New, stable beta-phosphorus-labelled pyrrolidine nitroxides from nitrones for magnetometry: an ESR investigation

The present study was devoted to the design and synthesis of new, prototypi cal nitroxides for use in dynamic nuclear polarization (DNP) magnetometers. Specifically, eight beta-phosphorylated pyrrolidine nitroxides were synthe sized. Their large (ca. 5.0 mT) beta-P-31 hyperfine splittings render them desirable for magnetometry applications. Seven of these free radicals are 2 -dialkylphosphityl-2,5,5-trialkyl- or 2-diarylphosphityl-2,5,5-trialkylpyrr olidine nitroxides. Structural and solvent effects on the ESR spectra of th ese nitroxides were explored. Hydrolysis of a phosphityl group creates a re markable eighth nitroxide. It exhibits an extremely pH-dependent ESR spectr um. Other nitroxides (e.g. beta-aminyls) are known to show pH-dependent ESR spectra (Delta a(N) approximate to 0.15 mT), and these have been proposed by Keana et al. as novel, sensitive pH indicators. Whereas they operate by changes in the nitroxide nitrogen hyperfine splitting, the present special nitroxide displays little change in the nitroxide nitrogen splitting (Delta a(N) approximate to 0.03 mT). A very robust variation in the beta-P-31 hyp erfine splitting (Delta a(N) approximate to 0.35 mT), however, is observed. Hence it may be a much more sensitive pH probe molecule than Keana et al.' s. It is proposed that this ESR variation for the present nitroxide is due to conformational changes that are pH dependent. Copyright (C) 1999 John Wi ley & Sons, Ltd.