Synthetic methodologies in organic chemistry involving incorporation of [¹⁷O] and [¹⁸O] isotopes

Development of Catalytic Site-Selective C-H Oxidation

Direct C-H bond oxygenation is a strong and useful tool for the construction of oxygen functional groups. After Chen and White's pioneering works, various non-heme-type iron and manganese complexes were introduced, leading to strong development in this area. However, for this method to become a truly useful tool for synthetic organic chemistry, it is necessary to make further efforts to improve site-selectivity, and catalyst durability. Recently, we found that non-heme-type ruthenium complex cis-1 presents efficient catalysis in C(sp3 )-H oxygenation under acidic conditions. cis-1-catalysed C-H oxygenation can oxidize various substrates including highly complex natural compounds using hypervalent iodine reagents as a terminal oxidant. Moreover, the catalyst system can use almost stoichiometric water molecules as the oxygen source through reversible hydrolysis of PhI(OCOR)2 . It is a strong tool for producing isotopic-oxygen-labelled compounds. Moreover, the environmentally friendly hydrogen peroxide can be used as a terminal oxidant under acidic conditions.

Preparation of 18O-labelled azaspiracids for accurate quantitation using liquid chromatography-mass spectrometry

Azaspiracids (AZAs) are a group of polyether marine algal toxins known to accumulate in shellfish, posing a risk to human health and the seafood industry. Analysis of AZAs is typically performed using LC-MS, which can suffer from matrix effects that significantly impact the accuracy of measurement results. While the use of isotopic internal standards is an effective approach to correct for these effects, isotopically labelled standards for AZAs are not currently available. In this study, 18O-labelled AZA1, AZA2, and AZA3 were prepared by reaction with H218O under acidic conditions, and the reaction kinetics and sites of incorporation were studied using LC-HRMS/MS aided by mathematical analysis of their isotope patterns. Analysis of the isotopic incorporation in AZA1 and AZA3 indicated the presence of four exchangeable oxygen atoms. Excessive isomerization occurred during preparation of 18O-labelled AZA2, suggesting a role for the 8-methyl group in the thermodynamic stability of AZAs. Neutralized mixtures of 18O-labelled AZA1 and AZA3 were found to maintain their isotopic and isomeric integrities when stored at -20 °C and were used to develop an isotope-dilution LC-MS method which was applied to reference materials of shellfish matrices containing AZAs, demonstrating high accuracy and excellent reproducibility. Preparation of isotopically labelled compounds using the isotopic exchange method, combined with the kinetic analysis, offers a feasible way to obtain isotopically labelled internal standards for a wide variety of biomolecules to support reliable quantitation.

Ex Situ Generation of 18O2 and 17O2 from Endoperoxides for *O-Labeling and Mechanistic Studies of Oxidations by Dioxygen

Near-stoichiometric amounts of ¹⁸O₂ and ¹⁷O₂ were generated ex situ from endoperoxides in a two-chamber glassware to oxidize various substrates. This strategy gave [*O₂]endoperoxides, [*O₁]quinones, [*O₁]phenols, and [*O_x]artemisin in moderate to good yields and high isotopic enrichments (up to 84%) at affordable costs. Moreover, mass spectrometry and ¹⁷O NMR of the [*O]products provided valuable information about the chemical mechanisms involved.

Algal ciguatoxin identified as source of ciguatera poisoning in the Caribbean

Ciguatera poisoning (CP) is a severe seafood-borne disease, caused by the consumption of reef fish contaminated with Caribbean ciguatoxins (C-CTXs) in the Caribbean and tropical Atlantic. However, C-CTXs have not been identified from their presumed algal source, so the relationship to the CTXs in fish causing illness remains unknown. This has hindered the development of detection methods, diagnostics, monitoring programs, and limited fundamental knowledge on the environmental factors that regulate C-CTX production. In this study, in vitro and chemical techniques were applied to unambiguously identify a novel C-CTX analogue, C-CTX5, from Gambierdiscus silvae and Gambierdiscus caribaeus strains from the Caribbean. Metabolism in vitro by fish liver microsomes converted algal C-CTX5 into C-CTX1/2, the dominant CTX in ciguatoxic fish from the Caribbean. Furthermore, C-CTX5 from G. silvae was confirmed to have voltage-gated sodium-channel-specific activity. This finding is crucial for risk assessment, understanding the fate of C-CTXs in food webs, and is a prerequisite for development of effective analytical methods and monitoring programs. The identification of an algal precursor produced by two Gambierdiscus species is a major breakthrough for ciguatera research that will foster major advances in this important seafood safety issue.

Fast and Cost-Efficient 17 O-Isotopic Labeling of Carboxylic Groups in Biomolecules: From Free Amino Acids to Peptide Chains

¹⁷ O NMR spectroscopy is a powerful technique, which can provide unique information regarding the structure and reactivity of biomolecules. However, the low natural abundance of ¹⁷ O (0.04 %) generally requires working with enriched samples, which are not easily accessible. Here, we present simple, fast and cost-efficient ¹⁷ O-enrichment strategies for amino acids and peptides by using mechanochemistry. First, five unprotected amino acids were enriched under ambient conditions, consuming only microliter amounts of costly labeled water, and producing pure molecules with enrichment levels up to ∼40 %, yields ∼60-85 %, and no loss of optical purity. Subsequently, ¹⁷ O-enriched Fmoc/tBu-protected amino acids were produced on a 1 g/day scale with high enrichment levels. Lastly, a site-selective ¹⁷ O-labeling of carboxylic functions in peptide side-chains was achieved for RGD and GRGDS peptides, with ∼28 % enrichment level. For all molecules, ¹⁷ O ssNMR spectra were recorded at 14.1 T in reasonable times, making this an important step forward for future NMR studies of biomolecules.

Production and stability of Oxygen-18 labeled Caribbean ciguatoxins and gambierones

Ciguatoxins (CTXs) and gambierones are ladder-shaped polyethers associated with ciguatera poisoning and Gambierdiscus spp. Several of these compounds contain carbonyl or hemiketal groups, which have the potential to exchange with ¹⁸O-labeled water under acidic conditions. The effects of solvent composition and acid on the rate of exchange and on the stability of the labels at various pH values were assessed to optimize the incorporation of ¹⁸O into Caribbean ciguatoxin-1 and -2 (C-CTX1/2), gambierone, and 44-methylgambierone. LC-HRMS results showed that ¹⁸O-labeling occurred at the hydroxy group of the hemiketal at C-56 in C-CTX1/2, and at the hydroxy group of the hemiketal at C-4 and the ketone at C-40 in gambierones. Labeling occurred very rapidly (complete in <30 min) for C-CTX1/2, and more slowly (complete in ca. 16 h) for both gambierones. Labeled C-CTX1/2 was reduced with sodium borohydride to produce ¹⁸O-labeled C-CTX3/4. The incorporated ¹⁸O labels in the gambierones and C-CTXs were retained in aqueous solvent mixtures under neutral conditions in a short-term stability study, demonstrating that these ¹⁸O-labeled toxins have the potential to be used in isotope dilution and metabolism studies.

Quantification of H217O by 1H-MR imaging at 3 T: a feasibility study

Background

Indirect ¹H-magnetic resonance (MR) imaging of ¹⁷O-labelled water allows imaging in vivo dynamic changes in water compartmentalisation. Our aim was to describe the feasibility of indirect ¹H-MR methods to evaluate the effect of H₂¹⁷O on the MR relaxation rates by using conventional a 3-T equipment and voxel-wise relaxation rates.

Methods

MR images were used to calculate the R1, R2, and R2* relaxation rates in phantoms (19 vials with different H₂¹⁷O concentrations, ranging from 0.039 to 5.5%). Afterwards, an experimental animal pilot study (8 rats) was designed to evaluate the in vivo relative R2 brain dynamic changes related to the intravenous administration of ¹⁷O-labelled water in rats.

Results

There were no significant changes on the R1 and R2* values from phantoms. The R2 obtained with the turbo spin-echo T2-weighted sequence with 20-ms echo time interval had the higher statistical difference (0.67 s⁻¹, interquartile range 0.34, p < 0.001) and Spearman correlation (rho 0.79). The R2 increase was adjusted to a linear fit between 0.25 and 5.5%, represented with equation R2 = 0.405 concentration + 0.3215. The highest significant differences were obtained for the higher concentrations (3.1–5.5%). The rat brain MR experiment showed a mean 10% change in the R2 value after the H₂¹⁷O injection with progressive normalisation.

Conclusions

Indirect ¹H-MR imaging method is able to measure H₂¹⁷O concentration by using R2 values and conventional 3-T MR equipment. Normalised R2 relative dynamic changes after the intravenous injection of a H₂¹⁷O saline solution provide a unique opportunity to map water pathophysiology in vivo, opening the analysis of aquaporins status and modifications by disease at clinically available 3-T proton MR scanners.

Catalytic Highly Regioselective C-H Oxygenation Using Water as the Oxygen Source: Preparation of 17O/18O-Isotope-Labeled Compounds

We found that the oxygen atom of water is activated to iodosylbenzene derivatives via reversible hydrolysis of PhI(OOCR)₂ and can be used to the oxygen source for ruthenium(bpga)-catalyzed site-selective C-H oxygenation. Ru(bpga)/PhI(OOCR)₂/H₂O system, sterically less bulky methinic and methylenic C-H bonds in various compounds can be converted to desired oxygen functional groups in a site-selective manner. Using this method, oxygen-isotope labeled compounds such as d-[3-¹⁷O/¹⁸O]-mannose can be prepared in a multigram scale.

18Oxygen Substituted Nucleosides Combined with Proton Beam Therapy: Therapeutic Transmutation In Vitro

Purpose

Proton therapy precisely delivers radiation to cancers to cause damaging strand breaks to cellular DNA, kill malignant cells, and stop tumor growth. Therapeutic protons also generate short-lived activated nuclei of carbon, oxygen, and nitrogen atoms in patients as a result of atomic transmutations that are imaged by positron emission tomography (PET). We hypothesized that the transition of ¹⁸O to ¹⁸F in an ¹⁸O-substituted nucleoside irradiated with therapeutic protons may result in the potential for combined diagnosis and treatment for cancer with proton therapy.

Materials and Methods

Reported here is a feasibility study with a therapeutic proton beam used to irradiate H₂¹⁸O to a dose of 10 Gy produced by an 85 MeV pristine Bragg peak. PET imaging initiated >45 minutes later showed an ¹⁸F decay signal with T_1/2 of ∼111 minutes.

Results

The ¹⁸O to ¹⁸F transmutation effect on cell survival was tested by exposing SQ20B squamous carcinoma cells to physiologic ¹⁸O-thymidine concentrations of 5 μM for 48 hours followed by 1- to 9-Gy graded doses of proton radiation given 24 hours later. Survival analyses show radiation sensitization with a dose modification factor (DMF) of 1.2.

Conclusions

These data support the idea of therapeutic transmutation in vitro as a biochemical consequence of proton activation of ¹⁸O to ¹⁸F in substituted thymidine enabling proton radiation enhancement in a cancer cell. ¹⁸O-substituted molecules that incorporate into cancer targets may hold promise for improving the therapeutic window of protons and can be evaluated further for postproton therapy PET imaging.

Synthesis of 18O-labelled alcohols from unlabelled alcohols

The synthesis of primary, secondary and tertiary 18O-enriched alcohols from readily available 16O-alcohols via a Mitsunobu esterification and hydrolysis is described. The method is further exemplified in the labelling of the active pharmaceutical ingredient, dropropizine and is shown to be tolerant of modern, separation friendly Mitsunobu reagents.

17O NMR studies of organic and biological molecules in aqueous solution and in the solid state

This review describes the latest developments in the field of ¹⁷O NMR spectroscopy of organic and biological molecules both in aqueous solution and in the solid state. In the first part of the review, a general theoretical description of the nuclear quadrupole relaxation process in isotropic liquids is presented at a mathematical level suitable for non-specialists. In addition to the first-order quadrupole interaction, the theory also includes additional relaxation mechanisms such as the second-order quadrupole interaction and its cross correlation with shielding anisotropy. This complete theoretical treatment allows one to assess the transverse relaxation rate (thus the line width) of NMR signals from half-integer quadrupolar nuclei in solution over the entire range of motion. On the basis of this theoretical framework, we discuss general features of quadrupole-central-transition (QCT) NMR, which is a particularly powerful method of studying biomolecules in the slow motion regime. Then we review recent advances in ¹⁷O QCT NMR studies of biological macromolecules in aqueous solution. The second part of the review is concerned with solid-state ¹⁷O NMR studies of organic and biological molecules. As a sequel to the previous review on the same subject [G. Wu, Prog. Nucl. Magn. Reson. Spectrosc. 52 (2008) 118-169], the current review provides a complete coverage of the literature published since 2008 in this area.

A versatile synthetic route to the preparation of 15 N heterocycles

A robust medium-scale (approximately 3 g) synthetic method for ¹⁵ N labeling of pyridine (¹⁵ N-Py) is reported based on the Zincke reaction. ¹⁵ N enrichment in excess of 81% was achieved with approximately 33% yield. ¹⁵ N-Py serves as a standard substrate in a wide range of studies employing a hyperpolarization technique for efficient polarization transfer from parahydrogen to heteronuclei; this technique, called SABRE (signal amplification by reversible exchange), employs a simultaneous chemical exchange of parahydrogen and a to-be-hyperpolarized substrate (e.g., pyridine) on metal centers. In studies aimed at the development of hyperpolarized contrast agents for in vivo molecular imaging, pyridine is often employed either as a model substrate (for hyperpolarization technique development, quality assurance, and phantom imaging studies) or as a co-substrate to facilitate more efficient hyperpolarization of a wide range of emerging contrast agents (e.g., nicotinamide). Here, the produced ¹⁵ N-Py was used for the feasibility study of spontaneous ¹⁵ N hyperpolarization at high magnetic (HF) fields (7 T and 9.4 T) of an NMR spectrometer and an MRI scanner. SABRE hyperpolarization enabled acquisition of 2D MRI imaging of catalyst-bound ¹⁵ N-pyridine with 75 × 75 mm² field of view (FOV), 32 × 32 matrix size, demonstrating the feasibility of ¹⁵ N HF-SABRE molecular imaging with 2.4 × 2.4 mm² spatial resolution.

17O MAS NMR Correlation Spectroscopy at High Magnetic Fields

The structure of two protected amino acids, FMOC-l-leucine and FMOC-l-valine, and a dipeptide, N-acetyl-l-valyl-l-leucine (N-Ac-VL), were studied via one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy. Utilizing ¹⁷O magic-angle spinning (MAS) NMR at multiple magnetic fields (17.6-35.2 T/750-1500 MHz for ¹H) the ¹⁷O quadrupolar and chemical shift parameters were determined for the two oxygen sites of each FMOC-protected amino acids and the three distinct oxygen environments of the dipeptide. The one- and two-dimensional, ¹⁷O, ¹⁵N-¹⁷O, ¹³C-¹⁷O, and ¹H-¹⁷O double-resonance correlation experiments performed on the uniformly ¹³C,¹⁵N and 70% ¹⁷O-labeled dipeptide prove the attainability of ¹⁷O as a probe for structure studies of biological systems. ¹⁵N-¹⁷O and ¹³C-¹⁷O distances were measured via one-dimensional REAPDOR and ZF-TEDOR experimental buildup curves and determined to be within 15% of previously reported distances, thus demonstrating the use of ¹⁷O NMR to quantitate interatomic distances in a fully labeled dipeptide. Through-space hydrogen bonding of N-Ac-VL was investigated by a two-dimensional ¹H-detected ¹⁷O R³-R-INEPT experiment, furthering the importance of ¹⁷O for studies of structure in biomolecular solids.

Solid-state 17O NMR study of 2-acylbenzoic acids and warfarin

We report synthesis and solid-state ¹⁷O NMR characterization of four site-specifically ¹⁷O-labeled 2-acylbenzoic acids (2-RC(O)C₆H₄COOH) where R=H and CH₃): 2-[3-¹⁷O]formylbenzoic acid, 2-[1,2-¹⁷O₂]formylbenzoic acid, 2-[3-¹⁷O]acetylbenzoic acid, and 2-[1,2,3-¹⁷O₃]acetylbenzoic acid. In the solid state, both 2-formyl- and 2-acetyl-benzoic acids exist as the cyclic phthalide form each containing a five-membered lactone ring and a cyclic hemiacetal/hemiketal group. Static and magic-angle-spinning ¹⁷O NMR spectra were recorded at 14.1 and 21.1T for these compounds, from which the ¹⁷O chemical shift and nuclear quadrupolar coupling tensors were determined for each oxygen site. These results represent the first time that ¹⁷O NMR tensors are fully characterized for lactone, cyclic hemiacetal, and cyclic hemiketal functional groups. We also report solid-state ¹⁷O NMR data for the cyclic hemiketal group an anticoagulant drug, warfarin. Experimental ¹⁷O NMR tensors in these compounds were compared with computational results obtained with a periodic DFT code BAND.

Can stable isotope mass spectrometry replace ‎radiolabelled approaches in metabolic studies?

Metabolic pathways and the key regulatory points thereof can be deduced using isotopically labelled substrates. One prerequisite is the accurate measurement of the labeling pattern of targeted metabolites. The subsequent estimation of metabolic fluxes following incubation in radiolabelled substrates has been extensively used. Radiolabelling is a sensitive approach and allows determination of total label uptake since the total radiolabel content is easy to detect. However, the incubation of cells, tissues or the whole plant in a stable isotope enriched environment and the use of either mass spectrometry or nuclear magnetic resonance techniques to determine label incorporation within specific metabolites offers the possibility to readily obtain metabolic information with higher resolution. It additionally also offers an important complement to other post-genomic strategies such as metabolite profiling providing insights into the regulation of the metabolic network and thus allowing a more thorough description of plant cellular function. Thus, although safety concerns mean that stable isotope feeding is generally preferred, the techniques are in truth highly complementary and application of both approaches in tandem currently probably provides the best route towards a comprehensive understanding of plant cellular metabolism.

Solid-State ¹⁷O NMR studies of organic and biological molecules: Recent advances and future directions

This Trends article highlights the recent advances published between 2012 and 2015 in solid-state (17)O NMR for organic and biological molecules. New developments in the following areas are described: (1) new oxygen-containing functional groups, (2) metal organic frameworks, (3) pharmaceuticals, (4) probing molecular motion in organic solids, (5) dynamic nuclear polarization, and (6) paramagnetic coordination compounds. For each of these areas, the author offers his personal views on important problems to be solved and possible future directions.

Heavy atom labeled nucleotides for measurement of kinetic isotope effects

Experimental analysis of kinetic isotope effects represents an extremely powerful approach for gaining information about the transition state structure of complex reactions not available through other methodologies. The implementation of this approach to the study of nucleic acid chemistry requires the synthesis of nucleobases and nucleotides enriched for heavy isotopes at specific positions. In this review, we highlight current approaches to the synthesis of nucleic acids enriched site specifically for heavy oxygen and nitrogen and their application in heavy atom isotope effect studies. This article is part of a special issue titled: Enzyme Transition States from Theory and Experiment.