Substituent effects on xenon binding affinity and solution behavior of water-soluble cryptophanes

A Cyclotriveratrylene Solvent-Dependent Chiral Switch

Chiral molecular switches are attracting attention as they could pave the way to chiral molecular machines. Herein, we report on the design and synthesis of a single molecule chiral switch based on a cyclotriveratrylene scaffold, in which the chirality inversion is controlled by the solvent. Hemicryptophanes are built around a C3 cyclotriveratrylene chiral unit, with either M or P handedness, connected to another tripod and usually displaying an "out" configuration. Here, we demonstrate that solvents are able to control the "in" and "out" configurations of the CTV unit, creating a chiral molecular switch from (M/P)"in" to (P/M)"out" handedness. The full characterization of the "in" and "out" configurations and of the chirality switch were made possible by combining NMR, HPLC, ECD, DFT and molecular dynamics. Interestingly, bulky aromatic solvents such as 2-t-butylphenol favor the "in" configuration while polar aprotic solvents such as acetone favor the "out" configuration. This chiral switch was found to be fully reversible allowing the system to oscillate between two different M and P configurations several times upon the action of solvents stimuli.

NMR chemical shift of confined 129Xe: coordination number, paramagnetic channels and molecular dynamics in a cryptophane-A biosensor

Advances in hyperpolarisation and indirect detection have enabled the development of xenon nuclear magnetic resonance (NMR) biosensors (XBSs) for molecule-selective sensing in down to picomolar concentration. Cryptophanes (Crs) are popular cages for hosting the Xe "spy". Understanding the microscopic host-guest chemistry has remained a challenge in the XBS field. While early NMR computations of XBSs did not consider the important effects of host dynamics and explicit solvent, here we model the motionally averaged, relativistic NMR chemical shift (CS) of free Xe, Xe in a prototypic CrA cage and Xe in a water-soluble CrA derivative, each in an explicit H2O solvent, over system configurations generated at three different levels of molecular dynamics (MD) simulations. We confirm the "contact-type" character of the Xe CS, arising from the increased availability of paramagnetic channels, magnetic couplings between occupied and virtual orbitals through the short-ranged orbital hyperfine operator, when neighbouring atoms are in contact with Xe. Remarkably, the Xe CS in the present, highly dynamic and conformationally flexible situations is found to depend linearly on the coordination number of the Xe atom. We interpret the high- and low-CS situations in terms of the magnetic absorption spectrum and choose our preference among the used MD methods based on comparison with the experimental CS. We check the role of spin-orbit coupling by comparing with fully relativistic CS calculations. The study outlines the computational workflow required to realistically model the CS of Xe confined in dynamic cavity structures under experimental conditions, and contributes to microscopic understanding of XBSs.

Energetics and exchange of xenon and water in a prototypic cryptophane-A biosensor structure

A microscopic description of the energetics and dynamics of xenon NMR biosensors can be experimentally difficult to achieve. We conduct molecular dynamics and metadynamics simulations of a prototypical Xe@cryptophane-A biosensor in an explicit water solvent. We compute the non-covalent Xe binding energy, identify the complexation mechanism of Xe, and calculate the exchange dynamics of water molecules between the solution and the host. Three distinct, hitherto unreported Xe exchange processes are identified, and water molecules initialize each one. The obtained binding energies support the existing literature. The residence times and energetics of water guests are reported. An empty host does not remain empty, but is occupied by water. The results contribute to the understanding and development of Xe biosensors based on cryptophane derivatives and alternative host structures.

Counteranions at Peripheral Sites Tune Guest Affinity for a Protonated Hemicryptophane

The affinity of small molecules for biomolecular cavities is tuned through a combination of primary and secondary interactions. It has been challenging to mimic these features in organic synthetic host molecules, however, where the cavities tend to be highly symmetric and nonpolar, and less amenable to chemical manipulation. Here, a host molecule composed of a TREN ligand and cyclotriveratrylene moiety was investigated. Size-matched polar guests were encapsulated within the cavity via triple protonation of the TREN moiety with various sulfonic acids. X-ray crystallography confirmed guest encapsulation and identified three methanesulfonates, p-toluenesulfonates, or 2-naphthalenesulfonates hydrogen-bonded with H3TREN at the periphery of the cavity. These structurally diverse counteranions were shown by 1H NMR spectroscopy to differentially regulate guest access at the three portals, and to undergo competitive displacement in solution. This work reveals "counteranion tuning" to be a simple and powerful strategy for modulating host-guest affinity, as applied here in a TREN-hemicryptophane.

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.

Enantiopure cryptophane derivatives: Synthesis and chiroptical properties

This review addresses the synthesis of enantiopure cryptophane and the study of their chiroptical properties. Cryptophane derivatives represent an important class of macrocyclic compounds that can bind a large range of species in solution under different conditions. The overwhelming majority of these host molecules is chiral, and their chiroptical properties have been thoroughly investigated. The first part of this review is dedicated to the optical resolution and the synthesis of enantiopure cryptophane derivatives. In a second part, the study of the chiroptical properties of these molecular hosts by different techniques such as electronic and vibrational circular dichroism and Raman optical activity is detailed. These techniques allow the determination of the absolute configuration of cryptophane derivatives and provide useful information about their conformation in different conditions.

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.

Mapping of Absolute Host Concentration and Exchange Kinetics of Xenon Hyper-CEST MRI Agents

Xenon magnetic resonance imaging (MRI) provides excellent sensitivity through the combination of spin hyperpolarization and chemical exchange saturation transfer (CEST). To this end, molecular hosts such as cryptophane-A or cucurbit[n]urils provide unique opportunities to design switchable MRI reporters. The concentration determination of such xenon binding sites in samples of unknown dilution remains, however, challenging. Contrary to 1H CEST agents, an internal reference of a certain host (in this case, cryptophane-A) at micromolar concentration is already sufficient to resolve the entire exchange kinetics information, including an unknown host concentration and the xenon spin exchange rate. Fast echo planar imaging (EPI)-based Hyper-CEST MRI in combination with Bloch-McConnell analysis thus allows quantitative insights to compare the performance of different emerging ultra-sensitive MRI reporters.

Decacationic Pillar[5]arene: A New Scaffold for the Development of 129Xe MRI Imaging Agents

A decacationic water-soluble pillar[5]arene possessing a nonsolvated hydrophobic core has been designed and synthesized. This supramolecular host is capable of binding xenon, as evidenced by hyperCEST depletion experiments. Fluorescence-based studies also demonstrate that xenon binds into the cavity of the pillararene with an association constant of 4.6 × 103 M-1. These data indicate that the water-soluble pillararene is a potential scaffold for building contrast agents that can be detected by xenon-129 magnetic resonance imaging.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Ultrasound Responsive Noble Gas Microbubbles for Applications in Image-Guided Gas Delivery

Noble gases, especially xenon (Xe), have been shown to have antiapoptotic effects in treating hypoxia ischemia related injuries. Currently, in vivo gas delivery is systemic and performed through inhalation, leading to reduced efficacy at the injury site. This report provides a first demonstration of the encapsulation of pure Xe, Ar, or He in phospholipid-coated sub-10 µm microbubbles, without the necessity of stabilizing perfluorocarbon additives. Optimization of shell compositions and preparation techniques show that distearoylphosphatidylcholine (DSPC) with DSPE-PEG5000 can produce stable microbubbles upon shaking, while dibehenoylphosphatidylcholine (DBPC) blended with either DSPE-PEG2000 or DSPE-PEG5000 produces a high yield of microbubbles via a sonication/centrifugation method. Xe and Ar concentrations released into the microbubble suspension headspace are measured using GC-MS, while Xe released directly in solution is detected by the fluorescence quenching of a Xe-sensitive cryptophane molecule. Bubble production is found to be amenable to scale-up while maintaining their size distribution and stability. Excellent ultrasound contrast is observed in a phantom for several minutes under physiological conditions, while an intravenous administration of a bolus of pure Xe microbubbles provides significant contrast in a mouse in pre- and post-lung settings (heart and kidney, respectively), paving the way for image-guided, localized gas delivery for theranostic applications.

Functionalized Lipopeptide Micelles as Highly Efficient NMR Depolarization Seed Points for Targeted Cell Labelling in Xenon MRI

Improving diagnostic imaging and therapy by targeted compound delivery to pathological areas and across biological barriers is of urgent need. A lipopeptide, P-CrA-A2, composed of a highly cationic peptide sequence (A2), an N-terminally attached palmitoyl chain (P) and cryptophane molecule (CrA) for preferred uptake into blood-brain barrier (BBB) capillary endothelial cells, was generated. CrA allows reversible binding of Xe for NMR detection with hyperpolarized nuclei. The lipopeptide forms size-optimized micelles with a diameter of about 11 nm at low micromolar concentration. Their high local CrA payload has a strong and switchable impact on the bulk magnetization through Hyper-CEST detection. Covalent fixation of CrA does not impede micelle formation and does not hamper its host functionality but simplifies Xe access to hosts for inducing saturation transfer. Xe Hyper-CEST magnetic resonance imaging (MRI) allows for distinguishing BBB endothelial cells from control aortic endothelial cells, and the small micelle volume with a sevenfold improved CrA-loading density compared to liposomal carriers allows preferred cell labelling with a minimally invasive volume (≈16 000-fold more efficient than ¹⁹ F cell labelling). Thus, these nanoscopic particles combine selectivity for human brain capillary endothelial cells with great sensitivity of Xe Hyper-CEST MRI and might be a potential MRI tool in brain diagnostics.

Can glycine betaine denature proteins?

Glycine betaine (GB) is a naturally occurring osmolyte that has been widely recognized as a protein protectant. Since GB consists of a methylated ammonium moiety, it can engage in strong cation-π interactions with aromatic amino acid sidechains. We hypothesize that such specific binding interactions would allow GB to decrease the stability of proteins that are predominantly stabilized by a cluster of aromatic amino acids. To test this hypothesis, we investigate the effect of GB on the stability of two β-hairpins (or mini-proteins) that contain such a cluster. We find that for both systems the stability of the folded state first decreases and then increases with increasing GB concentration. Such non-monotonic dependence not only confirms that GB can act as a protein denaturant, but also underscores the complex interplay between GB's stabilizing and destabilizing forces toward a given protein. While stabilizing osmolytes all have the tendency to be excluded from the protein surface which is the action underlying their stabilizing effect, our results suggest that in order to quantitatively assess the effect of GB on the stability of any given protein, specific cation-π binding interactions need to be explicitly considered. Moreover, our results show, consistent with other studies, that cation methylation can strengthen the respective cation-π interactions. Taken together, these findings provide new insight into the mechanism by which amino acid-based osmolytes interact with proteins.

Synthesis of Cyclotriveratrylene-Sucrose-Based Capsules

Cyclotriveratrylene (CTV) is a C₃-symmetrical macrocycle, which can be used as a chiral building block in the construction of supramolecular containers. Coupling of the CTV unit with a sucrose molecule gave enantiopure water-soluble (after deprotection) containers. The absolute configuration of the synthesized capsules was determined by NMR and ECD spectroscopies and DFT calculations.

Fully Collapsed Imploded Cryptophanes in Solution and in the Solid State

Cryptophanes with flexible linkers derived from (±)-tris-(4-formyl-phenyl)-cyclotriguaiacylene with either bisoxydi(ethylamine) or bis(aminopropyl)ether were isolated as single crystals, the crystal structures of which showed the proposed, but previously uncharacterised, out-in conformation, in which both cyclotriguaiacylene fragments adopt a crown conformation with one crown sitting inside the other. The usual cage-like out-out conformation of the cryptophanes was observed when crystals were dissolved upon heating, and the molecules collapsed back to the out-in isomers over time. In contrast, a cryptophane also derived from (±)-tris-(4-formyl-phenyl)-cyclotriguaiacylene but with rigid dibenzalhydrazine linkers was isolated as the more usual out-out isomer.

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.

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.

A cryptophane-based "turn-on" 129Xe NMR biosensor for monitoring calmodulin

We present the first cryptophane-based "turn-on" 129Xe NMR biosensor, employing a peptide-functionalized cryptophane to monitor the activation of calmodulin (CaM) protein in solution. In the absence of CaM binding, interaction between the peptide and cryptophane completely suppresses the hyperpolarized 129Xe-cryptophane NMR signal. Biosensor binding to Ca2+-activated CaM produces the expected 129Xe-cryptophane NMR signal.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

Infrared and fluorescence assessment of the hydration status of the tryptophan gate in the influenza A M2 proton channel

The M2 proton channel of the influenza A virus has been the subject of extensive studies because of its critical role in viral replication. As such, we now know a great deal about its mechanism of action, especially how it selects and conducts protons in an asymmetric fashion. The conductance of this channel is tuned to conduct protons at a relatively low biologically useful rate, which allows acidification of the viral interior of a virus entrapped within an endosome, but not so great as to cause toxicity to the infected host cell prior to packaging of the virus. The dynamic, structural and chemical features that give rise to this tuning are not fully understood. Herein, we use a tryptophan (Trp) analog, 5-cyanotryptophan, and various methods, including linear and nonlinear infrared spectroscopies, static and time-resolved fluorescence techniques, and molecular dynamics simulations, to site-specifically interrogate the structure and hydration dynamics of the Trp41 gate in the transmembrane domain of the M2 proton channel. Our results suggest that the Trp41 sidechain adopts the t90 rotamer, the χ2 dihedral angle of which undergoes an increase of approximately 35° upon changing the pH from 7.4 to 5.0. Furthermore, we find that Trp41 is situated in an environment lacking bulk-like water, and somewhat surprisingly, the water density and dynamics do not show a measurable difference between the high (7.4) and low (5.0) pH states. Since previous studies have shown that upon channel opening water flows into the cavity above the histidine tetrad (His37), the present finding thus provides evidence indicating that the lack of sufficient water molecules near Trp41 needed to establish a continuous hydrogen bonding network poses an additional energetic bottleneck for proton conduction.

An Expanded Palette of Xenon-129 NMR Biosensors

Molecular imaging holds considerable promise for elucidating biological processes in normal physiology as well as disease states, by determining the location and relative concentration of specific molecules of interest. Proton-based magnetic resonance imaging (¹H MRI) is nonionizing and provides good spatial resolution for clinical imaging but lacks sensitivity for imaging low-abundance (i.e., submicromolar) molecular markers of disease or environments with low proton densities. To address these limitations, hyperpolarized (hp) ¹²⁹Xe NMR spectroscopy and MRI have emerged as attractive complementary methodologies. Hyperpolarized xenon is nontoxic and can be readily delivered to patients via inhalation or injection, and improved xenon hyperpolarization technology makes it feasible to image the lungs and brain for clinical applications.

In order to target hp ¹²⁹Xe to biomolecular targets of interest, the concept of “xenon biosensing” was first proposed by a Berkeley team in 2001. The development of xenon biosensors has since focused on modifying organic host molecules (e.g., cryptophanes) via diverse conjugation chemistries and has brought about numerous sensing applications including the detection of peptides, proteins, oligonucleotides, metal ions, chemical modifications, and enzyme activity. Moreover, the large (∼300 ppm) chemical shift window for hp ¹²⁹Xe bound to host molecules in water makes possible the simultaneous identification of multiple species in solution, that is, multiplexing. Beyond hyperpolarization, a 10⁶-fold signal enhancement can be achieved through a technique known as hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST), which shows great potential to meet the sensitivity requirement in many applications.

This Account highlights an expanded palette of hyper-CEST biosensors, which now includes cryptophane and cucurbit[6]uril (CB[6]) small-molecule hosts, as well as genetically encoded gas vesicles and single proteins. In 2015, we reported picomolar detection of commercially available CB[6] via hyper-CEST. Inspired by the versatile host–guest chemistry of CB[6], our lab and others developed “turn-on” strategies for CB[6]-hyper-CEST biosensing, demonstrating detection of protein analytes in complex media and specific chemical events. CB[6] is starting to be employed for in vivo imaging applications. We also recently determined that TEM-1 β-lactamase can function as a single-protein reporter for hyper-CEST and observed useful saturation contrast for β-lactamase expressed in bacterial and mammalian cells. These newly developed small-molecule and genetically encoded xenon biosensors offer significant potential to extend the scope of hp ¹²⁹Xe toward molecular MRI.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

Utility of 5-Cyanotryptophan Fluorescence as a Sensitive Probe of Protein Hydration

Tryptophan (Trp) fluorescence has been widely used to interrogate the structure, dynamics, and function of proteins. In particular, it provides a convenient and site-specific means to probe a protein's hydration status and dynamics. Herein, we show that a tryptophan analogue, 5-cyanotryptophan (TrpCN), can also be used for this purpose, but with the benefit of enhanced sensitivity to hydration. This conclusion is reached based on measurements of the static and time-resolved fluorescence properties of 5-cyanoindole, TrpCN, and TrpCN-containing peptides in different solvents, which indicate that upon dehydration the fluorescence quantum yield (QY) and lifetime (τF) of TrpCN undergo a much greater change in comparison to those of Trp. For example, in H2O the QY of TrpCN is less than 0.01, which increases to 0.11 in 1,4-dioxane. Consistently, the fluorescence decay kinetics of TrpCN in H2O are dominated by a 0.4 ns component, whereas in 1,4-dioxane the kinetics are dominated by a 6.0 ns component. The versatile utility of TrpCN as a sensitive fluorescence reporter is further demonstrated in three applications, where we used it (1) to probe the solvent property of a binary mixture consisting of dimethyl sulfoxide and H2O, (2) to monitor the binding interaction of an antimicrobial peptide with lipid membranes, and (3) to differentiate two differently hydrated environments in a folded protein.

Xe affinities of water-soluble cryptophanes and the role of confined water

Simulations provide molecular insight on the aqueous binding of Xe to cryptophanes.

Given their relevance to drug design and chemical sensing, host–guest interactions are of broad interest in molecular science. Natural and synthetic host molecules provide vehicles for understanding selective molecular recognition in aqueous solution. Here, cryptophane–Xe host–guest systems are considered in aqueous media as a model molecular system that also has important applications. ¹²⁹Xe–cryptophane systems can be used in the creation of biosensors and powerful contrast agents for magnetic resonance imaging applications. Detailed molecular information on the determinants of Xe affinity is difficult to obtain experimentally. Thus, molecular simulation and free energy perturbation methods were applied to estimate the affinities of Xe for six water-soluble cryptophanes. The calculated affinities correlated well with the previously measured experimental values. The simulations provided molecular insight on the differences in affinities and the roles of conformational fluctuations, solvent, and counter ions on Xe binding to these host molecules. Displacement of confined water from the host interior cavity is a key component of the binding equilibrium, and the average number of water molecules within the host cavity is correlated with the free energy of Xe binding to the different cryptophanes. The findings highlight roles for molecular simulation and design in modulating the relative strengths of host–guest and host–solvent interactions.

Dispersive interactions in solution complexes

Dispersive interactions are known to play a major role in molecular associations in the gas phase and in the solid state. In solution, however, their significance has been disputed in recent years on the basis of several arguments. A major problem until now has been the separation of dispersive and hydrophobic effects, which are both maximized in water due the low polarizability of this most important medium. Analyses of complexes between porphyrins and systematically varied substrates in water have allowed us to discriminate dispersive from hydrophobic effects, as the latter turned out to be negligible for complexations with flat surfaces such as porphyrins. Also, for the first time, it has become possible to obtain binding free energy increments ΔΔG for a multitude of organic residues including halogen, amide, amino, ether, carbonyl, ester, nitro, sulfur, unsatured, and cyclopropane groups, which turned out to be additive. Binding contributions for saturated residues are unmeasurably small, with ΔΔG > 1 kJ/mol, but they increase to, e.g., ΔΔG = 5 kJ/mol for a nitro group, a value not far from, e.g., that of a stacking pyridine ring. Stacking interactions of heteroarenes with porphyrins depend essentially on the size of the arenes, in line with polarizabilities, and seem to be rather independent of the position of nitrogen within the rings. Measurements of halogen derivatives indicate that complexes with porphyrins, cyclodextrins, and pillarenes as hosts in different media consistently show increasing stability from fluorine to iodine as the substituent. This, and the observed sequence with other substrates, is in line with the expected increase in dispersive forces with increasing polarizability. Induced dipoles, which also would increase with polarizability, can be ruled out as providing the driving source in view of the data with halides: the observed stability sequence is opposite the change of electronegativity from fluorine to iodine. The same holds for the solvent effect observed in ethanol-water mixtures. Dispersive contributions vary not only with the polarizability of the used media but also with the interacting receptor sites; it has been shown that for cucurbiturils the polarizability inside the cavity is extremely low, which also explains why hydrophobic effects are maximized with these hosts. Complexations with other known host compounds, however, such as those between cryptands or cavitands with, e.g., noble gases, bear the signature of dominating dispersive forces. Some recent examples illustrate that such van der Waals forces can also play an important role in complexations with proteins. Again, a clue for this is the increase in ΔG for inhibitor binding by 7 kJ/mol for, e.g., a bromine in comparison to a fluorine derivative.

A "Smart" ¹²⁸Xe NMR Biosensor for pH-Dependent Cell Labeling

Here we present a "smart" xenon-129 NMR biosensor that undergoes a peptide conformational change and labels cells in acidic environments. To a cryptophane host molecule with high Xe affinity, we conjugated a 30mer EALA-repeat peptide that is α-helical at pH 5.5 and disordered at pH 7.5. The (129)Xe NMR chemical shift at room temperature was strongly pH-dependent (Δδ = 3.4 ppm): δ = 64.2 ppm at pH 7.5 vs δ = 67.6 ppm at pH 5.5, where Trp(peptide)-cryptophane interactions were evidenced by Trp fluorescence quenching. Using hyper-CEST NMR, we probed peptidocryptophane detection limits at low-picomolar (10(-11) M) concentration, which compares favorably to other NMR pH reporters at 10(-2)-10(-3) M. Finally, in biosensor-HeLa cell solutions, peptide-cell membrane insertion at pH 5.5 generated a 13.4 ppm downfield cryptophane-(129)Xe NMR chemical shift relative to pH 7.5 studies. This highlights new uses for (129)Xe as an ultrasensitive probe of peptide structure and function, along with potential applications for pH-dependent cell labeling in cancer diagnosis and treatment.

p-Cyanophenylalanine and selenomethionine constitute a useful fluorophore-quencher pair for short distance measurements: application to polyproline peptides

The C≡N stretching frequency and fluorescence quantum yield of p-cyanophenylalanine are sensitive to environment. As such, this unnatural amino acid has found broad applications, ranging from studying how proteins fold to determining the local electric field of membranes. Herein, we demonstrate that the fluorescence of p-cyanophenylalanine can be quenched by selenomethionine through an electron transfer process occurring at short distances, thus further expanding its spectroscopic utility. Using this fluorophore-quencher pair, we are able to show that short polyproline peptides (1-4 prolines) are not rigid; instead, they sample a bimodal conformational distribution.

Cryptophane-folate biosensor for (129)xe NMR

Folate-conjugated cryptophane was developed for targeting cryptophane to membrane-bound folate receptors that are overexpressed in many human cancers. The cryptophane biosensor was synthesized in 20 nonlinear steps, which included functionalization with folate recognition moiety, solubilizing peptide, and Cy3 fluorophore. Hyperpolarized (129)Xe NMR studies confirmed xenon binding to the folate-conjugated cryptophane. Cellular internalization of biosensor was monitored by confocal laser scanning microscopy and quantified by flow cytometry. Competitive blocking studies confirmed cryptophane endocytosis through a folate receptor-mediated pathway. Flow cytometry revealed 10-fold higher cellular internalization in KB cancer cells overexpressing folate receptors compared to HT-1080 cells with normal folate receptor expression. The biosensor was determined to be nontoxic in HT-1080 and KB cells by MTT assay at low micromolar concentrations typically used for hyperpolarized (129)Xe NMR experiments.

Biomedical imaging with hyperpolarized noble gases

Hyperpolarized noble gases (HNGs), polarized to approximately 50% or higher, have led to major advances in magnetic resonance (MR) imaging of porous structures and air-filled cavities in human subjects, particularly the lung. By boosting the available signal to a level about 100 000 times higher than that at thermal equilibrium, air spaces that would otherwise appear as signal voids in an MR image can be revealed for structural and functional assessments. This review discusses how HNG MR imaging differs from conventional proton MR imaging, how MR pulse sequence design is affected and how the properties of gas imaging can be exploited to obtain hitherto inaccessible information in humans and animals. Current and possible future imaging techniques, and their application in the assessment of normal lung function as well as certain lung diseases, are described.

The hydrophobic effect revisited--studies with supramolecular complexes imply high-energy water as a noncovalent driving force

Traditional descriptions of the hydrophobic effect on the basis of entropic arguments or the calculation of solvent-occupied surfaces must be questioned in view of new results obtained with supramolecular complexes. In these studies, it was possible to separate hydrophobic from dispersive interactions, which are strongest in aqueous systems. Even very hydrophobic alkanes associate significantly only in cavities containing water molecules with an insufficient number of possible hydrogen bonds. The replacement of high-energy water in cavities by guest molecules is the essential enthalpic driving force for complexation, as borne out by data for complexes of cyclodextrins, cyclophanes, and cucurbiturils, for which complexation enthalpies of up to -100 kJ mol(-1) were reached for encapsulated alkyl residues. Water-box simulations were used to characterize the different contributions from high-energy water and enabled the calculation of the association free enthalpies for selected cucurbituril complexes to within a 10% deviation from experimental values. Cavities in artificial receptors are more apt to show the enthalpic effect of high-energy water than those in proteins or nucleic acids, because they bear fewer or no functional groups in the inner cavity to stabilize interior water molecules.

Enantiopure Cryptophane-129Xe Nuclear Magnetic Resonance Biosensors Targeting Carbonic Anhydrase

The (+) and (-) enantiomers for a cryptophane-7-bond-linker-benzenesulfonamide biosensor (C7B) were synthesized and their chirality confirmed by electronic circular dichroism (ECD) spectroscopy. Biosensor binding to carbonic anhydrase II (CAII) was characterized for both enantiomers by hyperpolarized (hp) ¹²⁹Xe NMR spectroscopy. Our previous study of the racemic (+/-) C7B biosensor-CAII complex [Chambers, et al., J. Am. Chem. Soc. 2009, 131, 563-569], identified two "bound" ¹²⁹Xe@C7B peaks by hp ¹²⁹Xe NMR (at 71 and 67 ppm, relative to "free" biosensor at 64 ppm), which led to the initial hypothesis that (+) and (-) enantiomers produce diastereomeric peaks when coordinated to Zn²⁺ at the chiral CAII active site. Unexpectedly, the single enantiomers complexed with CAII also identified two "bound" ¹²⁹Xe@C7B peaks: (+) 72, 68 ppm and (-) 68, 67 ppm. These results are consistent with X-ray crystallographic evidence for benzenesulfonamide inhibitors occupying a second site near the CAII surface. As illustrated by our studies of this model protein-ligand interaction, hp ¹²⁹Xe NMR spectroscopy can be useful for identifying supramolecular assemblies in solution.

Bacterial spore detection and analysis using hyperpolarized 129Xe chemical exchange saturation transfer (Hyper-CEST) NMR

Previously, we reported hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (Hyper-CEST) NMR techniques for the ultrasensitive (i.e., 1 picomolar) detection of xenon host molecules known as cryptophane. Here, we demonstrate a more general role for Hyper-CEST NMR as a spectroscopic method for probing nanoporous structures, without the requirement for cryptophane or engineered xenon-binding sites. Hyper-CEST ¹²⁹Xe NMR spectroscopy was employed to detect Bacillus anthracis and Bacillus subtilis spores in solution, and interrogate the layers that comprise their structures. ¹²⁹Xe-spore samples were selectively irradiated with radiofrequency pulses; the depolarized ¹²⁹Xe returned to aqueous solution and depleted the ¹²⁹Xe-water signal, providing measurable contrast. Removal of the outermost spore layers in B. anthracis and B. subtilis (the exosporium and coat, respectively) enhanced ¹²⁹Xe exchange with the spore interior. Notably, the spores were invisible to hyperpolarized ¹²⁹Xe NMR direct detection methods, highlighting the lack of high-affinity xenon-binding sites, and the potential for extending Hyper-CEST NMR structural analysis to other biological and synthetic nanoporous structures.

Perspectives of hyperpolarized noble gas MRI beyond 3He

Nuclear Magnetic Resonance (NMR) studies with hyperpolarized (hp) noble gases are at an exciting interface between physics, chemistry, materials science and biomedical sciences. This paper intends to provide a brief overview and outlook of magnetic resonance imaging (MRI) with hp noble gases other than hp (3)He. A particular focus are the many intriguing experiments with (129)Xe, some of which have already matured to useful MRI protocols, while others display high potential for future MRI applications. Quite naturally for MRI applications the major usage so far has been for biomedical research but perspectives for engineering and materials science studies are also provided. In addition, the prospects for surface sensitive contrast with hp (83)Kr MRI is discussed.

Biomembrane interactions of functionalized cryptophane-A: combined fluorescence and 129Xe NMR studies of a bimodal contrast agent

Fluorescent derivatives of the (129)Xe NMR contrast agent cryptophane-A were obtained by functionalization with near infrared fluorescent dyes DY680 and DY682. The resulting conjugates were spectrally characterized, and their interaction with giant and large unilamellar vesicles of varying phospholipid composition was analyzed by fluorescence and NMR spectroscopy. In the latter, a chemical exchange saturation transfer with hyperpolarized (129)Xe (Hyper-CEST) was used to obtain sufficient sensitivity. To determine the partitioning coefficients, we developed a method based on fluorescence resonance energy transfer from Nile Red to the membrane-bound conjugates. This indicated that not only the hydrophobicity of the conjugates, but also the phospholipid composition, largely determines the membrane incorporation. Thereby, partitioning into the liquid-crystalline phase of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was most efficient. Fluorescence depth quenching and flip-flop assays suggest a perpendicular orientation of the conjugates to the membrane surface with negligible transversal diffusion, and that the fluorescent dyes reside in the interfacial area. The results serve as a basis to differentiate biomembranes by analyzing the Hyper-CEST signatures that are related to membrane fluidity, and pave the way for dissecting different contributions to the Hyper-CEST signal.

Utilizing a water-soluble cryptophane with fast xenon exchange rates for picomolar sensitivity NMR measurements

Hyperpolarized (129)Xe chemical exchange saturation transfer ((129)Xe Hyper-CEST) NMR is a powerful technique for the ultrasensitive, indirect detection of Xe host molecules (e.g., cryptophane-A). Irradiation at the appropriate Xe-cryptophane resonant radio frequency results in relaxation of the bound hyperpolarized (129)Xe and rapid accumulation of depolarized (129)Xe in bulk solution. The cryptophane effectively "catalyzes" this process by providing a unique molecular environment for spin depolarization to occur, while allowing xenon exchange with the bulk solution during the hyperpolarized lifetime (T(1) ≈ 1 min). Following this scheme, a triacetic acid cryptophane-A derivative (TAAC) was indirectly detected at 1.4 picomolar concentration at 320 K in aqueous solution, which is the record for a single-unit xenon host. To investigate this sensitivity enhancement, the xenon binding kinetics of TAAC in water was studied by NMR exchange lifetime measurement. At 297 K, k(on) ≈ 1.5 × 10(6) M(-1) s(-1) and k(off) = 45 s(-1), which represent the fastest Xe association and dissociation rates measured for a high-affinity, water-soluble xenon host molecule near rt. NMR line width measurements provided similar exchange rates at rt, which we assign to solvent-Xe exchange in TAAC. At 320 K, k(off) was estimated to be 1.1 × 10(3) s(-1). In Hyper-CEST NMR experiments, the rate of (129)Xe depolarization achieved by 14 pM TAAC in the presence of radio frequency (RF) pulses was calculated to be 0.17 μM·s(-1). On a per cryptophane basis, this equates to 1.2 × 10(4)(129)Xe atoms s(-1) (or 4.6 × 10(4) Xe atoms s(-1), all Xe isotopes), which is more than an order of magnitude faster than k(off), the directly measurable Xe-TAAC exchange rate. This compels us to consider multiple Xe exchange processes for cryptophane-mediated bulk (129)Xe depolarization, which provide at least 10(7)-fold sensitivity enhancements over directly detected hyperpolarized (129)Xe NMR signals.