NMR spectral analysis of strongly second-order 6-, 8-, 9- and 10- spin-systems (1H─19F, 19F─19F,and 13C─19F) in perfluorotoluyl- and tetrafluoro-pyridyl-aromatics using the lineshape method ANATOLIA

A simple to use nuclear magnetic resonance analysis method has been tested on complex ¹H, ¹⁹F, and ¹³C multiplets. This open-source line-shape analysis method analysis of total lineshape (ANATOLIA)¹ provides some significant advantages over traditional assign-iterate methods of NMR spectral analysis by avoiding false minima and progressing optimisation to the global minimum. The target molecules are 1-perfluorotol-4-yl-2-perfluorotol-4-yl-oxymethyl-1H-benzimidazole (molecule-I) and 1-tetrafluoropyrid-4-yl-2-tetrafluoropyrid-4-yl-thio-1H-benzimidazole (molecule-II) which were produced as part of a family of fluorinated drug scaffolds prepared for anticancer and antiparasitic screening. Spectra display significant second-order effects with ¹H Δδ = 3.68 and 4.67 Hz for the aromatic hydrogen “triplets”, with ¹⁹F ⁴J_AA', ⁴J_BB', ⁴J_XX', and ⁴J_YY' coupling constants range from +4.8 to −14.0 Hz and for ¹³C-isotopomers ¹⁹F Δδ of up to 111.56 Hz. A spin-system of six coupling nuclei (H_aH_bH_cH_d F_YF_Y') was analysed in 12 s, a spin-system of nine coupling fluorine nuclei (AA'BB'CCC-YY') was analysed within 2 min, and 10 coupling nuclei (XX'YY'ZZZ-BB'-H_d) was optimised in 6 min using a laptop computer. ANATOLIA was also robust enough to be able to yield accurate spectral values from inaccurate input values. In both compounds, a fluorine–fluorine coupling constant was identified between the two fluoro-aromatic rings (F_BB' and F_YY') of +4.05 and +4.67 Hz and attributed to a through-space interaction. Ab initio structure optimisations and coupling constant calculations provided useful input data for spectral analysis. A modern ¹⁹F nuclear magnetic resonance spectrum of perfluorotoluene (octafluorotoluene) and analysis from 1975 was used as a test data set to assess ANATOLIA.     

Chemical bonding and molecular dynamics in inclusion compounds of fluorinated graphite according to 19F NMR spectroscopy data

In Inclusion compounds of fluorinated graphite with chlorine trifluoride C₂. xClF₃ and hexafluoro-benzene C₂F. xC₆F₆ , the guest molecules are characterized by rotational mobility and weak bonds with the host matrix. ¹⁹F NMR chemical shift tensors are determined for the fluorine nuclei of the matrix and the guest molecules, including the structurally nonequivalent fluorine atoms ofClFj molecules [δ (Fl) = −700, δ(F1) = −280; δ|| (F2) = −440, δ±(F₂) = −220ppm relative to F2]. It is shown that C-F bonds in the host matrix are close to those in aromatic fluorocarbons. Translated from Zhumal Stmktumoi Khimii, Vol. 41, No. 1, pp. 80-85, January–February, 2000.     

Aromatic 19F–13C TROSY—[19F, 13C]-Pyrimidine Labeling for NMR Spectroscopy of RNA

We present the access to [5-¹⁹F, 5-¹³C]-uridine and -cytidine phosphoramidites for the production of site-specifically modified RNAs up to 65 nucleotides (nts). The amidites were used to introduce [5-¹⁹F, 5-¹³C]-pyrimidine labels into five RNAs—the 30 nt human immunodeficiency virus trans activation response (HIV TAR) 2 RNA, the 61 nt human hepatitis B virus ϵ (hHBV ϵ) RNA, the 49 nt SAM VI riboswitch aptamer domain from B. angulatum, the 29 nt apical stem loop of the pre-microRNA (miRNA) 21 and the 59 nt full length pre-miRNA 21. The main stimulus to introduce the aromatic ¹⁹F–¹³C-spin topology into RNA comes from a work of Boeszoermenyi et al., in which the dipole-dipole interaction and the chemical shift anisotropy relaxation mechanisms cancel each other leading to advantageous TROSY properties shown for aromatic protein sidechains. This aromatic ¹³C–¹⁹F labeling scheme is now transferred to RNA. We provide a protocol for the resonance assignment by solid phase synthesis based on diluted [5-¹⁹F, 5-¹³C]/[5-¹⁹F] pyrimidine labeling. For the 61 nt hHBV ϵ we find a beneficial ¹⁹F–¹³C TROSY enhancement, which should be even more pronounced in larger RNAs and will facilitate the NMR studies of larger RNAs. The [¹⁹F, ¹³C]-labeling of the SAM VI aptamer domain and the pre-miRNA 21 further opens the possibility to use the biorthogonal stable isotope reporter nuclei in in vivo NMR to observe ligand binding and microRNA processing in a biological relevant setting.     

The solid state 13C-NMR and 19F-NMR spectra of some graphite fluorides

The solid state ¹³C nuclear magnetic resonance spectra of fluorinated graphites show two resonances, one of which is assigned to aromatic carbon and the other to aliphatic carbon. The resonances are very broad with the high-field resonance centered at about 35 ppm below tetramethylsilane (TMS) and a low-field resonance centered at about 160 ppm below tetramethylsilane. The high-field resonance is typical of an sp³-like carbon and the low-field resonance is assigned to sp²-like carbons. It is found that the aromatic resonance in graphite decreases with an increase in fluorination of the graphite fluorides examined in this study. The ¹⁹F nuclear magnetic resonance spectra of C₄F and CF₁ each show one resonance. The fluorine resonance in C₄F is 180 ppm above CFCI₃ whereas the fluorine resonance in CF₁ is 55 ppm above CFCI₃. These peaks are in the range for fluorine bonded to aromatic and aliphatic carbons, respectively.     

Nucleophilic aromatic substitution by [<Superscript>18</Superscript>F]fluoride at substituted 2-nitropyridines

For the radiofluorination of benzenes and benzene derivatives, the electrophilic reaction with [¹⁸F]F₂ is a very common route. Yet, aromatic nucleophilic substitution (S_NAr) by n.c.a [¹⁸F]fluoride, which can be produced efficiently in high amounts, has been considered to be very desirable. However, to facilitate ¹⁸F-labelling via S_NAr at an electron rich aromatic system, an appropriate leaving group must be present together with an auxiliary group in ortho or para position to the leaving group. An interesting alternative for the auxiliary group is the heteroatom of a heteroaromatic system, for which pyridine is a leading example. Dolci et al. (J Label Compd Radiopharm 42:975–985, 1999) have evaluated the scope of the nucleophilic aromatic fluorination of 2-substituted pyridine rings using the activated K [¹⁸F]F-K₂₂₂ complex. As methyl and methoxy groups are known to enhance the electron density of an aromatic system by the +I and the +M effect, respectively, S_NAr is unlikely to occur. Until now, the effect of these substituents has not been studied towards the ¹⁸F-radiofluorination of substituted 2-nitropyridines by use of [¹⁸F]fluoride. Therefore, we have investigated the effect of methoxy and methyl groups in 2-nitropyridines. The results showed that 3-methoxy-2-nitropyridine and 3-methyl-2-nitropyridine can efficiently be substituted by [¹⁸F]fluoride with high RCY’s (70–89%) in short reaction times (1–30 min) at a reaction temperature of 140 °C. Moreover, 3-methoxy-6-methyl-2-[¹⁸F]fluoropyridine was obtained from the corresponding nitro-precursor in a high yield of 81 ± 1% after 30 min at 140 °C. In case of 2-nitropyridines data indicates the effect of methyl and methoxy groups on S_NAr to be of minor importance.     

Versatile Nickel(II) Scaffolds as Coordination-Induced Spin-State Switches for 19F Magnetic Resonance-Based Detection

¹⁹F magnetic resonance (MR) based detection coupled with well-designed inorganic systems shows promise in biological investigations. Two proof-of-concept inorganic probes that exploit a novel mechanism for ¹⁹F MR sensing based on converting from low-spin (S=0) to high-spin (S=1) Ni²⁺ are reported. Activation of diamagnetic NiL₁ and NiL₂ by light or β-galactosidase, respectively, converts them into paramagnetic NiL₀ , which displays a single ¹⁹F NMR peak shifted by >35 ppm with accelerated relaxation rates. This spin-state switch is effective for sensing light or enzyme expression in live cells using ¹⁹F MR spectroscopy and imaging that differentiate signals based on chemical shift and relaxation times. This general inorganic scaffold has potential for developing agents that can sense analytes ranging from ions to enzymes, opening up diverse possibilities for ¹⁹F MR based biosensing.     

19F NMR Study of Antimony(III) Fluoride Complexes in Solution

The state of SbF₃, Sb₃O₂F₅, MSb₃F₁₀, MSb₂F₇, M₃Sb₄F₁₅, MSbF₄, and M₂SbF₃, as well as HF and MF (M = Na, K, Rb, Cs, NH₄, and Tl), in a 0.25 M aqueous solution at room temperature was studied by ¹⁹F NMR. It is found that chemical shifts of the ¹⁹F NMR signals (relative to CFCl₃) for the compounds under investigation vary in the 73.3–157.5 ppm range. For the Sb(III) fluoride complexes, chemical shifts in the ¹⁹F NMR spectra vary in the 73.3–85.5 ppm range (relative to CFCl₃). Although the crystals of Sb(III) fluoride complexes contain different polyhedra that are joined in different ways, the NMR spectra of all compounds under study show one narrow signal. The spectra of aqueous solutions of Sb(III) fluoride complexes do not contain signals of free fluoride ions and oxofluoride Sb₃O₂F₅.     

Electrochemical radiofluorination: Labeling of benzene with [18F]fluoride by nucleophilic substitution

¹⁸F-labeling of aromatic compounds normally is achieved by electrophilic substitution. In that case [¹⁸F]fluoride cannot be applied although it is produced very efficiently at medical cyclotrons. By the use of electrochemical methods, however, benzene can be oxidized and thus, the electron density is reduced in a way that nucleophilic attack of [¹⁸F]fluoride occurs. For the first time benzene was shown to be labeled with [¹⁸F]fluoride after being electrochemically oxidized in a 2 ml electrolysis cell with 0.033M Et₃N^.3HF and 0.066M Et₃N^.HCl in CH₃CN and benzene in various concentrations. After 50 Coulombs (60-90 min) maximum of labeling was reached. With the highest concentration of aromatic compound (1.0M) the radiochemical yields were 16±9% with specific activities up to 27 GBq/mmol.     

Parahydrogen‐Induced Polarization Transfer to 19F in Perfluorocarbons for 19F NMR Spectroscopy and MRI

Fluorinated substances are important in chemistry, industry, and the life sciences. In a new approach, parahydrogen‐induced polarization (PHIP) is applied to enhance ¹⁹F MR signals of (perfluoro‐n‐hexyl)ethene and (perfluoro‐n‐hexyl)ethane. Unexpectedly, the end‐standing CF₃ group exhibits the highest amount of polarization despite the negligible coupling to the added protons. To clarify this non‐intuitive distribution of polarization, signal enhancements in deuterated chloroform and acetone were compared and ¹⁹F–¹⁹F NOESY spectra, as well as ¹⁹F T₁ values were measured by NMR spectroscopy. By using the well separated and enhanced signal of the CF₃ group, first ¹⁹F MR images of hyperpolarized linear semifluorinated alkenes were recorded.     

19F and 1H NMR of aldimines of fluoroamino acids with pyridoxal-5′-phosphate

¹H and ¹⁹F NMR spectra were obtained for six Schiff bases (aldimines) formed by pyridoxal-5′-phosphate (PLP) with four fluorinated or their two parent non-fluorinated α-amino acids (phenylalanine and α-aminobutyric acid). pK_A Values were derived from ¹H and ¹⁹F titration curves. The imine nitrogen of the aldimines is very basic (～13) and sensitive to the electron withdrawing effect of fluorine. The pyridine nitrogen is less basic in the aldimines (～7) than in PLP (8.12) and is less sensitive to the electron withdrawing effect of the fluorine than is the imine nitrogen. The phosphate group has a pK in the same range (～6) and the chemical shifts of some nuclei are sensitive to both pK values. Protonation of the aldimine causes the ¹H signal to shift downfield at the methyl protons of the pyridine ring and at the aldehydic proton of the aldimine for the high pK value (except for the aldimines prepared from the β-fluorophenylalanine), but upfield at the aromatic proton and at the aldehydic proton of the aldimine for the low pK. Protonation of the aldimine causes the ¹⁹F signal of an aryl fluorine to shift downfield but gives an upfield shift at a β-fluorine. These data are related to the highly conjugated electronic structure of the Schiff bases.     

Structure of Modified Polytetrafluoroethylene According to DFT Calculations and 19F NMR Spectroscopy

The molecular structure of n-C₇F₁₆ and the ¹⁹F nuclear magnetic screening tensors are calculated by density functional theory (DFT) methods. The results of calculations are compared with ¹⁹F NMR data, and it is shown that fine polytetrafluoroethylene (PTFE) contains the terminal CF₃ groups in its structure.     

19F coupling constants and chemical shifts in some dimethylamino and alkoxy derivatives of tellurium hexafluoride,cis TeF4XY

The spectra of A₂BC spin systems provided by the ¹⁹F nuclei in cis (RO) (MeO)TeF₄ and (RO) (Me₂N)TeF₄ (R = Me, Et, Pr, i-Pr) have been recorded and analysed. The geminal coupling constants ²J(F,F) range from 137–174 Hz and trends in the ¹⁹F chemical shifts permit complete assignment of the resonances. Stereospecific coupling between ¹⁹F and the protons of the N-methylamino groups is also observed.     

Rapid Aqueous Late‐Stage Radiolabelling of [GaF3(BnMe2‐tacn)] by 18F/19F Isotopic Exchange: Towards New PET Imaging Probes

A simple and rapid method for ¹⁸F radiolabelling of [GaF₃(BnMe₂‐tacn)] by ¹⁸F/¹⁹F isotopic exchange is described. The use of MeCN/H₂O or EtOH/H₂O (75:25) and aqueous [¹⁸F]F^? (up to 200 MBq) with heating (80 °C, 10 min) gave 66±4 % ¹⁸F incorporation at a concentration of 268 nm , and 37±5 % ¹⁸F incorporation at even lower concentration (27 nm ), without the need for a Lewis acid promoter. A solid‐phase extraction method was established to give [Ga¹⁸F¹⁹F₂(BnMe₂‐tacn)] in 99 % radiochemical purity in an EtOH/H₂O mixture.     

Aromatic 19F–13C TROSY—[19F, 13C]‐Pyrimidine Labeling for NMR Spectroscopy of RNA

We present the access to [5‐¹⁹F, 5‐¹³C]‐uridine and ‐cytidine phosphoramidites for the production of site‐specifically modified RNAs up to 65 nucleotides (nts). The amidites were used to introduce [5‐¹⁹F, 5‐¹³C]‐pyrimidine labels into five RNAs—the 30 nt human immunodeficiency virus trans activation response (HIV TAR) 2 RNA, the 61 nt human hepatitis B virus ? (hHBV ?) RNA, the 49 nt SAM VI riboswitch aptamer domain from B. angulatum, the 29 nt apical stem loop of the pre‐microRNA (miRNA) 21 and the 59 nt full length pre‐miRNA 21. The main stimulus to introduce the aromatic ¹⁹F–¹³C‐spin topology into RNA comes from a work of Boeszoermenyi et al., in which the dipole‐dipole interaction and the chemical shift anisotropy relaxation mechanisms cancel each other leading to advantageous TROSY properties shown for aromatic protein sidechains. This aromatic ¹³C–¹⁹F labeling scheme is now transferred to RNA. We provide a protocol for the resonance assignment by solid phase synthesis based on diluted [5‐¹⁹F, 5‐¹³C]/[5‐¹⁹F] pyrimidine labeling. For the 61 nt hHBV ? we find a beneficial ¹⁹F–¹³C TROSY enhancement, which should be even more pronounced in larger RNAs and will facilitate the NMR studies of larger RNAs. The [¹⁹F, ¹³C]‐labeling of the SAM VI aptamer domain and the pre‐miRNA 21 further opens the possibility to use the biorthogonal stable isotope reporter nuclei in in vivo NMR to observe ligand binding and microRNA processing in a biological relevant setting.     

19F Paramagnetic Relaxation Enhancement: A Valuable Tool for Distance Measurements in Proteins

Fluorine NMR paramagnetic relaxation enhancement was evaluated as a versatile approach for extracting distance information in selectively F‐labeled proteins. Proof of concept and initial applications are presented for the HIV‐inactivating lectin cyanovirin‐N. Single F atoms were introduced at the 4‐, 5‐, 6‐ or 7 positions of Trp49 and the 4‐position of Phe4, Phe54, and Phe80. The paramagnetic nitroxide spin label was attached to Cys residues that were placed into the protein at positions 50 or 52. ¹⁹F‐T₂ NMR spectra with different relaxation delays were recorded and the transverse ¹⁹F‐PRE rate, ¹⁹F‐Γ₂, was used to determine the average distance between the F nucleus and the paramagnetic center. Our data show that experimental ¹⁹F PRE‐based distances correspond to 0.93 of the ¹H_N‐PRE distances, in perfect agreement with the gyromagnetic γ¹⁹F/γ¹H ratio, thereby demonstrating that ¹⁹F PREs are excellent alternative parameters for quantitative distance measurements in selectively F‐labeled proteins.     

Solid‐state 1H → 19F/19F → 1H CP/MAS NMR study of poly(vinylidene fluoride)

Solid‐state ¹H → ¹⁹F and ¹⁹F → ¹H cross‐polarization magic angle spinning (CP/MAS) NMR spectra have been investigated for a semicrystalline fluoropolymer, namely poly(vinylidene fluoride) (PVDF). The ¹H → ¹⁹F CP/MAS spectra can be fitted by five Lorentzian functions, and the amorphous peaks were selectively observed by the DIVAM CP pulse sequences. Solid‐state spin‐lock experiments showed significant differences in T_1ρ^F and T_1ρ^H between the crystalline and amorphous domains, and the effective time constants, T_HF* and T_1ρ*, which were estimated from the ¹H → ¹⁹F CP curves, also clarify the difference in the strengths of dipolar interactions. Heteronuclear dipolar oscillation behaviour is observed in both standard CP and ¹H → ¹⁹F inversion recovery CP (IRCP) experiments. The inverse ¹⁹F → ¹H CP‐MAS and ¹H → ¹⁹F CP‐drain MAS experiments gave complementary information to the standard ¹H → ¹⁹F CP/MAS spectra in a manner reported in our previous papers for other fluoropolymers. The value of N_F/N_H (where N is a spin density) estimated from the CP‐drain curve is within experimental error equal to unity, which is consistent with the chemical structure. Copyright © 2001 John Wiley & Sons, Ltd.     

Concurrent 19F and 77Se or 19F and 125Te NMR T1 measurements for determination of 77Se and 125Te absolute shielding scales

⁷⁷Se and ¹²⁵Te absolute nuclear shieldings in SeF₆ and TeF⁶ are determined from simultaneous T₁ measurements of ⁷⁷Se and ¹⁹F (and ¹²⁵Te and ¹⁹F) in the gas phase.     

Gadolinium(III)-Based Dual 1H/19F Magnetic Resonance Imaging Probes

We present two novel octadentate cyclen-based ligands bearing one (L¹) or two (L²) phenylacetamide pendants with two CF₃ groups either at positions 3 and 5 (L¹) or 4 (L²). The corresponding Gd³⁺ complexes possess one coordinated water molecule, as confirmed by luminescence lifetime measurements on the Eu^III and Tb^III analogues. A detailed ¹H and ¹⁷O relaxometric characterization has revealed the parameters that govern the relaxivities of these complexes. The water-exchange rate of the mono-amide derivative GdL¹ (k_ex²⁹⁸=1.52×10⁶ s⁻¹) is faster than that determined for the bis-amide complex GdL² (k_ex²⁹⁸=0.73×10⁶ s⁻¹). ¹H and ¹⁹F NMR studies have indicated that the complexes are present in solution almost exclusively as the square-antiprismatic (SAP) isomers. ¹⁹F NMR relaxation studies indicated Gd ⋅⋅⋅ F distances of 7.4±0.1 and 9.1±0.1 Å for GdL¹ and GdL², respectively. Phantom MRI studies revealed the favorable properties of GdL² as a dual ¹H/¹⁹F magnetic resonance imaging (MRI) probe, whereas the shorter Gd ⋅⋅⋅ F distance of GdL¹ reduces the signal-to-noise ratio due to the very short transverse relaxation time of the ¹⁹F NMR signal.     

Partially Fluorinated Copolymers as Oxygen Sensitive 19F MRI Agents

Effective diagnosis of disease and its progression can be aided by ¹⁹F magnetic resonance imaging (MRI) techniques. Specifically, the inherent sensitivity of the spin–lattice relaxation time (T₁) of ¹⁹F nuclei to oxygen partial pressure makes ¹⁹F MRI an attractive non-invasive approach to quantify tissue oxygenation in a spatiotemporal manner. However, there are only few materials with the adequate sensitivity to be used as oxygen-sensitive ¹⁹F MRI agents at clinically relevant field strengths. Motivated by the limitations in current technologies, we report highly fluorinated monomers that provide a platform approach to realize water-soluble, partially fluorinated copolymers as ¹⁹F MRI agents with the required sensitivity to quantify solution oxygenation at clinically relevant magnetic field strengths. The synthesis of a systematic library of partially fluorinated copolymers enabled a comprehensive evaluation of copolymer structure–property relationships relevant to ¹⁹F MRI. The highest-performing material composition demonstrated a signal-to-noise ratio that corresponded to an apparent ¹⁹F density of 220 mm , which surpasses the threshold of 126 mm ¹⁹F required for visualization on a three Tesla clinical MRI. Furthermore, the T₁ of these high performing materials demonstrated a linear relationship with solution oxygenation, with oxygen sensitivity reaching 240×10⁻⁵ mmHg⁻¹s⁻¹. The relationships between material composition and ¹⁹F MRI performance identified herein suggest general structure–property criteria for the further improvement of modular, water-soluble ¹⁹F MRI agents for quantifying oxygenation in environments relevant to medical imaging.     

19F NMR study of mixed pentafluorochlorotantalum-trimethylphosphate adducts in solution

All twelve adducts comprising to the series TaF_5-iCl_i·(MeO)₃PO (O ≤ i ≤ 5) have been prepared in CH₂Cl₂ solution and characterised by ¹⁹F NMR Fourier Transform spectroscopy. The measured ΔG's of formation show that the mixed adducts are in slightly lower concentration than expected from a statistical distribution. The compounds are found to be more stable when fluorine rather than chlorine occupies an axial position; in addition, adducts with the same halide trans to each other are more stable than other isomers.