Imidazoline biradical spin labels with EPR spectral sensitivity to the local concentration of protons

EPR spectral sensitivity of the three biradicals of imidazoline and imidazolidine types to the local concentration of protons (pH) in aqueous solutions has been studied. Significant increase ing-factor (Δg ≈ 0.0002) and decrease in hyperfine interaction constant (ΔaN = 0.8−1.2 G) upon the protonation of atom N-3 of the radical heterocycle were observed. The pH dependence of aN andg- factor is described by conventional titration curve with one or two pK values. The latter case was explained by nonindependent protonation of two monoradical fragments closely located in the biradical structure. EPR spectra of protonated and deprotonated forms of the biradicals have significantly different intensities of “biradical” components, Ib, reflecting changes in spin exchange interaction. It has been found that activation energy of spin exchange, Ea, estimated from temperature dependence of Ib in glycerol/water solution is lower for the biradical conformation with larger Ib. This is in agreement with main contribution of direct collisions of monoradical fragments in spin exchange interaction so larger value of Ib corresponds to the conformation with less spatial restrictions for the motion of monoradical fragments (lower Ea). The potential use of the pH-sensitive biradicals as spin labels is discussed.