Combining Medium Effects and Cofactor Catalysis: Metal‐Coordinated Synzymes Accelerate Phosphate Transfer by 108

The systematic exploration of the modification of polyethylene imine with guanidinium and octyl groups has led to the identification of a catalyst, CD6, which accelerates the phosphate transfer reaction of HPNP (2‐hydroxypropyl‐4‐nitrophenyl phosphate) in the presence of divalent metals such as Zn²⁺, Co²⁺, Mg²⁺ or Ni²⁺. CD6 exhibits saturation kinetics that are described by Michaelis–Menten parameters K_m ranging from 2.5–8 mM and k_cat ranging from 0.0014–0.09 s^?1. For Zn^II–CD6 this corresponds to an overall acceleration k_cat/k_uncat of 3.8×10⁵ and a catalytic proficiency (k_cat/K_m)/k_uncat of 1.5×10⁸. Catalysis by Zn^II–CD6 is specifically inhibited by inorganic phosphate, allowing turnover regulation by product inhibition. This effect stands in contrast to Zn^II‐catalysed transesterification of HPNP in water or by the synzymes Co^II–CD6 and Ni^II–CD6, with which no such interference by product is observed. These characteristics render synzyme Zn^II–CD6 an efficient enzyme model that reflects enzyme‐like properties in a wide range of features.     

KINETICS OF FORMATION AND STABILITY CONSTANTS OF MONO AND BIS l-TYROSINE COMPLEXES OF Cu(II), Co(II), AND Ni(II)

Abstract

The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)^(3-n)+ _n-1 + HL- ? M(HL)^(2-n)+_n(k_n, forward rate constant; k_-n, reverse rate constant); where M=Cu, Co or Ni, HL^? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (K_n=k_n/k_-n) of the mono and bis complexes of Cu²⁺, Co²⁺ and Ni²⁺ with l-tyrosine, determined by potentiometric pH titration are: Cu²⁺, log K₁=7.90 ± 0.02, log K₂=7.27 ± 0.03; Co²⁺, log K₁=4.05 ± 0.02, log K₂=3.78 ± 0.04; Ni²⁺, log K₁=5.14 ± 0.02, log K₂=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu²⁺, k₁=(1.1 ± 0.1) × 10⁹ M ^?1 sec^?1, k_-1=(14 ± 3) sec^?1, k₂=(3.1 ± 0.6) × 10⁸ M ^?1 sec^?1, k^?2=(16 ± 4) sec^?1; Co²⁺, k₁=(1.3 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-1=(1.1 ± 0.2) × 10² sec^?1, k₂=(1.5 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-2=(2.5 ± 0.6) × 10² sec^?1; Ni²⁺, k₁=(1.4 ± 0.2) × 10⁴ M ^?1 sec^?1, k_-1=(0.10 ± 0.02) sec^?1, k₂=(2.4 ± 0.3) × 10⁴ M ^?1 sec^?1, k_-2=(0.94 ± 0.17) sec^?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.     

Fast evaluation of enantioselective drug metabolism by electrophoretically mediated microanalysis: Application to fluoxetine metabolism by CYP2D6

In this work, a capillary electrophoretic methodology for the enantioselective in vitro evaluation of drugs metabolism is applied to the evaluation of fluoxetine (FLX) metabolism by cytochrome 2D6 (CYP2D6). This methodology comprises the in‐capillary enzymatic reaction and the chiral separation of FLX and its major metabolite, norfluoxetine enantiomers employing highly sulfated β‐CD and the partial filling technique. The methodology employed in this work is a fast way to obtain a first approach of the enantioselective in vitro metabolism of racemic drugs, with the additional advantage of an extremely low consumption of enzymes, CDs and all the reagents involved in the process. Michaelis–Menten kinetic parameters (K_m and V_max) for the metabolism of FLX enantiomers by CYP2D6 have been estimated by nonlinear fitting of experimental data to the Michaelis–Menten equation. K_m values have been found to be 30 ± 3 μM for S‐FLX and 39 ± 5 μM for R‐FLX. V_max estimations were 28.6 ± 1.2 and 34 ± 2 pmol·min^?1·(pmol CYP)^?1 for S‐ and R‐FLX, respectively. Similar results were obtained using a single enantiomer (R‐FLX), indicating that the use of the racemate is a good option for obtaining enantioselective estimations. The results obtained show a slight enantioselectivity in favor of R‐FLX.     

Determination of Hg2+ in Tap Water Based on the Electrochemiluminescence of Ru(phen)32+ and Thymine at Bare and Graphene Oxide‐Modified Glassy Carbon Electrodes

The electrochemiluminescence (ECL) of the Ru(phen)₃²⁺/thymine (T) system at bare and graphene oxide (GO)‐modified glassy carbon (GC) electrodes was utilized to determine Hg²⁺ in tap water. The ECL intensity of Ru(phen)₃²⁺ was considerably enhanced by the addition of thymine because of the occurrence of ECL reaction between them. Subsequently, the ECL intensity of Ru(phen)₃²⁺/T system rapidly decreased with the addition of Hg²⁺ because of the formation of a T‐Hg²⁺‐T complex. A linear response (R²=0.9914) was obtained over a Hg²⁺ concentration range of 1.0×10^?9 mol/L to 1.0×10^?5 mol/L with a detection limit of 3.4×10^?10 mol/L at a bare GC electrode in 0.1 mol/L phosphate buffer (pH=8.0). The detection limit can be further reduced to 4.2×10^?12 mol/L after modification of the GC electrode by GO. To verify its applicability, the proposed method was utilized to determine Hg²⁺ in tap water and simulated wastewater. The method exhibited good reproducibility and stability and thus reveals the possibility of developing a novel ECL detection method for Hg²⁺.     

Application of capillary electrophoresis coupling with electrochemiluminescence detection to estimate activity of leucine aminopeptidas

A new method to estimate the leucine aminopeptidase (LAP, EC 3.4.11.1) activity using capillary electrophoresis coupled with electrochemiluminescence (ECL) is described. The liberated proline produced by LAP catalyzing the hydrolysis reaction of leucin–proline was used as an ECL coreagent to enhance Ru(bpy)₃²⁺ ECL signals efficiently. The detection limit for proline was 2.88 × 10^?6 m (signal‐to‐noise ratio 3), which was equal to 9.60 × 10^?8 units of LAP being used to catalyze leucin–proline for 1 min. The Michaelis constant K_m (2.07 × 10^?2 mol/L) and the maximum reaction velocity V_max (1.06 × 10^?5 mol/L/min) of LAP for leucin–proline are reported. The reaction conditions including the concentration of metal ions, incubation temperature and pH were optimized. This method was successfully applied to detect LAP activity in plasma and the results were in good agreement with that obtained by the clinical method. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluation of the Oxo‐bridged Dinuclear Ruthenium Ammine Complex as Redox Mediator in an Electrochemical Biosensor

The mediation of electron‐transfer by oxo‐bridged dinuclear ruthenium ammine [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ for the oxidation of glucose was investigated by cyclic voltammetry. These ruthenium (III) complexes exhibit appropriate redox potentials of 0.131–0.09 V vs. SCE to act as electron‐transfer mediators. The plot of anodic current vs. the glucose concentration was linear in the concentration range between 2.52×10^?5 and 1.00×10^?4 mol L^?1. Moreover, the apparent Michaelis‐Menten kinetic (K_M^app) and the catalytic (K_cat) constants were 8.757×10^?6 mol L^?1 and 1,956 s^?1, respectively, demonstrating the efficiency of the ruthenium dinuclear oxo‐complex [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ as mediator of redox electron‐transfer.     

Tubular Detector of Silver Solid Amalgam for Electrochemical Measurements in Flow Systems

This paper presents the application of the tubular detector based on silver solid amalgam (TD‐AgSA) for electrochemical determinations of reducible inorganic (Cd²⁺, Zn²⁺) and organic (4‐nitrophenol) compounds under flow injection analysis conditions. The newly developed TD‐AgSA is simple, robust and inexpensive. The limits of detections of Zn²⁺, Cd²⁺ and 4‐nitrophenol are 1.4×10^?6, 7.0×10^?7, and 5.0×10^?7 mol dm^?3, respectively (i.e. 0.09, 0.08 and 0.07 ppm). The obtained results proved the long‐term stability of the detector (RSD of the determination of Zn²⁺, Cd²⁺, and 4‐nitrophenol were 0.8, 0.9 and 0.8 % (n=10; c_Zn=7.7×10^?5 mol dm^?3, c_Cd=4.5×10^?5 mol dm^?3 and c_4‐NPh=3.6×10^?5 mol dm^?3), respectively and its applicability for cathodic measurements in aqueous solutions at potentials up to ?2 V.     

Formation and Dissociation of Tetrahedral Nickel(II) and Cobalt(II) Complexes of ‘Dithioimidodiphosphato’ Ligands,Measurement of Kinetically Determined Stability Constants,and Kinetic Investigations of the Rapid Formation of Tetrahedral Bis(ligand)copper(II) Complexes and Their Rates of Reduction to Trinuclear Copper(I) Species

The stopped‐flow technique was used to measure the rates of formation and dissociation of tetrahedral [ML₂] complexes (M²⁺=Ni²⁺ or Co²⁺) of four bidentate S₂‐donor ‘dithioimidodiphosphato’ ligands L^? (HL=[R¹R²P(?S)]NH[P(?S)R³R⁴], R¹ to R⁴=alkyl) at 25.0° in MeOH/H₂O 95 : 5 (v/v) solution and in the presence of either MOPS (=3‐(morpholin‐4‐yl)propane‐1‐sulfonic acid) or 2,6‐lutidine (=2,6‐dimethylpyridine) buffers. The kinetically determined equilibrium formation constants for [ML]⁺ ions (M=Ni or Co) are 10^?5 K=0.50±0.01 or 1.64±0.07 l mol^?1 for L=L³ (R¹=R²=Me(CH₂)₂CH(Me), R³=R⁴=Me₂CH), 1.27±0.02 or 7.93±0.09 l mol^?1 for L=L⁷ (R¹ to R⁴=Me₂CHCH₂), 0.88±0.04 or 3.84±0.13 l mol^?1 for L=L⁸ (R¹ to R⁴=Me₂CH), and in case of Ni²⁺ 1.88±0.04 l mol^?1 for L=L⁶ (R¹=R³=Bu, R²=R⁴=^tBu) (see Table 3; for L³ and L⁶–L⁸, see Table 1). Whereas the tetrahedral Ni²⁺ complexes dissociate more slowly than the analogous Co²⁺ species, in all cases, the Co²⁺ complexes are more stable than those of Ni²⁺ due to their larger formation rate constants (Table 3). Reactions of Cu²⁺ with eight ligands HL (R¹ to R⁴=alkyl, alkoxy, aryl, and aryloxy) show that formation of intensely colored tetrahedral [Cu^IIL₂] species is too fast be measured with the available stopped‐flow apparatus (t_1/2<2 ms), but the subsequent rates of reduction of [Cu^IIL₂] to give trinuclear products [Cu^I₃L₃] are measurable. An X‐ray analysis establishes the structure of one of the [Cu₃L₃] complexes, where R¹=R²=Me₂CHO and R³=R⁴=2‐(tert‐butyl)phenyl (L=L⁵), and a multiwavelength stopped‐flow kinetic experiment establishes the spectrum of a tetrahedral [Cu^IIL₂] species prior to the reduction reactions. The redox reactions proceed at 25.0° with first‐order rate constants in the range 0.285 s^?1 (R¹ to R⁴=PhO; L=L¹¹) to 2.58?10^?4 s^?1 (R¹ to R⁴=Me₂CHCH₂; L=L⁷) (Table 4).     

Synthesis of 1‐Methyl‐3‐aryl‐substituted Titanocene and Zirconocene Dichlorides and Crystal Structure of Bis[η5‐1‐methyl‐3‐(α, α‐dimethylbenzyl) cyclopentadienyl] titanium Dichloride

The syntheses of a series of l‐methyl‐3‐aryl‐substituted titanocene and zirconocene dichlorides are reported. These complexes are synthesized by the reaction of 2‐ and 3‐methyl‐6, 6‐dimethylfulvenes (1:4) with aryllithium, followed by the reaction with TiCl₄·2THF, ZrCl₄ and (CpTiCl₂)₂O respectively, to give complexes 1–5. The complex [η⁵‐1‐methyl‐3‐(α, α‐dimethylbenzyl) cyclopentadienyl] titanium dichloride has been studied by X‐ray diffraction. The red crystal of this complex is monoclinic, space group P2_t/C with unit cell parameters: a =6.973(6) × 10^?1 nm, b =36.91(2) × 10^?1 nm, c = 10.063(4) × 10^?1 nm, α=β= γ = 93.35(5)°, V = 2584(5) × 10^?3 nm³ and Z = 4. Refinement for 1004 observed reflections gives the final R of 0.088. There are four independent molecules per unit cell.     

Electrochemiluminescence of SDBS‐Ru(bpy)32+‐CPM System and Its Application

When the concentration of dodecyl benzene sulfonic acid sodium salt (SDBS) is 0.7 mmol·L^?1, the electrochemical and electrochemiluminescence (ECL) intensity of Ru(bpy)₃²⁺‐chlorpheniramine maleate (CPM) system at the Au electrode were studied. The results showed that compared with the absence of SDBS, enhancement of the ECL intensity was 14‐fold at Au electrode. Base on this, an ECL method was established for efficient and simple determination of CPM at Au electrode. Under the optimum experimental condition, the enhanced ECL intensities had good linear relationship with the concentration of CPM in the range of 1.0×10^?4–1.0×10^?7 mol·L^?1, and a linear regression equation was obtained as follows: I (counts)=48.805×10⁶c+394.03 (r=0.9975), the detection limit for CPM was 1.4×10^?8 mol·L^?1. The RSD for 5 times determinations of 1.0×10^?5 mol·L^?1 CPM was 3.2%. The results of recovery test were between 96.3%–102.5%, and the RSD of recovery test (n=5) was 2.7%. In addition, eleven kinds of tertiary amines‐Ru(bpy)₃²⁺ systems were investigated in the absence and presence of SDBS. The results showed that the enhancement of SDBS on ECL intensity of tertiary amines‐Ru(bpy)₃²⁺ systems was universal.     

Study on Dynamic Interfacial Tension of 3‐Dodecyloxy‐2‐hydroxypropyl Trimethyl Ammonium Chloride at n‐Decane/Water Interface

Dynamic interfacial tension (DIT) and interface adsorption kinetics at the n‐decane/water interface of 3‐dodecyloxy‐2‐hydroxypropyl trimethyl ammonium chloride (R₁₂TAC) were measured using spinning drop method. The effects of R_nTAC concentration and temperature on DIT have been investigated, the reason of the change of DIT with time has been discussed. The effective diffusion coefficient, D _a, and the adsorption barrier, ?_a, have been obtained with extended Word‐Tordai equation. The results show that the higher the concentration of surfactants is, and the smaller will be the DIT and the lower will be the curve of the DIT, and the R₁₂TAC solutions follow a mixed diffusion‐activation adsorption mechanism in this investigation. With increase of concentration in bulk solution of R₁₂TAC from 8×10^?4 mol · dm^?3 to 4×10^?3 mol · dm^?3, D _a decreases from 2.02×10^?10 m^?2 · s^?1 to 1.4×10^?11 m^?2 · s^?1 and ? _a increases from 2.60 kJ · mol^?1 to 9.32 kJ · mol^?1, while with increase of temperature from 30°C to 50°C, D _a increases from 2.02×10^?10 m^?2 · s^?1 to 5.86×10^?10 m^?2 · s^?1 and ε_a decreases from 2.60 kJ · mol^?1 to 0.73 kJ · mol^?1. This indicates that the diffusion tendency becomes weak with increase strength of the interaction between surfactant molecules and that the thermo‐motion of molecules favors interface adsorption.     

[CH3NH3]4Ga4SbS9S0.28O0.72H: A Three‐Dimensionally Open‐Framework Heterometallic Chalcogenidoantimonate Exhibiting Ni2+ Ion‐Exchange Property

An open‐framework chalcogenidoantimonate, namely, [CH₃NH₃]₄Ga₄SbS₉S_0.28O_0.72H ( 1 ), has been solvothermally synthesized and structurally characterized. Interestingly, 1 showed Ni²⁺ ion‐exchange properties and wide pH resistance, with a maximum exchange capacity of 76.9 mg g^?1. To the best of our knowledge, this is the first example of amine‐directed three‐dimensional (3D) heterometallic chalcogenidometalates for highly selective Ni²⁺ ion capture with a high distribution coefficient (K_d=1.65×10⁵ mL g^?1).     

Enhancing a Novel Robust Multicomposite Materials Platform for Glucose Biosensors

An effective, stable enzymatic glucose biosensor was fabricated on a glassy carbon electrode (GCE) surface using simple multicomposite materials (MCM): a solution of prepared poly(diallyldimethylammonium chloride)‐capped gold nanoparticles‐nickel ferrite particles‐carbon nanotubes‐chitosan (PDDA‐AuNPs‐NiFe₂O₄‐CNTs‐CHIT), electropolymerization of poly(o‐phenylenediamine) (PoPD) and immobilization of glucose oxidase (GOx). Biocompatibility and synergy of the MCM enhanced the immobilization and the reaction of GOx and as well as the electron transfer from an oxidation reaction of hydrogen peroxide in the system. The NiFe₂O₄ was synthesized by co‐precipitation and calcined at 700 °C. Characterization was carried out by field emission scanning electron microscopy (FE‐SEM), energy‐dispersive X‐ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and X‐ray diffraction (XRD) which presented both tetrahedral and octahedral metal stretching with a cubic NiFe₂O₄ crystal phase. The GOx/PoPD/MCM/GCE yielded a 0.77 s^?1 charge transfer rate constant (K_s), a 2.28×10^?6 cm² s^?1 diffusion coefficient value (D), a 0.21 mm² electroactive surface area (A_e) and a 1.93×10^?8 mol cm^?2 surface concentration ( ) as determined by cyclic voltammetry. The modified electrode showed a durable operation time (n=97, more than 50 % I), repeatability (%RSD=0.38, n=10), reproducibility (%RSD=1.60, n=10), high sensitivity (853.07 μA mM^?1 cm^?2), selectivity without effects of electroactive species (aspirin, uric acid, caffeine, cholesterol, ascorbic acid and dopamine) and two linear ranges from 0.5 to 10 μM (R²=0.998) and 10 to 15,000 μM (R²=0.991) with a low detection limit (0.35 μM, S/N=3). Its Michaelis‐Menten constant (K_m) was calculated as 93.51 μM with 46.30 μA maximum current (I_max). This proposed simple method was successfully applied for glucose determination in human blood samples.     

Hydrogen Peroxide Sensor Based on Hemoglobin Immobilized on Glassy Carbon Electrode with SiO2 Nanoparticles/Chitosan Film as Immobilization Matrix

Abstract

A novel amperometric sensor of hydrogen peroxide was constructed. Hemoglobin (Hb) was successfully immobilized on nanometer‐sized SiO₂, which was supported by chitosan. Chitosan was acted as dispersant. The determination of hydrogen peroxide was performed in the presence of an electron mediator hydroquinone. Hb immobilized on the SiO₂/chitosan composite film displayed excellent electrocatalytical activity to the reduction of H₂O₂. The linear range of detection towards H₂O₂ was from 6.25×10^?7 to 1.63×10^?4mol/L with a detection limit of 1.8×10^?7mol/L (S/N=3). The apparent Michaelis‐Menten constant (K ^app _M) was found to be 0.75mmol/L.     

Studies on the nuclease activity and interactions of the mixed‐polypyridyl [Ni2(1,3‐tpbd)(diimine)2(H2O)2]4+ complexes with thioredoxin reductase

Three new nickel(II) complexes formulated as [Ni₂(1,3‐tpbd)(diimine)₂(H₂O)₂]⁴⁺ [1,3‐tpbd = N,N,N′,N′‐tetrakis(2‐pyridylmethyl)benzene‐1,3‐diamine, where diimine is an N,N‐donor heterocyclic base like 1,10‐phenanthroline (phen),2,2′‐bipyridine (bpy), 4,5‐diazafluoren‐9‐one (dafo)], have been synthesized and structurally characterized by X‐ray crystallography: [Ni₂(1,3‐tpbd)(phen)₂(H₂O)₂]⁴⁺ (1), [Ni₂(1,3‐tpbd)(bpy)₂(H₂O)₂]⁴⁺(2) and [Ni₂(1,3‐tpbd)(dafo)₂(H₂O)₂]⁴⁺ (3). Single‐crystal diffraction reveals that the metal atoms in the complexes are all in a distorted octahedral geometry and in a trans arrangement around 1,3‐tpbd ligand. The interactions of the three complexes with calf thymus DNA (CT‐DNA) have been investigated by UV absorption, fluorescence spectroscopy, circular dichroism and viscosity. The apparent binding constant (K_app) values are calculated to be 1.91 × 10⁵ m ^?1 for 1, 1.18 × 10⁵ m ^?1 for 2, and 1.35 × 10⁵ m ^?1 for 3, following the order 1 > 3 > 2. The higher DNA binding affinity of 1 is due to the involvement in partial insertion of the phen ring between the DNA base pairs. A decrease in relative viscosities of DNA upon binding to 1–3 is consistent with the DNA binding affinities. These complexes efficiently display oxidative cleavage of supercoiled DNA in the presence of H₂O₂ (250 µ m ), with 3 exhibiting the highest nuclease activity. The rate constants for the conversion of supercoiled to nicked DNA are 5.28 × 10^?5 s^?1 (for 1), 6.67 × 10^?5 s^?1 (for 2) and 1.39 × 10^?4 s^?1 (for 3), also indicating that complex 3 shows higher catalytic activity than 1 and 2. Here the nuclease activity is not readily correlated to binding affinity. The inhibitory effect of complexes 1–3 on thioredoxin reductase has also been examined. The IC₅₀ values are calculated to be 26.54 ± 0.57, 31.03 ± 3.33 and 8.69 ± 2.54 µ m , respectively, showing a more marked inhibitory effect on thioredoxin reductase by complex 3 than the other two complexes. Copyright © 2012 John Wiley & Sons, Ltd.     

Highly active titanium(IV) dichloride FI catalysts bearing a diallylamino group for the synthesis of disentangled UHMWPE

A family of 16 salicylaldarylimine titanium(IV) dichloride complexes bearing diallylamino group, namely {2‐[3‐ or 4‐(CH₂?CH? CH₂)₂NC₆H₄N?CH]‐6‐R¹‐4‐R²‐C₆H₂O}₂TiCl₂ (R¹ = t‐Bu, CMe₂(Ph); R² = H, Me, OMe, t‐Bu) have been used for polymerization of ethylene in the presence of methylaluminoxane. The effects of reaction conditions on the polymerization were examined in detail. All the pre‐catalyst are highly active (up to 14.0 × 10⁶ g_(PE) mol_(Ti)^{?1 ?1} h^?1) for ethylene polymerization at 30°С to 60°С with the activities and MM correlating with the R¹‐substituent type and position of NAll₂‐group: CMe₂(Ph) > t‐Bu and meta‐NAll₂ > para‐NAll₂ for any R². Highly linear polyethylenes (T_m's as high as 141.0°С) can be obtained with high molecular weights in the range 0.70 to 4.10 × 10⁶ g mol^?1 with disentangled morphology, suitable for technologically more advanced and greeny way to produce high‐modulus high‐strength fibers of ultrahigh molecular weight polyethylene via solid‐state (solvent‐free) deformation processing.     

Magnetocaloric Properties of Heterometallic 3d–Gd Complexes Based on the [Gd(oda)3]3− Metalloligand

A series of heterometallic 3d–Gd³⁺ complexes based on a lanthanide metalloligand, [M(H₂O)₆][Gd(oda)₃] ? 3 H₂O [M=Cr³⁺ ( 1‐Cr )] (H₂oda=2,2′‐oxydiacetic acid), [M(H₂O)₆][MGd(oda)₃]₂ ? 3 H₂O [M=Mn²⁺ ( 2‐Mn ), Fe²⁺ ( 2‐Fe ) and Co²⁺ ( 2‐Co )], and [M₃Gd₂(oda)₆(H₂O)₆] ? 12 H₂O [M=Ni²⁺ ( 3‐Ni ), Cu²⁺ ( 3‐Cu ), and Zn²⁺ ( 3‐Zn )], are reported. Magnetic and heat‐capacity studies revealed a significant impact on the magnetocaloric effect depending on the anisotropy of the 3d transition metal ions, as confirmed by comparison of the observed maximum values of ?ΔS_m between complexes 2‐Co and 1‐Cr . In these two complexes, the 3d metal ions have the same spin (S=3/2 for Co²⁺ and Cr³⁺ ions), and the theoretical calculation suggested a larger ?ΔS_m value for 2‐Co (47.8 J K^?1 kg^?1) than 1‐Cr (37.5 J K^?1 kg^?1); however, the significant anisotropy of Co²⁺ ions in 2‐Co , which can result in smaller effective spins, gives a smaller value of ?ΔS_m for 2‐Co (32.2 J K^?1 kg^?1) than for 1‐Cr (35.4 J K^?1 kg^?1) at ΔH=9 T.     

Can the Theoretical Fitting of the Proton‐Nuclear‐Magnetic‐Relaxation‐Dispersion (Proton NMRD) Curves of Paramagnetic Complexes Be Improved by Independent Measurement of Their Self‐Diffusion Coefficients?

The self‐diffusion (D_c) coefficients of various lanthanum(III) diamagnetic analogues of open‐chain and macrocyclic complexes of gadolinium used as MRI contrast agents were determined in dilute aqueous solutions (3–31 mM ) by pulsed‐field‐gradient (PFG) high‐resolution ¹H‐NMR spectroscopy. The self‐diffusion coefficient of H₂O (D_w) was obtained for the same samples to derive the relative diffusion constant, a parameter involved in the outersphere paramagnetic‐relaxation mechanism. The results agree with an averaged relative diffusion constant of 2.5 (±0.1)×10^?9 and of 3.3 (±0.1)×10^?9 m² s^?1 at 25 and 37°, respectively, for 'small' contrast agents (M_r 500–750 g/mol), and with the value of bulk H₂O (2.2×10^?9 and 2.9×10^?9 m² s^?1 at 25° and at 37°, respectively) for larger complexes. The use of the measured values of D_c for the theoretical fitting of proton NMRD curves of gadolinium complexes shows that the rotational correlation times (τ_R) are very close to those already reported. However, differences in the electronic relaxation time (τ_SO) at very low field and in the correlation time (τ_V) related to electronic relaxation were found.     

Direct Electrochemistry and Electrocatalysis of Hemoglobin Based on Nafion‐Room Temperature Ionic Liquids‐Multiwalled Carbon Nanotubes Composite Film

A robust and effective composite film combined the benefits of Nafion, room temperature ionic liquid (RTIL) and multi‐wall carbon nanotubes (MWNTs) was prepared. Hemoglobin (Hb) was successfully immobilized on glassy carbon electrode surface by entrapping in the composite film. Direct electrochemistry and electrocatalysis of immobilized Hb were investigated in detail. A pair of well‐defined and quasi‐reversible redox peaks of Hb was obtained in 0.10 mol·L^?1 pH 7.0 phosphate buffer solution (PBS), indicating that the Nafion‐RTIL‐MWNTs film showed an obvious promotion for the direct electron transfer between Hb and the underlying electrode. The immobilized Hb exhibited an excellent electrocatalytic activity towards the reduction of H₂O₂. The catalysis current was linear to H₂O₂ concentration in the range of 2.0×10^?6 to 2.5×10^?4 mol·L^?1, with a detection limit of 8.0×10^?7 mol·L^?1 (S/N=3). The apparent Michaelis‐Menten constant (K_m^app) was calculated to be 0.34 mmol·L^?1. Moreover, the modified electrode displayed a good stability and reproducibility. Based on the composite film, a third‐generation reagentless biosensor could be constructed for the determination of H₂O₂.