Simple Test of the Effect of an Electric Field on the 14N-Hyperfine Coupling Constant in Nitroxide Spin Probes

The sensitivity of the 14N-hyperfine splitting of nitroxide spin to the local electric field, Eloc, has often been used to probe the polarity of microenvironments; however, two uncertainties have been faced at once as follows: (i) the value of Eloc in a complex microenvironment and (ii) the effect of Eloc on the hyperfine splitting. In this paper, we study nitroxide free radicals that may be charged (positively or negatively) by varying the pH. The additional charge produces an additional Eloc, whose value we may calculate with confidence, allowing us to focus attention on the effect that Eloc has on the hyperfine splitting. A simple theory is developed that provides a link between the shift in the 14N-hyperfine splitting, ΔAN, and the local electric field, Eloc. To test this approach, we have calculated the ΔAN for several charged nitroxide spin probes and found good agreement (both magnitude and sign) with the measured values using ΔAN = −3.8 × 10-8Eloc (ΔAN in Gauss and Eloc in V/cm), where Eloc is the component of Eloc directed along the N−O bond from the nitrogen to the oxygen.