Synthesis of deuterium-labeled cryptophane-A and investigation of Xe@cryptophane complexation dynamics by 1D-EXSY-NMR experiments

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.

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.

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.

Host-Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

Dynamics in complexes of porphyrin cage compounds and viologen-derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host-guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less-bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Paramagnetic Organocobalt Capsule Revealing Xenon Host-Guest Chemistry

We investigated Xe binding in a previously reported paramagnetic metal-organic tetrahedral capsule, [Co4L6]4-, where L2- = 4,4'-bis[(2-pyridinylmethylene)amino][1,1'-biphenyl]-2,2'-disulfonate. The Xe-inclusion complex, [XeCo4L6]4-, was confirmed by 1H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCo4L6]4-, koff = 4.45(5) × 102 s-1, was at least 40 times greater than that in the analogous [XeFe4L6]4- complex, highlighting the capability of metal-ligand interactions to tune the capsule size and guest permeability. The rapid exchange of 129Xe nuclei in [XeCo4L6]4- produced significant hyperpolarized 129Xe chemical exchange saturation transfer (hyper-CEST) NMR signal at 298 K, detected at a concentration of [XeCo4L6]4- as low as 100 pM, with presaturation at -89 ppm, which was referenced to solvated 129Xe in H2O. The saturation offset was highly temperature-dependent with a slope of -0.41(3) ppm/K, which is attributed to hyperfine interactions between the encapsulated 129Xe nucleus and electron spins on the four CoII centers. As such, [XeCo4L6]4- represents the first example of a paramagnetic hyper-CEST (paraHYPERCEST) sensor. Remarkably, the hyper-CEST 129Xe NMR resonance for [XeCo4L6]4- (δ = -89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFe4L6]4- (δ = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C(NH2)3)4[Xe0.7Co4L6]·75 H2O (1). Hydrogen bonding between capsule-linker sulfonate groups and exogenous guanidinium cations, (C(NH2)3)+, stabilized capsule-capsule interactions in the solid state and also assisted in trapping a Xe atom (∼42 Å3) in the large (135 Å3) cavity of 1. Magnetic susceptibility measurements confirmed the presence of four noninteracting, magnetically anisotropic high-spin CoII centers in 1. Furthermore, [Co4L6]4- was found to be stable toward aggregation and oxidation, and the CEST performance of [XeCo4L6]4- was unaffected by biological macromolecules in H2O. These results recommend metal-organic capsules for fundamental investigations of Xe host-guest chemistry as well as applications with highly sensitive 129Xe-based sensors.

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.

Remote detection of hyperpolarized 129Xe resonances via multiple distant dipolar field interactions with 1H

A remote detection scheme utilizing the distant dipolar field interaction between two different spin species was proposed by Granwehr et al. [J. Magn. Reson. 176(2), 125 (2005)]. In that sequence 1H spins were detected indirectly via their dipolar field interaction with 129Xe spins, which served as the sensing spins. Here we propose a modification of the proposed detection scheme that takes advantage of the longer T1 relaxation time of xenon to create a long lasting dipolar field with which the fast relaxing 1H spins are allowed to interact many times during a single acquisition. This new acquisition scheme improves detection sensitivity, but it also presents some challenges.

Improved hemicryptophane hosts for the stereoselective recognition of glucopyranosides

Four new enantiomerically and diastereomerically pure hemicryptophane hosts (M-SSS-2/P-SSS-2 and M-RRR-2/P-RRR-2 pairs) were designed for the recognition of sugar derivatives. Their absolute configuration was determined from the circular dichroism spectra and DFT calculations. The host molecules were then used for the stereoselective recognition of glucopyranosides. Binding constants were obtained from (1)H NMR titration experiments showing an increase of affinity for this class of receptors, associated with an improved diastereo- and enantio-differentiation.

Investigation of chloromethane complexes of cryptophane-A analogue with butoxy groups using ¹³C NMR in the solid state and solution along with single crystal X-ray diffraction

Host-guest complexes between cryptophane-A analogue with butoxy groups (cryptophane-But) and chloromethanes (chloroform, dichloromethane) were investigated in the solid state by means of magic-angle spinning (13)C NMR spectroscopy. The separated local fields method with (13)C-(1)H dipolar recoupling was used to determine the residual dipolar coupling for the guest molecules encaged in the host cavity. In the case of chloroform guest, the residual dipolar interaction was estimated to be about 19 kHz, consistent with a strongly restricted mobility of the guest in the cavity, while no residual interaction was observed for encaged dichloromethane. In order to rationalize this unexpected result, we performed single crystal X-ray diffraction studies, which confirmed that both guest molecules indeed were present inside the cryptophane cavity, with a certain level of disorder. To improve the insight in the dynamics, we performed a (13)C NMR spin-lattice relaxation study for the dichloromethane guest in solution. The system was characterized by chemical exchange, which was slow on the chemical shift time scale but fast with respect to the relaxation rates. Despite these disadvantageous conditions, we demonstrated that the data could be analyzed and that the results were consistent with an isotropic reorientation of dichloromethane within the cryptophane cavity.

Degeneracy in cryptophane–xenon complex formation in aqueous solution

Degenerate exchange prevails in the cryptophane-A–xenon host–guest system in aqueous solution.

NMR investigation of chloromethane complexes of cryptophane-A and its analogue with butoxy groups

Host-guest complexes between cryptophane-A as host and dichloromethane and chloroform as guests are investigated using (1)H and (13)C NMR spectroscopy. Moreover, a related cryptophane, with the methoxy groups replaced by butoxy units (cryptophane-But), and its complexes with the same guests were also studied. Variable temperature spectra showed effects of chemical exchange between the free and bound guests, as well as of conformational exchange of the host. The guest exchange was studied quantitatively by exchange spectroscopy or line shape analysis. Extraction of kinetic and thermodynamic parameters led to the characterization of the affinity between guests and hosts. On the other hand, the host exchange was investigated by means of (13)C Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion which aims at the determination of the transverse relaxation rate R2, the inverse of the transverse relaxation time T2, as a function of the repetition of the π pulses in a CPMG train. The variation of the measured transverse relaxation rate with the repetition rate νCPMG indicated conformational exchange occurring on the microsecond-millisecond time scale. Structural information was obtained through measurements of cross-relaxation rates, both within the host and between the host and the guest protons. The NMR results were supported by DFT calculations.

Explorations of caffeic acid derivatives: total syntheses of rufescenolide, yunnaneic acids C and D, and studies toward yunnaneic acids A and B

Yunnaneic acids A-D, isolated from the roots of Salvia yunnanensis , are hexameric (A and B) and trimeric (C and D) assemblies of caffeic acid that feature an array of synthetically challenging and structurally interesting domains. In addition to being caffeic acid oligomers, yunnaneic acids A and B are formally dimeric and heterodimeric adducts of yunnaneic acids C and D. Herein we report the first total syntheses of yunnaneic acids C and D featuring the formation of their bicyclo[2.2.2]octene cores in a single step from simple precursors via an oxidative dearomatization/Diels-Alder cascade that may have biogenetic relevance. In addition, exploitation of the key intermediate resulting from this cascade reaction has enabled rapid access to the structurally related caffeic acid metabolite rufescenolide through an unexpected Lewis acid-mediated reduction. Finally, we report the results of extensive model studies toward forming the dimeric yunnaneic acids A and B. These explorations indicate that the innate reactivities of the monomeric fragments do not favor spontaneous formation of the desired dimeric linkages. Consequently, enzymatic involvement may be required for the biosynthesis of these more complex family members.

Quantitative measurement of cancer metabolism using stimulated echo hyperpolarized carbon-13 MRS

PURPOSE

Magnetic resonance spectroscopy of hyperpolarized substrates allows for the observation of label exchange catalyzed by enzymes providing a powerful tool to investigate tissue metabolism and potentially kinetics in vivo. However, the accuracy of current methods to calculate kinetic parameters has been limited by T1 relaxation effects, extracellular signal contributions, and reduced precision at lower signal-to-noise ratio.

THEORY AND METHODS

To address these challenges, we investigated a new modeling technique using metabolic activity decomposition-stimulated echo acquisition mode. The metabolic activity decomposition-stimulated echo acquisition mode technique separates exchanging from nonexchanging metabolites providing twice the information as conventional techniques.

RESULTS

This allowed for accurate measurements of rates of conversion and of multiple T1 values simultaneously using a single acquisition.

CONCLUSION

The additional measurement of T1 values for the reaction metabolites provides further biological information about the cellular environment of the metabolites. The new technique was investigated through simulations and in vivo studies of transgenic mouse models of cancer demonstrating improved assessments of kinetic rate constants and new T1 relaxation value measurements for hyperpolarized (13) C-pyruvate, (13) C-lactate, and (13) C-alanine.

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.

Shorter synthesis of trifunctionalized cryptophane-A derivatives

Efficient syntheses of trisubstituted cryptophane-A derivatives that are versatile host molecules for many applications are reported. Trihydroxy cryptophane was synthesized in six or seven steps with yields as high as 9.5%. By a different route, trihydroxy cryptophane modified with three propargyl, allyl, or benzyl protecting groups was synthesized with yields of 4.1-5.8% in just six steps. Hyperpolarized (129)Xe NMR chemical shifts of 57-65 ppm were measured for these trisubstituted cryptophanes.

Encapsulation of small gas molecules by cryptophane-111 in organic solution. 1. Size- and shape-selective complexation of simple hydrocarbons

The reversible trapping of small hydrocarbons and other gases by cryptophane-111 (1) in organic solution was characterized with variable-temperature (1)H NMR spectroscopy. Characteristic spectral changes observed upon guest binding allowed kinetic and thermodynamic data to be readily extracted, permitting quantification and comparison of different host-guest interactions. Previous work (J. Am. Chem. Soc. 2007, 129, 10332) demonstrated that 1, the smallest cryptophane to date, forms a complex with xenon with remarkably high affinity. Presently, it is shown that 1 also exhibits slow exchange dynamics with methane at reduced temperatures (delta(bound) = -5.2 ppm) with an association constant K(a) = 148 M(-1) at 298 K. In contrast, ethane and ethylene are poorly recognized by 1 with K(a) values of only 2 M(-1) and 22 M(-1), respectively; moreover, chloromethane (whose molecular volume is similar to that of xenon, approximately 42 A(3)) is not observed to bind to 1. Separately, molecular hydrogen (H(2)) gas is observed to bind 1, but in contrast to other ligands presently studied, H(2) complexation is spectrally manifested by fast exchange throughout virtually the entire range of available conditions, as well as by a complex dependence of the guest (1)H resonance frequency upon temperature and host concentration. Taken together, these results establish 1 as a selective host for small gases, with implications for the design of size- and geometry-selective sensors targeted for various gas molecules.

Functionalized 129Xe contrast agents for magnetic resonance imaging

The concept of 'xenon biosensor' for magnetic resonance imaging (MRI) was first proposed by a Berkeley team in 2001, with evidence that hyperpolarized 129Xe bound to a biotin-labeled cryptophane can detect streptavidin at much lower concentrations (nM-microM) than is typical for contrast-enhanced MRI experiments. 129Xe biosensors have undergone many recent developments to address challenges in molecular imaging. For example, cryptophanes that exhibit 10-fold higher xenon affinity with distinct 129Xe magnetic resonance spectra have been synthesized. Also relevant are dendrimeric cryptophane assemblies and inorganic zeolites that localize many 129Xe atoms to rare targets. Finally, this article considers biosensors that produce measurable changes in 129Xe chemical shift based upon the activity of oligonucleotides, proteins, or enzymes, and includes the first cell studies.

Inclusion complexes of cryptophane-E with dichloromethane and chloroform: A thermodynamic and kinetic study using the 1D-EXSY NMR method

Complexation equilibria and kinetics of exchange of chloroform and dichloromethane molecules between the cavity of cryptophane-E and bulk solution were investigated using NMR methods. Using one-dimensional magnetization transfer (1D-EXSY type sequence), chemical exchange rates were measured in different temperature ranges, limited by the equilibrium constant values of the complexes and the boiling points of the guest substances. From the kinetic data, activation energies were calculated using the Arrhenius equation. From the temperature-dependence of the association constant data, the enthalpy and entropy of complexation were estimated and compared with values for similar complexes of other cryptophanes.

Conformational changes in cryptophane having C1-symmetry studied by vibrational circular dichroism

Vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations were used to obtain the absolute configuration of a cryptophane molecule having C1-symmetry (labeled cryptophane-H). This molecule exhibits chiroptical properties different from those published for cryptophane-A having D3-symmetry [Brotin et al. J. Am. Chem. Soc. 2006, 128, 5533-5540]. In particular, we have shown that the conformation of the aliphatic linkers is very dependent on the solvent used and its ability to enter (CDCl3 solution) or not (C2D2Cl4 solution) in the cryptophane cavity. Calculations performed at the DFT (B3PW91/6-31G*) level establish, besides the absolute configuration, the preferential anti and gauche conformations of the aliphatic linkers of the chloroform@cryptophane-H complex and the empty cryptophane-H molecule, respectively. Polarimetric measurements performed in several solvents reflect also the change of conformation of the bridges upon guest encapsulation.

Vibrational circular dichroism study of optically pure cryptophane-A

Vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations were used to obtain the absolute configuration of optically pure cryptophane-A molecule. This large molecule (120 atoms) that possess a globular shape, but no chiral centers, exceeds the molecular size of published structures for which VCD has been used to determine the absolute configuration. VCD spectra recorded in CDCl(3) solution for the two resolved enantiomers are near mirror images, and very good agreement between the observed IR and VCD spectra and intensity calculations performed at the DFT (B3PW91/6-31G) level establish, besides the absolute configuration, the preferential anti conformation of the aliphatic linkers of the chloroform-cryptophane-A complex. Experiments performed in CD(2)Cl(2) and C(2)D(2)Cl(4) solutions show no significant modifications in the IR and VCD spectra, indicating that the conformation of the aliphatic linkers is similar for empty (C(2)D(2)Cl(4) solution) and encaged (CDCl(3) and CD(2)Cl(2) solutions) cryptophane-A molecules.

Designing 129Xe NMR biosensors for matrix metalloproteinase detection

Xenon-129 biosensors offer an attractive alternative to conventional MRI contrast agents due to the chemical shift sensitivity and large nuclear magnetic signal of hyperpolarized (129)Xe. Here, we report the first enzyme-responsive (129)Xe NMR biosensor. This compound was synthesized in 13 steps by attaching the consensus peptide substrate for matrix metalloproteinase-7 (MMP-7), an enzyme that is upregulated in many cancers, to the xenon-binding organic cage, cryptophane-A. The final coupling step was achieved on solid support in 80-92% yield via a copper (I)-catalyzed [3+2] cycloaddition. In vitro enzymatic cleavage assays were monitored by HPLC and fluorescence spectroscopy. The biosensor was determined to be an excellent substrate for MMP-7 (K(M) = 43 microM, V(max) = 1.3 x 10(-)(8) M s(-1), k(cat)/K(M) = 7,200 M(-1) s(-1)). Enzymatic cleavage of the tryptophan-containing peptide led to a dramatic decrease in Trp fluorescence, lambda(max) = 358 nm. Stern-Volmer analysis gave an association constant of 9000 +/- 1,000 M(-1) at 298 K between the cage and Trp-containing hexapeptide under enzymatic assay conditions. Most promisingly, (129)Xe NMR spectroscopy distinguished between the intact and cleaved biosensors with a 0.5 ppm difference in chemical shift. This difference most likely reflected a change in the electrostatic environment of (129)Xe, caused by the cleavage of three positively charged residues from the C-terminus. This work provides guidelines for the design and application of new enzyme-responsive (129)Xe NMR biosensors.

Ferulic acid and benzothiazole dimer derivatives with high binding affinity to β-amyloid fibrils

New ferulic acid and benzothiazole dimer derivatives were synthesized and evaluated by in vitro competition assay using [(125)I]TZDM for their specific binding affinities to Abeta fibrils. In particular, 4a showed the most excellent binding affinity (K(i)=0.53 nM), compared to PIB (K(i)=0.77 nM), for benzothiazole binding sites of Abeta(1-42) fibrils. This result suggests a possibility of a potential AD diagnostic probe for detection of Abeta fibrils.

Diastereomeric Xe chemical shifts in tethered cryptophane cages

Cryptophane cages serve as host molecules to a Xe atom. Functionalization of cryptophane-A has permitted the development of Xe as a biosensor. Synthetic routes used to prepare cryptophanes result in racemic mixtures of the chiral cages. In the preparation of a tethered cryptophane-A cage for biosensor applications, some achiral and chiral substituents such as left-handed amino acids have been used. When the substituent is achiral, the NMR signal of the Xe atom in the functionalized cage in solution is a single isotropic peak, since the Xe shielding tensor components in the R and L cages differ by no more than the signs of the off-diagonal elements. Chiral substituents can split the cage-encapsulated Xe NMR signal into one or more sets of doublets, depending on the number of asymmetric centers in the substituent. We carry out quantum mechanical calculations of Xe nuclear magnetic shielding for the Xe atom at the same strategic position within an L cryptophane-A cage, under the influence of chiral potentials that represent r or l substituents outside the cage. Calculations of the Xe shielding response in the Lr and Ll diastereomeric pairs permit the prediction of the relative order of the Xe chemical shifts in solutions containing the Rl and Ll diastereomers. Where the substituent itself possesses two chiral centers, comparison of the calculated isotropic shielding responses in the Llr, Lrl, Rll, and Lrr systems, respectively, permits the prediction of the Xe spectrum of diastereomeric systems in solutions containing Llr, Rlr, Lll, and Rll systems. Assignment of the peaks observed in the experimental Xe NMR spectra is therefore possible, without having to undertake the difficult synthetic route that produces a single optically pure enantiomer.

Water Soluble Cryptophanes Showing Unprecedented Affinity for Xenon: Candidates as NMR-Based Biosensors

Cryptophanes bearing OCH(2)COOH groups in place of the methoxy groups represent a new class of xenon-carrier molecules soluble in water at biological pH. By using (1)H and (129)Xe NMR (thermally- and laser-polarized dissolved gas), the structural and dynamical behaviors of these host molecules as well as their interaction with xenon are studied. They are shown to exist in aqueous solution under different conformations in very slow exchange. A saddle form present for one of these conformations could explain the (1)H NMR spectra. Whereas the cryptophanes in such a conformation are unable to complex xenon, unprecedented high binding constants are found for cryptophanes in the other canonical crown-crown conformation. These host molecules could therefore be valuable candidates for biosensing using (129)Xe MRI.

Tris(pyridylmethylamino)cyclotriguaiacylene Cavitands: An Investigation of the Solution and Solid-State Behaviour of Metallo-Supramolecular Cages and Cavitand-Based Coordination Polymers

The synthesis of the three isomeric tris(pyridylmethylamino)cyclotriguaiacylene cavitands is reported, along with the crystal structures of the 2- and 4-pyridyl derivatives. The generality of a previously described [Ag(2){tris(3-pyridylmethylamino)cyclotriguaiacylene}(2)](2+) dimeric capsule motif and the [Ag(4){tris(4-pyridylmethylamino)cyclotriguaiacylene}(4)](4+) tetrahedron with several silver salts was confirmed in the solid state and the corresponding solution species were investigated by NMR spectroscopy. Host-guest interactions in these systems have been probed and these interactions are demonstrated to alter and influence the self-assembly outcome of the reaction. Notably, introduction of larger glutaronitrile guest molecules to the [Ag(4)L(4)](4+) tetrahedron system prevents formation of the tetrahedral structure, resulting instead in the formation of a 4.8(2) coordination network in the solid state. In the absence of templating acetonitrile guests in the [Ag(2)(3)(2)](2+) capsule system, formation of a cage-based one-dimensional coordination polymer is observed.

NMR studies of chloroform@cryptophane-A and chloroform@bis-cryptophane inclusion complexes oriented in thermotropic liquid crystals

Oriented inclusion complexes of chloroform@cryptophane-A and chloroform@bis-cryptophane were prepared using a nematic thermotropic liquid crystal (ZLI 1132), and the alignment and magnetic resonance properties of these host-guest systems were studied via (13)C NMR of the labeled guests. Large (1)H-(13)C dipolar splittings for (13)CHCl(3) guests indicated significantly enhanced (approximately 2-fold) ordering for the trapped vs. free ligands under all conditions studied, with similar ordering observed for the two complexes-despite significant differences in size and motional freedom between the hosts. For each environment, variable-temperature studies permitted the sign and magnitude of the order parameter for chloroform's C-H bond to be independently determined from the (13)C chemical shift anisotropy (CSA) shifts (via the gradient method) and the restored (1)H-(13)C dipolar couplings. In both systems, the results are consistent with overall alignment of the complexes such that the cage principal axis lies roughly perpendicular to the LC director.

A Six-Bowl Carceplex That Entraps Seven Guest Molecules

A six-bowl carceplex that entraps seven guest molecules, 5.(DMSO)7, was synthesized and characterized. The dynamics of the host shell was studied in solution in the absence and presence of water. A multiple-molecule template was found to drive the formation of 5.(DMSO)x.G(7-x) (G = DMA, DMF; x = 5-7). Higher selectivity was found for species containing greater numbers of DMSO molecules.

Improved Synthesis of Functional CTVs and Cryptophanes Using Sc(OTf)3 as Catalyst

Functional cyclotriveratrylene (CTV) and cryptophane derivatives are synthesized in the presence of scandium triflate [Sc(OTf)3]. This route allows the preparation of new derivatives that could not be prepared or easily obtained by using the previously reported experimental procedures. With a catalytic amount of scandium triflate (1% mol), CTVs were obtained with yields similar to or higher than those reported previously in reactions run under strong acidic conditions. Cryptophanes were also synthesized in fairly good yields by performing the ring-closure step in the presence of a stoichiometric amount of Sc(OTf)3. Interestingly, this novel approach strongly reduces the formation of side products and gives rise to novel functionalized molecules for the construction of supramolecular host-guest systems.

Chiral Trialkanolamine-Based Hemicryptophanes: Synthesis and Oxovanadium Complex

[reaction: see text] A novel class of chiral hemicryptophane hosts has been synthesized in diastereoisomerically pure form, namely, M-(R,R,R)-1a/P-(S,S,S)-1a and M-(S,S,S)-1b/P-(R,R,R)-1b. The C3-symmetrical precursor 9 was prepared, using either (R)- or (S)-glycidyl nosylate, repectively, as the chiral pool reactant and subsequently cyclized (trimerized) in the presence of Sc(OTf)3. The four stereoisomers were fully characterized and displayed two pairs of mirror-image CD spectra, which were used to determine their absolute configuration. The formation of the oxovanadium(V) complex of hemicryptophane 1a is also reported.

Development of a Functionalized Xenon Biosensor

NMR-based biosensors that utilize laser-polarized xenon offer potential advantages beyond current sensing technologies. These advantages include the capacity to simultaneously detect multiple analytes, the applicability to in vivo spectroscopy and imaging, and the possibility of "remote" amplified detection. Here, we present a detailed NMR characterization of the binding of a biotin-derivatized caged-xenon sensor to avidin. Binding of "functionalized" xenon to avidin leads to a change in the chemical shift of the encapsulated xenon in addition to a broadening of the resonance, both of which serve as NMR markers of ligand-target interaction. A control experiment in which the biotin-binding site of avidin was blocked with native biotin showed no such spectral changes, confirming that only specific binding, rather than nonspecific contact, between avidin and functionalized xenon leads to the effects on the xenon NMR spectrum. The exchange rate of xenon (between solution and cage) and the xenon spin-lattice relaxation rate were not changed significantly upon binding. We describe two methods for enhancing the signal from functionalized xenon by exploiting the laser-polarized xenon magnetization reservoir. We also show that the xenon chemical shifts are distinct for xenon encapsulated in different diastereomeric cage molecules. This demonstrates the potential for tuning the encapsulated xenon chemical shift, which is a key requirement for being able to multiplex the biosensor.

Interwoven 2-D Coordination Network Prepared from the Molecular Host Tris(isonicotinoyl)cyclotriguaiacylene and Silver(I) Cobalt(III) Bis(dicarbollide)

A new 3-fold symmetric molecular host, tris(isonicotinoyl)cyclotriguaiacylene, has been synthesized from cyclotriguaiacylene and isonicotinoyl chloride hydrochloride in 70% yield. Its crystal structure shows dimerization of the host molecules into a capsule-like arrangement. When reacted with Ag[Co(C(2)B(9)H(11))(2)], the host forms a 1-D [ML(2)](n) coordination polymer, which assembles into a 2-D interwoven network through a similar dimerization of the host moieties around acetonitrile guests. Thus, the network structure represents an elegant example whereby the solid state structure of the building blocks is echoed in the resulting supramolecular assembly. The 2-D sheets are further stabilized by pi...pi stacking interactions between pyridyl donors of alternate 1-D coordination polymers.