Integrated Stopped-Flow Device for the Study of Porous Materials Using Hyperpolarized 129Xe NMR.

A simple, low-cost, and efficient device is proposed for the study of porous materials via NMR using small gas probes. Mainly built through additive manufacturing and being equipped with a radiofrequency solenoid microcoil, it only requires tiny quantities of sample and/or gas and is particularly suited for hyperpolarized xenon. The performances of this device have been accessed on a commercial sample of MCM-41 exhibiting multiporosity. Both the delivery mode of hyperpolarized xenon and the stopped-flow system are judged as efficient according to 2D 129Xe self-diffusion and EXSY experiments.

A Water-Soluble Cryptophane Decorated with Aromatic Amine Groups Shows High Affinity for Cesium and Thallium(I).

An anti-cryptophane decorated with three aromatic amine and three phenol groups shows a high affinity for the cesium and thallium cations in LiOH/H2O (0.1 M). The formation of the complexes was studied by 133Cs NMR and by 205Tl NMR spectroscopy at different temperatures. Characteristic signals for caged cesium and thallium were observed at a high field with respect to the signals of the free cations present in the bulk. Isothermal titration calorimetric experiments performed in LiOH/H2O (0.1 M) and NaOH/KCl buffer (pH = 13) allowed us to determine the parameter of complexation and to ascertain the high affinity of this cryptophane for cesium and thallium. A comparison with other cryptophanes that bind these two cations shows that the introduction of nitrogen atoms into the cryptophane backbone has an effect on the binding properties. The affinity for cesium and thallium(I) ions is in the following order of substitution: OH > NH2 > OCH2COOH. This study paves the way to the design of new efficient host molecules for the extraction of these two cations in aqueous solution.

Three-Dimensional-Printed Device for In Situ Monitoring of an Organic Redox-Flow Battery via NMR/MRI.

A mini organic redox-flow battery pluggable on the basis of a high-resolution nuclear magnetic resonance probehead has been conceived and built mainly by 3D printing. This device allows the realization of all modern spectroscopy experiments as well as imaging experiments. It has been tested for the real-time monitoring of redox cycling of 9,10-anthraquinone-2,7-disulfonic acid disodium salt (2,7-AQDS) in acidic conditions, which has revealed the preponderant role of dimerization in the processes of oxidation and reduction. Determination of the thermodynamic properties of homo- and heterodimer formation through quantum chemical, multilevel modeling workflows confirms our hypotheses about the molecular processes occurring during charge and discharge.

In vivo detection of carnosine and its derivatives using chemical exchange saturation transfer.

PURPOSE: To detect carnosine, anserine and homocarnosine in vivo with chemical exchange saturation transfer (CEST) at 17.2 T. METHODS: CEST MR acquisitions were performed using a CEST-linescan sequence developed in-house and optimized for carnosine detection. In vivo CEST data were collected from three different regions of interest (the lower leg muscle, the olfactory bulb and the neocortex) of eight rats. RESULTS: The CEST effect for carnosine, anserine and homocarnosine was characterized in phantoms, demonstrating the possibility to separate individual contributions by employing high spectral resolution (0.005 ppm) and low CEST saturation power (0.15 µ$$ \mu $$ T). The CEST signature of these peptides was evidenced, in vivo, in the rat brain and skeletal muscle. The presence of carnosine and anserine in the muscle was corroborated by in vivo localized spectroscopy (MRS). However, the sensitivity of MRS was insufficient for carnosine and homocarnosine detection in the brain. The absolute amounts of carnosine and derivatives in the investigated tissues were determined by liquid chromatography-electrospray ionization-tandem mass spectrometry using isotopic dilution standard methods and were in agreement with the CEST results. CONCLUSION: The robustness of the CEST-linescan approach and the favorable conditions for CEST at ultra-high magnetic field allowed the in vivo CEST MR detection of carnosine and related peptides. This approach could be useful to investigate noninvasively the (patho)-physiological roles of these molecules.

Study of syn and anti Xenon-Cryptophanes Complexes Decorated with Aromatic Amine Groups: Chemical Platforms for Accessing New Cryptophanes.

We report the synthesis of C3-symmetric cryptophanes decorated with three aromatic amine groups on the same CTB cap and their interaction with xenon. The relative stereochemistry of these two stereoisomers syn and anti was assessed thanks to the determination of the X-ray structure of an intermediate compound. As previously observed with the tris-aza-cryptophanes analogs anti-1 and syn-2 (J. Org. Chem. 2021, 86, 11, 7648-7658), both compounds anti-5 and syn-6 show a slow in-out exchange dynamics of xenon at 11.7 T. Our work supports the idea that the presence of nitrogen atoms grafted directly onto the cryptophane backbone has a strong impact on the in-out exchange dynamics of xenon whatever their stereochemistry. This result contrasts with the case of other cryptophanes decorated solely with methoxy substituents. Finally, we demonstrate that these new derivatives can be used to design new anti/syn cryptophanes bearing suitable ligands in order to constitute potent 129Xe NMR-based sensors. An example is reported here with the synthesis of the tris-iodo derivatives anti-13 and syn-14 from compounds anti-5 and syn-6.

syn-Cryptophanes: macrocyclic compounds with optimized characteristics for the design of 129Xe NMR-based biosensors.

A new water-soluble xenon host system with great promise for the 129Xe NMR-based biosensing approach is presented: the syn-cryptophane-222-hexacarboxylate. It compares favorably with its already known anti diastereomer, on the one hand, and with cucurbit[6]uril, on the other hand, in particular in terms of xenon binding constant and xenon in-out exchange, a key parameter for the efficiency of the most sensitive HyperCEST method.

Are the Physical Properties of Xe@Cryptophane Complexes Easily Predictable? The Case of syn- and anti-Tris-aza-Cryptophanes.

We report the synthesis and optical resolution of C3-symmetrical tris-aza-cryptophanes anti-3 and syn-4, as well as the study of their interaction with xenon via hyperpolarized 129Xe NMR. These molecular cages are close structural analogues of the two well-known cryptophane-A (1; chiral) and cryptophane-B (2; achiral) diastereomers since these new compounds differ only by the presence of three nitrogen atoms grafted onto the same cyclotribenzylene unit. The assignment of their relative (syn vs anti) and absolute configurations was made possible, thanks to the combined use of quantum calculations at the density functional theory level and vibrational circular dichroism spectroscopy. More importantly, our results show that despite the large structural similarities with cryptophane-A (1) and -B (2), these two new compounds show a very different behavior in the presence of xenon in organic solutions. These results demonstrate that prediction of the physical properties of the xenon@cryptophane complexes, only based on structural parameters, remains extremely difficult.

Selective Capture of Thallium and Cesium by a Cryptophane Soluble at Neutral pH.

We report in this article the synthesis of an asymmetrical cryptophane derivative (possessing only C3-symmetry) bearing three phenol groups and three other carboxylic acid functions, each of these groups on the aromatic rings. Thanks to isothermal titration calorimetry experiments, we show that this compound binds large monovalent cations, such as Cs+ and Tl+, with a binding constant significantly lower than its congeners bearing a larger number of phenol groups grafted on the benzene rings. However, higher selectivity for Cs+ and Tl+ was observed with this compound since it does not show any affinity for other alkali cations. More importantly, due to the greater solubility of this derivative in pure water, we show for the first time that effective thallium(I) complexation takes place at neutral pH. This result demonstrates that cryptophane derivatives decorated with a higher number of phenol groups are promising host molecules for removing traces of thallium(I) from aqueous phases at neutral pH or above.

Enantiopure [Cs+/Xe⊂Cryptophane]⊂FeII4L4 Hierarchical Superstructures.

Hierarchically nested hosts offer new opportunities to control the guest binding of the inner host, functionalize the cavity of the outer host, and investigate communication between different layers. Here we report a self-assembled triazatruxene-based FeII4L4 capsule, which was able to encapsulate a covalent cage, cryptophane-111 (CRY). The resulting cage-in-cage complex was capable of accommodating a cesium cation or xenon atom with altered guest binding behavior compared to the CRY alone. A crystal structure of the Russian doll complex [Cs+⊂CRY]⊂FeII4L4 unambiguously demonstrated the unusual encapsulation of a cation within a capsule bearing a 8+ charge. Moreover, the binding of enantiopure CRY occurred with high enantioselectivity (530-fold) between the two enantiomers of the tetrahedron. This discrimination resulted in stereochemical information transfer from the inner covalent cage to the outer self-assembled capsule, leading to the formation of enantiopure [guest⊂cage]⊂cage complexes. The stereochemistry of the tetrahedron persisted even after displacement of CRY with an achiral guest.

Bimodal Detection of Proteins by 129 Xe NMR and Fluorescence Spectroscopy.

A full understanding of biological phenomena involves sensitive and noninvasive detection. Herein, we report the optimization of a probe for intracellular proteins that combines the advantages of fluorescence and hyperpolarized 129 Xe NMR spectroscopy detection. The fluorescence detection part is composed of six residues containing a tetracysteine tag (-CCXXCC-) genetically incorporated into the protein of interest and of a small organic molecule, CrAsH. CrAsH becomes fluorescent if it binds to the tetracysteine tag. The part of the biosensor that enables detection by means of 129 Xe NMR spectroscopy, which is linked to the CrAsH moiety by a spacer, is based on a cryptophane core that is fully suited to reversibly host xenon. Three different peptides, containing the tetracysteine tag and four organic biosensors of different stereochemistry, are benchmarked to propose the best couple that is fully suited for the in vitro detection of proteins.

Single-Scan Diffusion-Ordered NMR Spectroscopy of SABRE-Hyperpolarized Mixtures.

The analysis of complex mixtures of dissolved molecules is a major challenge, especially for systems that gradually evolve, e. g., in the course of a chemical reaction or in the case of chemical instability. 1D NMR is a fast and non-invasive method suitable for detailed molecular analysis, though of low sensitivity. Moreover, the spectral resolution of proton, the most commonly used and most sensitive stable isotope in NMR, is also quite limited. Spatially encoded (SPEN) experiments aim at creating in one acquisition a 2D data set by simultaneously performing different 1D sub-experiments on different slices of the NMR tube, at the price of an extra loss of sensitivity. Choosing translational diffusion coefficients as the additional dimension (the so-called DOSY approach) helps to recover proton spectra of each molecule in a mixture. The sensitivity limitation of SPEN NMR can, on the other hand, be addressed with hyperpolarization methods. Within hyperpolarization methods, signal amplification by reversible exchange (SABRE), based on parahydrogen, is the cheapest and the easiest one to set up, and allows multi-shot experiments. Here we show that the spectra of a mixture's components at millimolar concentration are resolved in few seconds by combining the SABRE, SPEN and DOSY concepts.

Synthesis of Cryptophane-223-Type Derivatives with Dual Functionalization.

In this article, we present the synthesis of new cryptophane-type hosts capable of binding xenon in aqueous media and that may be useful for the development of xenon-based magnetic resonance imaging derivatives. The synthetic route proposed was chosen to facilitate both the introduction of water-solubilizing substituents and the functionalization of the host with a single arm showing recognition properties that constitute two crucial steps. This was made possible by preparing new cryptophane-223 derivatives bearing two different chemical functions that can be easily modified at a later stage. Thus, subsequent reactions allowed the design of a new cryptophane host able to bind zinc or nickel cations. The ability of this molecule to bind cationic species was assessed by calorimetric titration experiments and hyperpolarized 129Xe NMR. The advantages and disadvantages of this approach are discussed.

Inductive Coupling and Flow for Increased NMR Sensitivity.

A device is proposed to enhance the NMR sensitivity of slowly relaxing nuclei, taking advantage of a controlled solution flow within a microfluidic circuit and microsized NMR detection. Unlike our previous work ( Carret et al. Anal. Chem. 2017 , 89 ( 5 ), 2995 - 3000 ), this setup can be easily installed on any commercial NMR probehead as it uses induction between the commercial antenna and the microcoil. Such a system leads to a significant gain in sensitivity per time unit for slowly relaxing nuclei while preserving the capabilities of the host probehead.

Accurate pH Sensing using Hyperpolarized 129 Xe†NMR Spectroscopy.

In the search for powerful non-invasive methods for pH measurement, NMR usually suffers from biases, especially for heterogeneous samples or tissues. In this Communication, using the signals of hyperpolarized 129 Xe encapsulated in a pair of water-soluble cryptophanes, we show that a differential pH measurement can be achieved, free from most of these biases, by monitoring the difference between their chemical shifts.

Synthesis of Cryptophane-B: Crystal Structure and Study of Its Complex with Xenon.

Whereas the synthesis of the anti-cryptophane-A (1) derivative has been known for nearly 40 years, the preparation of its diastereomer (cryptophane-B according to Collet's nomenclature) has never been reported. Thus, the synthesis of the cryptophane-B derivative represents a real challenge for chemists interested in the preparation of these hollow molecules. Herein, we describe a synthetic route that allows us to prepare cryptophane-B (2), albeit in a low yield. The X-ray crystallographic structure of this compound is described, and it reveals the presence of an ethanol molecule inside the cavity of the host. Finally, the ability of cryptophane-B to bind xenon in 1,1,2,2-tetrachloroethane- d2 is also studied via hyperpolarized 129Xe NMR.

Unsaturated cryptophanes: Toward dual PHIP/hyperpolarised xenon sensors.

Cryptophanes, cage-molecules constituted of aromatic bowls, are now well recognised as powerful xenon hosts in 129 Xe NMR-based biosensing. In the quest of a dual probe that can be addressed only by NMR, we have studied three cryptophanes bearing a tether with an unsaturated bond. The idea behind this is to build probes that can be detected both via hyperpolarised 129 Xe NMR and para-hydrogen induced polarisation 1 H NMR. Only two of the three cryptophanes experience a sufficiently fast hydrogenation enabling the para-hydrogen induced polarisation effect. Although the in-out xenon exchange properties are maintained after hydrogenation, the chemical shift of xenon encaged in these two cryptophanes is not strikingly modified, which impedes safe discrimination of the native and hydrogenated states via 129 Xe NMR. However, a thorough examination of the hyperpolarised 1 H spectra reveals some interesting features for the catalytic process and gives us clues for the design of doubly smart 1 H/129 Xe NMR-based biosensors.

Enhancing NMR of Nonrelaxing Species Using a Controlled Flow Motion and a Miniaturized Circuit.

In this article we show that circulation of the sample in a closed-loop circuit combined to microsized detection can lead to a significant signal NMR enhancement. We present an optimized NMR device based on a mini bubble-pump associated with fluidics and microdetection that can be installed on a commercial NMR spectrometer. In addition to a significant signal enhancement for slowly relaxing nuclei, we show that it enables more precise and frequent monitoring of chemical reactions. An additional modification leads to a stopped-flow system very efficient for instance for 2D NMR experiments with long mixing times.

Role of the Methoxy Groups in Cryptophanes for Complexation of Xenon: Conformational Selection Evidence from 129 Xe-1 H NMR SPINOE Experiments.

We report the laser-polarized 129 Xe and 1 H NMR spectra of a series of cryptophane derivatives that differ only by the number of methoxy groups attached on their benzene rings and the syn or anti arrangement of the linkers (compounds 6 a-s, 9 a-s, 12 a-s). All these compounds bind xenon even though the characteristic signal of the gas encapsulated in the cavity of the cage-molecule cannot always be detected. Interestingly, the exchange dynamics of xenon strongly depends on the degree of substitution and is different from that of the cryptophane derivatives studied previously. In solution, the 1 H NMR spectra of these derivatives show the presence of different conformations in a slow exchange regime that can be explained by a decrease of the flexibility of their skeleton. Thanks to 129 Xe-1 H dipolar cross-relaxation (SPINOE) spectra, we demonstrate that a single conformation present in solution can bind xenon.

Bimodal fluorescence/129Xe NMR probe for molecular imaging and biological inhibition of EGFR in Non-Small Cell Lung Cancer.

Although Non-Small Cell Lung Cancer (NSCLC) is one of the main causes of cancer death, very little improvement has been made in the last decades regarding diagnosis and outcomes. In this study, a bimodal fluorescence/129Xe NMR probe containing a xenon host, a fluorescent moiety and a therapeutic antibody has been designed to target the Epidermal Growth Factor Receptors (EGFR) overexpressed in cancer cells. This biosensor shows high selectivity for the EGFR, and a biological activity similar to that of the antibody. It is detected with high specificity and high sensitivity (sub-nanomolar range) through hyperpolarized 129Xe NMR. This promising system should find important applications for theranostic use.

Single-Scan Multidimensional NMR Analysis of Mixtures at Sub-Millimolar Concentrations by using SABRE Hyperpolarization.

Signal amplification by reversible exchange (SABRE) is a promising method to increase the sensitivity of nuclear magnetic resonance (NMR) experiments. However, SABRE-enhanced (1)H NMR signals are short lived, and SABRE is often used to record 1D NMR spectra only. When the sample of interest is a complex mixture, this results in severe overlaps for (1)H spectra. In addition, the use of a co-substrate, whose signals may obscure the (1) H spectra, is currently the most efficient way to lower the detection limit of SABRE experiments. Here, we describe an approach to obtain clean, SABRE-hyperpolarized 2D (1)H NMR spectra of mixtures of small molecules at sub-millimolar concentrations in a single scan. The method relies on the use of para-hydrogen together with a deuterated co-substrate for hyperpolarization and ultrafast 2D NMR for acquisition. It is applicable to all substrates that can be polarized with SABRE.