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.

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.

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.

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.

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.

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.

Smart detection of toxic metal ions, Pb2+ and Cd2+, using a 129Xe NMR-based sensor.

An approach for sensitive magnetic resonance detection of metal cations is proposed. Combining the use of hyperpolarized (129)Xe NMR and of a cage-molecule functionalized by a ligand able to chelate different cations, we show that simultaneous detection of lead, zinc, and cadmium ions at nanomolar concentration is possible in short time, thanks to fast MRI sequences based on the HyperCEST scheme.

Understanding a host-guest model system through ¹²9Xe†NMR spectroscopic experiments and theoretical studies.

Gaining an understanding of the nature of host-guest interactions in supramolecular complexes involving heavy atoms is a difficult task. Described herein is a robust simulation method applied to complexes between xenon and members of a cryptophane family. The calculated chemical shift of xenon caged in a H2O2 probe, as modeled by quantum chemistry with complementary-orbital, topological, and energy-decomposition analyses, is in excellent agreement with that observed in hyperpolarized (129)Xe NMR spectra. This approach can be extended to other van der Waals complexes involving heavy atoms.

"Clickable" hydrosoluble PEGylated cryptophane as a universal platform for 129Xe magnetic resonance imaging biosensors.

We describe the synthesis of a highly water-soluble cryptophane 1 that can be seen as a universal platform for the construction of (129)Xe magnetic resonance imaging (MRI)-based biosensors. Compound 1 is easily functionalized by Huisgen cycloaddition and exhibits excellent xenon-encapsulation properties. In addition, 1 is nontoxic at the concentrations typically used for hyperpolarized (129)Xe MRI.

129Xe ultra-fast Z spectroscopy enables micromolar detection of biosensors on a 1 T benchtop spectrometer.

The availability of a benchtop nuclear magnetic resonance (NMR) spectrometer, of low cost and easily transportable, can allow detection of low quantities of biosensors, provided that hyperpolarized species are used. Here we show that the micromolar threshold can easily be reached by employing laser-polarized xenon and cage molecules reversibly hosting it. Indirect detection of caged xenon is made via chemical exchange, using ultra-fast Z spectroscopy based on spatio-temporal encoding. On this non-dedicated low-field spectrometer, several ideas are proposed to improve the signal.

Secondary isotope effects in liquid chromatography behaviour of 2H and 3H labelled solutes and solvents.

The separation of solutes that differ only in the extent of isotopic substitution of their hydrogen atoms, using either mixtures of isotopically non-modified or perdeuterated solvents as mobile phases, is described. The occurrence of a secondary isotope effect is demonstrated in reversed-phase liquid chromatography, which is independent of the nature of the stationary phase (different octadecyl-bonded silicas, an embedded alkylamide-bonded silica, as well as one polymeric stationary phase were tested), and the water content and the nature of organic modifier of the mobile phase. The separation of 24 structurally different isotopologue pairs (apolar compounds and polar compounds with exchangeable or non-exchangeable hydrogen atoms) is examined using reversed-phase liquid chromatography. It is found that the greater the number of isotopically substituted hydrogen atoms in a given organic solute, the better is the separation of a particular isotopologue pair. The single secondary isotope effect is shown to be dependent on the number of isotopic substitutions. The greater the number of these substitutions, the smaller is the single isotope effect. The single secondary isotope effect is higher for aromatic hydrocarbons than for aliphatic hydrocarbons. A secondary isotope effect is also observed in chiral chromatography and normal-phase liquid chromatography, as well as on changing the nature of the substituting isotope, i.e.: tritium instead of deuterium. Thus, we have demonstrated that the total secondary isotopic effect for hydrogen/tritium is higher than for hydrogen/deuterium. This isotope effect involves only the consequences of changes in interactions due to nuclear motions. Overall this study confirms the predominance of hydrophobic effects in retention processes in reversed-phase liquid chromatography. In reversed-phase liquid chromatography, a secondary isotope effect related to mobile phase composition is also observed. The behaviour of deuterium oxide and water in mobile phases of the same composition (%, w/w) is compared. Independent of the nature of the organic modifier (methanol, acetonitrile or ethanol), the effect of replacing H2O with 2H2O in the mobile phase, is an increase in the retention factors and an improvement in the chromatographic resolution of isotopologue pairs. This increase in the resolution is not accompanied by a change in the chromatographic selectivity. The measurement of liquid-liquid extraction coefficients proves that the effect is mainly due to the modification of the phase ratio. In general the effect of 2H-labelled solvents (2H2O and C2H3CN) as mobile phase components, compared to their isotopically non-modified isomers, can be rationalized on the basis of their lower polarisabilities. Overall the use of perdeuterated rather than isotopically non-modified solvents as mobile phase components leads to the most efficient separation systems.

Rim-functionalized cryptophane-111 derivatives via heterocapping, and their xenon complexes.

Capping of cyclotriphenolene (3a) by the more available cyclotriguaiacylene (3c) or trisbromocyclotriphenolene (3b) gives the first rim-functionalized cryptophane-111 derivatives. Crystal structures of the xenon complexes reveal high cavity packing coefficients and unprecedentedly short Xe···C contacts.