Biothiol Xenon MRI Sensor Based on Thiol-Addition Reaction

Catalytic Syntheses of Thiol-End-Functionalized ROMP Polymers

Thiol-functionalized polymers have become a crucial class of materials due to their distinct chemical properties and versatile reactivity, leading to a broad spectrum of applications. Herein, we report the straightforward syntheses of a wide range of thiol-end-functionalized ring-opening metathesis polymerization (ROMP) polymers exploiting our previously reported catalytic ROMP mechanisms using suitable chain transfer agents. All the synthesized polymers were characterized via SEC, 1H NMR spectroscopy and MALDI-ToF mass spectrometry techniques. Furthermore, the existence of thiol groups on the polymer chains was verified through the well-established thiol coating reaction on gold nanoparticle surfaces. We believe this method of synthesizing thiol-end-functionalized ROMP polymers (using a reduced amount of ruthenium metal compared to conventional living ROMP) will be of great importance to materials science and biochemical research.

Dual-Signal Chemical Exchange Saturation Transfer (Dusi-CEST): An Efficient Strategy for Visualizing Drug Delivery Monitoring in Living Cells

We report a dual-signal chemical exchange saturation transfer (Dusi-CEST) strategy for drug delivery and detection in living cells. The two signals can be detected by operators in complex environments. This strategy is demonstrated on a cucurbit[6]uril (CB[6]) nanoparticle probe, as an example. The CB[6] probe is equipped with two kinds of hydrophobic cavities: one is found inside CB[6] itself, whereas the other exists inside the nanoparticle. When the probe is dispersed in aqueous solution as part of a hyperpolarized 129Xe NMR experiment, two signals appear at two different chemical shifts (100 and 200 ppm). These two resonances correspond to the NMR signals of 129Xe in the two different cavities. Upon loading with hydrophobic drugs, such as paclitaxel, for intracellular drug delivery, the two resonances undergo significant changes upon drug loading and cargo release, giving rise to a metric enabling the assessment of drug delivery success. The simultaneous change of Dusi-CEST likes a mobile phone that can receive both LTE and Wi-Fi signals, which can help reduce the occurrence of false positives and false negatives in complex biological environments and help improve the accuracy and sensitivity of single-shot detection.

A fluorescent probe based on a phenylalanine derivative is capable of sequential detection of Zn2+ and Cys/His

A facile and dual fluorescent chemosensor (named 7-IDF) based on a phenylalanine derivative with an indole group was designed and synthesized. 7-IDF can selectively and sensitively detect Zn²⁺ via obvious fluorescence enhancement in an aqueous solution. Remarkably, the 7-IDF-Zn complex with blue luminescence has higher selectivity toward cysteine (Cys) and histidine (His) than for other amino acids. Intriguingly, 7-IDF can also be used as an excellent probe to detect Zn²⁺ in real water samples. Moreover, 7-IDF and 7-IDF-Zn possess excellent biocompatibility and cell permeability, and 7-IDF can consecutively detect Zn²⁺ and Cys/His in Hela cells through fluorescence imaging experiments. This study suggests that the phenylalanine-based chemosensor possesses great potential applications for the sequential detection of Zn²⁺ and Cys/His in biosystems.

Ultrasensitive molecular building block for biothiol NMR detection at picomolar concentrations

Magnetic resonance imaging (MRI) provides structural and functional information, but it did not probe chemistry. Chemical information could help improve specificity of detection. Herein, we introduce a general method based on a modular design to construct a molecular building block Xe probe to help image intracellular biothiols (glutathione (GSH), cysteine (Cys) and homocysteine (Hcy)), the abnormal content of which is related to various diseases. This molecular building block possesses a high signal-to-noise ratio and no background signal effects. Its detection threshold was 100 pM, which enabled detection of intracellular biothiols in live cells. The construction strategy can be easily extended to the detection of any other biomolecule or biomarker. This modular design strategy promotes efficiency of development of low-cost multifunctional probes that can be combined with other readout parameters, such as optical readouts, to complement 129Xe MRI to usher in new capabilities for molecular imaging.

Cryptophane-xenon complexes for 129Xe MRI applications

The use of magnetic resonance imaging (MRI) and spectroscopy (MRS) in the clinical setting enables the acquisition of valuable anatomical information in a rapid, non-invasive fashion. However, MRI applications for identifying disease-related biomarkers are limited due to low sensitivity at clinical magnetic field strengths. The development of hyperpolarized (hp) ¹²⁹Xe MRI/MRS techniques as complements to traditional ¹H-based imaging has been a burgeoning area of research over the past two decades. Pioneering experiments have shown that hp ¹²⁹Xe can be encapsulated within host molecules to generate ultrasensitive biosensors. In particular, xenon has high affinity for cryptophanes, which are small organic cages that can be functionalized with affinity tags, fluorophores, solubilizing groups, and other moieties to identify biomedically relevant analytes. Cryptophane sensors designed for proteins, metal ions, nucleic acids, pH, and temperature have achieved nanomolar-to-femtomolar limits of detection via a combination of ¹²⁹Xe hyperpolarization and chemical exchange saturation transfer (CEST) techniques. This review aims to summarize the development of cryptophane biosensors for ¹²⁹Xe MRI applications, while highlighting innovative biosensor designs and the consequent enhancements in detection sensitivity, which will be invaluable in expanding the scope of ¹²⁹Xe MRI.

This review aims to summarize the development of cryptophane biosensors for ¹²⁹Xe MRI applications, while highlighting innovative biosensor designs and the consequent enhancements in detection sensitivity, which will be invaluable in expanding the scope of ¹²⁹Xe MRI.

Molecular Concentration Determination Using Long-Interval Chemical Exchange Inversion Transfer (CEIT) NMR Spectroscopy

Functionalized hyperpolarized xenon "cage" molecules have often been used for ultrasensitive detection of biomolecules and microenvironment properties. However, the rapid and accurate measurement of molecule concentration is still a challenge. Here, we report a molecule concentration measurement method using long-interval chemical exchange inversion transfer (CEIT) NMR spectroscopy. The molecule concentration can be quantitatively measured with only 2 scans, which shortens the acquisition time by about 10 times compared to conventional Hyper-CEST (chemical exchange saturation transfer) z-spectrum method. Moreover, we found that the accuracy of concentration determination would be the best when the CEIT effect is 1-1/e or close to it, and a relative deviation of CrA-(COOH)₆ less than ±1% has been achieved by only a one-step optimization of the number of cycles. The proposed method enables efficient and accurate determination of molecule concentration, which provides a potential way for rapid quantitative molecular imaging applications.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

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.

Photosensitive MRI biosensor for BCRP-Targeted uptake and light-induced inhibition of tumor cells

Riboflavin and its derivatives are the most important coenzymes in vivo metabolism, and are closely related to life activities. In this paper, the first photolysis ¹²⁹Xe biosensor was developed by combining cryptophane-A with riboflavin moiety, which showed photosensitivity recorded by hyperpolarized ¹²⁹Xe NMR/MRI technology with an obvious chemical shift change of 5.3 ppm in aqueous solution. Cellular fluorescence imaging confirmed that the biosensor could be enriched in MCF-7 cells, and MTT assays confirmed that the cytotoxicity was enhanced after irradiation. Findings suggested that the biosensor has a potential application in tumor targeting and the inhibition of tumor cell proliferation after photodegradation.

Monomeric Cryptophane with Record-High Xe Affinity Gives Insights into Aggregation-Dependent Sensing

Cryptophane host molecules provide ultrasensitive contrast agents for ¹²⁹Xe NMR/MRI. To investigate key features of cryptophane-Xe sensing behavior, we designed a novel water-soluble cryptophane with a pendant hydrophobic adamantyl moiety, which has good affinity for a model receptor, beta-cyclodextrin (β-CD). Adamantyl-functionalized cryptophane-A (AFCA) was synthesized and characterized for Xe affinity, ¹²⁹Xe NMR signal, and aggregation state at varying AFCA and β-CD concentrations. The Xe-AFCA association constant was determined by fluorescence quenching, K_A = 114,000 ± 5000 M^-1 at 293 K, which is the highest reported affinity for a cryptophane host in phosphate-buffered saline (pH 7.2). No hyperpolarized (hp) ¹²⁹Xe NMR peak corresponding to AFCA-bound Xe was directly observed at high (100 μM) AFCA concentration, where small cryptophane aggregates were observed, and was only detected at low (15 μM) AFCA concentration, where the sensor remained fully monomeric in solution. Additionally, we observed no change in the chemical shift of AFCA-encapsulated ¹²⁹Xe after β-CD binding to the adamantyl moiety and a concomitant lack of change in the size distribution of the complex, suggesting that a change in the aggregation state is necessary to elicit a ¹²⁹Xe NMR chemical shift in cryptophane-based sensing. These results aid in further elucidating the recently discovered aggregation phenomenon, highlight limitations of cryptophane-based Xe sensing, and offer insights into the design of monomeric, high-affinity Xe sensors.

Paramagnetic Shifts and Guest Exchange Kinetics in ConFe4-n Metal-Organic Capsules

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [Co_nFe_4-nL₆]^4- (n = 1-3), where L^2- = 4,4'-bis[(2-pyridinylmethylene)amino]-[1,1'-biphenyl]-2,2'-disulfonate. H bond networks between capsule sulfonates and guanidinium cations promote the crystallization of [Co_nFe_4-nL₆]^4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH₂)₃)₄[Co_1.8Fe_2.2L₆]·69H₂O (1) and (C(NH₂)₃)₄[Co_2.7Fe_1.3L₆]·73H₂O (2), and two Xe- and CFCl₃-encapsulated forms, (C(NH₂)₃)₄[(Xe)_0.8Co_1.8Fe_2.2L₆]·69H₂O (3) and (C(NH₂)₃)₄[(CFCl₃)Co_2.0Fe_2.0L₆]·73H₂O (4), respectively. Structural analyses reveal that Xe induces negligible structural changes in 3, while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl₃, in 4. These guest-encapsulated [Co_nFe_4-nL₆]^4- molecules reveal ¹²⁹Xe and ¹⁹F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin Fe^II by high-spin Co^II. Likewise, the temperature dependence of the ¹²⁹Xe and ¹⁹F NMR resonances increases by 0.1 and 0.06 ppm/K, respectively, with each additional paramagnetic Co^II center. The optimal temperature for hyperpolarized (hp) ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST) with [Co_nFe_4-nL₆]^4- capsules was found to be inversely proportional to the number of Co^II centers, n, which is consistent with the Xe chemical exchange accelerating as the portals expand. The systematic study was facilitated by the tunability of the [M₄L₆]^4- capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted ¹²⁹Xe resonances.

A Small Molecular Multifunctional Tool for pH Detection, Fluorescence Imaging, and Photodynamic Therapy

A smart multitool platform for theranostics would be useful for monitoring the administration of therapies in vivo. However, the integration of multiple functions into a single small-molecule platform remains a challenge. In this study, we developed a multifunctional probe based on a small-molecule platform. The properties of this probe were investigated via hyperpolarized ¹²⁹Xe NMR/MRI, fluorescence imaging in cells and in vivo, and photodynamic therapy (PDT) in tumor mouse models. This multifunctional probe shows good pH response across a broad range of pH values. It also exhibits excellent fluorescence in vivo for mapping its biodistribution. Additionally, it produces enough ¹O₂ radicals for in vivo PDT. The combination of these functionalities into a single small-molecule platform, rather than a bulky nanoconstruct, offers unique possibilities for molecular imaging and therapy.

Directly Functionalized Cucurbit[7]uril as a Biosensor for the Selective Detection of Protein Interactions by 129 Xe hyperCEST NMR

Advancement of hyperpolarized ¹²⁹ Xe MRI technology toward clinical settings demonstrates the considerable interest in this modality for diagnostic imaging. The number of contrast agents, termed biosensors, for ¹²⁹ Xe MRI that respond to specific biological targets, has grown and diversified. Directly functionalized xenon-carrying macrocycles, such as the large family of cryptophane-based biosensors, are good for localization-based imaging and provide contrast before and after binding events occur. Noncovalently functionalized constructs, such as cucurbituril- and cyclodextrin-based biosensors, benefit from commercial availability and optimal exchange dynamics for CEST imaging. In this work, we report the first directly functionalized cucurbituril used as a xenon biosensor. Biotinylated cucurbit[7]uril (btCB7) gives rise to a ¹²⁹ Xe hyperCEST response at the unusual shift of δ=28 ppm when bound to its protein target with substantial CEST contrast. We posit that the observed chemical shift is due to the deformation of btCB7 upon binding to avidin, caused by proximity to the protein surface. Conformational searches and molecular dynamics (MD) simulations support this hypothesis. This construct combines the strengths of both families of biosensors, enables a multitude of biological targets through avidin conjugation, and demonstrates the advantages of functionalized cucurbituril-based biosensors.

A PET-based turn-on fluorescent probe for sensitive detection of thiols and H2S and its bioimaging application in living cells, tissues and zebrafish

A two-photon turn-on fluorescent probe regulated by a PET mechanism has been engineered for detection of thiols and H₂S in living cells, tissues and zebrafish.

Cryptophane Nanoscale Assemblies Expand 129Xe NMR Biosensing

Cryptophane-based biosensors are promising agents for the ultrasensitive detection of biomedically relevant targets via 129Xe NMR. Dynamic light scattering revealed that cryptophanes form water-soluble aggregates tens to hundreds of nanometers in size. Acridine orange fluorescence quenching assays allowed quantitation of the aggregation state, with critical concentrations ranging from 200 nM to 600 nM, depending on the cryptophane species in solution. The addition of excess carbonic anhydrase (CA) protein target to a benzenesulfonamide-functionalized cryptophane biosensor (C8B) led to C8B disaggregation and produced the expected 1:1 C8B-CA complex. C8B showed higher affinity at 298 K for the cytoplasmic isozyme CAII than the extracellular CAXII isozyme, which is a biomarker of cancer. Using hyper-CEST NMR, we explored the role of stoichiometry in detecting these two isozymes. Under CA-saturating conditions, we observed that isozyme CAII produces a larger 129Xe NMR chemical shift change (δ = 5.9 ppm, relative to free biosensor) than CAXII (δ = 2.7 ppm), which indicates the strong potential for isozyme-specific detection. However, stoichiometry-dependent chemical shift data indicated that biosensor disaggregation contributes to the observed 129Xe NMR chemical shift change that is normally assigned to biosensor-target binding. Finally, we determined that monomeric cryptophane solutions improve hyper-CEST saturation contrast, which enables ultrasensitive detection of biosensor-protein complexes. These insights into cryptophane-solution behavior support further development of xenon biosensors, but will require reinterpretation of the data previously obtained for many water-soluble cryptophanes.

Nanomolar small-molecule detection using a genetically encoded 129Xe NMR contrast agent

Genetically encoded magnetic resonance imaging (MRI) contrast agents enable non-invasive detection of specific biomarkers in vivo.

Genetically encoded magnetic resonance imaging (MRI) contrast agents enable non-invasive detection of specific biomarkers in vivo. Here, we employed the hyper-CEST ¹²⁹Xe NMR technique to quantify maltose (32 nM to 1 mM) through its modulation of conformational change and xenon exchange in maltose binding protein (MBP). Remarkably, no hyper-CEST signal was observed for MBP in the absence of maltose, making MBP an ultrasensitive “smart” contrast agent. The resonance frequency of ¹²⁹Xe bound to MBP was greatly downfield-shifted (Δδ = 95 ppm) from the ¹²⁹Xe_(aq) peak, which facilitated detection in E. coli as well as multiplexing with TEM-1 β-lactamase. Finally, a Val to Ala mutation at the MBP–Xe binding site yielded 34% more contrast than WT, with ¹²⁹Xe resonance frequency shifted 59 ppm upfield from WT. We conclude that engineered MBPs constitute a new class of genetically encoded, analyte-sensitive molecular imaging agents detectable by ¹²⁹Xe NMR/MRI.

Mitochondria Targeted and Intracellular Biothiol Triggered Hyperpolarized 129Xe Magnetofluorescent Biosensor

Biothiols such as gluthathione (GSH), cysteine (Cys), homocysteine (Hcy), and thioredoxin (Trx) play vital roles in cellular metabolism. Various diseases are associated with abnormal cellular biothiol levels. Thus, the intracellular detection of biothiol levels could be a useful diagnostic tool. A number of methods have been developed to detect intracellular thiols, but sensitivity and specificity problems have limited their applications. To address these limitations, we have designed a new biosensor based on hyperpolarized xenon magnetic resonance detection, which can be used to detect biothiol levels noninvasively. The biosensor is a multimodal probe that incorporates a cryptophane-A cage as ¹²⁹Xe NMR reporter, a naphthalimide moiety as fluorescence reporter, a disulfide bond as thiol-specific cleavable group, and a triphenylphosphonium moiety as mitochondria targeting unit. When the biosensor interacts with biothiols, disulfide bond cleavage leads to enhancements in the fluorescence intensity and changes in the ¹²⁹Xe chemical shift. Using Hyper-CEST (chemical exchange saturation transfer) NMR, our biosensor shows a low detection limit at picomolar (10^-10 M) concentration, which makes a promise to detect thiols in cells. The biosensor can detect biothiol effectively in live cells and shows good targeting ability to the mitochondria. This new approach not only offers a practical technique to detect thiols in live cells, but may also present an excellent in vivo test platform for xenon biosensors.