18O Isotope Effects Research Articles

Remarkably Large 15N, 13C and 18O Isotope NMR Shifts of the Mono-coordinate Phosphorus Atoms in the Compounds (Me3 Si)iPrNCP, K+[iPrNCP]-and KOCP · 2 DME (DME = 1,2-Dimethoxyethane)

The 31P and 13C NMR spectra of the heteroatom-substituted λ3-phosphaalkynes (Me3Si)- iPrNCP (1), K+ [iPrNCPl- (2) and KOCP · 2 DME (3) are described (Me = methyl; iPr = isopropyl). In addition,15N NMR data of 1 and 2 as well as further NMR results of all accessible nuclei of 1 to 3 are reported. The absolute values of the coupling constants 1J(31P13C) of 1, 2 and 3, 18.2, 45.7 and 57.2 Hz, respectively, are considerably different; the geminal coupling constants 2J( 31P15N) of the first two compounds show a decrease in the reverse order, 15.1 and 3.3 Hz. 13C, 29Si,15N, and 18O isotope effects on the nuclear shielding of the 31P nucleus in the non isotope-enriched compounds 1, 2 or 3 are presented and discussed. The [mono-13C]isotopomers of 1, 2 and 3 exhibit extremely negative 31P13C one-bond isotope shifts varying from -211 to -223 ppb. Remarkably, for 1 and 2 large two-bond 31P15N isotope shifts of -32 and -84 ppb, respectively, are observed. An unexpectedly large two-bond 31P180 isotope effect of -124 ppb was assigned to the [mono-180]isotopomer of compound 3. A three-bond 31P13C isotope shift of -27 ppb was found in 2.

1,2-Migration in β-(acyloxy)ethyl radicals revisited—concerted or stepwise?

The acyloxy rearrangement in 2-(acetyloxy)-2-methyl-1-propyl radical (1a) and in 2-(trifluoroacetyloxy)-2-methyl-1-propyl radical (1b) has been investigated with a number of theoretical methods. In both systems the most favorable reaction pathway for 1,2-acyloxy rearrangement leads through a five-membered ring transition state in a concerted fashion. A second pathway through a three-membered ring transition state is only slightly less favorable, while the addition–elimination process through a cyclic 1,3-dioxolan-2-yl radical intermediate has significantly higher barriers. Stationary points corresponding to a contact ion pair could not be found. With respect to the most favorable reaction pathway, the barrier difference between substrates 1a and 1b amounts to 2.5 ± 0.1 kcal mol−1 at a variety of theoretical levels. Solvent effects for the concerted pathways in hydrocarbon solvents, alcohols, and water have been calculated using the PCM scheme and found to be of rather limited magnitude. The barrier difference for reaction of 1a in hydrocarbon solvents (cyclohexane, benzene) and in water was estimated to be around 1 kcal mol−1. Based on kinetic isotope effects calculated for the [3,2]- and [1,2]-acyloxy rearrangement processes a differentiation of pathways is most easily possible through the strongly inverse deuterium isotope effects for d2-labeling of the radical center in 1 and through the ratio of the 18O isotope effects on the carboxylate oxygen atoms.

Effect of Concomitant18O in13C-Urea on the Urea breath test

Helicobacter pylori infection in stomach has been extensively assessed by the 13C-urea breath test (UBT) with infra-red or mass spectrometer. However, analyses of commercially available 13C-urea for UBT showed about 10 atom% of 18O abundance. It is therefore very important to investigate the 18O-isotope effect on UBT. Using a mass spectrometer, breath samples of UBT were analyzed to investigate the change of mass spectra by the isotopic exchange reaction between 18O of carbon dioxide and 16O of water. As the result, the 18O content of 13C, 18O-carbon dioxide exhaled in UBT reached its natural abundance level immediately after the administration of 13C, 18O-urea. It was concluded that the 18O content of 13C, 18O-urea has no effect on UBT using either infra-red or mass spectroscopy. Copyright © 2000 John Wiley & Sons, Ltd.

Carbon-13 and Oxygen-17 Chemical Shifts, (16O/18O) Isotope Effects on13C Chemical Shifts, and Vibrational Frequencies of Carbon Monoxide in Various Solvents and of the Fe−C−O Unit in Carbonmonoxy Heme Proteins and Synthetic Model Compounds

13C shieldings, δ(13C), 17O shieldings, δ(17O), and 18O isotope effects on 13C shieldings, 1Δ13C(18O/16O), of carbon monoxide (99.7% 13C, 0.9% 17O, and 11.8% 18O enriched) in a variety of solvents and of the Fe−C−O unit of several carbonmonoxy hemoprotein models with varying polar and steric effects of the distal organic superstructure and constraints of the proximal side are reported. This enables, first, comparisons with hemoproteins, C−O vibrational frequencies, ν(C−O), and X-ray structural data to be made; second, to investigate whether polarizable CO is an adequate model for distal ligand effects in carbonmonoxy heme proteins and synthetic model compounds; third, to investigate the effect of electronic perturbation within the heme pocket and pocket deformation on δ(13C), δ(17O), 1Δ13C(18O/16O), and ν(C−O). A variety of solvents with varying dielectric constants and solvation abilities appears to have negligible effect on δ(17O), δ(13C), and 1Δ13C(18O/16O) and little direct effect on ν(C−O) of dissolv...

Analysis of concomitant18O in13C-urea

The 13C-urea breath test by infra-red absorption or mass spectra has been extensively used to detect. Helicobacter pylori infection. However, commercially available 13C-urea contains higher 18O, compared to natural urea. It is evidently important for this diagnosis to investigate the 18O-isotope effect on infra-red absorption or mass spectra, based on the quantitative analysis of 18O in 13C-urea. For this purpose, a new analytical method using mass spectroscopy has been established to determine 18O abundance in urea. Copyright © 1999 John Wiley & Sons, Ltd.

Nature of oxygen activation in glucose oxidase from Aspergillus niger: the importance of electrostatic stabilization in superoxide formation.

Glucose oxidase catalyzes the oxidation of glucose by molecular dioxygen, forming gluconolactone and hydrogen peroxide. A series of probes have been applied to investigate the activation of dioxygen in the oxidative half-reaction, including pH dependence, viscosity effects, 18O isotope effects, and solvent isotope effects on the kinetic parameter Vmax/Km(O2). The pH profile of Vmax/Km(O2) exhibits a pKa of 7.9 +/- 0.1, with the protonated enzyme form more reactive by 2 orders of magnitude. The effect of viscosogen on Vmax/Km(O2) reveals the surprising fact that the faster reaction at low pH (1.6 x 10(6) M-1 s-1) is actually less diffusion-controlled than the slow reaction at high pH (1.4 x 10(4) M-1 s-1); dioxygen reduction is almost fully diffusion-controlled at pH 9.8, while the extent of diffusion control decreases to 88% at pH 9.0 and 32% at pH 5.0, suggesting a transition of the first irreversible step from dioxygen binding at high pH to a later step at low pH. The puzzle is resolved by 18O isotope effects. 18(Vmax/Km) has been determined to be 1.028 +/- 0.002 at pH 5.0 and 1.027 +/- 0.001 at pH 9.0, indicating that a significant O-O bond order decrease accompanies the steps from dioxygen binding up to the first irreversible step at either pH. The results at high pH lead to an unequivocal mechanism; the rate-limiting step in Vmax/Km(O2) for the deprotonated enzyme is the first electron transfer from the reduced flavin to dioxygen, and this step accompanies binding of molecular dioxygen to the active site. In combination with the published structural data, a model is presented in which a protonated active site histidine at low pH accelerates the second-order rate constant for one electron transfer to dioxygen through electrostatic stabilization of the superoxide anion intermediate. Consistent with the proposed mechanisms for both high and low pH, solvent isotope effects indicate that proton transfer steps occur after the rate-limiting step(s). Kinetic simulations show that the model that is presented, although apparently in conflict with previous models for glucose oxidase, is in good agreement with previously published kinetic data for glucose oxidase. A role for electrostatic stabilization of the superoxide anion intermediate, as a general catalytic strategy in dioxygen-utilizing enzymes, is discussed.

Investigation of Lattice Effects in Rare Earth Manganites by 18O-isotope Exchange

The strong coupling between the electron spin and lattice arising from the Jahn-Teller effect of manganese ions plays an important role in the mechanism of colossal magnetoresistance and related properties of the rare earth manganites. The lattice effects in this class of oxides have been extensively studied through the application of hydrostatic as well as chemical pressures and magnetic fields. The recently observed giant 18O isotope effect provides direct evidence for the formation of lattice polarons in manganites. Here we report the preliminary results of our investigations on a variety of normal as well as 18O isotope-exchanged perovskite manganites exhibiting colossal magnetoresistance and charge ordering. The observed isotope shift of Tc as well as that of Tco is correlated with the key parameters controlling the lattice such as the Mn 3+ content, the average ionic radius of the A-site cation ⟨rA⟩ , and the A-site ionic size disorder σ2 .

13C shieldings, 18O isotope effects on 13C shieldings, and 57Fe-13C spin couplings of the Fe-C-O unit in superstructured hemoprotein models: Comparison with hemoproteins, C-O vibrational frequencies, and X-ray structural data

13C NMR spectra of several carbon monoxide (99.7% 13C and 11.8% 18O enriched) hemoprotein models with varying polar and steric effects of the distal organic superstructure and constraints of the proximal side are reported. This enables the 57Fe-13C(O) coupling constants (\(^{\text{1}} {\text{J}}_{{\text{57}}_{{\text{Fe}}} \_13_{\text{C}} }\)), 13C shieldings (δ(13C)), and 18O isotope effects on13 C shieldings (1Δ13C(18O/16O)) to be measured and hence comparisons with hemoproteins, C-O vibrational frequencies and X-ray structural data to be made. Negative polar interactions in the binding pocket and inhibition of Fe/→/CO back-donation or positive distal polar interactions with amide NH groups appear to have little direct effect on \(^{\text{1}} {\text{J}}_{{\text{57}}_{{\text{Fe}}} \_13_{\text{C}} }\) couplings. Similarly, the axial hindered base 1,2-dimethylimidazole has a minor effect on the \(^{\text{1}} {\text{J}}_{{\text{57}}_{{\text{Fe}}} \_13_{\text{C}} }\) values despite higher rates of CO desorption being observed for such complexes. On the contrary,13 C shieldings vary widely and an excellent correlation was found between the infrared C-O vibrational frequencies (ν(C-O)) and13 C shieldings and a reasonable correlation with18 O isotope effects on 13C shieldings. This suggests that δ(13C), ν(C-O) and1 Δ13 C(18O/16O) are accurate monitors of the multiple mechanisms by which steric and electronic interactions are released in superstructured heme model compounds. The 13C shieldings of heme models cover a 4.0 ppm range which is extended to 7.0 ppm when several HbCO and MbCO species at different pH values are included. The latter were found to obey a similar linear δ(13 (13C) versus ν(C-O) relationship, which proves that both heme models and heme proteins are homogeneous from the structural and electronic viewpoint. Our results suggest that ν(C-O), δ(13C) and 1Δ13C(18O/16O) measurements reflect similar interaction which is primarily the modulation of π back-bonding from the Fe dπ to the CO π* orbital by the distal pocket polar interactions. The lack of correlation between1 Δ13 C(18O/16O) and crystallographic CO bond lengths (r(C-O)) reflects significant uncertainties in the X-ray determination of the carbon and oxygen positions.

Comparative Study of 17O and 18O Isotope Effects As a Probe for Dioxygen Activation: Application to the Soybean Lipoxygenase Reaction

We present the first comparative study of 17O and 18O isotope effects in a dioxygen-dependent enzymatic reaction. The binding of dioxygen to soybean lipoxygenase is expected to be near the diffusive limit, introducing the possibility of a magnetic isotope effect with 17O reacting faster than 16O. However, the experimental data indicate a purely mass effect among the 16O, 17O, and 18O isotopes. A reaction mechanism for soybean lipoxygenase is proposed in which dioxygen undergoes reversible reaction with a substrate derived radical, followed by a slow release of the bound lipid hydroperoxide.

Site-selective oxygen isotope effect in highly doped YBa 2Cu 3O x

New measurements of the oxygen ( 16O vs. 18O) isotope effect (OIS) in optimally doped YBa 2Cu 3O x ( x=6.95) are presented. A new two-step exchange process allowed the simultaneous preparation of fully isotope exchanged YBa 2Cu 3O x samples and site-selective substituted samples. this allows a direct comparison of the OIS of the different oxygen sites (site-selective OIS). In contract to a recent OIS study by Nickel et al., our results demonstrate a conventional site-selective OIS. The observed site-selective OIS indicates that the CuO 2 plane sites play a significant role in the phonon-mediated part of the electron pairing.

Oxygen‐18 labeling of cyclic and acyclic sulfinate esters

AbstractBoth cyclic and acyclic sulfinate esters were labeled with 18O at the sulfinyl oxygen by acid‐catalyzed isotope exchange with H218O or alkaline hydrolysis in H218O followed by re‐esterification. Long‐range 18O isotope effects on the 13C NMR chemical shifts were observed.

Transition-state structures for enzymatic and alkaline phosphotriester hydrolysis.

The primary and secondary 18O isotope effects for the alkaline (KOH) and enzymatic (phosphotriesterase) hydrolysis of two phosphotriesters, O,O-diethyl p-nitrophenyl phosphate (I) and O,O-diethyl O-(4-carbamoylphenyl) phosphate (II), are consistent with an associative mechanism with significant changes in bond order to both the phosphoryl and phenolic leaving group oxygens in the transition state. The synthesis of [15N, phosphoryl-18O]-, [15N, phenolic-18O]-, and [15N]-O,O-diethyl p-nitrophenyl phosphate and O,O-diethyl O-(4-carbamoylphenyl)phosphate is described. The primary and secondary 18O isotope effects for the alkaline hydrolysis of compound I are 1.0060 and 1.0063 +/- 0.0001, whereas for compound II they are 1.027 +/- 0.002 and 1.025 +/- 0.002, respectively. These isotope effects are consistent with the rate-limiting addition of hydroxide and provide evidence for a SN2-like transition state with the absence of a stable phosphorane intermediate. For the enzymatic hydrolysis of compound I, the primary and secondary 18O isotope effects are very small, 1.0020 and 1.0021 +/- 0.0004, respectively, and indicate that the chemical step in the enzymatic mechanism is not rate-limiting. The 18O isotope effects for the enzymatic hydrolysis of compound II are 1.036 +/- 0.001 and 1.0181 +/- 0.0007, respectively, and are comparable in magnitude to the isotope effects for alkaline hydrolysis, suggesting that the chemical step is rate-limiting. The relative magnitude of the primary 18O isotope effects for the alkaline and enzymatic hydrolysis of compound II reflect a transition state that is more progressed for the enzymatic reaction.

ChemInform Abstract: Secondary 18O Isotope Effects as a Tool for Studying Reactions of Phosphate Mono‐, Di‐, and Triesters

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Secondary oxygen-18 isotope effects for hexokinase-catalyzed phosphoryl transfer from ATP

Secondary 18O isotope effects in the gamma-position of ATP have been measured on phosphoryl transfer catalyzed by yeast hexokinase in an effort to deduce the structure of the transition state. The isotope effects were measured by the remote-label method with the exocyclic amino group of adenine as the remote label. With glucose as substrate, the secondary 18O isotope effect per 18O was 0.9987 at pH 8.2 and 0.9965 at pH 5.3, which is below the pK of 6.15 seen in the V/K profile for MgATP. With the slow substrate 1,5-anhydro-D-glucitol, the value was 0.9976 at pH 8.2. While part of the inverse nature of the isotope effect may result from an isotope effect on binding, the more inverse values when catalysis is made more rate limiting by decreasing the pH or switching to a slower substrate suggest a dissociative transition state for phosphoryl transfer, in agreement with predictions from model chemistry. The 18O equilibrium isotope effect for deprotonation of HATP3- is 1.0156, while Mg2+ coordination to ATP4- does not appear to be accompanied by an 18O isotope effect larger than 1.001.

Investigations utilizing the 18O isotope shift in 13C nuclear magnetic resonance spectroscopy. 1. Correlation with 17O NMR chemical shifts

AbstractA survey of the one‐bond upfield 18O isotope effects on 13C NMR chemical shifts shows that a linear relationship exists between the magnitude of the isotope shift and the 17O NMR chemical shift for the corresponding oxygen atom in the molecule. The relationship holds over the entire range of functional groups to a significance level of >99.999% for 66 pairs of data points. This relationship permits an estimation with confidence of the general magnitude of an 18O isotope shift in 13C NMR, simply from a knowledge of the 17O NMR chemical shift for an oxygen atom of interest. Extrapolation of this relationship anticipates observation of downfield isotope shifts and may assist in an explanation of this rare and unusual phenomenon in NMR spectroscopy.

Secondary 18 O isotope effects as a tool for studying reactions of phosphate mono‐, di‐, and triesters 1

Secondary 18O isotope effects have been developed as a tool for determining transition state structures in enzymatic and nonenzymatic phosphoryl transfer reactions. 18O substitution in the nonbridge oxygens of a phosphoryl group makes the reaction go faster when the bond order is higher to these oxygens in the transition state than in the reactant, whereas the reaction goes slower if the bond order is less. The isotope effects are measured by the remote label method, using an isotope ratio mass spectrometer for analysis. The bond order to p-nitrophenolate ion when it is the leaving group is indicated by the secondary 15N isotope effect in the nitro group, with a value of 1.0028 representing nearly complete bond cleavage. It appears that the transition states for phosphoryl transfer have no more than one negative charge on the nonbridge oxygens, so that reactions of monoesters are dissociative, reactions of triesters are associative, and reactions of diesters are SN2 with half bond order to entering and leaving groups.

à 1B2–X̃ 1A1 26v transitions of 18O-enriched tropolone

Laser excitation spectra with v=0, 2, 4, and 6 in the à 1B2–X̃ 1A1 26v0 progression of jet-cooled 18O/16O isotopomers of tropolone are reported. The isotope shift for ν26, an out-of-plane deformation mode at 39 cm−1 in the à state, is 2% for tropolone-18O18O. This large 18O isotope effect indicates that Q26 for tropolone resembles the analogous normal mode of tropone, which is a ring deformation towards the boat conformation of 2, 4, 6-cycloheptatriene accompanied by a large O atom displacement. Tunneling by tropolone in the à state is quenched by exciting the 26v overtone states and a mechanism for this quenching is proposed in terms of the indicated normal coordinate. Tunneling splittings are <0.3 cm−1 for the zero point levels of the X̃ state of the symmetrical isotopomers. In contrast, vibrational isotope effects dominate the tunneling interactions to split the corresponding levels of tropolone-16O18O by 1.7 cm−1. In the à state of this isotopomer the tunneling interactions are dominant. Because they are determined by the overlap between localized and delocalized wave functions, the Franck–Condon factors of tropolone-16O18O are smaller than those of the symmetrical isotopomers.

18O isotope effect on Tc in La2-xSrxCuO4 as a function of X

The 18O/16O isotope effect on Tc in La2-xSrxCuO4 is a strong function of x with α ≊ (0.4−0.06) at x < 0.15 and α ≊ 0.1 at x ≥ 0.15.

Evidence for phonon-mediated coupling in superconducting Ba1−xKxBiO3

Superconducting Ba1−xKxBiO3, with a Tc of 30K, shows a large 18O isotope effect which indicates that phonons are involved in the pairing mechanism. Superconducting energy gap measurements from IR reflectivity and tunneling are consistent with moderate coupling (2Δ/kTc = 3.5 ± 0.5). A characteristic phonon energy of about 40 meV would be required to obtain the high Tc. Neuron scattering measurements show a large density of phonons in the range 40 to 80 meV and strong coupling of electrons to these modes is indicated in tunneling spectroscopy. Additional results are reported, including the structural phase diagram, which suggest that superconductivity is phonon mediated.

Isotope effects on the crotonase reaction.

The primary, alpha-secondary, beta-secondary, and beta'-secondary deuterium and primary 18O kinetic isotope effects on V/K for the dehydration of [(3S)-3-hydroxybutyryl]pantetheine by bovine liver crotonase (enoyl-CoA hydratase, EC 4.2.1.17) have been determined by the equilibrium perturbation method. The primary deuterium and 18O kinetic isotope effects are 1.61 and 1.051, respectively. The secondary deuterium effects at C-2, C-3, and C-4 are 1.12, 1.13, and 1.00 per H, respectively. The large 18O isotope effect suggests C-O bond cleavage is largely rate determining but is consistent with either an E1cb or E2 mechanism with a large amount of carbanion character. The beta-secondary effect is a factor of 1.05 greater than the equilibrium isotope effect, indicating that this C-H bond is less stiff in the affected transition state or that its motion is coupled to the reaction coordinate motion. Analytical solutions to the differential equations describing uni-uni equilibrium perturbations are presented.