Intramolecular Hydrogen Bond Reverting the Solvent Effect on Phosphorus Hyperfine Coupling Constants of ß-Phosphorylated Nitroxides.

Recently, we reported a dramatic solvent effect on the phosphorus hyperfine coupling constant aP of ß-phosphorylated six-membered ring nitroxides, that is, approximately 25 G of difference in aP from n-hexane to water (Org. Biomol. Chem. 2016, 14, -1228-1292). In this article, we report on the effect of intramolecular hydrogen bonding (IHB) in three nitroxides exhibiting IHB between the hydroxyl and diethylphosphoryl groups and one exhibiting IHB between the hydroxyl group and the nitroxyl moiety. It is observed that for the first three nitroxides, aP increases with increasing polarity/polarizability and hydrogen bond donor (HBD) properties of the solvent (π* and α, respectively)-in sharp contrast to the data reported in the literature-and for the last nitroxide, aP decreases with π* and α. In fact, the occurrence of IHB induces a large strain, its suppression by hydrogen bond acceptor (HBA) solvents affords an increase in aP .

The ß-phosphorus hyperfine coupling constant in nitroxides: 6. Solvent effects in non-cyclic nitroxides.

In two recent articles (Org. Biomol. Chem., 2015 and 2016), we showed that changes in the phosphorus hyperfine coupling constant aP at position ß in ß-phosphorylated nitroxides can be dramatic. Such changes were applied to the titration of water in organic solvents and conversely of organic solvents in water. One of the molecules tested was a non-cyclic nitroxide meaning that a thorough investigation of the solvent effect on the EPR hyperfine coupling constant is timely due. In this article, we show that the aP of persistent non-cyclic ß-phosphorylated nitroxides decrease with the normalized polarity Reichardt's constant E(N)T. The Koppel-Palm and Kalmet-Abboud-Taft relationships were applied to gain deeper insight into the effects influencing aN and aP: polarity/polarizability, hydrogen bond donor properties, and the structuredness of the cybotactic region.

Solvent effect in ß-phosphorylated nitroxides. Part 4: detection of traces of water by electron paramagnetic resonance.

For decades, the nitrogene hyperfine coupling constant aN of nitroxides has been applied to probe their environment using EPR. However, the small changes observed (≈2 G from n-pentane to water) with the solvent polarity allow only a qualitative discussion. A stable ß-phosphorylated nitroxide exhibiting a small change in aN (≈3 G from n-pentane to water) and a striking change (≈25 G from n-pentane to water) in phosphorus hyperfine coupling constant aP with the polarity of solvent was prepared and used to develop the first procedure for the titration of water in THF by EPR, down to 0.1% v/v.

The ß-phosphorus hyperfine coupling constant in nitroxide: part 3: titration of water by electron paramagnetic resonance.

Recently, we showed that the phosphorus hyperfine coupling constant aPß of persistent cyclic nitroxides decreased with the normalized polarity Reichardt's constant E. Thus, we investigated the changes in aPß in binary mixtures of solvents. The sensitivity of aPß to the solvent was high enough to allow us to perform water titration in THF, 1,4-dioxane, and acetonitrile by EPR. Accuracies of a few percent were achieved.

Enzymatically Shifting Nitroxides for EPR Spectroscopy and Overhauser-Enhanced Magnetic Resonance Imaging.

In vivo investigations of enzymatic processes using non-invasive approaches are a long-lasting challenge. Recently, we showed that Overhauser-enhanced MRI is suitable to such a purpose. A ß-phosphorylated nitroxide substrate prototype exhibiting keto-enol equilibrium upon enzymatic activity has been prepared. Upon enzymatic hydrolysis, a large variation of the phosphorus hyperfine coupling constant (Δa(P)=4 G) was observed. The enzymatic activities of several enzymes were conveniently monitored by electronic paramagnetic resonance (EPR). Using a 0.2 T MRI machine, in vitro and in vivo OMRI experiments were successfully performed, affording a 1200% enhanced MRI signal in vitro, and a 600% enhanced signal in vivo. These results highlight the enhanced imaging potential of these nitroxides upon specific enzymatic substrate-to-product conversion.

Chemically triggered C-ON bond homolysis of alkoxyamines. 8. Quaternization and steric effects.

The C-ON bond homolysis in alkoxyamine 2a was chemically triggered by quaternization of the 1-(pyridin-2-yl)ethyl fragment using protonation, acylation, and oxidation into the N-oxide. The solvent effect was also investigated, and DFT calculations were performed to explore this chemical activation. Alkoxyamines 2a-d were also compared to the 1-(pyridin-4-yl)ethyl analogues 3a-d.