Functional in vivo EPR Spectroscopy and Imaging Using Nitroxide and Trityl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 554

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Figure16.11 (a) L-band pH mapping. Cross-sectional 3-D (1-spectral/2-spatial)image of a phantom prepared
using capillary tubes of 2mm diameter filled with 1mM solutions of the radical ATI prepared at different pH
(indicated in the figure). The intensity bar to the right of the image quantifies pH values. The data acquisition
parameters were: acquisition time, 5min; projections, 64; maximum gradient, 30G/cm; field of view, 24mm.
(b) pH mapping by functional PEDRI of a phantom prepared using capillary tubes of 5mm diameter filled with
1mM aqueous solution of ATI prepared atdifferentpH (indicatedin the figure).The intensitybar to the rightof
the image quantifies pH values. The pH map was calculated from two PEDRI images acquired at the operating
EPR frequency567MHz and differentEPR excitationfields,214.16G and 214.88G, which correspond to peak
amplitudesof DNP spectraof the RH + and R forms of the ATIprobe, respectively.Total acquisitiontime, 8.8s.

Fieldofview,30mm × 30mm,withresolution64 × 64.

Figure 16.11b demonstrates the capacity of pH mapping using the recently proposed functional PEDRI 122
approach. Normally, PEDRI is acquired at fixed EPR field, resulting in loss of EPR spectral information.
In the proposed functional approach, PEDRI images were acquired at two different EPR excitation fields
which correspond to peak amplitudes of DNP spectra of the RH + and R forms of the pH probe. This permits
extraction of the [RH + ]/[R] ratio and, therefore, the pH values from the NMR signal enhancements observed
at each pixel of only two PEDRI images. Note that pH mapping using functional PEDRI (Figure 16.11b)
allows for about a 30-fold decrease in acquisition time compared with EPRI (Figure 16.11a) for the
phantoms with the same NR probe and similar spatial and functional resolutions. This is particularly
important for in vivo applications where the experimental window and stability of the NRs are limited.
In addition, PEDRI allows for functional (pH map) and anatomical resolution in one experimental set-up
otherwise available only in EPR/NMR co-imaging. 121

16.5.2 Dual function pH- and oxygen-sensitive trityl radicals

TAMs containing ionizable groups may demonstrate pH-sensitive magnetic resonance parameters similar
to the phenomenon described for pH-sensitive NRs. The first observations of the pH sensitivity of EPR
spectra of TAMs were reported for the derivatives containing carboxyl groups. 46