Functional in vivo EPR Spectroscopy and Imaging Using Nitroxide and Trityl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 555

3522.55

3522.50

3522.45

3522.40

pKa = 2.6

3522.35

3522.30

3522.25

0 1234

pH

56789

(a)

12124.8

12124.6

12124.4

12124.2

pKa = 2.6

12124.0

0 1 2 3 4 5 6

pH

(b)

Figure 16.12 pH dependence of the EPR spectral line position of 50 M solution of Oxo63 TAM in 1.5mM

sodiumcitratebuffermeasuredbyX-band(a)andQ-band(b)EPRspectroscopy.Thesymbols(

µ

) denote
the data obtained upon titration from alkaline pH into direction to acidic pH and in the reverse direction,
respectively. The values of g-factors for neutral RH and deprotonated (R − ) forms of the radical were found
to be equal to 2.00329 and 2.00315, correspondingly. (Reprinted with permission from [46]. Copyright 2007
AmericanChemicalSociety.)

)and( ◦

The pH-dependent shift of the EPR line position of Oxo63 TAM (Scheme 16.2) measured at two different
EPR frequencies is shown in Figure 16.12. This line shift is reversible in agreement with the reversible
deprotonation of the COOH group. The value of the line shift is proportional to the EPR frequency being
equal to 0.27 G and 0.81 G for 10 GHz X-band and for 35 GHz Q-band spectra, respectively, which is
characteristic for the changes in g-factor. The low pK a value of Oxo63 derivative limits its application to