Wyniki wyszukiwania - Biblioteka Nauki

1

Sequence-specific Ni(II)-dependent peptide bond hydrolysis in a peptide containing threonine and histidine residues

100%

Krężel A. , Mylonas M. , Kopera E. , Bal W.

Acta Biochimica Polonica

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2006

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tom 53

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nr 4

721-727

EN

Previously we demonstrated that Ni(II) complexes of Ac-Thr-Glu-Ser-His-His-Lys-NH2 hexapeptide, representing residues 120-125 of human histone H2A, and some of its analogs undergo E-S peptide bond hydrolysis. In this work we demonstrate a similar coordination and reactivity pattern in Ni(II) complexes of Ac-Thr-Glu-Thr-His-His-Lys-NH2, its threonine analogue, studied using potentiometry, electronic absorption spectroscopy and HPLC. For the first time we present the detailed temperature and pH dependence of such Ni(II)-dependent hydrolysis reactions. The temperature dependence of the rate of hydrolysis yielded activation energy Ea = 92.0 kJ mol-1 and activation entropy ΔS≠ = 208 J mol-1 K-1. The pH profile of the reaction rate coincided with the formation of the four-nitrogen square-planar Ni(II) complex of Ac-Thr-Glu-Thr-His-His-Lys-NH2. These results expand the range of protein sequences susceptible to Ni(II) dependent cleavage by those containing threonine residues and permit predictions of the course of this reaction at various temperatures and pH values.

2

Quinolinium fluorochromate oxidation of alpha-hydroxy acids in perchloric acid medium

100%

Meenakshisundaram S. , Sathiyendiran V.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 10

2261-2266

EN

The kinetics of oxidation of alpha-hydroxy acids (mandelic, lactic and glycolic) with a recently developed new mild Cr(VI) regent quinolinium fluorochromate (QFC) has been investigated in perchloric acid medium. First-order depedence with respect to [oxidant] is observed. Increase in [substrate] increases the rate proportionally. The acidity dependence is fractional order throughout the entire range of acidities studied. Decrease in dielectric constant of the medium increases the oxidation rate considerably. Kinetic solvent isotope effect studies indicate a pre-equilibrium fast proton transfer. Absence of polymerization with acrylonitrile rules out a free radical mechanism. An intermediate formed between protonated oxidant and substrate decomposes in a slow rate-limiting step giving the products.

3

Kinetics of homogeneous decomposition of asymmetric dialkyl peroxides

100%

Baj S. , Dawid M.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 9

2151-2158

EN

The homogeneous decomposition of various asymmetric dialkyl peroxides ROOR' (R=C6H5(CH3)2; R'=Et, Pr, Bu, Bu(i), Bu(s), Bu(t), n-heptyl, n-dodecyl and allyl) is examined by the measurement of autooxidation of isopropylbenzene in the presence of peroxides as initiators. Rate constants and activation parameters are determined and the size effect and type of alkyl group R' on stability of O-O bond is discussed.

4

Kinetics of oxidation of oxine by pyridinium dichromate

100%

Palaniappan A. , Bharathi K. , Sekar K. G.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 1

122-126

EN

The kinetics of oxidation of 8-hydroxyquinoline (oxine) by pyridinium dichromate has been studied in 60% acetic acid-water (v/v) medium. The reaction order is one with respect to oxidant, second with respect to the substrate and zero with respect to hydrogen ion concentration. Decrease of the dielectric constant of the medium, reduces the rate of the reaction. Increase in ionic strength has no effect on the reaction rate. The reaction does not induce the polymerization of acrylonitrile. From the kinetic data obtained, the activation parameters have been calculated and a plausible mechanism has been proposed.

5

Kinetics of oxidation of phenoxyacetic acids by quinolinium fluorochromate

88%

Karunakaran K.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 5

916-924

EN

The kinetics of oxidation of twenty one phenoxyacetic acids by quinolinium fluorochromate have been studied in binary solvent mixtures. Acrylonitrile has no effect on the rates of oxidation in dipolar protic solvent but in a aprotic solvent it decreases the oxidation rates. In both solvent systems there exists an equilibrium prior to the rate limiting step, followed by irreversible decomposition of the complex. The calculated rate constants correlate well with Hammett delta values. Suitable mechanisms have been proposed.

6

Synthesis and physico-Chemical Characterization of Chelates Derived from 2-Hydroxy-5-methylacetophenone-S-benzyldithiocarbazate

88%

Aswar A. , Mahale R. , Makode J.

Polish Journal of Chemistry

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2000

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tom Vol. 74, nr 4

469-475

EN

Chelates of Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), VO(IV) and UO2)VI= with Schiff base ligand. The thermogravimetric data have been analyzed for kinetic parameters using Freeman-Carroll and Sharp-Wentworth methods.

7

Kinetics and mechanism of the acid-catalyzed hydrolysis of [carbonatotetra(1-methylimidazole)cobalt(III)](+) ion

88%

Chatłas J. , Danilczuk E. , Nasiadko M.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 4

777-785

EN

The kinetics of acod-catalyzed hydrolysis of the [Co(1-Meim)4(CO3)](+) ion follows the rate law -d ln[complex]/d t=k(1)K[H(+)]/(1 + K[H(+)]) (25:45 degree C, [H(+)]=0.05:1.0 M and I=1.0 M(NaClO4)). The reaction course consists of rapid pre-equilibrium protonation followed by a rate determining ring opening process and the subsequent fast relase of monodentate carbonato ligand. Kinetic parameters, k(1) and K, at 25 degree C are 5.1x 10(-3)s(-1) ans 0.48 M(-1) respectively, and activation parameters for k(1) are Delta H(1)=84.4+:-0.8kJ mol(-1) and DeltaS(1)=-5.8+:-3.7J mol(-1) K(-1). The hydrolysis rate increases with the ionic strengh. The results are compared with those for similar cobalt(III) complexes.

8

Kinetics of the reaction of 2,3,5,6-tetrachloro-1,4-benzoquinone with imidazole in acetonitrile and benzene

88%

Dworniczak M. , Przybyszewska M. , Jarczewski A.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 1

108-112

EN

Nucleophilic vinylic substitution of 2,3,5,6-tetrachloro-1,4-benzoquinone by imidazole in acetonitrile and benzene has been studied spectrophotometrically. The reaction product is the diaminodichlorosubstituted quinone. The values of the activation parameters indicate that there is a considerable charge separation in the transition state. The reaction mechanism is discussed.

9

Kinetics and mechanism of oxidation of L-phenylalanine by alkaline permanganate

88%

Panari R. G. , Chougale R. B. , Nandibewoor S. T.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 1

99-107

EN

The data suggest that the oxidation of L-phenylalanine proceeds via the formation of a complex between L-phenylalanine and permanganate which decomposes slowly, followed by a fast reaction between the free radical of L-phenylalanine and another molecule of permanganate to give the products. The reaction constants involved in the mechanism are derived. There is a good agreement between the observed and calculated rate contants at varying conditions of experiments. The activation parameters are computed with respect to the slow step of the mechanism.

10

The Kinetics of Chloromethyloxirane reaction with Alcohols and Their Adducts in the presence of Stannic Chloride

88%

Chlebicki J. , Poźniak R.

Polish Journal of Chemistry

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2000

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tom Vol. 74, nr 1

79-88

EN

Reaction is not first order in respect to epichlorohydrine and is particularly fast at the beginning and its rate constants are proportional to the concentration of the catalyst. The rate constants increase in the homogolous series of C1-C10 alcohols.

11

Kinetics and Mechanism of Oxidative Degradation of L-Proline by Alkaline Diperiodatonickelate(IV)- A Free Radical Intervention and Decarboxylation

88%

Mulla R. , Gurubasavaraj H. , Nandibewoor S.

Polish Journal of Chemistry

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2003

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tom Vol. 77 / nr 12

1833-1840

EN

The kinetics of oxidation of L-proline by diperiodatonickelate(IV) (DPN) in aqueous alkaline medium at a constant ionic strength of 0.50 mol dm-3 was studied spectrophotometrically. The reaction is of first order in [DPN], zero order in [alkali] and less than unit order in [L-proline]. Addition of periodate has no effect on the rate of reaction. Effect of added products, ionic strength and dielectric constant of the reaction medium have been investigated. The main products were identified by spot test and IR spectra.Amechanism involving the diperiodatonickelate(IV) (DPN) as the reactive species of the oxidant has been proposed. The reaction constants involved in the different steps of mechanism are calculated. The activation parameters with respect to slow step of the mechanism are computed and discussed and thermodynamic quantities are also calculated. The isokinetic temperature was determined and discussed.

12

Acid-Catalyzed Cleavage of Some Chromone, Coumarin and Pyrone Derivaties of Aminomethylphosphonic Acid. Products and Kinetics of the ReactionB

75%

Boduszek B. , Latajka R. , Walkowiak U.

Polish Journal of Chemistry

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2001

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tom Vol. 75, nr 1

63-69

EN

Treatment of chromone-2, coumarin-4 and pyrone-2 derivatives of N-benzylaminomethylphosphonic acid with strong mineral acids leads to formation of the corresponding heterocyclic amines and phosphoric acid. Kinetic studies of this cleavage reaction demonstrate that protonation has a remarkable influence on a cleavage of C-P bonds. In aq. H2SO4, cleavage of the acids 1-3 exhibits a kinetic dependence on [H+]. The measured solvent isotope effect (kH/kD) was about 1.5 for the 1 and 2 and only 1.1 for the 3. The existence of the isotope effect shows that protons are involved on the rate-determining step. The data obtained suggest that the protonated phosphonate molecule is split by a dissociative mechanism with A-SE2 character and this is combined with an elimination of the phosphonate group as a positive-charged phosphorus moiety.

13

Synthesis and Properties of Azoles and Their Derivatives : Part XLI : Stereoselectivity and Kinetic Study of the [2+3]Cycloaddition of E-B-Nitrostyrene to Z-C,N-Diphenylnitrone

75%

Barański A.

Polish Journal of Chemistry

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1999

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tom Vol. 73, nr 10

1711-1717

EN

Stereoselectivity and the kinetics of the reaction of E-B-nitrostyrene to Z-C,N-diphenylnitrone at different temperatures and in several solvents have been studied. From the results obtained, it is evident that the reaction shows the properties of a one-step concerted process, so that it can be considered a typical [2+3]cycloaddition process.

14

(13)C kinetic isotope effects in the decarboxylation of phenylpropiolic acid (PPA) assisted with formic acid and (13)C KIE in the decarbonylation of formic acid in the presence of PPA

75%

Zieliński M. , Zielińska A. , Paul H. , Papiernik-Zielińska H.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 12

2551-2555

EN

Carbon-13 kinetic isotope effects, (13)C-KIE, in the decarboxylation of phenylpropiolic acid (carboxyl C-13) in formic acid medium and (13)C KIE in the decarbonylation of liquid formic acid assisted with PPA and acetophenone have been studied in the 70-100 degree C temperature interval. The kinetic and isotope results have been discussed and interpreted as indicating that the formation of C alpha-H bond, preceded by the protonation of acetylenic bond of PPA, is the rate determining step followed by carbon dioxide splitting. The (13)CO-KIE in the carbon monoxide generation assisted with PPA is much larger than the (13)CO-KIE observed in the generation of CO in the presence of phenylacetylene. Thus the decaeboxylation of PPA and decarbonylation of formic acid are interrelated processes proceeding in the reaction cage.

15

Mechanism of Ring Opening Reaction of 4-Benzylidene-2-methyl-5-oxazolone. Part I. Reaction with n-Butylamine

75%

Betłakowska B. , Banecki B. , Łankiewicz L. , Wiczk W.

Polish Journal of Chemistry

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2001

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tom Vol. 75, nr 3

401-410

EN

The reaction between 4-benzylidene-2-methyl-5-oxazolone (Ox, c = 5.25 _ 10-5 M) with n-butylamine (nBuA, c = 0.026 to 1.66 M) in acetonitrile was studied by UV-Vis spectroscopy using steady-state and stopped-flow apparatus. The amine to Ox molar ratio higher than 500 allows to apply pseudo-first order approximation, whose rate constants depend on amine concentration according to a parabolic equation: kobs = 4.63 _ 0.02 _ CnBuA 2 , indicating a complex reaction mechanism. On the other hand, singular value decomposition (SVD) analysis, as well as global analysis of the data obtained from stopped-flow measurements support a simple A B reaction model. Nonlinear dependence of the pseudo- first order rate constant on amine concentration can be explained, assuming that the oxazolone ring opening reaction by n-butylamine in acetonitrile proceeds as parallel reactions: A + B C and A+2B C. Based on our experimental results and theoretical calculations of the oxazolone ring opening reaction, a mechanism of the reaction between 4-benzylidene-2-methyl-5-oxazolone and n-butylamine is proposed.

16

Studies on kinetics and mechanism of oxidation of pyridoxal by dichromates in aqueous HClO4 solutions

75%

Kita E. , Kita P. , Pietkiewicz A. , Wrzeszcz G. , Rozpłoch F.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 3

573-581

EN

Oxidation of pyridoxal (PL) by Cr2O7(2-) ion has been studied using an excess of the aldehyde under air or argon atmospheres. The reaction leads mainly to two Cr(III) complexes: [Cr(PA)(H2O)4](2+) and [Cr(H2O)6](3+) and to uncoordinated pyridoxic acid (PA). The rate of PL oxidation follows a mixed third order rate law: first order in concentrations of Cr(VI), Pl and H3O(+). The reduction of Cr(VI) toCr(III) proceeds through chromium(V) intermediate complex, which has been detected by the EPR method. Mechanism of the reaction has been discussed.

17

C-13 isotope effects in the decarboxylation of phenylpropiolic acid (PPA) in water solution of formic acid (FA)

75%

Zieliński M. , Zielińska A. , Ogrinc N. , Kobal I. , Paul H. , Bernasconi S. , Papiernik-Zielińska H.

Polish Journal of Chemistry

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1999

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tom vol. 73 nr 5

813-820

EN

Kinetics and the carbon-13 kinetic isotope effects in the decarboxylation of phenylpropiolic acid in HCOOH/H2O, 1:1/V:V, solution have been examined between 80-140.13oC. 13C KIE in the decarbonylation of formic acid, proceeding with measurable rate in this medium between 130-150oC, has determined also and compared with corresponding values found in the pure HCOOH/H2O solution. Kinetics and C-13 KIE in the decarboxylation of phenylpropiolic acid in pure water have been investigated subsequently between 100-143oC in all glass reaction vessels sealed under vacuum. The enthalpy of activation of decarboxylation of PPA in pure water, 30.20 kcal/mol, and the entropy of activation, DS* = -3.7 e.u., are by 6.7 kcal/mol and 12.5 e.u., respectively, higher than the DH* and DS* values found in the decarboxylation of PPA in pure formic acid. The C-13 KIE equal to 1.004-1.005 between 70-100oC in the pure HCOOH medium increased to C-13 KIE of 1.020 in the case of decarboxylation of PPA at 133.7oC in the initially pure water.

18

Proton Abstraction from 4-Nitrophenyl[bis(ethylsulphonyl)]methane with TMG, TBD and MTBD Bases in Acetonitrile

63%

Jarczewski A. , Binkowska I.

Polish Journal of Chemistry

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2001

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tom Vol. 75, nr 12

1895-1903

EN

4-Nitrophenyl[bis(ethylsulphonyl)]methane has been synthesized and used in kinetic studies of proton abstraction induced by 1,1,3,3-tetramethylguanidine (TMG), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyI-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) bases in acetonitrile. The pK, values of this carbon acid in water and in acetonitrile solvents are 10.08 and 22.8 respectively. The electronic spectra of 4-nitrophenyl[bis(ethylsulphonyl)]methane and its anion ar well defined and temperaturę dependent. The rates of proton abstraction are large as the reaction occurs in the range of microseconds. The relaxation times were studied by spectrophotometric temperature-jump technique. The rate constants for proton transfer reaction promoted by TMG, TBD and MTBD bases in acetonitrile are: 1.39x10 5-2.1 Ix10 5; 8.8x10 6-19.2x10 6; 0.84x105-2.43x 10S [dm3 mol-1 s-1] respectively between 20-40°C. The enthalpies of activation are: deltaH# = 18.1, 28.7 and 40.0 [kJ mol-1] for TMG, TBD and MTBD respectively. The entropies of activation are all negative: deltaS# = -84.9, -13.6, -14.3 [J mol-1 deg-1] for the same sequence of bases reacting with 4-nitrophenyl[bis(ethylsulphonyl)]methane in acetonitrile solvent. The general discussion of the results obtained and their comparison with those for proton transfer reaction carried out with "normal" C-acids is given.

19

Volumetric, Viscometric, and Refractive Index Studies of Some alfa-Amino Acids in Aqueous Tartrazine at Different Temperatures

63%

Ali A. , Patel R. , Shahjahan , Tasneem S.

Polish Journal of Chemistry

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2009

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tom Vol. 83, nr 7

1353-1363

EN

The densities (r), viscosities (h), and refractive indices (nD) of (0.10, 0.15, 0.20, 0.25, and 0.30) m glycine, L-alanine, and L-valine were measured in 0.01 m aqueous tartrazine at 288.15, 293.15, 298.15, 303.15, 308.15, and 313.15 K. The measured data were utilized to evaluate various use ful parame ters, viz., apparent molar volume, (fi ni ), partial molar volume, (fi ni o), partial molar isobaric expansibility, (fi i E o ), transfer volume (fi ni o (tr)), A and B – coefficients of viscosity, free energy of activation of viscous flow per mole of solvent (Delta mi 1 o*) and solute (Delta mi 2o*) and molar refractivity (RD). The linear varia ion of fi ni o with the number of carbon atoms (nC) present in the alkyl chain of amino acids was also considered for the evaluation of the contribution of zwitterionic end groups (NH3 + , COO–) and methylene groups to fi ni o. The results were used to discussion–ion, ion–nonpolar and nonpolar–nonpolar interactions and the presence of structural hydration in the present systems.