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1

Nitrogen-15 Kinetic Isotope Effects in the Decomposition of Nitric Oxide on Copper

100%

Polič S. , Senegačnik M. , Kobal I. , Zieliński M.

Polish Journal of Chemistry

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2002

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tom Vol. 76 / nr 7

1009-1018

EN

Isotopic rate constant ratios, k14/k15, have been determined for the decomposition of nitric oxide over copper wire at initial NO pressures 40-60 kPa over temperature range 725-825 K. They were interpreted using the Bigeleisen formalism. Under these experimental conditions the reaction was found to be second order in NO. On the basis of the temperature independence, k14/k15 = 1.010 š 0.001 at all temperatures studied, a cis ONNO transition state was shown to account satisfactorily for the experimental k14/k15 values.

2

Carbon and Chlorine Kinetic Isotope Effects and Solvent Effects on the Hydrolysis of Chloroformates

100%

Swanson S. , Rudziński J. , Paneth P. , O'Leary M. H.

Polish Journal of Chemistry

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2002

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tom Vol. 76 / nr 12

1721-1731

EN

Carbon and chlorine kinetic isotope effects and pseudo-first order rate constants were measured for the hydrolysis of ethyl, benzyl, and isopropyl chloroformate in various mixtures of the binary solvent water-1,4-dioxane. Rate constants decreased as the percentage of 1,4-dioxane increased for all three chloroformates. Carbon-13 kinetic isotope effects (k12/k13) for hydrolysis of these compounds were 1.039-1.042 for ethyl chloroformate (0-75% 1,4-dioxane), 1.034-1.039 for benzyl chloroformate (15-75% 1,4-dioxane), and 1.025-1.037 for isopropyl chloroformate (25-75% 1,4-dioxane). Chlorine-37 kinetic isotope effects (k35/k37) were measured for benzyl and isopropyl chloroformates in 25% 1,4-dioxane (1.0088, benzyl; 1.0080 isopropyl) and 75% 1,4-dioxane (1.0090, benzyl; 1.0102 isopropyl). These data are consistent with an associative mechanism for benzyl and ethyl chloroformates, but hydrolysis of isopropyl chloroformate appears to be dissociative in polar solvents.

3

Elucidation of reaction mechanisms using kinetic isotope effects of short-lived radionuclides

88%

Matsson O.

Nukleonika

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2002

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tom Vol. 47,suppl.1

43-45

EN

The use of the short-lived radionuclides 11C and 18F for the elucidation of organic reaction mechanisms is described. Examples of the different mechanistic problems that are discussed include concerted vs. stepwise base catalysed elimination (E2 or E1cB) and finding the rate limiting step for nucleophilic aromatic substitution (SNAr). The use of radionuclides to learn details about transition state structure for concerted nucleophilic aliphatic substitution (SN2) is also described.

4

DFT/PM3 study of the enoyl-CoA hydratase catalyzed reaction

75%

Pawlak J. , Bahnson B. , Anderson V.

Nukleonika

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2002

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tom Vol. 47,suppl.1

33-36

EN

The enoyl-CoA hydratase catalyzed hydration of alfa,beta-unsaturated thiolesters has been modeled by using the crystal structure of 4-(N,N-dimethylamino)cinnamoyl-CoA bound at the active site. The quantum chemical calculation used the ONIOM mixed level procedure to permit the substrate thiolester and water molecule to be modeled using B3LYP/6-31G(d) level of theory and the active site residues modeled at a semiempirical level using the PM3 Hamiltonian. The results permitted the identification of a stable thiolester enolate intermediate, supporting a stepwise reaction mechanism. The calculation also suggests that the same proton removed from the nucleophilic water molecule is transferred to C alfa in the subsequent protonation of the enolate intermediate. This observation reconciles the stepwise mechanism with the previously reported double isotope effect study [3].

5

Carbon-14 kinetic isotope effects in the debromination of p-nitrodibromocinnamic acid

75%

Bukowski J. , Kanski R. , Kanska M.

Nukleonika

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2002

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tom Vol. 47, nr 4

133-136

EN

The kinetic isotope effect (KIE) method was applied to study the mechanism of elimination of bromine from p-nitro-erythro-alfa,beta-dibromocinnamic acid labeled at the alfa-carbon. This compound was obtained starting from [2-14C]malonic acid via [2-14C]cinnamic acid and subsequent addition of bromide. The value of 14C KIE determined for alfa-position of side chain of p-nitro-erythro-alfa,beta-dibromocinamic acid proves that elimination of bromine leading to formation of (E)-p-nitrocinnamic acid proceeds via E2 mechanism.

6

Structural aspects of the inhibition and catalytic mechanism of thymidylate synthase

63%

Hardy L. W.

Acta Biochimica Polonica

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1995

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tom 42

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nr 4

EN

Thymidylate synthase (TS) is a target for anticancer drugs, due to its unique role in the biosynthesis of an essential DNA precursor. The X-ray structures available for several bacterial enzymes have been used to design novel inhibitors of TS, to structurally analyze the binding mode of existing inhibitors, and to propose catalytic roles for amino-acid residues on the protein. The first part of this paper describes some aspects of structure-based drug design, including a recent result from the groups of Montfort and Maley emphasizing the importance of conformational changes in inhibitor binding. The second part of the paper describes the work of the author on the TS mechanism, especially the catalytic roles of active site amino acids Asnl77 and Glu58 in TS from Escherichia coli. An important function for Glu58 is proposed to be preventing the excessive stabilization of a covalent intermediate. The use of isotope effects to probe the mechanistic basis for stimulation of E. coli TS by magnesium ions, and to identify differences between the E. coli and human enzymes, is described. The hypothesis that N5 of tetrahydrofolate provides the basicity for deprotonation of the nucleotide is also discussed.

7

Carbon isotope distribution along pine needles [Pinus nigra Arnold]

51%

Barszczowska L. , Jedrysek M. O.

Acta Societatis Botanicorum Poloniae

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2005

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tom 74

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nr 2

EN

In this paper we show spatial carbon isotope variations in black pine (Pinus nigra Arnold) needles, collected in spring 2001 and autumn 2003, from trees at the coast of south-western Croatia island (Žirje) and southern Spain (Benalmadena near Malaga), respectively. Needles were segmented perpendicularly to the longer axis (base, middle and top) and each segment was analysed separately. d13C values in needles from Croatia varied between -26.65‰ to -24.43‰ (2 months old needles) and from -28.25‰ to -25.21‰ (1 year old needles), while d13C values in needles from Spain varied from -27.58‰ to -25.27‰. The difference between d13Cb (base) and d13Ct (top) in the same needle (D13Cb-t) varied from 1.85 to 2.05‰ (in young needles from Croatia), from 0,02 to 1,80‰ (young needles from Spain), and 1.16 to 2.32‰, (in old needles from Croatia). The average D13Cb-t values were 0.78‰ and 1.73‰ in Spain and Croatia, respectively. In each needle the base of the needle was always 13C-enriched as compared to the top of the same needle. This evidences that carbon isotopes are not retranslocated after its fixation into the leaf structure (after the growth process is over). Temperature variation was most probably negligible for the discovered intraneedle carbon isotope distribution. Although, the intraneedle carbon isotope inhomogeneity can be partly the result of seasonal variation in d13C of atmospheric CO2, most probably the remarkably high D13Cb-t values, and regular pattern, are predominantly resulting from isotopic and chemical composition of primary and secondary products contained in the growin part of needle and kinetic isotope fractionation during decomposition of storage materials at the base.