The interaction of pyridoxal (PL) with pyridoxamine (PM) in the presence or absence of CU(II) has been studied in acidic aqueous solutions. We conclude that a ternary complex is formed prior to the interaction of pyridoxal with pyridoxamine in the presence of Cu(II) ions. Although the rate of interaction of excess PL with the Cu(II)-PM system of composition 1:1 followed pseudo-first-order kinetics, this was not so with composition ratios of PM to Cu(II), greater than unity. The observed rate constant (kobs) has the following form for Cu(II):PM:PL in the ratio 1:1:10 (or more): 相似文献
L-Cysteinatogold(I) was prepared by the reaction of L-cysteine with KAuBr4 in acidic media and its solubility determined from pH 4 to 10. The solubility at pH 7.4 and 37 degrees C is 1 microM. In the presence of excess cysteine, the solubility increases because of formation of bis(L-cysteinato)gold(I). The equilibrium-constant for formation of the bis complex is 2.1 +/- 0.4 X 10(-3), which at pH 7.4 CORRESPONDs to an apparant formation constant of 4.4 X10(4). The formation of the bis adduct was confirmed by chromatographic separation of the products of the reaction between [35S]-L-cysteine and Na2AuTM. This complex elutes with Kav = 1.15 which allows it to be distinguished from other gold thiolates that might form in vivo. The bis(cysteinato)gold(I) complex is shown to be present in kidney cytosol isolated from rats given Na2AuTM in vivo. When additional cysteine is added to the cytosol in vitro, the peak at 1.15 is increased, but if glutathione is added, the low molecular weight gold elutes at Kav = 1.00, which is taken as evidence for the existence of bis(cysteinato)gold(I) in the cytosol preparation. The amount of gold present as bis(cysteinato)gold(I) after 4 different dose schedules has been measured and found to increase with the total cytosol gold concentration. L-Cysteinatogold(I) does not dissolve in the presence of bovine serum albumin to form an adduct. 相似文献
[NEt4]3[Fe6M2S8(SEt)9] (M = Mo or W) compounds are isomorphous and contain molybdenum and tungsten atoms in an essentially identical environment. These complexes undergo an irreversible one-electron oxidation at −0.46 V (Mo) and −0.51 V (W) and two one-electron reductions at −1.56 and −1.76 V (Mo) and −1.52 and −1.84 V (W), in DMSO solution versus
(0.1 M). The only distinction between the behavior of these molybdenum and tungsten complexes identified thus far is that, for the former the reductions are reversible whereas for the latter they are irreversible. This difference may be relevant to the low activity found for nitrogenases reconstituted with tungsten in place of molybdenum. 相似文献
1H and 13C NMR studies of Zn(II) binding to oxidized glutathione (GSSG) in aqueous solution over the pH range 4-11 show that it forms a complex with a 1:1 Zn:GSSG stoichiometry. At pH values between 6 and 11 the metal ligands are the COO- and NH2 groups of the glutamate residues. Below pH 5 the glycine end of the molecule also binds to the metal ions. EPR and visible absorption spectra of Cu(II) GSSG solutions suggest that similar complexes are formed with Cu(II). The solid products obtained from these solutions are shown by analysis and EPR to be primarily binuclear with Cu2GSSG stoichiometry, although the structures depend on the pH and stoichiometry of the solution from which they were obtained. 相似文献
Mercury(II) bridge complexes of the type [Nuc-Hg-Nuc] (Nuc = thymidine or guanosine), and methylmercury(II) complexes of thymidine and guanosine of the type [CH3Hg(Nuc)], have been prepared under appropriate conditions of pH and reactant's stochiometry in acqueous soluton. The various complexes have been characterized by 1H and 13C NMR and used as probes, in competition and exchange studies, to establish the relative affinities of Hg(II) and CH3Hg(II) towards the nucleosides guanosine and thymidine. These studies have confirmed that Hg(II) and CH3Hg(II) bind to N3 of thymidine in preference to N1 of guanosine. The studies further show that reactions of mercury(II) with the nucleosides are thermodynamically controlled; the preperential binding reflects the relative stabilities of the respective complexes. 相似文献
Fast reaction kinetic experiments on the electron transfer reaction between azurin and cytochrome c551 isolated from Pseudomonas aeruginosa confirmed the existence of two redox forms of reduced azurin previously reported. The pH dependence of the amplitudes of the relaxation processes observed in temperature jump experiments indicate that these two redox forms are in pH dependent equilibrium. The pH independence of the overall equilibrium constant indicates that redox active and inactive forms of cytochrome c551 may also exist. Evidence that reduced cytochrome c551 undergoes a pH transition is given by optical spectrophotometry. The nature of the transition is discussed in the context of recent nmr studies and in terms of the Marcus theory of electron transfer. The metabolic consequences of these transitions are also discussed. 相似文献
The two major species of slow reacting substance (SRS) contain either a glutathionyl or cysteinyl-glycyl side chain. Incubation of these SRS's with undiluted or diluted (usually 1:10 or 1:50) human plasma at 37°C resulted in marked losses of smooth muscle contracting activity due primarily to conversion of their oligopeptide side chains to cysteine. 相似文献
Yeast enolase binds one mole of 113Cd2+ per subunit at a site that consists of all oxyligands in a distorted octahedral environment. This "conformational" metal ion's environment undergoes further distortion on addition of substrate/product or analogs. At pH's below the optimum value the shifted resonance tends to break up into several, suggesting the existence of several slowly exchanging intermediate forms. At acid pH's, on addition of one additional mole/subunit of 113Cd2+, which greatly increases catalysis, "conformational" resonance(s) further broadens, suggesting that the second, "catalytic" metal ion increases the rates of interconversion between "conformational" species. At more alkaline pH's, near the optimum pH, the "conformational" peak is sharpened, which suggests that very fast interconversion is occurring. The position of the "catalytic" metal ion resonance also suggests all oxyligands in a distorted octahedral geometry. The "catalytic" resonance is often broadened to the point where it cannot be seen, suggesting rapid changes in its geometry due to interconversion of substrate and product. 相似文献
Complexation in the H+-Si(OH)4-tropolone (HL) system was studied in 0.6 M (Na)Cl medium at 25° C. Speciation and formation constants were determined from potentiometric (glass electrode) and 29Si-NMR data. Experimental data cover the ranges 1.5 ? - log[H+] ? 8.4, 0.002 ? B ? 0.012 M, and 0 ? C ? 0.060 M (B and C stand for the total concentration of Si and tropolone, respectively). In acid solutions (-log[H+] ? 3) a hexacoordinated cationic complex, SiL3+, is formed with log K(Si(OH)4 + 3HL + H+ XXX SiL3+ + 4H2O) = 7.08 ± 0.03. Furthermore, the formation of a disilicic acid was established from 29Si-NMR data. The dimerization constant of Si(OH)4 was found to be 10 exp (1.2 ± 0.1). In model calculations the solubility of quartz and amorphous SiO2 in the presence of tropolone is demonstrated. Data were analyzed using the least-squares computer program LETAGROPVRID. 相似文献
Poly(1-vinyl-2-methylimidazole) and poly(1-vinyl-2-methylimidazole-co-1-vinylpyrrolidone) predominantly form five-coordinate heme complexes in aqueous solution. The apparent formation constants (K) of the heme complexes were estimated spectroscopically. The K values of the polymer-heme complexes were about 102 to 103 times those of the corresponding monomeric ones. These large K values were canceled by adding poly(1-vinylpyrrolidone), alcohol, or dimethylformamide. The viscometric measurement of the polymer-heme solution showed that the polymer complex took a compact shape. These results indicate a hydrophobic interaction of heme with the polymer-ligand. Poly(1-vinyl-2-methylimidazole) could form the five-coordinate heme complex, even in the presence of a large amount of imidazole. 相似文献
The reduction of cytochrome c by a series of ferrous EDTA-like complexes was analyzed by the relative Marcus and Tunneling theories. All of the rate constants were calculated at infinite ionic strengths by the use of the Wherland-Gray equation to eliminate electrostatic effects. Both of the theories predicted the rate constants quite well for most of the complexes. But, for those complexes that cannot completely fill all the coordination sites of the iron (HEDTA, NTA), the rate constants were anomalously low. This would be consistent with an increased tunneling distance due to an increase in the size of the complex due to hydrogen bonding between the coordinated hydroxide and the solvent, or due to poorer interaction between the protein and the more hydrophobic mediators. 相似文献
The preparation and molecular and crystal structure of the complex [(ethylenediamine)bis(7,9,-dimethylhypoxanthine)platinum(II)] hexafluorophosphate, [Pt(C2H8N2)(C7H8N4O)2] (PF6)2, are reported. The complex crystallizes in the monoclinic system, space group C2/c, with a = 12.334(2)Å, b = 10.256(2)Å, c = 22.339(3)Å, β = 101.31(1)°, V = 2771.0Å3, Z = 4, Dmeasd = 2.087(3) g cm?3, Dcalc = 2.094 g cm?3. Intensities for 3992 symmetry-averaged reflections were collected in the θ-2o scan mode on an automated diffractometer employing graphite-monochromatized MoKα radiation. The structure was solved by standard heavy-atom Patterson and Fourier methods. Full matrix least-squares refinement led to a final R value of 0.051. Both the ethylenediamine chelate and the PF6? anion are disordered. The primary coordination sphere about the Pt(II) center is approximately square planar with the bidentate ethylenediamine ligand and the N(1) atoms [Pt(II) ? N(1) = 2.020(5)Å] of two 7,9-dimethylhypoxanthine bases (related by a crystallographic twofold axis of symmetry) occupying the four coordination sites. The exocyclic O(6) carbonyl oxygen atoms of the two 7,9-dimethylhypoxanthine ligands participate in intracomplex hydrogen bonding with the amino groups of the ethylenediamine chelate [N(ethylenediamine) ? O(6) = 2.89( )Å]. The observed Pt ? O(6) intramolecular distances of 3.074(6)Å are similar to those found in other Pt(II) N(1)-bound 6-oxopurine complexes and in several Pt(II) N(3)-bound cytosine systems. 相似文献
The hydrolysis of 4-nitrophenyl acetate by metal complexes Co(en)2(imH)H2O3+, Co(en)2(bzmH)H2O3+, and Co(en)2(imCH3)H2O3+ (imH = imidazole, bzmH = benzimodazole, imCH3 = methyl imidazole) has been investigated in the pH range 5.4-8.9. The small difference in nucleophilic reactivity in the pH range 5.4-6.7 is assumed to be due to hydrogen bonding abilities of the imidazole and substituted imidazole ligands and small pKa differences (k2(imH) = 2.2 X 10(-2) M-1 sec-1, k2(bzmH) = 5.68 X 10(-2) M-1 sec-1, k2(imCH3) = 1.35 X 10(-2) M-1 sec-1, 40 degrees C, 1 = 0.3 NaClO4, pKa(imH) = 6.2, pKa(imCH3) = 6.2 and pKa(bzmH) = 5.9). In the pH range 7.8-8.9, the differences in nucleophilic reactivity (k3(imH) = 85.5 X 10(-2) M-1 sec-1, k3(bzmH) = 33.4 X 10(-2) M-1 sec-1, 40 degrees C, I = 0.3 NaClO4) are reconciled with a significant steric factor outweighing the acidity of the benzimidazole complex. In the pH region 6.7-7.7, the deviation from linearity is presumably due to both hydroxo and imido ligands functioning as nucleophiles, the latter being about 40 times stronger than the former. 相似文献
Activation of yeast enolase by Cd2+ exhibits properties similar to activation by the physiological cofactor Mg2+. The activity is weakly stimulated, then inhibited by increasing ionic strength. The activity increases, then falls with increasing Cd2+ concentration. The effect of pH on activity produced by Cd2+ is very similar to that produced by Mg2+, except that the Cd2+ profile is shifted one pH unit to more alkaline values, and the maximum activity of the Cd2+-enzyme is about 10% of that of the Mg2+-enzyme. The apparent kinetic parameters of Cd2+ activation show little effect of pH except for inhibition by high concentrations of Cd2+: the apparent Ki increases sharply with pH. This is interpreted as the result of Cd2+ being a less effective "catalytic" metal ion, and Cd2+ being more effective in stabilizing the enzyme at alkaline pH's. The similarity of effects of ionic strength, divalent cation, and pH may be due to interaction with the same six sites per mole of enzyme. We also characterized the dependence of what is believed to be the enzyme-catalyzed enolization of a substrate analog, D-tartronate semialdehyde-2-phosphate (TSP) on similar parameters of pH, ionic strength, etc. The putative enolization is dependent on catalytic metal ion, although the TSP binds to the conformational Cd2+-enzyme complex. The reaction is very slow and very pH dependent, increasing with pH with a midpoint of reaction velocity at pH 8.7. There is a strong qualitative correlation between pH dependencies of reaction velocity of substrate conversion and TSP enolization and absorbance of the enzyme-bound TSP enolate, whether with Mg2+ or Cd2+ as cofactor. The slowness of the Cd2+-TSP reaction is not limited by proton release or any reaction involving covalent bonds to hydrogen. The apparent reaction rate constant increases linearly with Cd2+ concentration. Addition of excess ethylenediaminetetraacetic acid reverses the TSP reaction, but again very slowly. The binding of Cd2+ to the catalytic sites is characterized by low association and dissociation rate constants. 相似文献
CH3Hg(II)OH forms complexes at pH 8 with tyrosine and with tyrosine ethyl ester (TEE) that are detected by ultraviolet difference absorption spectra. With Kf defined by CH3HgOH + HB CH3HgB + H2O, we find log Kf = 3.61 (tyrosine) and 3.36 (TEE). A heavy-atom effect is observed in frozen glasses of the complexes; this indicates a close interaction between Hg and the chromophore. No UV difference spectrum or heavy-atom effect is observed with N-acetyl tyrosine ethyl ester, indicating that complexing at the phenol O does not occur, and suggesting that binding occurs at the amine N. Zero field optically detected magnetic resonance (ODMR) measurements of the CH3Hg(II)-tyrosine triplet state give (D, E) = (0.129, 0.047) or (0.134, 0.041) cm?1 depending upon assignment of transitions. D of tyrosine is relatively unaffected, but E is reduced by CH3Hg(II) complexing. Low-temperature kinetic measurements show that the shortest lived sublevel of the complex is Tz, where z lies along the phenol long axis in tyrosine. A dominant 11.6-msec component in the 77 K decay of the phosphorescence is consistent with the individual sublevel lifetimes obtained by ODMR. 相似文献
The effect of divalent cations on the near ultraviolet circular dichroism (CD) spectrum of yeast enolase showed that calcium, magnesium, and nickel ions produced identical changes. This was interpreted as indicating that the cations bound to the same sites on the enzyme and produced identical changes in tertiary structure. There was no effect of magnesium ion on the far ultraviolet spectrum. Evidently magnesium ion has no effect on the secondary structure. Substrate bound to the enzyme when the above cations were present although calcium permits no enzymatic activity. The CD spectral difference produced by the substrate was nearly the reverse of that produced by the metal ions. Glycolic acid phosphate, a competitive inhibitor lacking carbon-3, produced no effect, indicating carbon-3 was necessary for the CD spectral changes. The CD and visible absorption spectra of nickel and cobalt bound to various sites on the enzyme showed that the binding sites were octahedral or distorted octahedral in coordination and that the ligands appeared to be oxyligands: water molecules, hydroxyl or carboxyl groups. Examination of the effects of substrate and two compounds thought to be "transition state analogues" showed that these perturbed the "conformational" sites of the enzyme. The "catalytic" and "inhibitory" sites did not appear to be very CD active. 相似文献
The complexation reactions of O-phospho-DL-serine with Ni(II) or Co(II) were studied at 25°C and ionic strength 0.2 M (KNO3) by temperature-jump. The observed rate constants for formation of the Ni2+ and Co+2 monocomplexes were (1.32 ± 0.09) × 105 and (1.73 ± 0.33) × 107 M?1 sec?1, respectively. Complexation is postulated to involve formation of a monocoordinated steady state intermediate followed by rate-determining chelate ring closure. 相似文献
13C nmr studies of gold thioglucose, AuSTg, and solutions containing added β-1-D-thioglucose, TgSH, have been conducted at PD 7.4 and interpreted in terms of complexation and ligand exchange reactions that are consistent with the known preference of gold(I) for linear two-coordinate structures. The upper limit of the half-life for ligand exchange between 0.25 M Au(STg)2? and TgSH at pD 7.4 is 2.2 msec. The 13C nmr spectra of various thioglucose derivatives have been assigned. A novel oxidation reduction reaction was discovered that leads to the formation of metallic gold and a product tentatively identified as the sulfinic acid derivative of thioglucose. The presence of sulfinic acid in AuSTg was indicated by the infrared absorption at 1050 cm?1. The same product was formed by slow hydrolysis of thioglucose disulfide. A mechanism for the formation of the sulfinic acid derivative from AuSTg is proposed. 相似文献
Our reported data on the cortical inhibitory actions of prostaglandin F2α (PGF2α) and the diversity of data in the literature on cerebral PG actions are examined here in the light of intracellular recording which provides the requisite membrane data for the first time. Thus, 1) intracellular recording from the cat cerebral cortex is obtained for the actions of PGF2α and for norepinephrine (NE) and serotonin (5HT). 2) The parallel changes in firing and polarization and the simultaneous transmembrane conductance changes are qualitatively identical for PGF2α, NE and 5HT. 3) The reduction in firing accompanied by hyperpolarization indicates that PGF2α, NE and 5HT all inhibit these cells. 4) The ionic species responsible for this inhibition is such that it increased the transmembrane resistance, and this was true for all three. 5) The changes in membrane parameters, identical in direction for PGF2α and NE, but stronger for the latter, constitute conditions that can lead to competitive inhibition and therefore conote, presumably, actions at the same or related receptors. Such competition with evoked cortical field potentials is shown in the preceding paper. 相似文献
The chelating behavior of a new multidentate ligand with tuberculostatic activity toward Cu(II) and Ni(II) ions has been studied. This ligand 3-(2-carboxyhydrazine)phenylimino-2-oximobutane(H2C POB) is found to chelate the above metal ions in both its keto and enol forms. The probable structures of all the complexes and the location of the bonding sites have been established through magnetic and spectroscopic (infrared, electronic) studies. The Cu(II) complex of the enol form exhibits subnormal magnetic moment at room temperature, indicating the probable existence of some sort of super exchange phenomenon in the system. The ligand itself and a few of its Cu(II) complexes have been found to exert powerful in vitro antibacterial activity toward some tuberculosis mycobacteria, such as Mycobacterium flae, Mycobacterium smegmatis, and Mycobacterium H37Rv. 相似文献