First hyperpolarization of quaternary pyridinium salts using PHIP
Frederike Euchner1, Rainer Ringleb1, Joachim Bargon2, Ute Bommerich1, Johannes Bernarding1, and Markus Plaumann1

1Department for Biometrics and Medical Informatics, Otto-von-Guericke University Magdeburg, Magdeburg, Germany, 2Institute for Physical and Theoretical Chemistry, University of Bonn, Bonn, Germany


Quaternary ammonium substrates are of huge interest in the pharmaceutical field. Here, the hyperpolarization of new fluorinated pyridinium ions was examined. The effect of the positive charged nitrogen relating to the polarization transfer inside the molecule was proven and compared with the 1H, 13C and 19F hyperpolarization examinations of 2-(3-fluoro-phenyl)-3-buten-2-ol. The observed effect can be used for localization of polarization and for the synthesis of extened MR signal enhancements.


It is well known that quaternary ammonium ions can enrich in cell membranes of living organisms and affect the function of the cell membrane. But less is known about the pharmaceutical effect of pyridinium ions and their metabolism. The structural modification of these water soluble compounds can be manifold. Some of them, like 12-methacryloyloxydodecylpyridiniumbromide (MDPB)1 and cetylpyridiniumchloride (CPC),2 have cytotoxic properties. Others are necessary in the daily life (e.g. NAD+) or will be discussed for inhibiting cancer metastases.3 Because of this wide field of application; we decided to synthesize new fluorinated pyridinium derivatives for future cell labeling and metabolism experiences. In general, the use of fluorinated derivatives is of great interest in medical chemistry and diagnostics especially for the investigation of metabolism studies and protein-ligand interactions.4 The very low natural abundance in living organisms and the similarity of 19F with 1H with respect to van der Waals radius and bond length makes fluorine ideally suited for 19F-MRS and MRI studies. The disadvantage of low spin density in vivo, which results in low signals, can be overcome by hyperpolarization methods such as Parahydrogen Induced Polarization (PHIP).5 In comparison to further 19F hyperpolarization studies of our group,6,7 these new fluorinated pyridinium derivatives are water soluble and allow examination of a positive charged ion affect in the target molecule concerning polarization transfer.


The unsaturated fluorinated pyridinium precursors (five derivatives), which are necessary for PHIP experiments were synthesized by reaction of 1-bromo-2-butyne with the corresponding pyridine or nicotinic acid derivative in acetonitrile (see example of 3-fluoropyridine in Fig.1). After purification and characterization, the unsaturated pyridinium salts were dissolved in a) CD3OD or b) in D2O for hyperpolarization examinations. The hydrogenation reactions occur with 50 % enriched parahydrogen (6 bar pressure) in the vented solvents and in presence of a Rh(I) based catalysts. The hyperpolarization of the pyridinium salts was realized by hydrogenation in earth field (ALTADENA) and subsequent transport into high field (7T). 1H NMR spectra were detected by using a single pulse experiment with a 45° excitation pulse on a Bruker wide bore ultra shielded 300 MHz spectrometer. In contrast to this, the 13C and 19F NMR spectra were measured under same reaction conditions, but using a 90° excitation pulse. The obtained signal enhancements (SEs) were calculated from signal-to-noise ratios of the thermal and the hyperpolarized spectra.


The synthesis of the pyridinium ions achieved without byproducts. As an example, here the single-scan 1H NMR spectrum (section of enhanced signals) recorded directly after hydrogenation (Fig. 2, blue spectrum) shows two enhanced signals which can be attributed to the added hydrogen atoms at 5.8 ppm (CH) and 6.2 ppm (CH). Both signals were enhanced with around factor 40. It should be mentioned that the nuclei of the ring system were not enhanced. Smaller enhancements were calculated for the CH2 and CH3 group neighboring to the new formed double bond (not shown in Fig. 2). 19F-NMR spectra confirm the formation of the hydrogenation product. Until now, in our measurements, no 19F signal enhancement could be detected if a pyridinium ion was used as precursor for hydrogenation. For comparison, 2-(3-fluoro-phenyl)-3-butyn-2-ol was hydrogenated in different organic solvents as well as D2O. Here, a hyperpolarization of 19F could be observed. Furthermore, despite of the less water solubility, the 1H NMR spectra of the hyperpolarization experiments show a weak polarization transfer also to the aromatic protons (Fig. 3).


The positive charge of the nitrogen has a significant effect to polarization transfer in the molecule. Despite the negative results relating to the hyperpolarization of fluorine in the new substrates, these results are helpful for the aim of the creation of polarization. In the case of the pyridinium ions, the polarization was localized on the added hydrogen atoms and their near surrounding. Furthermore, a closer examination to the measurements shows an extended time of the observed signal enhancement. The reason therefore is based on the separated spin systems.


These new results demonstrates the effect of a positive charged ion (here nitrogen) in hyperpolarization examinations. It can be used for localization of polarization and within an extension of the time of signal enhancements, which is very important for application in MRS or MRI measurement.


This work was supported by the Deutsche Forschungsgemeinschaft.


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Fig. 1: Example for the synthesis of fluorinated pyridinium ions.

Fig. 2: Example reaction scheme of the hydrogenation of the synthesized 3-fluoropyridinium ion and the corresponding 1H NMR spectra of its hydrogenation measured in methanol-d4: hyperpolarized 3-fluoropyridinium ion derivative (above) and its spectrum in thermal equilibrium (below).

Fig. 3: a) 1H NMR detected from parahydrogenation of 2-(3-fluorophenyl)-3-butyn-2-ol in acetone-d6. PHIP spectrum after ca. 10s of hydrogenation (bottom); spectrum after achievement of thermal equilibrium (top); b) corresponding 13C NMR using ALTADENA conditions (bottom) and field cycling (top); c) 19F NMR detected from an analogous reaction solution; d) 1H NMR detected from hydrogenation in D2O.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)