Single Fluorinated Agent for Multiplexed 19 F-MRI with Micromolar Detectability Based on Dynamic Exchange.

The weak thermal polarization of nuclear spins limits the sensitivity of MRI, even for MR-sensitive nuclei as fluorine-19. Therefore, despite being the source of inspiration for the development of background-free MRI for various applications, including for multiplexed imaging, the inability to map very low concentrations of targets using 19 F-MRI raises the need to further enhance this platform's capabilities. Here, we employ the principles of CEST-MRI in 19 F-MRI to obtain a 900-fold signal amplification of a biocompatible fluorinated agent, which can be presented in a "multicolor" fashion. Capitalizing on the dynamic interactions in host-guest supramolecular assemblies in an approach termed GEST, we demonstrate that an inhalable fluorinated anesthetic can be used as a single 19 F-probe for the concurrent detection of micromolar levels of two targets, with potential in vivo translatability. Further extending GEST with new designs could expand the applicability of 19 F-MRI to the mapping of targets that have so-far remained non-detectable.

Elucidating dissociation activation energies in host-guest assemblies featuring fast exchange dynamics.

The ability to mediate the kinetic properties and dissociation activation energies (E a) of bound guests by controlling the characteristics of "supramolecular lids" in host-guest molecular systems is essential for both their design and performance. While the synthesis of such systems is well advanced, the experimental quantification of their kinetic parameters, particularly in systems experiencing fast association and dissociation dynamics, has been very difficult or impossible with the established methods at hand. Here, we demonstrate the utility of the NMR-based guest exchange saturation transfer (GEST) approach for quantifying the dissociation exchange rates (k out) and activation energy (E a,out) in host-guest systems featuring fast dissociation dynamics. Our assessment of the effect of different monovalent cations on the extracted E a,out in cucurbit[7]uril:guest systems with very fast k out highlights their role as "supramolecular lids" in mediating a guest's dissociation E a. We envision that GEST could be further extended to study kinetic parameters in other supramolecular systems characterized by fast kinetic properties and to design novel switchable host-guest assemblies.

Dynamic Interactions in Synthetic Receptors: A Guest Exchange Saturation Transfer Study.

The accumulated knowledge regarding molecular architectures is based on established, reliable, and accessible analytical tools that provide robust structural and functional information on assemblies. However, both the dynamicity and low population of noncovalently interacting moieties within studied molecular systems limit the efficiency and accuracy of traditional methods. Herein, the use of a saturation transfer-based NMR approach to study the dynamic binding characteristics of an anion to a series of synthetic receptors derived from bambusuril macrocycles is demonstrated. The exchange rates of BF4 - are mediated by the side chains on the receptor (100 s-1