₃N)₄Ru(bptz)Ru(NH₃)₄]⁵⁺, reflecting the competition between CN⁻ and bptz for the π-electron density of the metals. Comparison with several other bptz-bridged diruthenium(II,III) complexes reveals an approximate correlation between K_c and the diminishing effective π acceptor capacity of the ancillary terminal ligands. In addition to the intense MLCT absorption at λ_max = 624 nm, the main IVCT (intervalence charge transfer) band of 1³⁻ was detected by spectroelectrochemistry at λ_max = 1695 nm (in CH₃CN; ε = 3200 M⁻¹ cm⁻¹). The experimental band width at half-height, Δν_1/2 = 2700 cm⁻¹, is slightly smaller than the theoretical value Δν_1/2 = 3660 cm⁻¹, calculated from the Hush approximation for Class II mixed-valent species. In agreement with comparatively moderate metal-metal coupling, the mixed-valent intermediate 1³⁻ was found to be EPR silent even at 4 K. The unsymmetrical mixed-valent complex [(bpy)₂Ru^II(μ-bptz)Ru^III(CN)₄]⁺, obtained in situ by bromine oxidation of 2 in CH₃CN/H₂O, displays a broad NIR absorption originating from an IVCT transition at λ_max = 1075 nm (ε ≈ 1000 M⁻¹ cm⁻¹, Δν_1/2 ≈ 4000 cm⁻¹). In addition, the lifetime of the excited-state of the mononuclear precursor complex [Ru(bptz)(CN)₄]²⁻ was measured in H₂O by laser flash photolysis; the obtained value of τ = 19.6 ns reveals that bptz induces a metal-to-ligand electronic delocalisation effect intermediate between that induced by bpy and bpz (bpz = 2,2′-bipyrazine) in analogous tetracyanoruthenium complexes.     

Effect of high external pressures on the vibrational spectra of crystalline dichloro(1,5-cycloctadiene)platinum(II), Pt(η-C8H12)Cl2

The effects of high external pressures on the principal IR and Raman bands of crystalline dichloro(1,5-cycloctadiene)platinum(II), Pt(COD)Cl₂ (COD = η⁴-C₈H₁₂), have been investigated for pressures up to ∼30 kbar by diamond-anvil cell microspectroscopy. This square-planar Pt(II) complex does not undergo any pressure-induced structural change throughout the pressure range investigated and the pressure dependences (dν/dP) for the vibrational modes range from −0.18 to 0.79 cm⁻¹ kbar⁻¹. The negative dν/dP value observed for the IR band at 1426 cm⁻¹ (−0.18 cm⁻¹ kbar⁻¹) suggests that this band is chiefly associated with a CC stretching mode of the Pt-COD group. This observation provides yet another indication that high-pressure vibrational spectroscopy may indeed be a general method for establishing the presence of π-backbonding in organometallic complexes, such as metal carbonyls and alkenes.     

Metal-carboxylate interactions in metal-alginate complexes studied with FTIR spectroscopy

FTIR spectroscopy was used in order to obtain information about metal-carboxylate interactions in metal-alginate complexes of alginic acid and sodium alginate from the brown algae Laminaria digitata after crosslinking with Ca²⁺, Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺ and Pb²⁺. From the frequencies of the characteristic peaks for asymmetric COO stretching vibration (ν_asym(COO⁻) and symmetric COO stretching vibration (ν_sym(COO⁻)) a ‘pseudo bridged’ unidentate coordination with intermolecular hydrogen bonds is proposed for the metal-carboxylate complexes in polyguluronic regions while for the polymannuronic regions the bidentate bridging coordination was proposed. The PIB factor introduced previously as a relationship between metal sorption and frequencies of the asymmetric vibrations was found not to correlate with sorption capacity or any other physical property of the metal-alginate complexes studied.     

Structural features and bioactivities of the chitosan

Fourier transform infrared (FT-IR) spectroscopic studies (3500-600 cm⁻¹) showed some different bands of chitosan. The absorption at 3439 cm⁻¹ is stretching vibration of -OH and -NH₂ bonds, indicating the association of the hydrogen-bond between them. The bands at 1659, 1599 and 1321 cm⁻¹ are attributable to the peaks of stretching vibrations of amide I (ν_(CO)), II (δ_(N-H)), and the peak of stretching and bending vibrations of III (ν_(C-N)) (δ_(N-H)). The chitosan showed strong free radical scavenging activities. Pretreatment with chitosan significantly prevented the decrease of antioxidant enzymes activities and the increase of p-JNK at 3 h after renal ischemia and reduced renal tubular epithelial cell apoptosis.     

Investigations of vibrational coherence in the low-frequency region of ferric heme proteins

Femtosecond coherence spectroscopy is applied to a series of ferric heme protein samples. The low-frequency vibrational spectra that are revealed show dominant oscillations near 40 cm⁻¹. MbCN is taken as a typical example of a histidine-ligated, six-coordinate, ferric heme and a comprehensive spectroscopic analysis is carried out. The results of this analysis reveal a new heme photoproduct species, absorbing near 418 nm, which is consistent with the photolysis of the His⁹³ axial ligand. The photoproduct undergoes subsequent rebinding/recovery with a time constant of ∼4 ps. The photoproduct lineshapes are consistent with a photolysis quantum yield of 75-100%, although the observation of a relatively strong six-coordinate heme coherence near 252 cm⁻¹ (assigned to ν₉ in the MbCN Raman spectrum) suggests that the 75% lower limit is much more likely. The phase and amplitude excitation profiles of the low-frequency mode at 40 cm⁻¹ suggest that this mode is strongly coupled to the MbCN photoproduct species and it is assigned to the doming mode of the transient penta-coordinated material. The absolute phase of the 40 cm⁻¹ mode is found to be π/2 on the red side of 418 nm and it jumps to 3π/2 as excitation is tuned to the blue side of 418 nm. The absolute phase of the 40 cm⁻¹ signal is not explained by the standard theory for resonant impulsive stimulated Raman scattering. New mechanisms that give a dominant momentum impulse to the resonant wavepacket, rather than a coordinate displacement, are discussed. The possibilities of heme iron atom recoil after photolysis, as well as ultrafast nonradiative decay, are explored as potential ways to generate the strong momentum impulse needed to understand the phase properties of the 40 cm⁻¹ mode.     

Development of cesium phosphotungstate salt and chitosan composite membrane for direct methanol fuel cells

A novel composite membrane has been developed by doping cesium phosphotungstate salt (Cs_xH_3−xPW₁₂O₄₀ (0 ≤ x ≤3), Cs_x-PTA) into chitosan (CTS/Cs_x-PTA) for application in direct methanol fuel cells (DMFCs). Uniform distribution of Cs_x-PTA nanoparticles has been achieved in the chitosan matrix. The proton conductivity of the composite membrane is significantly affected by the Cs_x-PTA content in the composite membrane as well as the Cs substitution in PTA. The highest proton conductivity for the CTS/Cs_x-PTA membranes was obtained with x = 2 and Cs₂-PTA content of 5 wt%. The value is 6 × 10⁻³ S cm⁻¹ and 1.75 × 10⁻² S cm⁻¹ at 298 K and 353 K, respectively. The methanol permeability of CTS/Cs₂-PTA membrane is about 5.6 × 10⁻⁷, 90% lower than that of Nafion-212 membrane. The highest selectivity factor (φ) was obtained on CTS/Cs₂-PTA-5 wt% composite membrane, 1.1 × 10⁴/S cm⁻³ s. The present study indicates the promising potential of CTS/Cs_x-PTA composite membrane as alternative proton exchange membranes in direct methanol fuel cells.     

Reaction of platinum(II) diamine and triamine complexes with selenomethionine

Four new coordination polymers namely {[Mn₂(BT)(DPS)₂(H₂O)₆]·10H₂O}_n (MnBTDPS), {[Co₂(BT)(DPS)₂(H₂O)₆]·10H₂O}_n (CoBTDPS), {[Cu₂(BT)(DPS)(H₂O)₄]·5H₂O}_n (CuBTDPS) and {[Zn₂(BT)(DPS)₂]·6H₂O}_n (ZnBTDPS), where BT = 1,2,4,5-benzenetetracarboxylate and DPS = di(4-pyridyl) sulfide, were synthesized and characterized by thermal analysis, vibrational spectroscopy (Raman and infrared) and single crystal X-ray diffraction analysis. In all compounds, the DPS ligands are coordinated to metal sites in a bridging mode and the carboxylate moiety of BT ligands adopts a monodentate coordination mode, as indicated by the Raman spectra data through the Δν (ν_asym(COO) − ν_sym(COO)) value. According to X-ray diffraction analysis, MnBTDPS and CoBTDPS are isostructural and in these cases, the metal centers exhibit a distorted octahedral geometry. In CuBTBPP, the Cu²⁺ centers geometries are best described as square-pyramids, according to the trigonality index τ = 0.14 for Cu1 and τ = 0.10 for Cu2. On the other hand, in ZnBTDPS, the Zn²⁺ sites adopt a tetrahedral geometry. Finally, the four compounds formed two-dimensional sheets that are connected to each other through hydrogen bonding giving rise to three-dimensional supramolecular arrays.     

Determination of the excitation migration time in Photosystem II: Consequences for the membrane organization and charge separation parameters

The fluorescence decay kinetics of Photosystem II (PSII) membranes from spinach with open reaction centers (RCs), were compared after exciting at 420 and 484 nm. These wavelengths lead to preferential excitation of chlorophyll (Chl) a and Chl b, respectively, which causes different initial excited-state populations in the inner and outer antenna system. The non-exponential fluorescence decay appears to be 4.3 ± 1.8 ps slower upon 484 nm excitation for preparations that contain on average 2.45 LHCII (light-harvesting complex II) trimers per reaction center. Using a recently introduced coarse-grained model it can be concluded that the average migration time of an electronic excitation towards the RC contributes ~ 23% to the overall average trapping time. The migration time appears to be approximately two times faster than expected based on previous ultrafast transient absorption and fluorescence measurements. It is concluded that excitation energy transfer in PSII follows specific energy transfer pathways that require an optimized organization of the antenna complexes with respect to each other. Within the context of the coarse-grained model it can be calculated that the rate of primary charge separation of the RC is (5.5 ± 0.4 ps)^− 1, the rate of secondary charge separation is (137 ± 5 ps)^− 1 and the drop in free energy upon primary charge separation is 826 ± 30 cm^− 1. These parameters are in rather good agreement with recently published results on isolated core complexes [Y. Miloslavina, M. Szczepaniak, M.G. Muller, J. Sander, M. Nowaczyk, M. Rögner, A.R. Holzwarth, Charge separation kinetics in intact Photosystem II core particles is trap-limited. A picosecond fluorescence study, Biochemistry 45 (2006) 2436-2442].     

Characterization of the primary photochemistry of proteorhodopsin with femtosecond spectroscopy

Proteorhodopsin is an ion-translocating member of the microbial rhodopsin family. Light absorption by its retinal chromophore initiates a photocycle, driven by trans/cis isomerization, leading to transmembrane translocation of a proton toward the extracellular side of the cytoplasmic membrane. Here we report a study on the photoisomerization dynamics of the retinal chromophore of proteorhodopsin, using femtosecond time-resolved spectroscopy, by probing in the visible- and in the midinfrared spectral regions. Experiments were performed both at pH 9.5 (a physiologically relevant pH value in which the primary proton acceptor of the protonated Schiff base, Asp⁹⁷, is deprotonated) and at pH 6.5 (with Asp⁹⁷ protonated). Simultaneous analysis of the data sets recorded in the two spectral regions and at both pH values reveals a multiexponential excited state decay, with time constants of ∼0.2 ps, ∼2 ps, and ∼20 ps. From the difference spectra associated with these dynamics, we conclude that there are two chromophore-isomerizaton pathways that lead to the K-state: one with an effective rate of ∼(2 ps)⁻¹ and the other with a rate of ∼(20 ps)⁻¹. At high pH, both pathways are equally effective, with an estimated quantum yield for K-formation of ∼0.7. At pH 6.5, the slower pathway is less productive, which results in an isomerization quantum yield of 0.5. We further observe an ultrafast response of residue Asp²²⁷, which forms part of the counterion complex, corresponding to a strengthening of its hydrogen bond with the Schiff base on K-state formation; and a feature that develops on the 0.2 ps and 2 ps timescale and probably reflects a response of an amide II band in reaction to the isomerization process.     

Non-covalent interactions in blue copper protein probed by Met16 mutation and electronic and resonance Raman spectroscopy of Achromobacter cycloclastes pseudoazurin

We have used low-temperature (77 K) resonance Raman (RR) spectroscopy as a probe of the electronic and molecular structure to investigate weak π-π interactions between the metal ion-coordinated His imidazoles and aromatic side chains in the second coordination sphere of blue copper proteins. For this purpose, the RR spectra of Met16 mutants of Achromobacter cycloclastes pseudoazurin (AcPAz) with aromatic (Met16Tyr, Met16Trp, and Met16Phe) and aliphatic (Met16Ala, Met16Val, Met16Leu, and Met16Ile) amino acid side chains have been obtained and analyzed over the 100-500 cm⁻¹ spectral region. Subtle strengthening of the Cu(II)-S(Cys) interaction on replacing Met16 with Tyr, Trp, and Phe is indicated by the upshifted (0.3-0.8 cm⁻¹) RR bands involving ν(Cu-S)_Cys stretching modes. In contrast, the RR spectra of Met16 mutants with aliphatic amino acids revealed larger (0.2-1.8 cm⁻¹) shifts of the ν(Cu-S)_Cys stretching modes to a lower frequency region, which indicate a weakening of the Cu(II)-S(Cys) bond. Comparisons of the predominantly ν(Cu-S)_Cys stretching RR peaks of the Met16X = Tyr, Trp, and Phe variants, with the molar absorptivity ratio ε₁/ε₂ of σ(∼455 nm)/π(∼595 nm) (Cys)S → Cu(II) charge-transfer bands in the optical spectrum and the axial/rhombic EPR signals, revealed a slightly more trigonal disposition of ligands about the copper(II) ion. In contrast, the RR spectra of Met16Z = Ala, Val, Leu, and Ile variants with aliphatic amino acid side chains show a more tetrahedral perturbation of the copper active site, as judged by the lower frequencies of the ν(Cu-S)_Cys stretching modes, much larger values of the ε₁/ε₂ ratio, and the increased rhombicity of the EPR spectra.     

Electronic spectrum and magnetic properties of a dinuclear nickel(II) complex with two nickel(II) ions of C2-twisted octahedral geometry

A dinickel(II) complex [Ni₂(sym-hmp)₂](BPh₄)₂·3.5DMF·0.5(2-PrOH) (1) was synthesized with a dinucleating ligand, 2,6-bis[(2-hydroxyethyl)methylaminomethyl]-4-methyl-phenol [H(sym-hmp)]. The complex 1 (C₉₀H_118.50B₂N_7.50Ni₂O₁₀) crystallized in the triclinic space group with dimensions a = 14.7446(4) Å, b = 15.4244(4) Å, c = 18.7385(6) Å, α = 86.9495(9)°, β = 76.7263(10)°, γ = 86.5370(8)°, and V = 4136.8(2) Å³ and with Z = 2; this is isomorphous to a previous cobalt(II) complex [Co₂(sym-hmp)₂](BPh₄)₂. Single-crystal X-ray analysis revealed a bis(μ-phenoxo)dinickel(II) core structure containing two distorted octahedral nickel(II) ions of C₂ symmetry. The order of the coordination bond lengths is Ni-O(phenoxo) < Ni-O(hydroxy) < Ni-N. The electronic spectrum of 1 was typical for the octahedral nickel(II) complexes, but the axial elongation and the C₂-twist of the equatorial plane were found after a detailed analysis. The bond angles obtained by the electronic spectrum agreed with the crystallographically obtained bond angles within 2.3°. The order of the AOM parameters was e_{σ,O(phenoxo)} > e_{σ,O(hydroxy)} > e_σ,N, which was consistent with the order of the coordination bond lengths. Magnetic susceptibility data for 1 were fitted well with the parameters 2J = −69.7 cm⁻¹, D = 0.00 cm⁻¹, g = 2.17, and TIP = 265 × 10⁻⁶ cm³ mol⁻¹. The result indicates significant antiferromagnetic exchange interaction and negligible zero-field splitting, while the isostructural cobalt(II) complex showed an anisotropic behavior.     

Formation and decay of P680 (PD1–PD2)PheoD1 radical ion pair in photosystem II core complexes

Under physiological conditions (278 K) femtosecond pump-probe laser spectroscopy with 20-fs time resolution was applied to study primary charge separation in spinach photosystem II (PSII) core complexes excited at 710 nm. It was shown that initial formation of anion radical band of pheophytin molecule (Pheo⁻) at 460 nm is observed with rise time of ~ 11 ps. The kinetics of the observed rise was ascribed to charge separation between Chl (chlorophyll a) dimer, primary electron donor in PSII (P680*) and Pheo located in D1 protein subunit (Pheo_D1) absorbing at 420 nm, 545 nm and 680 nm with formation of the ion-radical pair P680⁺Pheo_DI⁻. The subsequent electron transfer from Pheo_D1⁻ to primary plastoquinone electron acceptor (Q_A) was accompanied by relaxation of the 460-nm band and occurred within ~ 250 ps in good agreement with previous measurements in Photosystem II-enriched particles and bacterial reaction centers. The subtraction of the P680⁺ spectrum measured at 455 ps delay from the spectra at 23 ps or 44 ps delay reveals the spectrum of Pheo_DI⁻, which is very similar to that measured earlier by accumulation method. The spectrum of Pheo_DI⁻ formation includes a bleaching (or red shift) of the 670 nm band indicating that Chl-670 is close to Pheo_D1. According to previous measurements in the femtosecond–picosecond time range this Chl-670 was ascribed to Chl_D1 [Shelaev, Gostev, Vishnev, Shkuropatov, Ptushenko, Mamedov, Sarkisov, Nadtochenko, Semenov and Shuvalov, J. Photochemistry and Photobiology, B: Biology 104 (2011) 45–50]. Stimulated emission at 685 nm was found to have two decaying components with time constants of ~ 1 ps and ~ 14 ps. These components appear to reflect formation of P680⁺Chl_D1⁻ and P680⁺Pheo_D1⁻, respectively, as found earlier. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

Syntheses, structures and vibrational spectroscopy of some unusual silver(I) (pseudo-) halide/unidentate nitrogen base polymers

The meagre (structurally defined) array of 1:2 silver(I) (pseudo-)halide:unidentate nitrogen base adducts is augmented by the single-crystal X-ray structural characterization of the 1:2 silver(I) thiocyanate:piperidine (‘pip’) adduct. It is of the one-dimensional ‘castellated polymer’ type previously recorded for the chloride: ?Ag(pip)₂(μ-SCN)Ag(pip)₂? a single bridging atom (S) linking successive silver atoms. By contrast, in its copper(I) counterpart, also a one-dimensional polymer, the thiocyanate bridges as end-bound SN-ambidentate: ?CuSCNCuSCN? A study of the 1:1 silver(I) bromide:quinoline (‘quin’) adduct is recorded, as the 0.25 quin solvate, isomorphous with its previous reported ‘saddle polymer’ chloride counterpart.Recrystallization of 1:1 silver(I) iodide:tris(2,4,6-trimethoxyphenyl)phosphine (‘tmpp’) mixtures from py and quinoline (‘quin’)/acetonitrile solutions has yielded crystalline materials which have also been characterized by X-ray studies. In both cases the products are salts, the cation in each being the linearly coordinated silver(I) species [Ag(tmpp)₂]⁺, while the anions are, respectively, the discrete [Ag₅I₇(py)₂]²⁻ species, based on the already known but unsolvated [Cu₅I₇]²⁻ discrete, and the polymeric, arrays, and polymeric . The detailed stereochemistry of the [Ag(tmpp)₂]⁺ cation is a remarkably constant feature of all structures, as is its tendency to close-pack in sheets normal to their P-Ag-P axes.The far-IR spectra of the above species and of several related complexes have been recorded and assigned. The vibrational modes of the single stranded polymeric AgX chains in [XAg(pip)₂]_(∞|∞) (X = Cl, SCN) are discussed, and the assignments ν(AgX) = 155, 190 cm⁻¹ (X = Cl) and 208 cm⁻¹ (X = SCN) are made. The ν(AgX) and ν(AgN) modes in the cubane tetramers [XAg(pip)]₄ (X = Br, I) are assigned and discussed in relation to the assignments for the polymeric AgX:pip (1:2) complexes, and those for the polymeric [XAg(quin)]_(∞|∞) (X = Cl, Br) compounds. The far-IR spectra of [Ag(tmpp)₂]₂[Ag₅I₇(py)₂] and its corresponding 2-methylpyridine complex show a single strong band at about 420 cm⁻¹ which is assigned to the coordinated tmpp ligand in [Ag(tmpp)₂]⁺, and a partially resolved triplet at about 90, 110 and 140 cm⁻¹ which is assigned to the ν(AgI) modes of the [Ag₅I₇L₂]²⁻ anion. An analysis of this pattern is given using a model which has been used previously to account for unexpectedly simple ν(CuI) spectra for oligomeric iodocuprate(I) species.     

A high-pressure, Raman spectroscopic study of bis(pyrophosphato)tetrazinc(II) and -tetramanganese(II) decahydrate salts

The Raman spectra of crystalline bis(pyrophosphato)tetrazinc(II) and -manganese(II) decahydrates, M₄(P₂O₇)₂ · 10H₂O (M = Zn and Mn), have been investigated at room temperature. The Mn(II) salt readily underwent photodecomposition upon extended exposure to 785-nm laser irradiation and so could not be examined under high pressures in a diamond-anvil cell. The Zn(II) salt was much more stable and was studied successfully at pressures up to ∼35 kbar. The pressure dependences of the δ_sPO₂, ν_sPOP, ν_asPOP, ν_sPO₂, and ν_asPO₂ internal modes of the pyrophosphate group and of an external H₂O mode indicated the occurrence of a pressure-induced phase transition in the 15-20 kbar region. Further confirmation of this structural change came from the appearance of four initially forbidden modes in the 19-26 kbar range. This phase transition proved to be reversible upon release of the applied pressure. The pressure dependences before and after the phase transition were −0.22 to 0.66 and 0.06 to 0.33 cm⁻¹/kbar, respectively.     

Hydrolysis of the soluble fluorescent molecule carboxyumbelliferyl-beta-D-glucuronide by E. coli beta-glucuronidase as applied in a rugged, in situ optical sensor

Techniques utilizing β-glucuronidase (GUS) activity as an indicator of Escherichia coli (E. coli) presence use labeled glucuronides to produce optical signals. Carboxyumbelliferyl-β-d-glucuronide (CUGlcU) is a fluorescent labeled glucuronide that is soluble and highly fluorescent at natural water pHs and temperatures and, therefore, may be an ideal reagent for use in an in situ optical sensor. This paper reports for the first time the Michaelis-Menten kinetic parameters for the binding of E. coli GUS with CUGlcU as K_m = 910 μM, V_max = 41.0 μM min⁻¹, V_max/K_m 45.0 μmol L⁻¹ min⁻¹, the optimal pH as 6.5 ± 1.0, optimal temperature as 38 °C, and the Gibb's free energy of activation as 61.40 kJ mol⁻¹. Additionally, it was found CUGlcU hydrolysis is not significantly affected by heavy solvents suggesting proton transfer and solvent addition that occur during hydrolysis are not limiting steps. Comparison studies were made with the more common fluorescent molecule methylumbelliferyl-β-d-glucuronide (MUGlcU). Experiments showed GUS preferentially binds to MUGlcU in comparison to CUGlcU. CUGlcU was also demonstrated in a prototype optical sensor for the detection of E. coli. Initial bench testing of the sensor produced detection of low concentrations of E. coli (1.00 × 10³ CFU/100 mL) in 230 ± 15.1 min and high concentrations (1.05 × 10⁵ CFU/100 mL) in 8.00 ± 1.01 min.     

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.     

Synthesis, structure, and magnetic properties of dinuclear nickel(II) complexes with a phenol-based dinucleating ligand with four methoxyethyl chelating arms

Dinuclear nickel(II) complexes [Ni₂(bomp)(MeCO₂)₂]BPh₄ (1) and [Ni₂(bomp)(PhCO₂)₂]BPh₄ (2) were synthesized with the dinucleating ligand 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)]. X-Ray analysis revealed that the complex 1 · 0.5CHCl₃ contains two nickel(II) ions bridged by phenolic oxygen and two acetate groups, forming a μ-phenoxo-bis(μ-acetato)dinickel(II) core. Electronic spectra were investigated for 1 and 2 in the range of 400-1800 nm, and the data were typical for the octahedral high-spin nickel(II) complexes. Obtained spectral components were well simulated based on the angular overlap model assuming the trigonally distorted octahedral geometry. Magnetic susceptibility was measured for 1 and 2 over a temperature range of 4.5-300 K. The optimized magnetic data were J = 1.75 cm⁻¹, zJ′ = −0.234 cm⁻¹, g = 2.21, D = 15.1 cm⁻¹, and TIP = 370 × 10⁻⁶ cm⁻¹ for complex 1 and J = 3.55 cm⁻¹, zJ′ = −0.238 cm⁻¹, g = 2.23, D = 21.8 cm⁻¹, and TIP = 470 × 10⁻⁶ cm⁻¹ for complex 2. The data revealed ferromagnetic interactions between the two nickel(II) ions.     

ESR study of sublevel structure of anionic bis(phthalocyaninato)gadolinium(III)

Sublevel structure of the ⁸S_7/2 electronic ground state of anionic bis(phthalocyaninato)gadolinium(III) has been determined by simulation analysis of an ESR spectrum in frozen solution. The simplex multidimensional minimization algorithm was employed to find the zero-field-splitting parameter set giving the minimum RMS error from the observed spectrum. The parameter set {B₂⁰,B₄⁰,B₆⁰} for the potential of D_4d symmetry has been determined to be ±{(1.54 ± 0.01)× 10⁻² cm⁻¹, (0.9 ± 0.1)× 10⁻⁴ cm⁻¹, (−0.6 ± 0.9)× 10⁻⁶ cm⁻¹}. The energy difference between the lowest and highest sublevels has been found to be about 0.5 cm⁻¹.     

Kinetic and Vibrational Isotope Effects of Proton Transfer Reactions in Channelrhodopsin-2

Channelrhodopsins (ChRs) are light-gated cation channels. After blue-light excitation, the protein undergoes a photocycle with different intermediates. Here, we have recorded transient absorbance changes of ChR2 from Chlamydomonas reinhardtii in the visible and infrared regions with nanosecond time resolution, the latter being accomplished using tunable quantum cascade lasers. Because proton transfer reactions play a key role in channel gating, we determined vibrational as well as kinetic isotope effects (VIEs and KIEs) of carboxylic groups of various key aspartic and glutamic acid residues by monitoring their C=O stretching vibrations in H₂O and in D₂O. D156 exhibits a substantial KIE (>2) in its deprotonation and reprotonation, which substantiates its role as the internal proton donor to the retinal Schiff base. The unusual VIE of D156, upshifted from 1736 cm⁻¹ to 1738 cm⁻¹ in D₂O, was scrutinized by studying the D156E variant. The C=O stretch of E156 shifted down by 8 cm⁻¹ in D₂O, providing evidence for the accessibility of the carboxylic group. The C=O stretching band of E90 exhibits a VIE of 9 cm⁻¹ and a KIE of ∼2 for the de- and the reprotonation reactions during the lifetime of the late desensitized state. The KIE of 1 determined in the time range from 20 ns to 5 ms is incompatible with early deprotonation of E90.     

Crystal structure, optical, magnetic, and photochemical properties of the complex pentakis(dimethyl sulfoxide)nitrosylchromium(2+) hexafluorophosphate

The nitrosyl complex [Cr(dmso)₅(NO)](PF₆)₂ (1) (dmso = dimethyl sulfoxide) has been prepared by the solvolysis of [Cr(NCCH₃)₅(NO)](PF₆)₂ in neat dmso. The optical absorption spectrum of 1 in dmso shows maxima at 734, 567, 450, 413, and 337 nm. Continuous photolysis of 1 with λ = 365-580 nm light in dmso solution results in a release of NO with quantum yield, Φ, in the range 0.034-0.108 mol Einstein⁻¹. Irradiation of a deoxygenated CH₃CN solution of [Cr(NCCH₃)₅(NO)](PF₆)₂ in the presence of excess of [Fe(S₂CNEt₂)₂] results in a transfer of NO to the iron centre as shown from the characteristic EPR spectrum of [Fe(S₂CNEt₂)₂(NO)] with A_iso(¹⁴N) = 12.2 × 10⁻⁴ cm⁻¹. The EPR parameters of 1 were determined: g_iso, g_‖ and g_⊥ : 1.96725, 1.91881(4) and 1.992763(2); A_iso(⁵³Cr), A_‖ (⁵³Cr) and A_⊥(⁵³Cr): 22.8 × 10⁻⁴, 39 × 10⁻⁴ and 15.8 × 10⁻⁴ cm⁻¹; A_iso(¹⁴N), A_‖ (¹⁴N) and A_⊥(¹⁴N): 5.9 × 10⁻⁴, 2 × 10⁻⁴ and 7.540(4) × 10⁻⁴ cm⁻¹.