Immobilized citrate synthase

Kinetic properties of pig heart citrate synthase immobilized on Sepharose were determined. Compared to the free enzyme the K_m values for both acetyl-CoA and oxalacetate were increased. The kinetic pattern of the Lineweaver-Burk plots of both substrates for the immobilized enzyme was that of lines intersecting on thex axis. This is the same as that obtained for the free enzyme and indicates that there is no change in the kinetic mechanism of the reaction. The pH response and the Arrhenius plot of both the immobilized and free enzyme were the same. The enzymes show a break in their Arrhenius plots. The immobilized enzyme exhibits greater heat stability than does the free enzyme.     

Cryoscopic studies of aqueous solutions of tartaric acid,sodium hydrogen tartrate,potassium tartrate,sodium dihydrogen citrate,potassium dihydrogen citrate,disodium hydrogen citrate,sodium citrate and potassium citrate

Freezing temperature lowerings of aqueous solutions of tartaric acid, sodium hydrogen tartrate, sodium dihydrogen citrate, potassium dihydrogen citrate, disodium hydrogen citrate, sodium citrate and potassium citrate were determined. These values and those taken from the literature for potassium tartrate were used in the determination of the osmotic and activity coefficients in the studied systems, via the numerical integration of the Gibbs–Duhem equation.     

Electrospray ionisation with selected reaction monitoring for the determination of Mn‐citrate,Fe‐citrate,Cu‐citrate and Zn‐citrate

Citrate complexes of Mn and Fe, and potentially those of Cu and Zn, are considered as important low molecular mass species in human serum and cerebrospinal fluid (CSF). For example, Mn is supposed to enter the brain under excess exposure as Mn‐citrate leading to neurotoxic effects. Mn‐citrate has been characterised in human CSF using chromatography and electrophoresis online with inductively coupled plasma mass spectrometry, but not yet with molecular mass spectrometry. Therefore, this study explores the potential of electrospray ionisation (ESI) with selected reaction monitoring (SRM) for the detection of metal‐citrate complexes, in particular Mn‐citrate. The collision‐induced dissociation of precursor ions with various metal:citrate stoichiometries was studied for Mn‐citrate, Fe‐citrate, Cu‐citrate and Zn‐citrate. High selectivity was achieved for Mn(II)‐citrate even in respect to Fe(III)‐citrate which forms isobaric precursor ions. The limit of detection for Mn‐citrate was estimated to be around 250 µg L^?1 (referring to the total Mn content in the standard) using flow injection. The sensitivity was sufficient for the determination of Mn‐citrate in standard solutions and in an extract of an Mn‐citrate‐containing supplement. An improved ESI source design is expected to reduce the limits of detection significantly. The developed ESI‐SRM method has the potential to provide complementary data for the quality control of current separation methods for metal citrates using element‐selective detection, with application to biomedical samples and further matrices. Copyright © 2009 John Wiley & Sons, Ltd.     

Size control of gold nanocrystals in citrate reduction: the third role of citrate

Growth kinetics and temporal size/shape evolution of gold nanocrystals by citrate reduction in boiling water were studied systematically and quantitatively. Results reveal that the size variation and overall reaction mechanism were mostly determined by the solution pH that was in turn controlled by the concentration of sodium citrate (Na3Ct) in the traditional Frens's synthesis. This conclusion was further confirmed by the reactions with variable pH but fixed concentrations of the two reactants, HAuCl4 and Na3Ct. Two substantially different reaction pathways were identified, with the switching point at pH = 6.2-6.5. The first pathway is for the low pH range and consists of three overlapping steps: nucleation, random attachment to polycrystalline nanowires, and smoothing of the nanowires via intra-particle ripening to dots. The second pathway that occurred above the pH switching point is consistent with the commonly known nucleation-growth route. Using the second pathway, we demonstrated a new synthetic route for the synthesis of nearly monodisperse gold nanocrystals in the size range from 20 to 40 nm by simply varying the solution pH with fixed concentrations of HAuCl4 and Na3Ct. The switching of the reaction pathways is likely due to the integration nature of water as a reaction medium. In the citrate reduction, the solution pH was varied by changing the initial HAuCl4/Na3Ct ratio. Consequently, when pH was higher than about 6.2, the very reactive [AuCl3(OH)]- would be converted to less reactive [AuCl2(OH)2]- and [AuCl(OH)3]-.     

Vibrational study of tamoxifen citrate polymorphism

The trans isomer of (Z)-2-[p-(1,2-diphenyl-butenyl)phenoxy]-N,N-dimethyletylamine (tamoxifen) is well known for its endocrine activity as an antiestrogenic agent. Its citrate salt, a widely used pharmaceutical agent, appears in three main polymorphic forms, two of which are well known (I and II) and another form not yet well evidenced.

A vibrational study has been conducted for identifying the two known polymorphic forms of tamoxifen citrate (I and II) and for characterising the other form (form III) examined in this study.

Other techniques for the characterization of the different polymorphs, such as XRDP, have been used.     

分光光度法定量测定柠檬酸及其盐的含量

酸性介质中,柠檬酸(或盐)与Fe3+形成的黄色络合物在光照条件下发生光致变色现象,生成紫红色络合物,采用分光光度法可定量测定柠檬酸及其盐的含量.在冰水浴中,日光照射20 min,阴凉处静置30 min,检测波长为490nm.线性范围为(0～300)mg/L,相对标准偏差(RSD)为0.46％(n=6),检测限为3.46...     

Electrodeposition of iron-molybdenum coatings from citrate electrolyte

Specific features of the electrodeposition of iron-molybdenum coatings from a citrate electrolyte based on iron(III) sulfate and sodium molybdate in dc and unipolar pulsed modes were studied. It was demonstrated that bright compact coatings with varied content of molybdenum can be produced by varying the relative concentrations of salts of the alloy-forming components and the solution pH. The current density ranges providing the high efficiency of the galvanostastic electrolysis were determined and it was shown that using the pulsed mode makes smaller the amount of nonmetallic impurities in a coating, diminishes its roughness and leads to formation of surface layers with a more uniform composition, The iron-molybdenum coatings exhibit a high corrosion resistance in corrosive media and physicomechanical properties improved as compared with the base metal, which makes it possible to recommend these coatings for protection from corrosive-mechanical disintegration and restoration of the surface of worn articles.     

Platinum states in citrate sols by EXAFS

Platinum sols have been prepared by citrate reduction in the temperature range of 343-363 K. The Pt state in the solution was examined by EXAFS (extended X-ray absorption fine-structure spectroscopy). It did not show any PtPt bonding, a characteristic for reduced Pt sols. EXAFS model fitting further proved the presence of PtO with 4 oxygen neighbors, which suggests a tetraplanar coordination configuration. The possibility of neighboring Pt sharing oxygen ligand or the formation of PtO(x) is rejected by EXAFS model fitting. Citrate was found to be the most likely ligand to orient its oxygen end toward a charged Pt center. Thus we have revealed that the citrate treatment at this temperature range was clearly insufficient to reduce H2PtCl(6(aq)). Neither an extended period of reaction time nor an excess citrate reduced the Pt precursor. It is therefore highly recommended that the citrate sols should be carefully prepared and used.     

Cobalt(II) citrate cubane single-molecule magnet

An investigation of the magnetic properties of the cobalt(II) citrate cubane [C(NH 2) 3] 8{Co 4(cit) 4}.4H 2O reveals that the cluster is a new cobalt(II) single-molecule magnet, with an energy barrier to reorientation of the magnetization, Delta E/ k B = 21 K, and tau 0 = 8 x 10 (-7) s. The compound displays distinct, frequency-dependent peaks in the out-of-phase (chi') component of the ac magnetic susceptibility and magnetization versus field hysteresis loops that are temperature and sweep rate dependent. The hysteresis loops collapse at zero field due to very fast quantum tunneling of the magnetization (QTM).     

Characterisation of the citrate synthase reaction with propionyl-CoA

Experiments with propionyl-CoA stereoselectively deuteriated in the propionyl moiety demonstrate that the formation of (2S,3S)-methylcitric acid (1) catalysed by citrate (si)-synthase occurs with inversion of configuration in the propionyl moiety; the absolute configurations of the methylcitric acids 1 and 2 indicate a si attack on oxaloacetate. Deuterium in the pro-S position is exchanged for protium 60 times faster than deuterium in the pro-R position. Experiments with (R,S)-(2-2H1)propionyl-CoA allowed the determination of isotope effects. For the enzymatic formation of 1, a primary deuterium isotope effect kH/kD = 1.8 and a secondary alpha-deuterium isotope effect kH/kD = 0.99 were calculated; both are effects on Vmax/KM.     

Manganese citrate chemistry: syntheses, spectroscopic studies, and structural characterizations of novel mononuclear, water-soluble manganese citrate complexes

The first two mononuclear manganese citrate complexes, (NH4)4[MnII(C6H5O7)2] (1) and (NH4)5[MnIII(C6H4O7)2].2H2O (2) were synthesized in aqueous solutions near physiological pH values. They were isolated in their pure crystalline forms and characterized by elemental analyses and spectroscopic techniques, including UV/visible, electron paramagnetic resonance, Fourier transformed infrared, and magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.777(1) A, b = 13.656(3) A, c = 9.162(2) A, beta = 113.62(2) degrees, V = 1006.2(6) A3, and Z = 2. Compound 2 crystallizes in the triclinic space group P1, with a = 9.606(3) A, b = 9.914(3) A, c = 7.247(3) A, alpha = 91.05(1) degrees, beta = 105.60(1) degrees, gamma = 119.16(1) degrees, V = 571.3(3) A3, and Z = 1. The X-ray crystal structures of 1 and 2 revealed that, in both cases, the manganese ion is six-coordinate and is bound by two citrate ligands in a distorted octahedral fashion. In the case of complex 1, the citrate ion binds to Mn2+ as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound 2, the citrate ligand coordinates to Mn3+ as a fully deprotonated entity. Compound 2 contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn(3+)-citrate assembly. There are significant contributions to the stabilities of the assembled lattices in 1 and 2 arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn2+ and Mn3+ species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5-300 K. Complexes 1 and 2 present clear cases of mononuclear manganese citrate species relevant to manganese speciation in biological media and potentially related to the beneficial as well as toxic effects of manganese on humans.     

Unprecedented oxo-titanium citrate complex precipitated from aqueous citrate solutions, exhibiting a novel bilayered Ti8O10 structural core

Aqueous titanium citrate solutions were prepared from the reaction of citric acid with titanium 2-propoxide in a range of molar ratios. Solutions containing two or fewer citrates per titanium resulted in the slow crystallization of an insoluble titanium oxo-citrate complex. Single-crystal X-ray analysis identified the species as Ti(8)O(10)(citrate)(4)(H(2)O)(12).14H(2)O.3HOPr(i)(), crystallized in the tetragonal space group I4(1)/a, with a = 30.775(7) A, c = 14.528(7) A, V = 13 759(8) A(3), and Z = 8. The trianionic citrate ligands supply both carboxylate and alkoxide coordination and stabilize the structure using simultaneous chelating and bridging modes of attachment. The compound is a neutral species, exhibiting titanium in three contrasting environments. Laser Raman microscopy and (13)C CPMAS solid-state NMR data were consistent with those of the X-ray crystal structure. When exposed to air, the crystals rapidly lost water and became a powder. The dehydrated powder was noncrystalline to X-rays and insoluble, but (13)C NMR results demonstrated retention of the carboxylate linkages.     

Spectrophotometric methods for the determination of tamoxifen citrate

Three simple and sensitive spectrophotometric methods for the determination of tamoxifen citrate have been developed. They are based on the formation of an ion-association complex between the drug and a dye, Erioglaucine A, which is extractable into chloroform and has an absorption maximum at 625 nm (method A), oxidation with excess potassium permanganate and the determination of unconsumed permanganate using Fast Green FCF (method B), or by the formation of a coloured cobalt thiocyante coordination complex which is extracted into benzene and measured at 635 nm (method C). Beer's law limits for methods A, B, and C are 0.5-3.0 mug ml(-1), 1.0-6.0 mug ml(-1) and 100-500 mug ml(-1), respectively. No interference was observed from tableting additives and the applicability of the methods was examined by analysing tablets containing tamoxifen. The quantities determined were 99.0-100.03% of the exptected values.     

Dilithium (citrate) crystals and their relatives

New compounds of the type LiMHC₆H₅O₇ (M = Li, Na, K, Rb) have been prepared from the metal carbonates and citric acid in solution. The crystal structures have been solved and refined using laboratory powder X‐ray diffraction data, and optimized using density functional techniques. The compounds crystallize in the triclinic space group P and are nearly isostructural. The structures are lamellar, with the layers in the ab plane. The boundaries of the layers consist of hydrophobic methylene groups and very strong intermolecular O—H…O hydrogen bonds. The O…O distances range from 2.666 Å for M = Li to 2.465 Å for M = Rb. The Li—O bonds exhibit significant covalent character, while the heavier M—O bonds are ionic. The Li atoms are four‐, five‐, or six‐coordinate, while the coordination numbers of the larger cations are higher, i.e. eight for Na and nine for K and Rb. The citrate anion occurs in the trans,trans conformation, one of the two low‐energy conformations of an isolated citrate anion. The crystal structure of LiRbHC₆H₅O₇·H₂O was also solved and refined. It consists of the same layers as in the anhydrous M = Rb compound, with interlayer water molecules and a different hydrogen‐bonding pattern.     

柠檬酸法制备复氧化物材料的配位结构化学

柠檬酸法在复氧化物材料制备中占有重要地位,它具有分散均匀、制备温度低、产物粒径小的特点.以特定组成和结构的柠檬酸络合物为前驱体,直接分解可得到纯度较高的复氧化物,从中可以了解复氧化物的形成过程.本文从配位结构化学的角度,评述以钛、钒、钼和钨柠檬酸络合物为前驱体制备复氧化物材料的研究进展.     

Isolation and characterization of a polymeric lanthanum citrate

The first lanthanide citrate coordination polymer with the formula [La(Hcit)(H(2)O)](n) (Hcit(3-) = C(OH)(COO(-))(CH(2)COO(-))(2)) was prepared from La(2)O(3) and citric acid at pH = 2.2-2.5 under hydrothermal conditions at 120 degrees C. The compound was characterized by elemental analysis, IR, TG-DTA, and X-ray crystallography. It is thermally stable up to 158 degrees C and insoluble in common solvents. The compound crystallizes in the monoclinic space group C2/c with a = 16.765(3) A, b = 8.822(2) A, c = 14.048(3) A, beta = 120.64(3) degrees , and Z = 8. The structure consists of chains of La(III) cations bridged by O--C--O groups with pendant Hcit anions forming a pillar structure. The Hcit ligand is involved in six La--O bonds to five different La centers in a very compact 3D structure.