ICE Issue 9

19 Scientific Article The Israel Chemist and Chemical Engineer Issue 9 · January 2023 · Tevet 5783 the exchangeable proton in 1H-CEST (compare Figures 1a and 1c). The two different chemical shifts obtained for a bound 19F-agent is a result of the diversity of macrocycles that can be used and the saturation transfer capability is a direct benefit of the reversible dynamic interactions of supramolecular hostguest assemblies that allow the amplification of the 19F-MR signals of extremely low concentrations of complexes. More specifically, the 19F-agent (or 19F-guest) fluroxene is capable of generating significant 19F-GEST contrast in the presence of two different molecular hosts with two opposite Dw values, either downfield, when incorporated into cucurbit[7]uril (CB7, obtaining a fluroxene@CB7 complex), or upfield, when incorporated into octa-acid (OA, obtaining a fluroxene@OA complex) relative to the resonance of unbound fluroxene (Figure 4b). While the GEST effect of fluroxene@ OA was at Dw = -1.6 ppm and represented as a green color, the GEST effect of fluroxene@CB7 was obtained at Dw = 2.2 ppm and is represented as magenta in the GEST map (Figure 4c-g). Importantly, the existence of either the host or the guest or their complexes does not affect the 19F-MRI appearance and the 19F-GEST contrast is obtained “on demand” upon the application of the saturation pulse at the Dw of the host-guest complex and can be presented as a dualcolored MRI map (Figure 4e). Note that the 19F-GEST effect can be observed from the intensity of the free 19F-guest at the 19F-NMR spectrumwhen acquired from a defined voxel using Guest exchange saturation transfer (19F-GEST) The mi l l imolar sensitivity of 19F-probes restricts the implementation of 19F-MRI to study and map biologically relevant low-concentration targets and cal ls for novel developments. The maturity of the CEST contrast for MRI [21] opened opportunities to indirectly detect low concentrations of solutes, exploiting both dynamic proton exchange processes and magnetization transfer capabilities. The establishment of the CEST approach created opportunities to implement the CEST principles into 19F-MRI [11, 22] as first demonstrated with fluorinated chelates for sensing andmapping of metal ions [23]. This demonstration of 19F-CEST applicability provided a diverse platform for the development of novel strategies for multicolor MRI. For that purpose, and inspired by the hyperCEST [24] methodology (used with hyperpolarized 129Xe gas as the guest), we developed the 19F-GEST (guest exchange saturation transfer) approach [25-27]. Capitalizing on the different chemical environments of the inner cavity of two different macrocyclic hosts that induce different chemical shift offsets (Dw) of a rapidly exchanging complexed 19F-guest (either upfield or downfield relative to the non-complexed agent), we were able to demonstrate the use of a single 19F-agent for dual-color 19F-MRI with micromolar detectability based on GEST (Figure 4a). Analogously to 1H-CEST, the large 19FMR signal of the free 19F-agent mimics the 1H 2O signal and the 19F-agent that is bound to the molecular host resembles Figure 4. 19F-GEST using host-guest molecular pairs. (a) 19F-NMR spectrum of an aqueous solution of the 19F-guest fluroxene (G, 5 mM) and two molecular hosts (50 μM each), CB7 (H2) and OA (H7). (b) Schematic z-spectrum representation of an H-G systemdepicting the opposite GEST effect of a single guest in the presence of two different molecular hosts. (c) 1H-MRI, (d) 19F-MRI, and (e) 19F-GEST map of 5 mM fluroxene and 5 μM of either H2 (CB7) or H7 (OA). (f, g) Localized spectroscopy-GEST data of two voxels, as labeled in d.