Deuterium isotope effects on noncovalent interactions between molecules

Water slowing down drives the occurrence of the low temperature dynamical transition in microgels

The protein dynamical transition marks an increase in atomic mobility and the onset of anharmonic motions at a critical temperature (T d), which is considered relevant for protein functionality. This phenomenon is ubiquitous, regardless of protein composition, structure and biological function and typically occurs at large protein content, to avoid water crystallization. Recently, a dynamical transition has also been reported in non-biological macromolecules, such as poly(N-isopropyl acrylamide) (PNIPAM) microgels, bearing many similarities to proteins. While the generality of this phenomenon is well-established, the role of water in the transition remains a subject of debate. In this study, we use atomistic molecular dynamics (MD) simulations and elastic incoherent neutron scattering (EINS) experiments with selective deuteration to investigate the microscopic origin of the dynamical transition and distinguish water and PNIPAM roles. While a standard analysis of EINS experiments would suggest that the dynamical transition occurs in PNIPAM and water at a similar temperature, simulations reveal a different perspective, also qualitatively supported by experiments. From room temperature down to about 180 K, PNIPAM exhibits only modest changes of dynamics, while water, being mainly hydration water under the probed extreme confinement, significantly slows down and undergoes a mode-coupling transition from diffusive to activated. Our findings therefore challenge the traditional view of the dynamical transition, demonstrating that it occurs in proximity of the water mode-coupling transition, shedding light on the intricate interplay between polymer and water dynamics.

Long-lasting, biochemically modified mRNA, and its frameshifted recombinant spike proteins in human tissues and circulation after COVID-19 vaccination

According to the CDC, both Pfizer and Moderna COVID-19 vaccines contain nucleoside-modified messenger RNA (mRNA) encoding the viral spike glycoprotein of severe acute respiratory syndrome caused by corona virus (SARS-CoV-2), administered via intramuscular injections. Despite their worldwide use, very little is known about how nucleoside modifications in mRNA sequences affect their breakdown, transcription and protein synthesis. It was hoped that resident and circulating immune cells attracted to the injection site make copies of the spike protein while the injected mRNA degrades within a few days. It was also originally estimated that recombinant spike proteins generated by mRNA vaccines would persist in the body for a few weeks. In reality, clinical studies now report that modified SARS-CoV-2 mRNA routinely persist up to a month from injection and can be detected in cardiac and skeletal muscle at sites of inflammation and fibrosis, while the recombinant spike protein may persist a little over half a year in blood. Vaccination with 1-methylΨ (pseudouridine enriched) mRNA can elicit cellular immunity to peptide antigens produced by +1 ribosomal frameshifting in major histocompatibility complex-diverse people. The translation of 1-methylΨ mRNA using liquid chromatography tandem mass spectrometry identified nine peptides derived from the mRNA +1 frame. These products impact on off-target host T cell immunity that include increased production of new B cell antigens with far reaching clinical consequences. As an example, a highly significant increase in heart muscle 18-flourodeoxyglucose uptake was detected in vaccinated patients up to half a year (180 days). This review article focuses on medical biochemistry, proteomics and deutenomics principles that explain the persisting spike phenomenon in circulation with organ-related functional damage even in asymptomatic individuals. Proline and hydroxyproline residues emerge as prominent deuterium (heavy hydrogen) binding sites in structural proteins with robust isotopic stability that resists not only enzymatic breakdown, but virtually all (non)-enzymatic cleavage mechanisms known in chemistry.

Anti-plasticizing effect of water on prilocaine and lidocaine - the role of the hydrogen bonding pattern

It is generally accepted that water, as an effective plasticizer, decreases the glass transition temperatures (Tgs) of amorphous drugs, potentially resulting in physical instabilities. However, recent studies suggest that water can also increase the Tgs of the amorphous forms of the drugs prilocaine (PRL) and lidocaine (LID), thus acting as an anti-plasticizer. To further understand the nature of the anti-plasticizing effect of water, interactions with different solvents and the resulting structural features of PRL and LID were investigated by Fourier transform infrared spectroscopy (FTIR) and quantum chemical simulations. Heavy water (deuterium oxides) was chosen as a solvent, as the deuterium and hydrogen atoms are electronically identical. It was found that substituting hydrogen with deuterium showed a minimal impact on the anti-plasticization of water on PRL. Ethanol and ethylene glycol were chosen as solvents to compare the hydrogen bonding patterns occurring between the hydroxyl groups of the solvents and PRL and LID. Comparison of the various Tgs showed a weaker anti-plasticizing potential of these two solvents on PRL and LID. The frequency shifts of the amide CO groups of PRL and LID due to the interactions with water, heavy water, ethanol, and ethylene glycol as observed in the FTIR spectra showed a correlation with the binding energies calculated by quantum chemical simulations. Overall, this study showed that the combination of weak hydrogen bonding and strong electrostatic contributions in hydrated PRL and LID could play an important role in inducing the anti-plasticizing effect of water on those drugs.

Fractionation of Methane Isotopologues during Preparation for Analysis from Ambient Air

Preconcentration of methane (CH4) from air is a critical sampling step in the measurement of singly and doubly substituted isotopologue ratios. We demonstrate the potential for isotope fractionation during preconcentration onto and elution from the common trapping material HayeSep-D and investigate its significance in laser spectroscopy measurements. By altering the trapping temperature for adsorption, the flow direction of CH4 through the trap and the time at which CH4 is eluted during a desorption temperature ramp, we explain the mechanisms behind fractionation affecting δ13C(CH4) and δ2H(CH4). The results highlight that carbon isotope fractionation is driven by advection and diffusion, while hydrogen isotope fractionation is driven by the interaction of CH4 with the adsorbing material (tending to smaller isotopic effects at higher temperatures). We have compared the difference between the measured isotope ratio of sample gases (compressed whole air and a synthetic mixture of CH4 at ambient amount fraction in an N2 matrix) and their known isotopic composition. An open-system Rayleigh model is used to quantify the magnitude of isotopic fractionation affecting measured δ13C(CH4) and δ2H(CH4), which can be used to calculate the possible magnitude of isotopic fractionation given the recovery percentage. These results provide a quantitative understanding of isotopic fractionation during the sample preparation of CH4 from ambient air. The results also provide valuable insights applicable to other cryogenic preconcentration systems, such as those for measurements that probe the distribution of rarer isotopologues.

Synthesis and initial evaluation of radioiodine-labelled deuterated tropane derivatives targeting dopamine transporter

The dopamine transporter (DAT) is closely related to a variety of neurological disorders including Parkinson's disease (PD) and other neurodegenerative diseases. In vivo imaging of DAT with radio-labelled tracers has become a powerful technique in related disorders. The radioiodine-labelled tropane derivative [123I]FP-CIT ([123I]1a) is widely used in clinical single photon emission computed tomography (SPECT) imaging as a DAT imaging agent. To develop more metabolically stable DAT radioligands for accurate imaging, this work compared two novel deuterated tropane derivatives ([131I]1c-d) with non-deuterated tropane derivatives ([131I]1a-b). [131I]1a-d were obtained in high radiochemical purity (RCP) above 99 % with molar activities of 7.0-10.0 GBq/μmol. The [131I]1a and [131I]1c exhibited relatively higher affinity to DAT (Ki: 2.0-3.12 nM) than [131I]1b and [131I]1d. Biodistribution results showed that [131I]1c consistently exhibited a higher ratio of the target to non-target (striatum/cerebellum) than [131I]1a. Furthermore, metabolism studies indicated that the in vivo metabolic stability of [131I]1c was superior to that of [131I]1a. Ex vivo autoradiography showed that [131I]1c selectively localized on DAT-rich striatal regions and the specific signal could be blocked by DAT inhibitor. These results indicated that [131I]1c might be a potential probe for DAT SPECT imaging in the brain.

Separation of undeuterated and partially deuterated enantioisotopologues of some amphetamine derivatives on achiral and polysaccharide-based chiral columns in high-performance liquid chromatography

The different behavior of enantiomers of chiral compounds in non-isotropic environments (among them in living organism) is well known. On the other hand, the importance of a kinetic isotope effect in the biomedical field has become evident during past few decades. Thus, separation of both, enantiomers and isotopologues is now critical. Only very few published studies have attempted the simultaneous separation of enantioisotopologues. In this article we report baseline separation of partially deuterated isotopologues of a few amphetamine derivatives in high-performance liquid chromatography (HPLC) using achiral columns. In addition, the simultaneous separations of enantiomers and isotopologues (i.e. enantioisotopologues) were attempted on polysaccharide-based chiral columns. For several compounds the isotope effect was tunable and could be switched from a "normal" to "inverse" by making changes to the mobile-phase composition. A stronger isotope effect was observed in acetonitrile-containing mobile phases compared to methanol-containing ones with both chiral and achiral columns. In a separation system where both "normal" and "inverse" isotope effects were observed the "normal" isotope effect was favored in polar organic solvents while increasing content of the aqueous component in the reversed-phase (RP) mobile phase favored an "inverse" isotope effect. This observation indicates that polar, hydrogen bonding-type noncovalent interactions are involved in the "normal" isotope effect, while apolar hydrophobic-type interactions are mostly responsible for the "inverse" isotope effect.

HDPairFinder: A data processing platform for hydrogen/deuterium isotopic labeling-based nontargeted analysis of trace-level amino-containing chemicals in environmental water

The combination of hydrogen/deuterium (H/D) formaldehyde-based isotopic methyl labeling with solid-phase extraction and high-performance liquid chromatography-high resolution mass spectrometry (HPLC-HRMS) is a powerful analytical solution for nontargeted analysis of trace-level amino-containing chemicals in water samples. Given the huge amount of chemical information generated in HPLC-HRMS analysis, identifying all possible H/D-labeled amino chemicals presents a significant challenge in data processing. To address this, we designed a streamlined data processing pipeline that can automatically extract H/D-labeled amino chemicals from the raw HPLC-HRMS data with high accuracy and efficiency. First, we developed a cross-correlation algorithm to correct the retention time shift resulting from deuterium isotopic effects, which enables reliable pairing of H- and D-labeled peaks. Second, we implemented several bioinformatic solutions to remove false chemical features generated by in-source fragmentation, salt adduction, and natural 13C isotopes. Third, we used a data mining strategy to construct the AMINES library that consists of over 38,000 structure-disjointed primary and secondary amines to facilitate putative compound annotation. Finally, we integrated these modules into a freely available R program, HDPairFinder.R. The rationale of each module was justified and its performance tested using experimental H/D-labeled chemical standards and authentic water samples. We further demonstrated the application of HDPairFinder to effectively extract N-containing contaminants, thus enabling the monitoring of changes of primary and secondary N-compounds in authentic water samples. HDPairFinder is a reliable bioinformatic tool for rapid processing of H/D isotopic methyl labeling-based nontargeted analysis of water samples, and will facilitate a better understanding of N-containing chemical compounds in water.

Discovery and In Vitro Characterization of SPL028: Deuterated N,N-Dimethyltryptamine

The psychedelic N,N- dimethyltryptamine (DMT) is in clinical development for the treatment of major depressive disorder. However, when administered via intravenous infusion, its effects are short-lived due to rapid clearance. Here we describe the synthesis of deuterated analogues of DMT with the aim of prolonging the half-life and decreasing the clearance rate while maintaining similar pharmacological effects. The molecule with the greatest degree of deuteration at the α-carbon (N,N-D2-dimethyltryptamine, D2-DMT) demonstrated the longest half-life and intrinsic clearance in hepatocyte mitochondrial fractions when compared with DMT. The in vitro receptor binding profile of D2-DMT was comparable to that of DMT, with the highest affinity at the 5-HT1A, 5-HT2A, and 5-HT2C receptors. D2-DMT was therefore the preferred candidate to consider for further evaluation.

Solvent Polarity under Vibrational Strong Coupling

Vibrational strong coupling (VSC) occurs when molecular vibrations hybridize with the modes of an optical cavity, an interaction mediated by vacuum fluctuations. VSC has been shown to influence the rates and selectivity of chemical reactions. However, a clear understanding of the mechanism at play remains elusive. Here, we show that VSC affects the polarity of solvents, which is a parameter well-known to influence reactivity. The strong solvatochromic response of Reichardt's dye (RD) was used to quantify the polarity of a series of alcohol solvents at visible wavelengths. We observed that, by simultaneously coupling the OH and CH vibrational bands of the alcohols, the absorption maximum of Reichardt's dye redshifted by up to ∼15.1 nm, corresponding to an energy change of 5.1 kJ·mol^-1. With aliphatic alcohols, the magnitude of the absorption change of RD was observed to be related to the length of the alkyl chain, the molecular surface area, and the polarizability, indicating that dispersion forces are impacted by strong coupling. Therefore, we propose that dispersion interactions, which themselves originate from vacuum fluctuations, are impacted under strong coupling and are therefore critical to understanding how VSC influences chemistry.

Synthesis and Chiroptical Properties of a Chiral Isotopologue of syn-Cryptophane-B

We report the synthesis and absolute configuration (AC) of a chiral isotopologue of syn-cryptophane-B. Low chiral signatures were measured by polarimetry and electronic circular dichroism, whereas most significant chiroptical effects were observed by vibrational circular dichroism (VCD) and Raman optical activity (ROA). The comparison of experimental VCD and ROA spectra with those predicted by DFT calculations allows the determination of the AC of the two enantiomers as (-)₅₈₉-MP-syn-2 and (+)₅₈₉-PM-syn-2.

Targeting DNA topoisomerase IIα (TOP2A) in the hypoxic tumour microenvironment using unidirectional hypoxia-activated prodrugs (uHAPs)

The hypoxic tumour microenvironment (hTME), arising from inadequate and chaotic vascularity, can present a major obstacle for the treatment of solid tumours. Hypoxic tumour cells compromise responses to treatment since they can generate resistance to radiotherapy, chemotherapy and immunotherapy. The hTME impairs the delivery of a range of anti-cancer drugs, creates routes for metastasis and exerts selection pressures for aggressive phenotypes; these changes potentially occur within an immunosuppressed environment. Therapeutic strategies aimed at the hTME include targeting the molecular changes associated with hypoxia. An alternative approach is to exploit the prevailing lack of oxygen as a principle for the selective activation of prodrugs to target cellular components within the hTME. This review focuses on the design concepts and rationale for the use of unidirectional Hypoxia-Activated Prodrugs (uHAPs) to target the hTME as exemplified by the uHAPs AQ4N and OCT1002. These agents undergo irreversible reduction in a hypoxic environment to active forms that target DNA topoisomerase IIα (TOP2A). This nuclear enzyme is essential for cell division and is a recognised chemotherapeutic target. An activated uHAP interacts with the enzyme-DNA complex to induce DNA damage, cell cycle arrest and tumour cell death. uHAPs are designed to overcome the shortcomings of conventional HAPs and offer unique pharmacodynamic properties for effective targeting of TOP2A in the hTME. uHAP therapy in combination with standard of care treatments has the potential to enhance outcomes by co-addressing the therapeutic challenge presented by the hTME.

Stable carbon and hydrogen isotope fractionation of volatile organic compounds caused by vapor-liquid equilibrium

Several types of laboratory experiments were conducted to evaluate isotope fractionation caused by phase transfer process for a selection of common environmental contaminants. Carbon and hydrogen isotope fractionation caused by vaporization of non-aqueous phase liquid (NAPL), by volatilization from water and by dissolution into an organic solvent (tetraethylene glycol dimethylether or TGDE) under equilibrium conditions was investigated with closed system experimental setups to isolate the air-liquid partitioning process. A selection of aromatic, aliphatic and chlorinated compounds along with one fuel oxygenate (methyl tert-butyl ether or MTBE) were evaluated to determine isotope enrichment factor related to respective phase transfer process. During NAPL vaporization, the residual mass of aromatic compounds, aliphatic compounds and MTBE became progressively depleted in heavy carbon and hydrogen isotopes. In contrast, during volatilization from water, the residual mass of aromatic compounds and MTBE dissolved in the water became progressively enriched in heavy hydrogen isotopes, whereas no significant change in carbon isotope was observed, except for MTBE showing a significant depletion. For the air-TGDE partitioning process, most of the aromatic compounds tested led to no significant carbon (except ethylbenzene) or hydrogen (except toluene and o-xylene) isotope fractionation. In contrast, significant carbon isotope fractionation was observed for aliphatic and chlorinated compounds and hydrogen isotope fractionation for aliphatic compounds, and are comparable to progressive NAPL vaporization in direction and magnitude. The isotope fractionation factors determined in this study are key for interpreting the change in isotope ratios when assessing the fate of gas-phase VOCs present in the soil air or when gas-phase VOCs are sampled using TGDE as the sink matrix. The results of this study contribute to expand the list of common environmental contaminants that can be assessed by the compound-specific isotope analysis (CSIA) method deployed in the frame of gas-phase studies.

Effect of deuteration on the single dose pharmacokinetic properties and postoperative analgesic activity of methadone

Although methadone is effective in the management of acute pain, the complexity of its absorption-distribution-metabolism-excretion profile limits its use as an opioid of choice for perioperative analgesia. Because deuteration is known to improve the pharmacokinetic, pharmacodynamic and toxicological properties of some drugs, here we characterized the single dose pharmacokinetic properties and post-operative analgesic efficacy of d9-methadone. The pharmacokinetic profiles of d9-methadone and methadone administered intravenously to CD-1 male mice revealed that deuteration leads to a 5.7- and 4.4-fold increase in the area under the time-concentration curve and maximum concentration in plasma, respectively, as well as reduction in clearance (0.9 ± 0.3 L/h/kg vs 4.7 ± 0.8 L/h/kg). The lower brain-to-plasma ratio of d9-methadone compared to that of methadone (0.35 ± 0.12 vs 2.05 ± 0.62) suggested that deuteration decreases the transfer of the drug across the blood-brain barrier. The estimated LD50 value for a single intravenous dose of d9-methadone was 2.1-fold higher than that for methadone. Moreover, d9-methadone outperformed methadone in the efficacy against postoperative pain by primarily activating peripheral opioid receptors. Collectively, these data suggest that the replacement of three hydrogen atoms in three methyl groups of methadone altered its pharmacokinetic properties, improved safety, and enhanced its analgesic efficacy.

Deuterated Drugs and Biomarkers in the COVID-19 Pandemic

Coronavirus disease 2019 (COVID-19) is a highly contagious disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Initially identified in Wuhan (China) in December 2019, COVID-19 rapidly spread globally, resulting in the COVID-19 pandemic. Carriers of the SARS-CoV-2 can experience symptoms ranging from mild to severe (or no symptoms whatsoever). Although vaccination provides extra immunity toward SARS-CoV-2, there has been an urgent need to develop treatments for COVID-19 to alleviate symptoms for carriers of the disease. In seeking a potential treatment, deuterated compounds have played a critical role either as therapeutic agents or as internal MS standards for studying the pharmacological properties of new drugs by quantifying the parent compounds and metabolites. We have identified >70 examples of deuterium-labeled compounds associated with treatment of COVID-19. Of these, we found 9 repurposed drugs and >20 novel drugs studied for potential therapeutic roles along with a total of 38 compounds (drugs, biomarkers, and lipids) explored as internal mass spectrometry standards. This review details the synthetic pathways and modes of action of these compounds (if known), and a brief analysis of each study.

Creating enzyme-mimicking nanopockets in metal-organic frameworks for catalysis

Numerous efforts are being made toward constructing artificial nanopockets inside heterogeneous catalysts to implement challenging reactions that are difficult to occur on traditional heterogeneous catalysts. Here, the enzyme-mimetic nanopockets are fabricated inside the typical UiO-66 by coordinating zirconium nodes with terephthalate (BDC) ligands and monocarboxylate modulators including formic acid (FC), acetic acid (AC), or trifluoroacetic acid (TFA). When used in transfer hydrogenation of alkyl levulinates with isopropanol toward γ-valerolactone (GVL), these modulators endow zirconium sites with enhanced activity and selectivity and good stability. The catalytic activity of UiO-66FC is ~30 times that of UiO-66, also outperforming the state-of-the-art heterogeneous catalysts. Distinct from general consensus on electron-withdrawing or electron-donating effect on the altered activity of metal centers, this improvement mainly originates from the conformational change of modulators in the nanopocket to assist forming the rate-determining six-membered ring intermediate at zirconium sites, which are stabilized by van der Waals force interactions.

Cultivation of the microalgae Chlamydomonas reinhardtii and Desmodesmus quadricauda in highly deuterated media: Balancing the light intensity

The production of organic deuterated compounds in microalgal systems represents a cheaper and more versatile alternative to more complicated chemical synthesis. In the present study, we investigate the autotrophic growth of two microalgae, Chlamydomonas reinhardtii and Desmodesmus quadricauda, in medium containing high doses of deuterated water, D₂O. The growth of such cultures was evaluated in the context of the intensity of incident light, since light is a critical factor in the management of autotrophic algal cultures. Deuteration increases the light sensitivity of both model organisms, resulting in increased levels of singlet oxygen and poorer photosynthetic performance. Our results also show a slowdown in growth and cell division processes with increasing D₂O concentrations. At the same time, impaired cell division leads to cell enlargement and accumulation of highly deuterated compounds, especially energy-storing molecules. Thus, considering the specifics of highly deuterated cultures and using the growth conditions proposed in this study, it is possible to obtain highly deuterated algal biomass, which could be a valuable source of deuterated organic compounds.

Preparation of Deuterium-Labeled Armodafinil by Hydrogen–Deuterium Exchange and Its Application in Quantitative Analysis by LC-MS

Armodafinil, the R enantiomer of modafinil, was approved in 2007 by the US Food and Drug Administration as a wake-promoting agent for excessive sleepiness treatment. Due to its abuse by students and athletes, there is a need of its quantification. Quantitative analysis by liquid chromatography-mass spectrometry, however, though very common and sensitive, frequently cannot be performed without isotopically labeled standards which usually have to be specially synthesized. Here we reported our investigation on the preparation of deuterated standard of armodafinil based on the simple and inexpensive hydrogen–deuterium exchange reaction at the carbon centers. The obtained results clearly indicate the possibility of introduction of three deuterons into the armodafinil molecule. The introduced deuterons do not undergo back exchange under neutral and acidic conditions. Moreover, the deuterated and non-deuterated armodafinil isotopologues revealed co-elution during the chromatographic analysis. The ability to control the degree of deuteration using different reaction conditions was determined. The proposed method of deuterated armodafinil standard preparation is rapid, cost-efficient and may be successfully used in its quantitative analysis by LC-MS.

Insights into Extended Structures and Their Driving Force: Influence of Salt on Polyelectrolyte/Surfactant Mixtures at the Air/Water Interface

This paper addresses the effect of polyelectrolyte stiffness on the surface structure of polyelectrolyte (P)/surfactant (S) mixtures. Therefore, two different anionic Ps with different intrinsic persistence length lP are studied while varying the salt concentration (0-10-2 M). Either monosulfonated polyphenylene sulfone (sPSO2-220, lP ∼20 nm) or sodium poly(styrenesulfonate) (PSS, lP ∼1 nm) is mixed with the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB) well below its critical micelle concentration and studied with tensiometry and neutron reflectivity experiments. We kept the S concentration (10-4 M) constant, while we varied the P concentration (10-5-10-3 M of the monomer, denoted as monoM). P and S adsorb at the air/water interface for all studied mixtures. Around the bulk stoichiometric mixing point (BSMP), PSS/C14TAB mixtures lose their surface activity, whereas sPSO2-220/C14TAB mixtures form extended structures perpendicular to the surface (meaning a layer of S with attached P and additional layers of P and S underneath instead of only a monolayer of S with P). Considering the different P monomer structures as well as the impact of salt, we identified the driving force for the formation of these extended structures: compensation of all interfacial charges (P/S ratio ∼1) to maximize the gain of entropy. By increasing the flexibility of P, we can tune the interfacial structures from extended structures to monolayers. These findings may help improve applications based on the adsorption of P/S mixtures in the fields of cosmetic or oil recovery.

New Design of a Sample Cell for Neutron Reflectometry in Liquid–Liquid Systems and Its Application for Studying Structures at Air–Liquid and Liquid–Liquid Interfaces

Knowledge of interfacial structures in liquid–liquid systems is imperative, especially for improving two-phase biological and chemical reactions. Therefore, we developed a new sample cell for neutron reflectometry (NR), which enables us to observe the layer structure around the interface, and investigated the adsorption behavior of a typical surfactant, sodium dodecyl sulfate (SDS), on the toluene-d8-D2O interface under the new experimental conditions. The new cell was characterized by placing the PTFE frame at the bottom to produce a smooth interface and downsized compared to the conventional cell. The obtained NR profiles were readily analyzable and we determined a slight difference in the SDS adsorption layer structure at the interface between the toluene-d8-D2O and air-D2O systems. This could be owing to the difference in the adsorption behavior of the SDS molecules depending on the interfacial conditions.

ESI-MS reveals preferential complex formation of carbohydrates with Z-sinapinic acid compared with the E-isomer

ZSA + carbohydrate complex preferential formation and higher stability (ESI) support the previously proposed model for ZSA differential efficiency as the MALDI-MS matrix.

Pharmacokinetic, pharmacological, and genotoxic evaluation of deuterated caffeine

Altering caffeine's negative physiological effects and extending its duration of activity is an active area of research; however, deuteration as a means of achieving these goals is unexplored. Deuteration substitutes one or more of the hydrogen atoms of a substance with deuterium, a stable isotope of hydrogen that contains an extra neutron. Deuteration can potentially alter the metabolic profile of a substance, while maintaining its pharmacodynamic properties. d9-Caffeine is a deuterated isotopologue of caffeine with the nine hydrogens contained in the 1, 3, and 7 methyl groups of caffeine substituted with deuterium. d9-Caffeine may prove to be an alternative to caffeine that may be consumed with less frequency, at lower doses, and with less exposure to downstream active metabolites of caffeine. Characterization of d9-caffeine's genotoxic potential, pharmacodynamic, and pharmacokinetic behavior is critical in establishing how it may differ from caffeine. d9-Caffeine was non-genotoxic with and without metabolic activation in both a bacterial reverse mutation assay and a human mammalian cell micronucleus assay at concentrations up to the ICH concentration limits. d9-Caffeine exhibited a prolonged systemic and brain exposure time in rats as compared to caffeine following oral administration. The adenosine receptor antagonist potency of d9-caffeine was similar to caffeine.

Are chlorine isotopologues of polychlorinated organic pollutants binomially distributed? Theoretical evaluation, numerical simulation, experimental evidences and implications for chlorine isotope analysis and source identification

Relative abundances of chlorine isotopologues of polychlorinated organic compounds (POCs) are commonly recognized to comply with binomial distribution. This study investigated whether chlorine isotopologue distributions of polychlorinated organic pollutants are binomial and evaluated implications of the distributions to relevant analytical and environmental research by theoretical derivation, numerical simulation and experiment. Chlorine kinetic isotope effects and equilibrium isotope effects vary in stepwise chlorination reactions, leading to inconsistent chlorine isotope ratios on different reaction positions of products, which results in non-binomial chlorine isotopologue distributions of the products. After physical changes and dechlorination, chlorine isotopologues of POCs are unlikely binomially distributed. The experimental results demonstrated that the chlorine isotopologue distributions of perchloroethylene, trichloroethylene, methyl-triclosan, and 2,3,7,8-tetrachlorodibenzofuran in standards and four polychlorinated biphenyls in both standard solutions and sediments were non-binomial. The patterns of chlorine isotope ratios derived from pairs of neighboring chlorine isotopologues of POCs from different sources were different, implying different isotopologue distributions, which might cause biases in compound-specific isotope analysis of chlorine (CSIA-Cl) and source identification. A complete-isotopologue scheme for isotope ratio calculation is recommended to CSIA-Cl for obtaining accurate data. Gas chromatography-double focusing magnetic-sector high resolution mass spectrometry is a promising instrument for CSIA-Cl that uses the complete-isotopologue scheme due to its excellent sensitivity, selectivity and ruggedness. This study yields new insights into chlorine isotopologue distributions of polychlorinated organic pollutants and proposes practicable solutions to improve CSIA-Cl that uses gas chromatography-mass spectrometry and facilitate source identification of polychlorinated organic pollutants.

Sorption- and diffusion-induced isotopic fractionation in chloroethenes

Isotopic fractionation of groundwater contaminants can occur due to degradation, diffusion and sorption. Of these, only degradation has been extensively explored, yet diffusive isotopic fractionation (DIF) and sorptive isotopic fractionation (SIF) can have significant effects on the isotopic enrichment of groundwater contaminants. Understanding how to mathematically describe and model these processes is vital to the correct interpretation of compound-specific isotope analysis (CSIA) data in the field. Here, models for these physical fractionation processes are developed and described, including the definition of a sorption enrichment factor. These models are then implemented numerically using inverse and finite-element methods to investigate two scenarios, diffusion-sorption and diffusion-sorption-advection, that have been measured in the laboratory. Concentration, δ³⁷Cl, and δ²H data from cis-dichloroethene (cDCE) and trichloroethene (TCE) are used as inputs to the models. Unknown transport parameters including diffusive fractionation exponents are determined from an inverse modelling approach. DIF is shown to have a stronger influence on chlorine isotopologues than on hydrogen isotopologues. For both cDCE and TCE, the sorption enrichment factor of chlorine is found to be negative while that of hydrogen is positive. The presented approach and results provide novel tools and insight into DIF and SIF and underline that these processes should be taken into account when using CSIA to assess contaminant fate.

Deuterium equilibrium isotope effects in a supramolecular receptor for the hydrochalcogenide and halide anions

We highlight a convenient synthesis to selectively deuterate an aryl C-H hydrogen bond donor in an arylethynyl bisurea supramolecular anion receptor and use the Perrin method of competitive titrations to study the deuterium equilibrium isotope effects (DEIE) of anion binding for HS-, Cl-, and Br-. This work highlights the utility and also challenges in using this method to determine EIE with highly reactive and/or weakly binding anions.

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

Isotopic Consequences of Host-Guest Interactions; Noncovalent Chlorine Isotope Effects

Although weak intermolecular interactions are the essence of most processes of key importance in medicine, industry, environment, and life cycles, their characterization is still not sufficient. Enzymatic dehalogenations that involve chloride anion interaction within a host-guest framework is one of the many examples. Recently published experimental results on host-guest systems provided us with models suitable to assess isotopic consequences of these noncovalent interactions. Herein, we report the influence of environmental and structural variations on chlorine isotope effects. We show that these effects, although small, may obscure mechanistic interpretations, as well as analytical protocols of dehalogenation processes.

Elucidating degradation mechanisms of florfenicol in soil by stable-isotope assisted nontarget screening

Antibiotics in soil environments are a growing concern. Identifying transformation products is key to elucidating degradation pathways and mechanisms of antibiotics and other organic micropollutants. The primary challenge of transformation product identification is the interference of matrices. In this study, stable-isotope assisted nontarget screening was used to identify biodegradation products of florfenicol in soil. A total of 74 candidates were prioritized from thousands of mass features observed by a tiered peak filtering approach. Moreover, with the support of in silico prediction tools, the structures of 12 transformation products were elucidated, and 9 of them were reported for the first time. A biodegradation map of florfenicol consisting of amide hydrolysis, dechlorination, dehydration, defluorination, and sulfone reduction was established based on these identified products. A total of 8 products were also found in 6 field soil samples with manure application. Because of the structural similarity to florfenicol, some transformation products might still keep antimicrobial activity toward a variety of bacterial species. The strategies demonstrated in this study provide a basis for efficient identification of transformation products of other organic micropollutants in a variety of environmental matrices. The results also shed light on the degradation mechanisms, risk assessments, and regulations of these compounds.

Intramolecular non-covalent isotope effects at natural abundance associated with the migration of paracetamol in solid matrices during liquid chromatography

Position-specific isotope analysis by Nuclear Magnetic Resonance spectrometry was employed to study the ¹³C intramolecular isotopic fractionation associated with the migration of organic substrates through different stationary phases chromatography columns. Liquid chromatography is often used to isolate compounds prior to their isotope analysis and this purification step potentially alters the isotopic composition of target compounds introducing a bias in the later measured data. Moreover, results from liquid chromatography can yield the sorption parameters needed in reactive transport models that predict the transport and fate of organic contaminants to in the environment. The aim of this study was to use intramolecular isotope analysis to study both ¹³C and ¹⁵N isotope effects associated with the elution of paracetamol (acetaminophen) through different stationary phases and to compare them to effects observed previously for vanillin. Results showed very different intramolecular isotope fractionation profiles depending on the chemical structure of the stationary phase. The data also demonstrate that both the amplitude and the distribution of measured isotope effects depend on the nature of the non-covalent interactions involved in the migration process. Results provided by theoretical calculation performed during this study also confirmed the direct link between observed intramolecular isotope fractionation and the nature of involved intermolecular interactions. It is concluded that the nature of the stationary phase through which the substrate passes has a major impact on the intramolecular isotopic composition of organic compounds isolated by chromatography methods..

Sensory Perception of Non-Deuterated and Deuterated Organic Compounds

The chemical background of olfactory perception has been subject of intensive research, but no available model can fully explain the sense of smell. There are also inconsistent results on the role of the isotopology of molecules. In experiments with human subjects it was found that the isotope effect is weak with acetone and D6 -acetone. In contrast, clear differences were observed in the perception of octanoic acid and D15 -octanoic acid. Furthermore, a trained sniffer dog was initially able to distinguish between these isotopologues of octanoic acid. In chromatographic measurements, the respective deuterated molecule showed weaker interaction with a non-polar liquid phase. Quantum chemical calculations give evidence that deuterated octanoic acid binds more strongly to a model receptor than non-deuterated. In contrast, the binding of the non-deuterated molecule is stronger with acetone. The isotope effect is calculated in the framework of statistical mechanics. It results from a complicated interplay between various thermostatistical contributions to the non-covalent free binding energies and it turns out to be very molecule-specific. The vibrational terms including non-classical zero-point energies play about the same role as rotational/translational contributions and are larger than bond length effects for the differential isotope perception of odor for which general rules cannot be derived.

Proton-deuterium exchange of acetone catalyzed in imidazolium-based ionic liquid-D2O mixtures

The reaction of the proton-deuterium exchange of acetone in imidazolium-based ionic liquid (IL)-deuterium oxide mixtures was studied in detail via NMR spectroscopy. Certain ILs exhibit considerable catalytic properties and contribute to the course of reaction up to the complete deuteration. The efficiency of deuterium exchange crucially depends on the features of ILs; the type of anion and chain length of cation. The linear secondary isotope effects on the NMR chemical shifts of the 13C atoms in acetone were observed depending on the deuteration level of the molecule.

The application of isotopically labeled analogues for the determination of small organic compounds by GC/MS with selected ion monitoring

The study aimed to show the limitations and advantages of the use of stable isotope labeled internal standards (SILISs) for the quantification of small polar compounds (ibuprofen, diclofenac, metoprolol, bisphenol A, 17β-estradiol) in water samples by GC/MS with selected ion monitoring (SIM). The selection of SIM ions shows that for some of the analytes, in the form of trimethylsilylated derivatives, it is problematic to have a minimum of two qualifiers because of the poor mass spectra, high impact of the background or overlapping of the peaks from the analytes and SILISs. The "isotope effect" was observed on GC chromatograms. During the development of the SIM method, special attention was given to the qualifier/quantifier ratio, for which a variety of acceptance criteria can be found in the official guidelines. The values of the solid-phase extraction efficiency, as well as the matrix effect and absolute recovery were the same for the analytes and their corresponding SILISs. The developed method was used for the quantification of analytes in wastewaters.

Bioisosteric Replacement as a Tool in Anti-HIV Drug Design

Bioisosteric replacement is a powerful tool for modulating the drug-like properties, toxicity, and chemical space of experimental therapeutics. In this review, we focus on selected cases where bioisosteric replacement and scaffold hopping have been used in the development of new anti-HIV-1 therapeutics. Moreover, we cover field-based, computational methodologies for bioisosteric replacement, using studies from our group as an example. It is our hope that this review will serve to highlight the utility and potential of bioisosteric replacement in the continuing search for new and improved anti-HIV drugs.

Polarizability and isotope effects on dispersion interactions in water

True understanding of dispersion interaction in solution remains elusive because of difficulty in the precise evaluation of its interaction energy. Here, the effect of substituents with different polarizability on dispersion interactions in water is discussed based on the thermodynamic parameters determined by isothermal titration calorimetry for the formation of discrete aggregates from gear-shaped amphiphiles (GSAs). The substituents with higher polarizability enthalpically more stabilize the nanocube, which is due to stronger dispersion interactions and to the hydrophobic effect. The differences in the thermodynamic parameters for the nanocubes from the GSAs with CH3 and CD3 groups are also discussed to lead to the conclusion that the H/D isotope effect on dispersion interactions is negligibly small, which is due to almost perfect entropy-enthalpy compensation between the two isotopomers.

Single-Neuron Comparison of the Olfactory Receptor Response to Deuterated and Nondeuterated Odorants

The mammalian olfactory receptors (ORs) constitute a large subfamily of the Class A G-protein coupled receptors (GPCRs). The molecular details of how these receptors convert odorant chemical information into neural signal are unknown, but are predicted by analogy to other GPCRs to involve stabilization of the activated form of the OR by the odorant. An alternative hypothesis maintains that the vibrational modes of an odorant's bonds constitute the main determinant for OR activation, and that odorants containing deuterium in place of hydrogen should activate different sets of OR family members. Experiments using heterologously expressed ORs have failed to show different responses for deuterated odorants, but experiments in the sensory neuron environment have been lacking. We tested the response to deuterated and nondeuterated versions of p-cymene, 1-octanol, 1-undecanol, and octanal in dissociated mouse olfactory receptor neurons (ORNs) by calcium imaging. In all, we tested 23 812 cells, including a subset expressing recombinant mouse olfactory receptor 2 ( Olfr2/OR-I7 ), and found that nearly all of the 1610 odorant-responding neurons were unable to distinguish the D- and H-odorants. These results support the conclusion that if mammals can perceive deuterated odorants differently, the difference arises from the receptor-independent steps of olfaction. Nevertheless, 0.81% of the responding ORNs responded differently to D- and H-odorants, and those in the octanal experiments responded selectively to H-octanal at concentrations from 3 to 100 μM. The few ORs responding differently to H and D may be hypersensitive to one of the several H/D physicochemical differences, such as the difference in H/D hydrophobicity.

Stable Isotope Labeling-Assisted Metabolite Probing for Emerging Contaminants in Plants

Biotransformation is a notable modulator of the fate, bioaccumulation, and toxicity of contaminants in the environment. However, it is often formidable to identify unknown biotransformation products in the absence of reference standards, and this analytical challenge is particularly true for contaminants of emerging concern (CECs) that are mostly polar molecules without characteristic structures (e.g., Cl and Br) and in complex matrices such as plants. In this study, using the fibrate drug gemfibrozil as a model CEC and Arabidopsis thaliana as a model plant, we developed and demonstrated a novel analytical framework coupling deuterium stable isotope labeling with high-resolution mass spectrometry (SILAMS) in identifying plant biotransformation products. When exposed in A. thaliana cells, gemfibrozil was quickly taken up into the cells and extensively metabolized. The use of nonlabeled and deuterated gemfibrozil at a 3:1 ratio created unique diagnostic patterns in mass spectra, enabling the identification of 11 novel phase II amino acid/peptide conjugates. Similarity in mass fragmentation patterns and chromatographic behaviors was then employed to establish the probable structures. Two major metabolites were further confirmed as glutamate and glutamine conjugates using authentic standards. Most of the identified conjugates were also detected in the whole A. thaliana plant. Therefore, SILAMS offers unique advantages by excluding false matrix positives and helping discern unknown metabolites, including polar conjugates with endogenous biomolecules, with a high degree of confidence. This novel framework may be readily applied to other CECs for high-throughput metabolite screening in plants to improve our understanding of their food safety and human health risks and potential deleterious effects on other species living on plants.

Saponin Adsorption at the Air-Water Interface-Neutron Reflectivity and Surface Tension Study

Saponins are a large group of glycosides present in many plant species. They exhibit high surface activity, which arises from a hydrophobic scaffold of triterpenoid or steroid groups and attached hydrophilic saccharide chains. The diversity of molecular structures, present in various plants, gives rise to a rich variety of physicochemical properties and biological activity and results in a wide range of applications in foods, cosmetics, medicine, and several other industrial sectors. Saponin surface activity is a key property in such applications and here the adsorption of three triterpenoid saponins, escin, tea saponins, and Quillaja saponin, is studied at the air-water interface by neutron reflectivity and surface tension. All these saponins form adsorption layers with very high surface visco-elasticity. The structure of the adsorbed layers has been determined from the neutron reflectivity data and is related to the molecular structure of the saponins. The results indicate that the structure of the saturated adsorption layers is governed by densely packed hydrophilic saccharide groups. The tight molecular packing and the strong hydrogen bonds between the neighboring saccharide groups are the main reasons for the unusual rheological properties of the saponin adsorption layers.

Deuterium- and Tritium-Labelled Compounds: Applications in the Life Sciences

Hydrogen isotopes are unique tools for identifying and understanding biological and chemical processes. Hydrogen isotope labelling allows for the traceless and direct incorporation of an additional mass or radioactive tag into an organic molecule with almost no changes in its chemical structure, physical properties, or biological activity. Using deuterium-labelled isotopologues to study the unique mass-spectrometric patterns generated from mixtures of biologically relevant molecules drastically simplifies analysis. Such methods are now providing unprecedented levels of insight in a wide and continuously growing range of applications in the life sciences and beyond. Tritium (³ H), in particular, has seen an increase in utilization, especially in pharmaceutical drug discovery. The efforts and costs associated with the synthesis of labelled compounds are more than compensated for by the enhanced molecular sensitivity during analysis and the high reliability of the data obtained. In this Review, advances in the application of hydrogen isotopes in the life sciences are described.

Isotope fractionation in phase-transfer processes under thermodynamic and kinetic control - Implications for diffusive fractionation in aqueous solution

Diffusive isotope fractionation of organic compounds in aqueous solution was investigated by means of liquid-liquid and liquid-gas partitioning experiments under kinetic control. The two-film model was used to describe phase-transfer kinetics. It assumes the diffusion of solutes across a stagnant water boundary layer as the rate-controlling step. For all investigated solutes (benzene-D₀ and -D₆, toluene-D₀, -D₅, and -D₈, cyclohexane-D₀ and -D₁₂), there was no significant observable fractionation effect between nondeuterated and perdeuterated isotopologues, resulting in a ratio of diffusion coefficients D_light: D_heavy=1.00±0.01. In addition, isotope fractionation due to equilibrium partitioning of solutes between water and n-octane or gas phase was measured. The deuterated compounds are more hydrophilic than their light isotopologues in all cases, giving rise to fractionation coefficients αH_part=K_{octane/water,H}: K_{octane/water,D}=1.085 to 1.15. Thus, thermodynamic fractionation effects are much larger than diffusion fractionation effects. Methodical and environmental implications of these findings are discussed.

Chemical Tagging with tert-Butyl and Trimethylsilyl Groups for Measuring Intermolecular Nuclear Overhauser Effects in a Large Protein-Ligand Complex

Intermolecular ¹ H-¹ H nuclear Overhauser effects (NOE) present a powerful tool to assess contacts between proteins and binding partners, but are difficult to identify for complexes of high molecular weight. This report shows that intermolecular NOEs can readily be observed following chemical labeling with tert-butyl or trimethylsilyl (TMS) groups. Proteins can be furnished with tert-butyl or TMS groups site-specifically using genetically encoded unnatural amino acids or by chemical modification of single cysteine residues. No isotope labeling is required. The approach is demonstrated with the 95 kDa complex between tetrameric E. coli single-stranded DNA binding protein (SSB) and single-stranded DNA.