129Xe NMR Spectroscopy of Deuterium-Labeled Cryptophane-A Xenon Complexes: Investigation of Host−Guest Complexation Dynamics

NMR-Based Biosensing with Optimized Delivery of Polarized129Xe to Solutions

Laser-enhanced (LE) 129Xe nuclear magnetic resonance (NMR) is an exceptional tool for sensing extremely small physical and chemical changes; however, the difficult mechanics of bringing polarized xenon and samples of interest together have limited applications, particularly to biological molecules. Here we present a method for accomplishing solution 129Xe biosensing based on flow (bubbling) of LE 129Xe gas through a solution in situ in the NMR probe, with pauses for data acquisition. This overcomes fundamental limitations of conventional solution-state LE 129Xe NMR, e.g., the difficulty in transferring hydrophobic xenon into aqueous environments, and the need to handle the sample to refresh LE 129Xe after an observation pulse depletes polarization. With this new method, we gained a factor of >100 in sensitivity due to improved xenon transfer to the solution and the ability to signal average by renewing the polarized xenon. Polarized xenon in biosensors was detected at very low concentrations, </=250 nanomolar, while retaining all the usual information from NMR. This approach can be used to simultaneously detect multiple sensors with different chemical shifts and is also capable of detecting signals from opaque, heterogeneous samples, which is a unique advantage over optical methods. This general approach is adaptable for sensing minute quantities of xenon in heterogeneous in vitro samples, in miniaturized devices and should be applicable to certain in-vivo environments.

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.

Applications of laser-polarized 129xe to biomolecular assays

The chemical shift sensitivity and significant signal enhancement afforded by laser-polarized 129Xe have motivated the application of 129Xe NMR to biological imaging and spectroscopy. Recent research done by our group has used laser-polarized 129Xe in biomolecular assays that detect ligand-binding events and distinguish protein conformations. The successful application of unfunctionalized and functionalized 129Xe NMR to in vitro biomolecular assays suggests the potential future use of a functionalized xenon biosensor for in vivo imaging.

Kinetic and thermodynamic consequences of the substitution of SMe for OMe substituents of cryptophane hosts on the binding of neutral and cationic guests

To investigate the origin of the high selectivity of cryptophane-E (1) towards Me3NH+, Me4N+, and CHCl3, and particularly to discriminate the different contributions that stabilize the supramolecular complexes, we have synthesized the new cryptophane 2 bearing six MeS groups instead of MeO groups in 1. This led to a decrease of the negative charge density in the equatorial region of 2 without affecting notably the size of the molecular cavity. The binding properties of 1 and 2 towards the three guests were examined in solution and showed a slight decrease of the deltaGa favoring the complexes of 1, accompanied by a significant modification of the deltaHa vs. deltaSa balance. The binding of the ammonium guests to 1 and 2 was strongly entropy driven, while that of CHCl3 was purely enthalpy driven. A combination of spectroscopic and computational techniques was used to assign the main intermolecular interactions that occurred during the inclusion process. The neutral CHCl3 molecule is more stabilized in the less negatively charged CTV cap of 1. The different behavior towards the ammonium cations can be explained in term of interactions with the electronegative heteroatoms and cation-pi interactions. Moreover, this study revealed a considerable slowing down of the guest exchange kinetics with host 2, for which the association and dissociation rates are reduced by a factor 10(3) to 10(4) with respect to 1. For example, at room temperature, the Me4N+@2 complex exhibits a half-life of ca. 2 years, instead of a few hours for the corresponding complex of 1.

Supramolecular recognition: on the kinetic lability of thermodynamically stable host-guest association complexes

A molecular receptor consisting of a spacer bearing two cofacially disposed terpyridyl-palladium-ligand (terpy-Pd-L) units rigidly separated by about 7 A has been investigated for molecular recognition of planar aromatic molecules. It is found that although the receptor forms stable 1:2 host-guest association complexes with 9-methylanthracene (9-MA), the guest undergoes very rapid site exchange within the receptor and with external free 9-MA. A crystal structure of the 2:1 adduct shows one 9-MA in the molecular cleft defined by the two terpy-Pd-L units and the other resides on an outside face of one terpy-Pd-L unit. To establish the site residency time of the guests, a number of tethered molecules were prepared. These involve an anthracene molecule tethered to a pyridine ligand bound to the palladium atoms to form intramolecular host-guest adducts. Rotating-frame Overhauser effects were used to infer the site residency of the anthracene guests in the receptor. Variable-temperature (1)H NMR spectroscopy of the intramolecular host-guest complexes has revealed that the site residency time of the anthracene guests is 1.6 x 10(-5) sec at 20 degrees C and 1.3 sec at -90 degrees C in acetone solution. Whereas the guests are thermodynamically stable, they are kinetically very labile. A crystal structure of one of the tethered host-guest adducts reveals the expected structure which is the same as that determined in solution by (1)H rotating-frame Overhauser enhancement spectroscopy experiments.

A model for sequential threading of alpha-cyclodextrin onto a guest: a complete thermodynamic and kinetic study in water

The first variable-temperature and variable-pressure stopped-flow spectrophotometric study of the sequential threading of alpha-cyclodextrin (alpha-CD) onto the guest dye Mordant Orange 10, S, is reported. Complementary (1)H one-dimensional (1D) variable-temperature kinetic studies and two-dimensional (2D) rotating-frame nuclear Overhauser effect spectroscopy (ROESY) and EXSY NMR studies are also reported. In aqueous solution at 298.2 K, the first alpha-CD threads onto S to form a 1:1 complex S.alpha-CD with a forward rate constant k(1,f) = 15 200 +/- 200 M(-1) s(-1) and dethreads with a reverse rate constant k(1,r) = 4.4 +/- 0.3 s(-1). Subsequently, S.alpha-CD isomerizes to S.alpha-CD (k(3,f) = 0.158 +/- 0.006 s(-1), k(3,f) = 0.148 +/- 0.006 s(-1)). This process can be viewed as a thermodynamically controlled molecular shuttle. A second alpha-CD threads onto S.alpha-CD to form a 1:2 complex, S.(alpha-CD)(2), with k(2,f) = 98 +/- 2 M(-1) s(-1) and k(2,r) = 0.032 +/- 0.002 s(-1). A second alpha-CD also threads onto S.alpha-CD to form another 1:2 complex, S.(alpha-CD)(2), characterized by k(4,f) = 9640 +/- 1800 M(-1) s(-1) and k(4,r) = 61 +/- 6 s(-1). Direct interconvertion between S.(alpha-CD)(2) and S.(alpha-CD)(2) was not detected; instead, they interconvert by dethreading the second alpha-CD and through the isomerization equilibrium between S.alpha-CD and S.alpha-CD. The reaction volumes, DeltaV(0), were found to be negative for the first three equilibria and positive for the fourth equilibrium. For the first three forward and reverse reactions, the volumes of activation are substantially more negative, indicating a compression of the transition state in comparison with the ground states. These data were used in conjunction with DeltaH, DeltaH degrees, DeltaS, and DeltaS degrees data to deduce the dominant mechanistic threading processes, which appear to be largely controlled by changes in hydration and van der Waals interactions, and possibly by conformational changes in both S and alpha-CD. The structure of the four complexes were deduced from (1)H 2D ROESY NMR studies.

Formation of cryptophanes from their precursors as viewed by liquid secondary ion mass spectrometry

The formation of cryptophane-A (C1) and the deuterated cryptophanes C2-C6 from their respective precursors P1-P6 in a mass spectrometer ion-source was evidenced by liquid secondary ion mass spectrometry (LSIMS). Mass-analyzed ion kinetic energy experiments performed on the precursor molecular ions suggested that cryptophane formation occurred mainly in the liquid-matrix before desorption rather than in the gas phase. In addition, we observed that the presence of cations, such as lithium or sodium ions, inhibited the formation of the cryptophane molecular ions. In the light of these results we used the LSIMS technique to investigate the formation of the new cryptophanes C7-C13. All the data collected support the idea that a direct comparison can be made between these experimental findings and chemistry in solution.

Functionalized xenon as a biosensor

The detection of biological molecules and their interactions is a significant component of modern biomedical research. In current biosensor technologies, simultaneous detection is limited to a small number of analytes by the spectral overlap of their signals. We have developed an NMR-based xenon biosensor that capitalizes on the enhanced signal-to-noise, spectral simplicity, and chemical-shift sensitivity of laser-polarized xenon to detect specific biomolecules at the level of tens of nanomoles. We present results using xenon "functionalized" by a biotin-modified supramolecular cage to detect biotin-avidin binding. This biosensor methodology can be extended to a multiplexing assay for multiple analytes.

Dynamics of an inclusion complex of chloroform and cryptophane-E: evidence for a strongly anisotropic van der Waals bond

The motional dynamics of a van der Waals inclusion complex of cryptophane-E and chloroform has been investigated by a combined NMR exchange and relaxation study. The kinetics of exchange of chloroform between the bulk solution and the complex was investigated by means of proton EXSY measurements. The carbon-13 relaxation of the cryptophane-E host and of the bound chloroform guest was analyzed using the Lipari-Szabo "model-free" approach. For interpretation of the carbon-13 relaxation measurements for chloroform, the chemical-exchange process of complex formation and dissociation had to be taken into account in terms of the modified Bloch equations. It was found that the complex behaves as a single molecule without any significant guest chloroform motion inside the host's cavity.

Creation of novel chiral cryptophanes by a self-assembling method utilizing a pyridyl-Pd(II) interaction

[structure: see text]. This Letter demonstrates the molecular design of novel self-assembled chiral cryptophanes. Mediated by square-planar Pd(II) complexes, racemic pyridyl cyclotriveratrylene derivative rac-2 self-assembles into mixtures of racemic chiral cryptophanes and meso cryptophanes (1), which interconvert with each other, and the rates are remarkably enhanced by the addition of a slight excess of rac-2. On the other hand, optically resolved P-2 or M-2 self-assembles into the chiral cryptophane as the only product.

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

We present the synthesis of a series of deuterated cryptophanes 2-6 by a slightly modified procedure used for cryptophane-A. We show that for [Xe@cryptophane] complexes the use of variable-temperature one-dimensional 129Xe magnetization transfer (1D-EX-SY) allows the measurement of exchange rates. From these data the decomplexation activation energy Ea has been estimated to be 37.5+/-2 kJ mol(-1). The decomplexation activation enthalpy, deltaH(++) = 35.5+/-2 kJ mol(-1), and entropy, deltaS(++) = -60+/-5 J mol(-1) K(-1), have also been calculated. The calculated negative activation entropy suggests that the activated complex associated with decomplexation is conformationally more strained than the complex in its ground state.