The palladium(II)-promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine have been studied at 25 °C and I=0.1 M in the pH range 4–5. At a 1:1 metal to ligand ratio the peptide esters act as tridentate ligands, donation occurring via the terminal amino group, the deprotonated amide nitrogen, and the carbonyl group of the ester. Due to the high Lewis acidity of Pd(II) rapid hydrolysis of the ester function by water and hydroxide ion occurs. Rate constants kOH and kH2O have been obtained for base hydrolysis and water hydrolysis of the coordinated peptide esters at 25 °C. The rate constants for base hydrolysis are 3.4 X 106 M−1 s−1 (L-alaglyOMe), 6.4 X 106 M−1 s−1 (gly-L-alaOMe) and 2.3 X 107 M−1 s−1 (gly-L-leuOMe). Base hydrolysis of the coordinated peptide esters is at least 106 times that of the free unprotonated ligand. Activation parameters have been obtained for both water and base hydrolysis of the Pd(II) complex of methyl L-alanylglycinate and possible mechanisms for the hydrolyses are considered. 相似文献
The kinetics of rapid CO substitution by PPh3 in Co4(CO)12 and Rh4(CO)12 have been examined by stopped-flow and low temperature FT-IR methods. In Co4(CO)12 rapid (kobs ∼ 1.8 s−1) substitution of CO occurs after a 1–15 s induction period at 28 °C in C6H5Cl solvent by a catalytic process. Addition of PPh3 to Rh4(CO)12 yields Rh4(CO)11(PPh3) according to a predominantly second order rate law k1[Rh4- (CO)12] + k2[Rh4(CO)12][PPh3] with k1 = 25 ± 11 s−1 and k2 = 2.97 ± 0.27 X 104 M−1 s−1 at 28 °C. Substitution of a second CO ligand also occurs rapidly with k1 = 0.15 ± 0.09 s−1 and k2 = 6.54 ± 0.07 X 102 M−1 s−1 at 28 °C. The reactivity of Rh4(CO)12 toward associative substitution is 104– 1011 faster than for the Co and Ir analogues, In Rh4(CO)11(PPh3) the increase in CO substitution rates over Co and Rh analogues is 102–107. The ordering of associative substitution rates Co << Rh >>> Ir in these clusters exaggerates the trend seen in mononuclear metal complexes. 相似文献
The reactions of PtCl2en or cis-Pt(NH3)2Cl2 and their aqua species with adenine and adenosine were studied by means of ion-pair HPLC. From the chromatograms, it was found that the first binding site of Pt(II) was the N(7) site of adenine under both acidic and neutral conditions. The rates of Pt(II) binding at the (N7) site of adenosine and deoxyadenosine were measured. The rate constants, k1, were obtained for the reactions of PtCl2en or cis-Pt(NH3)2Cl2 with adenosine and deoxyadenosine at pH 3 and 7 over the temperature range 9–25 °C. The k1 values were 6.8–7.7 × 10−4 dm3 mol−1 s−1 at 25 °C. For the aqua species, the rate of [cis-Pt(NH3)2ClH2O]+ with adenosine N(7) was measured. The rate constants, k2 which were found to be smaller than those of hydrolysis, kh, were calculated at pH 3 over the temperature range 25–40 °C. The k2 value obtained at 25 °C was 1.1 × 10−2 dm3 mol−1 s−1, 15 time larger than k1. The activation parameters were also calculated. 相似文献
The reaction of CrCl3 · 6H2O (dehydrated in DMSO) with 1,5,9-triazanonane (3,3-tri) gives mer- CrCl3(3,3-tri), the configuration being established by isomorphism with the corresponding Co(III) complex. This non-electrolyte is hydrolyzed in aqueous acidic solution and mer-[CrCl2(3,3-tri)- (OH2)]ClO4 can be isolated by anation with HCl in the presence of HClO4. Reaction of mer-CrCl3- (3,3-tri) in DMF with diamines produces complexes of the type [CrCl(diamine)(3,3-tri)] Cl2 [diamine= 1,2-diaminoethane (en), 1.2-diaminopropane (pn), 1,3-diaminopropane (tn), 2,2-dimethyl-1,3-diaminopropane (Me2tn) and cyclohexanediamine (chxn, cis plus trans mixture; two isomers A and B)] and these have been characterized as the ZnCl42− salts. The configuration of the triamine ligand in these complexes has been established as mer-(H↓)- by a single crystal X-ray analysis of [CrCl(en)(3,3-tri)]- ZnCl4, monoclinic, P21, a=7.932, b= 14.711, c= 8.312 Å, β=104.6° and Z=2, refined to a conventional R factor of 0.034. The kinetics of the Hg2+- assisted chloride release from [CrCl(diamine)(3,3- tri)]ZnCl4 salts were measured spectrophotometrically (μ=1.0 M HClO4 or HNO3) over 15 K temperature ranges to give, in order, 104kHg (298.2 K) (M−1 s−1), Ea(kJ mol−1), ΔS# (J K−1 mol−1): en- (HClO4): 5.95, 78.1, -53; pn(HClO4); 5.24, 81.2; -44; tn(HClO4): 26.7, 85.6, -15; Me2tn(HClO4): 21.8, 78.6, -40; A-chxn(HNO3): 7.60, 81.0,-41; B-chxn(HNO3): 18.3, 56.8, -115. A ‘non-replaced ligand effect’ on the rate is observed for the first time in this series of homologous Cr(III) complexes. The kinetics of the thermal aquation (kH, 0.1 M HClO4) were measured titrimetrically for CrCl(diamine) (3,3-tri)2+ to give the following kinetic parameters: diamine=en: 107kH (298.2)=5.34 s−1, Ea=99.2 kJ mol−1, ΔS#=-40 J K−1 mol-1; diamine =tn: 107kH (298.2)=5.04 s−1, Ea= 82.8, ΔS#= -96. 相似文献
Nonenzymatic rates of hydrolytic deamination of adenosine and cytidine by acids and bases analogous to side chains of naturally occurring amino acids are compared with the rates of uncatalyzed deamination in water and with the rates of the hydroxide- and hydrogen ion-catalyzed reactions. For adenosine, hydroxide ion is an effective catalyst, with a second-order rate constant of 7.5 × 10−6 m−1 s−1 at 85°C and an energy of activation of 19.9 kcal/mol. Acid-catalyzed deamination of adenine proceeds with a second-order rate constant of 1.5 × 10−6 m−1 s−1 at 85°C. At concentrations of 1 m and at pH values corresponding to their respective pKa values, dimethylamine, acetate, selenide, imidazole, phosphate, and zinc(II) do not enhance the rate of deamination of adenosine beyond that observed in water, and 2-mercaptoethanol produces only a modest rate enhancement. The uncatalyzed rate of adenosine deamination in water is 8.6 × 10−9 s−1 at 85°C: extrapolation to 37°C and comparison with kcat for rat hepatoma adenosine deaminase yield a rate enhancement by the enzyme of approximately 2 × 1012-fold. 1,6-Dimethyladenosine, the conjugate acid of which has a pKa value much higher than that of adenosine, is not readily deaminated, suggesting that the uncatalyzed deamination of adenosine does not proceed by hydroxide ion attack on the rare protonated form of adenosine, but rather by attack on the neutral species. Deamination of cytidine is catalyzed most effectively by hydroxide ion, with a second-order rate constant of 4.5 × 10−4 m−1 s−1 at 85°C and an energy of activation of 28.5 kcal/mol. The uncatalyzed rate of deamination of cytidine in water, which also exhibits an energy of activation of 28.5 kcal/mol, is 8.8 × 10−8 s−1 at 85°C. Comparison of the rate extrapolated to 25°C with kcat for bacterial cytidine deaminase gives a rate enhancement for the enzyme of 4 × 1011-fold. The C-5 proton of the pyrimidine ring of cytidine does not exchange with solvent during alkaline hydrolysis, suggesting that deamination under these conditions does not involve prior addition of water across the 5,6 double bond. 相似文献
Analkali tolerant α-l-rhamnosidase has been purified to homogeneity from the culture filtrate of a new fungal strain, Fusarium moniliforme MTCC-2088, using concentration by ultrafiltration and cation exchange chromatography on CM cellulose column. The molecular mass of the purified enzyme has been found to be 36.0 kDa using SDS-PAGE analysis. The Km value using p-nitrophenyl-α-l-rhamnopyranoside as the variable substrate in 0.2 M sodium phosphate buffer pH10.5 at50 °C was 0.50 mM. The catalytic rate constant was15.6 s−1giving the values of kcat/Km is 3.12 × 104M−1 s−1. The pH and temperature optima of the enzyme were 10.5 and 50 °C, respectively. The purified enzyme had better stability at 10 °C in basic pH medium. The enzyme derhamnosylated natural glycosides like naringin to prunin, rutin to isoquercitrin and hesperidin to hesperetin glucoside. The purified α-l-rhamnosidase has potential for enhancement of wine aroma. 相似文献
The kinetics and mechanism of a linear trihydroxamic acid siderophore (deferriferrioxamine B, H4DFB+) ligand exchange with Al(H2O)63+ to form mono(deferriferrioxamine B)aluminum(III) (Al(H2O)4H3DFB)3+ have been investigated at 25 °C over the [H+] range 0.001−1.0 M and I = 2.0 M (HClO4/NaClO4) by 27Al NMR. Kinetic results are consistent with Al(H2O)4(H3DFB)3+ formation and dissociation proceeding through a parallel path mechanistic scheme involving Al(H2O)63+(k2/k−1) and Al(H2O)5(OH)2+(k2/k−2) where k1 = 0.13 M−1 s−1, k−1 = 8.7 × 10−3 M−1 s−1, k2 = 2.7 × 103 M−1 s−1, and k−2 = 9.6 × 10−4 s−1. Relative complex formation rates at Al(H2O)63+ and Al(H2O)5OH2+, and comparison with kinetic data for a series of synthetic hydroxamic acids, suggest that an interchange mechanism is operative. These results are also discussed in relation to kinetic data for the corresponding iron(III)-deferriferrioxamine B system. 相似文献
The kinetics of the formation of the thiomolybdate ions MoOS32− and MoS42− were determined spectroscopically from the addition of excess sulphide to MoO2S22− in pH buffered media (6–8) at 30 °C. The reverse (hydrolysis) reactions of MoO2S22− and MoOS32− were measured under the same conditions. The reaction rates measured are shown below: Values of the rate-constants (s−1) obtained at pH 7.0 were k10 2.4 × 10−3, k21 1.5 × 10−5, k30 2.1 × 10−5, k23 6.0 × 10−4, and k34 1.9 × 10−5; where the results are comparable they are in good agreement with those obtained by earlier workers, although different conditions were used. However, in this work it was found that certain reactions had to be mathematically treated as two consecutively occurring reactions. There is also a difference in interpretation of the mechanism of the hydrolysis reactions of the tri- and tetrathio ions. In general the lability towards further S replacement of O atoms, and the reverse reaction, decreased with increased S substitution. All reaction rates increased with increasing H+ ion concentration, mostly this was a linear relationship over the limited pH range examined. The effect of the H+ ion is interpreted in terms of protonation of the oxythiomolybdate ions at an O atom leading to increased lability. 相似文献
Tissue kallikrein may play a role in processing precursor polypeptide hormones. We investigated whether hydrolysis of natural enkephalin precursors, peptide F and bovine adrenal medulla docosapeptide (BAM-22P), by hog pancreatic kallikrein is consistent with this concept. Incubation of peptide F with this tissue kallikrein resulted in the release of Met5-enkephalin and Met5-Lys6-enkephalin. Met5-Lys6-enkephalin was the main peptide released, indicating that the major cleavage site was between two lysine residues. At 37°C and pH 8.5, the KM values for formation of Met5-enkephalin and Met5-Lys6-enkephalin were 129 and 191 μM, respectively. Corresponding kcat values were 0.001 and 0.03 s−1 and kcat/KM ratios were 8 and 1.6·102 M−1 · s−1, respectively. Cleavage of peptide F at acidic pH (5.5) was negligible. When BAM-22P was used as a substrate, Met5-Arg6-enkephalin was released, thus indicating cleavage between two arginine residues. At pH 8.5, KM was 64 μM, kcat was 4.5 s−1, and the kcat/KM ratio was 7 · 104 M−1 · s−1. At 5.5, the pH of the secretory granules, KM, kcat and kcat/KM were 184 μM, 1.9 s−1 and 104 M−1 · s−1, respectively. It is unlikely that peptide F could be a substrate for kallikrein in vivo; however, tissue kallikrein could aid in processing proenkephalin precursors such as BAM-22P by cleaving Arg-Arg peptide bonds. 相似文献
The hydrolysis of the ester 2,4-dinitrophenyl- ethyl methylphosphonate has been examined by both stop-flow spectrophotometric and pH-stat techniques. These reactions have been carried out in the presence of several nucleophiles including simple non-labile (w.r.t. substitution) mono-aquo metal ion complexes. Comparison of reaction rates of the metal complexes with sterically hindered organic nucleophiles has led to the conclusion that the metal ions function predominantly as general base catalysts in dilute aqueous solution. Reaction rates for the various nucleophiles studied are tabulated together with solvolysis constants for hydroxide ion and water at 35 °C and I=0.1 mol dm−3 (KNO3). These later two values are respectively 32.7 mol−1 dm3 s−1 and 1.37 x 10−4 s−1. A Brönsted β value of 0.52 for the phosphonate ester studied has also been derived. 相似文献
Pulse radiolytic studies of α-tocopherol (αTH) oxidation-reduction processes were carried out with low doses (5 Gy) of high-energy electrons in O2−, N2−, and air-saturated ethanolic solutions. Depending on the concentration of oxygen in solution, two different radicals, A· and B·, were observed. The first, A·, was obtained under N2 and results from aTH reaction with solvated electron (kaTH+csolv− = 3.4 × 108 mol−1 liter s−1) and with H3C-ĊH-OH, (R·) (kaTH + R· = 5 × 105 mol−1 liter s−1). B·, observed under O2, is produced by αTH reaction with RO2 peroxyl radicals (kaTH + RO2. = 9.5 × 104 mol−1 liter s−1). 相似文献
The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal)2H2O]2−(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps (k1 and k2: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)]2−. The rates for each step depended linearly on the concentrations of free oxalate species, Hox− and ox2−. The second-order rate constants for the replacement by ox2− were much larger in the k1 step than in the k2 step and the activation parameters were determined as follows: ΔH≠= 43.5 ± 5.6 kJ mol−1, ΔS±-53 ± 19 J K−1 mol−1 and ΔH≠= 43.6 ± 0.5 kJ mol−1, δS≠ = -62 ± 2 J K−l mol−1 for the k1 and k2 steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm3 mol−1 at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants. 相似文献
Racemic mer-[CoCl(en)(NH2CH2CHNCH2CH2NH2)]ZnCl4, which contains no dissymetric chelate rings, no asymmetric carbon centers and no asymmetric nitrogen centers, has been resolved using sodium arsenic(III)—(+)-tartrate. The chirality arises by virtue of the coordinated unsymmetric tridentate ligand and the less soluble diastereoisomeride is associated with the (+)-cation. The rate of base hydrolysis was measured spectrophotometrically using tris buffers. Kinetic parameters (25 °C, \3m; ∼ 0.04 M) are kOH = 1.28 X 103 M−3 s−1, Ea = 87.0 ± 0.7 kJ mol−1 and ΔS#; = +98 ± 1.4 J K−1 mol−1. Complete racemisation accompanies the base hydrolysis reaction and the rate of loss of optical activity is 0.5 times that of base hydrolysis. These data are interpreted in terms of the formation of a symmetrical trigonal bipyramid intermediate generated from the conjugate base. 相似文献
Magnesium binding to cation-depleted blue bacteriorhodopsin (b-bR) was studied spectrophotometrically as well as by following stopped-flow kinetics. There exist three kinetically different steps in the binding process, yielding purple bacteriorhodopsin (p-bR). Since only the firtst step is dependent on the concentration of the reactants, the reaction scheme can be proposed as the simplest model, with MgbR being the first intermediate and ΣI denoting a set of successive intermediates. According to this model k1, k−1 and k2 are calculated to be 2.8 × 104 M−1 · s−1, 5.0 × 10 s−1 and 1 × 10−2 s−1, respectively. 相似文献
Protein tyrosine phosphatase (PTP) targeted, peptide based chemical probes are valuable tools for studying this important family of enzymes, despite the inherent difficulty of developing peptides targeted towards an individual PTP. Here, we have taken a rational approach to designing a SHP-2 targeted, fluorogenic peptide substrate based on information about the potential biological substrates of SHP-2. The fluorogenic, phosphotyrosine mimetic phosphocoumaryl aminopropionic acid (pCAP) provides a facile readout for monitoring PTP activity. By optimizing the amino acids surrounding the pCAP residue, we obtained a substrate with the sequence Ac-DDPI-pCAP-DVLD-NH2 and optimized kinetic parameters (kcat = 0.059 ± 0.008 s−1, Km = 220 ± 50 µM, kcat/Km of 270 M−1s−1). In comparison, the phosphorylated coumarin moiety alone is an exceedingly poor substrate for SHP-2, with a kcat value of 0.0038 ± 0.0003 s−1, a Km value of 1100 ± 100 µM and a kcat/Km of 3 M−1s−1. Furthermore, this optimized peptide has selectivity for SHP-2 over HePTP, MEG1 and PTPµ. The data presented here demonstrate that PTP-targeted peptide substrates can be obtained by optimizing the sequence of a pCAP containing peptide. 相似文献
[125I]α-Bungarotoxinisusedasaprobetostudythenicotinic-cholinergicreceptorinmembrane preparations of the cockroach brain. Binding is restricted mainly to particulate fractions of brain homogenates, is time dependent and is saturable above 2 nM with very low non-specific binding. Scatchard analysis indicates that binding is associated with a single affinity site (Kd = 1.09 nM) having a Bmax of 8926 fmol/mg protein which is the highest concentration of binding sites yet reported in insects. Association kinetics are best fit by a mono-exponential model with a kobs = 4.37 × 10−3s−1. Dissociation is best described by a bi-exponential model giving dissociation constants of 1.18 × 10−5 and 9.94 × 10−5s−1. The Kds calculated from kinetic data are 0.029 and 0.25 nM suggesting the possibility of heterogeneous binding sites not detected by saturation studies. Displacement studies indicate that binding follows a nicotinic pharmacology and demonstrate the high affinity of methyllycaconitine and the anthelmintics, morantel and pyrantel. Displacement by neuronal bungarotoxin shows the presence of two distinct binding sites not differentiated by α-bungarotoxin. Autoradiographic studies show α-bungarotoxin to be binding to neuropile regions of the brain, to be displaced from these regions by agents effective in binding studies and demonstrate that the neuronal bungarotoxin binding sites can be regionally localized. 相似文献
The kinetics of the reaction catalyzed by arginine kinase have been determined at 9.5 and 23°C for in vivo leg muscle of Carcinus maenas (the common shore crab) using the noninvasive technique of 31P-NMR spectroscopy. Concentrations of mobile phosphorus metabolites were the same at both temperatures: 78.7 mM for arginine phosphate, 9.0 mM for adenosine triphosphate (ATP), and 2.6 mM for inorganic phosphate (Pi), as estimated from NMR resonance intensities and literature values for ATP concentration as assayed by traditional biochemical methods. Apparent unidirectional rate constants for formation of ATP from arginine phosphate and ADP were 0.09 s?1 at 9.5°C and 0.27 s?1 at 23°C. Pseudo-first-order rate constants for arginine phosphate generation from Arg and ATP were 0.38 and 1.10 s?1 at 9.5 and 23°C, respectively. In vivo Q10 for the arginine kinase reaction between 9.5 and 23°C was thus 2.2 for both directions. When the kinetic data are analyzed using the Arrhenius equation, activation energies of 126 kJ/mol for ATP formation and 105 kJ/mol for arginine phosphate formation are found. The measured chemical fluxes through arginine kinase in the forward reaction (arginine phosphate hydrolysis) were twice those in the reverse reaction, consistent with either compartmentation of substrates or participation of substrates in alternative metabolic pathways. 相似文献
Invertase converts sucrose to glucose and fructose. The reaction mechanism for the formation of glucose and fructose was studied by stopped flow spectrophotometer and circular dichroism. The reaction mechanism follows biphasic mode with rate constants of k10.0053 s?1?±?0.001 s?1 and k2 0.030 s?1?±?0.01 s?1 for 25 mM concentration of sucrose. Far UV circular dichroic spectrum of invertase in presence of sucrose shows 18 % increase in β conformation as a function of time. Taken together, the invertase hydrolysis follows biphasic mode where it undergoes conformational changes followed by hydrolysis of the sucrose. 相似文献
Complex formation between Pd(II), Pt(II) and iodide has been studied at 25 °C for an aqueous 1.00 M perchloric acid medium. Measurements of the solubility of PdI2(s) in aqueous mercury(II) perchlorate and of AgI(s) and PdI2(s) in aqueous solutions of Pd2+(aq) and Ag+(aq) gave the solubility product of PdI2(s) as Kso=(7±3) × 10−32 M3, which is much smaller than previous literature values.The stability constants β1=[MI(H2O)3+]/([M(H2O)42+][I−]) for the two systems were obtained as the ratio between rate constants for the forward and reverse reactions of (i). The following values of k1 (s−1 M−1), k−1 (s−1) and β1 (M−1) were obtained at 25 °C: (1.14±0.11) × 106, (0.92±0.18), (12±4) × 105 for MPd, and (7.7±0.4), (8.0±0.7) × 10−5, (9.6±1.3) × 104 for MPt. Combination with previous literature data gives the following values of log(β1 (M−1)) to log(β4 (M−4)): 6.08, ∼22, 25.8 and 28.3 for MPd, and 4.98, ∼25, ∼28, and ∼30 for MPt. The present results show that the large overall stability constants β4 observed for the M2+I− systems are most likely due to a very large stability of the second complex MI2(H2O)2, which is probably a cis-isomer. A distinct plateau in the formation curve for mean ligand number 2 is obtained both for MPd and Pt. The other iodo complexes are not especially stable compared to those of chloride and bromide.ΔH≠ (kJ mol−1) and ΔS≠ (JK−1 mol−1) for the forward reaction of (i), MPd, are (17.3±1.7) and (−71±5), and for the reverse reaction of (i) MPd, (45±3) and (−95±6), respectively. The kinetics are compatible with associative activation (Ia). The contribution from bond-breaking in the formation of the transition state seems to be less important for Pd than for Pt. 相似文献