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

Abstract

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 in g-factor (Ag 0.0002) and decrease in hyperfine interaction constant (AaN = 0.8-1.2 G) upon the protonation of atom N-3 of the radical heterocycle were observed. The pH dependence of a N and gfactor 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 I b . 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.

Continuous wave electron paramagnetic resonance of nitroxide biradicals in fluid solution

“…The spectra of these species do not exhibit the alternating line widths that characterize flexible dinitroxides in which the rates of interconversion are designated as fast on the EPR timescale (section 2.2). pH dependence of J was also observed for imidazoline dinitroxides with ‐NH‐ linkages, with the protonated form exhibiting negligible exchange …”

“…pH dependence of J was also observed for imidazoline dinitroxides with -NH-linkages, with the protonated form exhibiting negligible exchange. 64…”

Heme Binding Constricts the Conformational Dynamics of the Cytochrome b559′ Heme Binding Cavity

“…Generally, the temperature dependence of the spin exchange rate in long chain biradicals (polymers) can be described by the Arrhenius equation. The activation energy of spin exchange is obtained from the Arrhenius plot of the intensities or the line widths of the spin exchange ( I b ) and the monoradical ( I m ) components of the EPR spectrum versus inverse temperature. ,,, A low activation energy for spin exchange is related to more direct, less hindered collisions of the monoradical fragments. Furthermore, when line narrowing and increasing intensities of the “biradical” component (spectral component I b ) can be observed with an increasing temperature, this is indicative of faster spin exchange. , Figure A shows CW EPR spectra of the spin-labeled PsbF peptides at different temperatures.…”