Proton exchange in stable nitroxyl radicals. EPR study of the pH of aqueous solutions

Abstract The influence of the pH on EPR spectra of nitroxyl radicals was studied. Hyperfine splitting constants( a N ) and g factors differ in protonated (RH + ) and non-protonated (R) forms. Proton exchange rates between R and RH + are fast or slow on the EPR time scale, depending on the radical pK a . These radicals may be used as “ pH probes”, biological systems included.