Dl. Haire et al., New, stable beta-phosphorus-labelled pyrrolidine nitroxides from nitrones for magnetometry: an ESR investigation, MAGN RES CH, 37(4), 1999, pp. 251-258
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.