Fluorine-containing compounds, which play a key role in pharmaceutical and other industries, are a class of molecules with unparalleled significance and versatility underscoring the significance of fluorine chemistry in shaping the future of scientific and technological progress. The unique electronic and physicochemical properties of fluorine render these compounds invaluable for drug design [1], [2], [3]. Among many benefits, they enhance pharmacokinetic and pharmacodynamic profiles, metabolic stability, receptor-binding affinities, and physicochemical properties, such as permeability, and solubility [4], [5]. Beyond pharmaceuticals, fluorine-containing compounds are widely applied in various industrial sectors specializing in agrochemicals [6], materials science [7], [8], and catalysis [9]. The incorporation of fluorine atoms into organic molecules offers many advantages, such as increased lipophilicity and altered reactivity, which in turn influence the overall efficacy and safety of the end products.

The preparation of deoxyfluorinated saccharide derivatives has been the subject of much focus [10], [11], [12], [13]. For example, it has been shown that deoxyfluorination can significantly modify the binding of saccharides to galectins [12], [14]. Generally, oligosaccharides are conformationally flexible [15], and can undergo conformational changes upon binding to better fit the protein binding site [16]. Therefore, obtaining information about oligosaccharide conformation in free and bound states is important for understanding the binding energetics and design of high-affinity ligands.

After hydrogen 1H, fluorine 19F is the second most sensitive nucleus in nuclear magnetic resonance (NMR) spectroscopy. 19F NMR spectroscopy is an excellent tool for investigating fluorinated molecules, exhibiting 100 % natural abundance, a spin quantum number I = 1/2, and a large chemical shift dispersion of hundreds of ppm. The most important NMR parameters in solution are chemical shifts and J-couplings, manifested as line splitting. The chemical shift reflects the electron density in the vicinity of the nucleus, while the J-coupling reflects the electronic structure between the coupled nuclei. J-coupling is also called indirect coupling because it is mediated by electrons, typically bonding electrons, on the pathway between the coupled nuclei. Therefore, J-coupling is closely associated with molecular conformation. For example, in 1H NMR spectra, the coupling between hydrogen atoms separated by three covalent bonds strongly depends on the dihedral angle, a relationship famously described in the Karplus equation [17], [18], [19].

Although J-coupling is often considered proof of the presence of covalent bonds between coupled nuclei, the covalent interaction is generally not necessary for the coupling [20]. For example, commonly observed through-hydrogen-bond J-couplings provide information about the hydrogen bond [20], [21], [22], [23]. Similarly, through-halogen-bond coupling has been observed [24]. Notably, J-coupling can even occur in nuclei close in space that are not connected via a specific covalent or non-covalent interaction. These couplings are called through-space couplings (TSCs) [25], [26].

TSCs most often occur between two fluorine atoms due to an overlap of fluorine lone-pair orbitals [27], [28], [29], [30]. Notably, fluorine–fluorine TSCs have been identified between two fluorine-labeled tryptophan residues in dihydrofolate reductase. In this case, although the two fluorine atoms were formally separated by 89 covalent bonds, a coupling of 17 Hz was observed between them [31]. However, the overlap of lone-pair orbitals is not a necessary condition for the occurrence of TSCs. Indeed, fluorine–carbon [32], [33], fluorine–nitrogen [34], [35], fluorine–hydrogen [27], [36], phosphorus–phosphorus [37], [38], carbon–phosphorus [39], [40], and hydrogen–hydrogen [41], [42], [43] TSCs have all been reported. TSCs have also been investigated in a number of computational studies [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57]. Importantly, the magnitudes of TSCs depend not only on the distance between the coupled nuclei, but also on the geometry of the molecular fragments involved. However, TSCs are not commonly used in conformational analysis.

In a recent study, we prepared a complete series of methyl β-glycosides of monodeoxyfluorinated N-acetyllactosamine (LacNAc) analogues, which we employed as 19F NMR-active probes to study galectin recognition of LacNAc [14]. LacNAc is a disaccharide composed of d-galactose and N-acetyl-d-glucosamine joined by a β1-4 glycosidic linkage (Galβ1-4GlcNAc). LacNAc and its linear oligomers [-3Galβ1-4GlcNAcβ1-] occur as common glycan extensions attached to the core structures of N- and O-glycans and glycolipids [58]. Soluble protein galectins recognize and bind to monomeric and oligomeric LacNAc units, an interaction that plays a key role in a wide range of (patho)physiological pathways [59]. When characterizing deoxyfluorinated disaccharide derivatives, we noticed that some of the carbon signals in NMR spectra exhibited TSCs with fluorine atoms in the second monosaccharide unit. Encouraged by this finding, we speculated that these couplings might be used to reveal the conformation of the disaccharides.

Herein, we investigate the use of fluorine–carbon TSCs as a means of elucidating disaccharide conformation. First, we conducted a conformational analysis of the deoxyfluorinated disaccharides using a combination of experimental NMR data and molecular modeling. Second, we benchmarked quantum chemical calculations of the TSCs using a simple fluoromethane⋯methane model. Finally, we demonstrated that calculated fluorine–carbon TSC values for the disaccharides corresponded with experimental data only for conformers identified as the most probable structures by independent methods. This suggests that these TSCs might be reliably utilized in the structural analysis of (bio)organic compounds.