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PubMed2002

Isotope effects and the mechanism of an electron-transfer-catalyzed Diels-Alder reaction.

The electron-transfer-catalyzed Diels-Alder reaction of indole and 1,3-cyclohexadiene was studied by a combination of experimental and theoretical methods. The (13)C kinetic isotope effects were determined at natural abundance by NMR methodology. B3LYP/6-31G* calculations allow for the first time a quantitatively accurate description of the different possible pathways and provide the basis for an analysis of the experimentally observed isotope effects. The computational results, in conjunction with experimental observations, show that the reaction has a stepwise mechanism that is initiated by attack of the diene into the 3-position of the indole. Numerical simulation of the experimentally observed isotope effects shows that the first step is rate-determining and that the electron exchange in the reactant contributes partially to the overall isotope effect. The combination of electronic structure theory, experimental isotope effects, and numerical simulation thus allows a detailed analysis of a complex reaction mechanism.

Carbon Isotopes↗
PubMed2005

Isotope effects, dynamics, and the mechanism of solvolysis of aryldiazonium cations in water.

The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant (13)C kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C(2)-C(3) and C(5)-C(6) bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple S(N)1 heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N(2) by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the (13)C isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between S(N)2Ar and S(N)1 mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.

Carbon Isotopes↗
PubMed2001

Metabolic fate of isotopes during the biological transformation of carbohydrates to 2,5-dimethyl-4-hydroxy-3(2h)-furanone in strawberry fruits.

Isotopically labeled D-glucose, D-fructose, 1-deoxy-D-fructose, and 6-deoxyhexoses were applied to detached ripening strawberry (Fragaria x ananassa) fruits, and the incorporation of the isotopes into the key strawberry aroma compounds 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF, 1) and 2,5-dimethyl-4-methoxy-3(2H)-furanone (DMMF, 2) was determined by gas chromatography-mass spectrometry. In contrast to previous reports the data clearly showed that 6-deoxy-D-fructose/6-deoxy-D-glucose and 1-deoxy-D-fructose are not natural precursors of the furanones. However, isotopically labeled 1 and 2 were observed after the application of [1-(2)H]-, [2-(2)H]-, and [6,6-(2)H(2)]-D-glucose as well as [U-(13)C(6)]-, [1-(13)C]-, [1-(2)H]-, [6,6-(2)H(2)]-D-fructose. The isotope label of [4-(2)H]-D-glucose was not recovered in the furanones. In contrast, [2-(2)H]-D-glucose was converted to [1- or 6-(2)H]-1 and [1- or 6-(2)H]-2 by the strawberry fruits. The observed isotope shift can be explained by the catalysis of phosphohexose isomerase in the course of the biogenesis of the hydroxyfuranone (1) and the methoxyfuranone (2) from D-glucose. Thus, the applied D-glucose is metabolized to D-fructose-6-phosphate prior to the transformation into the furanones.

Fruit↗
PubMed2000

13C and (2)H kinetic isotope effects and the mechanism of Lewis acid-catalyzed ene reactions of formaldehyde.

13C and (2)H kinetic isotope effects were determined for the ene reaction of formaldehyde with 2-methyl-2-butene at natural abundance catalyzed by diethylaluminum chloride. The reactive methyl group exhibits a k(12)(C)/k(13)(C) of 1.006-1.009 and a k(H)/k(D) of approximately 1.22-1.23. The latter represents a combination of primary and secondary effects and is consistent with a significant primary deuterium isotope effect. A very close correspondence of the other isotope effects with the equilibrium isotope effects predicted for formation of a model intermediate cation is observed. An intermolecular deuterium isotope effect of 2.0-2.5 was observed under several reaction conditions in the Lewis acid-catalyzed reaction of formaldehyde with d(0)/d(12)-tetramethylethylene. The results are interpreted as supporting the reversible formation of an essentially classical open cation followed by rate-limiting proton transfer.

Acids↗
PubMed2006

Magnetic isotope effect in the photolysis of organotin compounds.

Photolysis of organotin molecules RSnMe3 is shown to be a spin selective radical reaction accompanied by fractionation of magnetic, (117,119)Sn, and nonmagnetic, (118,120)Sn, isotopes between starting reagents and products. A primary photolysis process is a homolytic cleavage of the C-Sn bond and generation of a triplet radical pair as a spin-selective nanoreactor. Nuclear spin dependent triplet-singlet conversion of the pair results in the tin isotope fractionation. Experimentally detected isotope distribution unambiguously demonstrates that the classical, mass-dependent isotope effect is negligible in comparison with magnetic, spin-dependent isotope effect.

Free Radicals↗
PubMed2006

In-Gel Stable-Isotope Labeling (ISIL): a strategy for mass spectrometry-based relative quantification.

Most proteomics approaches for relative quantification of protein expression use a combination of stable-isotope labeling and mass spectrometry. Traditionally, researchers have used difference gel electrophoresis (DIGE) from stained 1D and 2D gels for relative quantification. While differences in protein staining intensity can often be visualized, abundant proteins can obscure less abundant proteins, and quantification of post-translational modifications is difficult. A method is presented for quantifying changes in the abundance of a specific protein or changes in specific modifications of a protein using In-gel Stable-Isotope Labeling (ISIL). Proteins extracted from any source (tissue, cell line, immunoprecipitate, etc.), treated under two experimental conditions, are resolved in separate lanes by gel electrophoresis. The regions of interest (visualized by staining) are reacted separately with light versus heavy isotope-labeled reagents, and the gel slices are then mixed and digested with proteases. The resulting peptides are then analyzed by LC-MS to determine relative abundance of light/heavy isotope pairs and analyzed by LC-MS/MS for identification of sequence and modifications. The strategy compares well with other relative quantification strategies, and in silico calculations reveal its effectiveness as a global relative quantification strategy. An advantage of ISIL is that visualization of gel differences can be used as a first quantification step followed by accurate and sensitive protein level stable-isotope labeling and mass spectrometry-based relative quantification.

Chromatography, Liquid↗
PubMed1999

Martian stable isotopes: volatile evolution, climate change and exobiological implications.

Measurements of the ratios of stable isotopes in the martian atmosphere and crust provide fundamental information about the evolution of the martian volatile and climate system. Current best estimates of the isotope ratios indicate that there has been substantial loss of gases to space and exchange of gases between the atmosphere and the crust throughout geologic time; exchange may have occurred through circulation of water in hydrothermal systems. Processes of volatile evolution and exchange will fractionate the isotopes in a manner that complicates the possible interpretation of isotopic data in terms of any fractionation that may have been caused by martian biota, and must be understood first. Key measurements are suggested that will enhance our understanding of the non-biological fractionation of the isotopes and of the evolution of the martian volatile system.

Climate↗
PubMed1998

The use of stable carbon isotope analysis to detect the abuse of testosterone in cattle.

The use of stable carbon isotope analysis to detect the administration of anabolic steroids to cattle was investigated. Samples were extracted by solid-phase extraction on C18 cartridges. Stable isotope ratios (13C:12C) were measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) of the underivatised extracts. A programmed temperature vaporiser (PTV) injector was installed in the GC-IRMS system, which conferred a number of advantages. First, it allowed large volumes of sample to be injected whilst the injector liner was cool. The solvent was subsequently vented to the atmosphere prior to transfer of the sample to the GC column. Thus a significantly greater amount of sample could be presented for analysis, thereby increasing the sensitivity. Second, by this means virtually all the solvent could be removed prior to analysis. This eliminates solvent peak tailing, which can be a major problem in GC-IRMS. Finally, the PTV allowed the use of higher initial GC oven temperatures, which in turn facilitated the analysis of underivatised steroids by reducing the GC run time and improving the chromatographic peak shape. The carbon isotope composition of 5 beta-androstane-3 alpha,17 alpha-diol, the major metabolite of testosterone found in bovine bile, was measured in bile samples from untreated cattle and from cattle injected intramuscularly with testosterone or a mixture of testosterone esters. There was considerable inter-animal variation in the values obtained and there was no significant difference between samples from treated and untreated animals. However, when the isotopic composition of the metabolite was normalised with respect to that of an endogenous reference compound (cholesterol) in the same sample, the difference between treated and untreated animals become statistically significant.

Anabolic Agents↗
PubMed2001

Large carbon isotope fractionation associated with oxidation of methyl halides by methylotrophic bacteria.

The largest biological fractionations of stable carbon isotopes observed in nature occur during production of methane by methanogenic archaea. These fractionations result in substantial (as much as approximately 70 per thousand) shifts in delta(13)C relative to the initial substrate. We now report that a stable carbon isotopic fractionation of comparable magnitude (up to 70 per thousand) occurs during oxidation of methyl halides by methylotrophic bacteria. We have demonstrated biological fractionation with whole cells of three methylotrophs (strain IMB-1, strain CC495, and strain MB2) and, to a lesser extent, with the purified cobalamin-dependent methyltransferase enzyme obtained from strain CC495. Thus, the genetic similarities recently reported between methylotrophs, and methanogens with respect to their pathways for C(1)-unit metabolism are also reflected in the carbon isotopic fractionations achieved by these organisms. We found that only part of the observed fractionation of carbon isotopes could be accounted for by the activity of the corrinoid methyltransferase enzyme, suggesting fractionation by enzymes further along the degradation pathway. These observations are of potential biogeochemical significance in the application of stable carbon isotope ratios to constrain the tropospheric budgets for the ozone-depleting halocarbons, methyl bromide and methyl chloride.

Bacteria↗
PubMed2002

Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates.

Nearly all chemical processes fractionate 17O and 18O in a mass-dependent way relative to 16O, a major exception being the formation of ozone from diatomic oxygen in the presence of UV radiation or electrical discharge. Investigation of oxygen three-isotope behavior during thermal decomposition of naturally occurring carbonates of calcium and magnesium in vacuo has revealed that, surprisingly, anomalous isotopic compositions are also generated during this process. High-precision measurements of the attendant three-isotope fractionation line, and consequently the magnitude of the isotopic anomaly (delta17O), demonstrate that the slope of the line is independent of the nature of the carbonate but is controlled by empirical factors relating to the decomposition procedure. For a slope identical to that describing terrestrial silicates and waters (0.5247 +/- 0.0007 at the 95% confidence level), solid oxides formed during carbonate pyrolysis fit a parallel line offset by -0.241 +/- 0.042 per thousand. The corresponding CO2 is characterized by a positive offset of half this magnitude, confirming the mass-independent nature of the fractionation. Slow, protracted thermolysis produces a fractionation line of shallower slope (0.5198 +/- 0.0007). These findings of a 17O anomaly being generated from a solid, and solely by thermal means, provide a further challenge to current understanding of the nature of mass-independent isotopic fractionation.

Calcium Carbonate↗
PubMed2004

Stable isotope-labelled vitamin C as a probe for vitamin C absorption by human subjects.

Factors affecting absorption of physiological doses of vitamin C in man have not been widely studied, partly because few suitable tools exist to distinguish recently absorbed vitamin C from endogenous vitamin. Stable isotope-labelled vitamin C provides such a tool. Fifteen healthy non-smoking subjects aged 26-59 years were studied. Each received 30 mg l-[1-(13)C]ascorbic acid orally on two occasions, 3-4 weeks apart. The ascorbate was given alone or with Fe (100 mg as ferrous fumarate) or with red grape juice, which is rich in polyphenols. Blood was collected at frequent intervals for 1 h, and then each hour for a further 3 h. Total concentration of vitamin C was measured fluorometrically and its (13)C-isotope enrichment was measured by GC-MS after conversion to volatile trimethylsilyl esters. Peak plasma enrichment occurred within 25-50 min. No kinetic variables were significantly altered by the iron fumarate supplement. Grape juice attenuated vitamin C absorption, reaching significance at the 20 min time point. There were weak correlations between isotope enrichment and body weight or endogenous ascorbate concentration. The increment in total plasma ascorbate was smaller if calculated from isotope enrichment than from vitamin C concentration increase. The dilution pool was much larger than the plasma ascorbate pool. Further studies are needed to resolve these paradoxes. Stable isotope-labelled ascorbate is potentially useful for measurement of vitamin C absorption by human subjects.

Absorption↗
PubMed2002

Sulfur isotopic composition of H2S and SO4(2-) from mineral springs in the Polish Carpathians.

A number of springs in Carpathian Mts. contain dissolved H2S and SO4(2-) in concentrations above 10 mg/dm3. In this study we have investigated the sulfur isotope composition (delta34S) of the dissolved sulfur species in the springs from the flysch area in the Carpathian Mts. along the tectonic dislocation. It is believed that some of these springs may carry a major fraction of dissolved sulfur species of extremely deep sulfur (of mantle origin), which is subjected to SO4(2-)-H2S isotope exchange at high temperatures. The original isotopic compositions may be modified by reduction/oxidation at low temperatures and by admixture of sulfur from other sources. In order to distinguish the sulfur of mantle origin we investigated delta34S of dissolved sulfide and sulfate and on the basis of known concentrations we calculated delta34S of total dissolved sulfur. The isotope fractionation between sulfate and sulfide helped to distinguish the sulfur origin. Evaluating the sulfur isotope exchange, we selected 4 springs which likely have only weakly disturbed sulfur of mantle origin.

Air Pollutants↗
PubMed2004

Stable isotope biogeochemistry of the sulfur cycle in modern marine sediments: I. Seasonal dynamics in a temperate intertidal sandy surface sediment.

A biogeochemical and stable isotope geochemical study was carried out in surface sediments of an organic-matter poor temperate intertidal sandy surface sediment (German Wadden Sea of the North Sea) to investigate the activity of sulfate-reducing bacteria and the dynamics of the vertical partitioning of sedimentary sulfur, iron, and manganese species in relation to the availability of total organic carbon (TOC) and mud contents. The contents and stable isotopic compositions ((34)S/(32)S) of total reduced inorganic sulfur species (TRIS) and dissolved sulfate were measured. Maximum oxygen penetration depths were estimated from the onset of a blackening of the sediments due to FeS accumulation and ranged from 5 to 10 mm below surface (mmbsf). A zone of relatively moderate relative organic-matter enrichment was found between 5 and 20 mmbsf leading to enhanced activities of sulfate-reducing bacteria with sulfate-reduction rates (SRR) up to 350 nmol cm(-3) d(-1). Below this zone, microbial SRR dropped significantly. Depth integrated SRR seem to depend not only on temperature but also on the availability of reactive organic matter. The sulfur-isotopic composition of TRIS was depleted in (34)S by 33-40 per thousand with respect to coexisting dissolved sulfate (constant at about +21 per thousand vs. Vienna-Canyon Diablo Troilite (V-CDT)). Since sulfate reduction is not limited by dissolved sulfate (open system), depth variations of the isotopic composition of TRIS reflect changes in overall isotope effect due to superimposed microbial and abiotic reactions. Most of the solid-phase iron and manganese was bonded to (non-reactive) heavy minerals. However, a layer of reactive Fe(III) and Mn(IV) oxi(hydroxi)des was found in the uppermost sediment section due to re-oxidation of dissolved Fe(II) and Mn(II) species at the sediment-water interface. Metal cycling below the surface is at least partially coupled to intense sulfur cycling.

Carbon↗
PubMed1992

The analysis of stable isotopes in urine to determine the fractional absorption of zinc.

We measured isotopic enrichment in urine after oral and intravenous administration of stable isotopes of zinc to determine fractional absorption (FA). 68Zn and 70Zn were administered orally and intravenously to four normal adults. Subsequently, urine and fecal samples were collected for 7 and 14 d, respectively, ashed, and passed through ion-exchange columns to separate zinc from other elements. Samples were analyzed by fast-atom-bombardment mass spectrometry. From 32 h onwards the enrichment of 68Zn and 70Zn in urine declined proportionately so that FA could be determined as follows: FA = enrichment (oral/iv) x dose (iv/oral). FA determinations from urine and feces (cumulative excretion) were, respectively, for subject ZK1, urine 0.79 +/- 0.03 and feces 0.70 +/- 0.01; ZK2, 0.79 +/- 0.05 and 0.69 +/- 0.02; ZK3, 0.26 +/- 0.01 and 0.25 +/- 0.01; and ZK4, 0.41 +/- 0.02 and 0.37 +/- 0.02. ZK1 and ZK2 received the oral isotope while fasting whereas ZK3 and ZK4 received the oral isotope with meals. FA of zinc can be determined by measurement of isotope enrichment in urine.

Absorption↗
PubMed2004

Isotopica: a tool for the calculation and viewing of complex isotopic envelopes.

The web application Isotopica has been developed as an aid to the interpretation of ions that contain naturally occurring isotopes in a mass spectrum. It allows the calculation of mass values and isotopic distributions based on molecular formulas, peptides/proteins, DNA/RNA, carbohydrate sequences or combinations thereof. In addition, Isotopica takes modifications of the input molecule into consideration using a simple and flexible language as a straightforward extension of the molecular formula syntax. This function is especially useful for biomolecules, which are often subjected to additional modifications other than normal constituents, such as the frequently occurring post-translational modification in proteins. The isotopic distribution of any molecule thus defined can be calculated by considering full widths at half maximum or mass resolution. The combined envelope of several overlapping isotopic distributions of a mixture of molecules can be determined after specifying each molecule's relative abundance. The results can be displayed graphically on a local PC using the Isotopica viewer, a standalone application that is downloadable from the sites below, as a complement to the client browser. The m/z and intensity values can also be obtained in the form of a plain ASCII text file. The software has proved to be useful for peptide mass fingerprinting and validating an observed isotopic ion distribution with reference to the theoretical one, even from a multi-component sample. The web server can be accessed at http://bioinformatica.cigb.edu.cu/isotopica and http://coco.protein.osaka-u.ac.jp/isotopica [correction].

Amino Acid Sequence↗
PubMed2003

Metabolic flux analysis of the phenylpropanoid pathway in wound-healing potato tuber tissue using stable isotope-labeled tracer and LC-MS spectroscopy.

The metabolic flux of two phenylpropanoid metabolites, N-p-coumaroyloctopamine (p-CO) and chlorogenic acid (CGA), in the wound-healing potato tuber tissue was quantitatively analyzed by a newly developed method based upon the tracer experiment using stable isotope-labeled compounds and LC-MS. Tuber disks were treated with aqueous solution of L-phenyl-d(5)-alanine, and the change in the ratio of stable isotope-labeled compound to non-labeled (isotope abundance) was monitored for p-CO and CGA in the tissue extract by LC-MS. The time-dependent change in the isotope abundance of each metabolite was fitted to an equation that was derived from the formation and conversion kinetics of each compound. Good correlations were obtained between the observed and calculated isotope abundances for both p-CO and CGA. The rates of p-CO formation and conversion (i.e. fluxes) were 1.15 and 0.96 nmol (g FW)(-1) h(-1), respectively, and for CGA, the rates 4.63 and 0.42 nmol (g FW)(-1) h(-1), respectively. This analysis enabled a direct comparison of the biosynthetic activity between these two compounds.

Algorithms↗
PubMed2004

Tracing metabolic routes of feed ingredients in tissues of broiler chickens using stable isotopes.

The present study aimed to quantify the proportion of 13C from energy and protein feed ingredients that follow the metabolic routing of the liver and muscle in broiler chickens. A stable isotope of carbon technique was used that is based on the isotopic discrimination that occurs in the plants during the photosynthesis process. One-day-old male chicks were subjected to treatments based on free choice of energy and protein sources. Rice bran (R) and soybean meal (S), C3 plants, have higher isotopic ratios than corn (C), a C4 plant, and corn gluten meal (G). Choices were R+S, C+G, R+G, C+S, or R+C+G+S. A complete feed (CF) was a sixth treatment. Feed intake and BW were measured at 30 d of age, when liver and breast muscle were collected for isotopic analysis. Treatments affected the amount of feed intake and the choices of energy or protein sources. Complete feed had the largest intake, differing from the other treatments that had free-choice feeding. Final BW was a direct reflection of consumption by these birds in all treatments. The isotopic results indicated that the 13C/12C ratio was generally higher in breast muscle than in liver, probably because of higher protein content. Moreover, in the liver, the proportion of 13C retained from the energy ingredient was greater than the proportion from the protein ingredient. That is in contrast to muscle, where the proportion of 13C retained from the protein ingredient was greater than from the energy ingredient that was self-selected.

Animals↗
PubMed1995

Determination of iron absorption using erythrocyte iron incorporation of two stable isotopes of iron (57Fe and 58Fe) in very low birthweight premature infants.

Due to limited erythropoiesis, iron needs for the premature infant during the first 2 months of life are low. With the potential for increased use of erythropoietin in the preterm infant, iron requirements may become markedly higher. The ability of the preterm infant to absorb iron, therefore, becomes crucial. Previous studies using single stable isotopes of iron without metabolic balances were unable to quantitatively determine iron absorption since the percentage of absorbed iron appearing in the erythrocyte could not be measured. The objective of the current study was to determine iron absorption using the method of erythrocyte iron incorporation of two stable isotopes of iron given by the enteral and parenteral route to very low birth weight (VLBW) infants. Results obtained were compared to iron absorption values from studies using single isotopes and metabolic balance techniques. Six VLBW premature infants (gestational age 26.8 +/- 1.7 weeks, postnatal age 3.6 +/- 1.9 weeks, birth weight 863 +/- 117 g) were studied. Iron dosages were (i.v.) 0.15 mg/kg (57FeSO4) and (enteral) 1.5 mg/kg (58FeSO4). Erythrocyte isotope ratios, 57Fe/54Fe and 58Fe/54Fe, were determined by inductively coupled plasma mass spectrometry (ICP/MS) from single blood samples (100 microliters) collected before and after concurrent enteral (58Fe) and parenteral (57Fe) administration of isotopes. Only 17.8% of the i.v.-infused iron dose was incorporated into hemoglobin on day 15. Using a correction factor based on the percentage of i.v. iron (57Fe) incorporated into erythrocytes, the corrected incorporation of 58Fe was calculated to be 26.3 +/- 13.0% of the enteral dose of 58FeSO4.

Absorption↗