Pulsed, Fourier-transform, and continuous-wave electron paramagnetic resonance methods are used to study the g=2.0319 EPR signal in synthetic diamond crystals. This signal is from Ni which is found to be located at a substitutional site in the diamond lattice without detectable nearby charge compensation. The effective spin state of S=(3/2 is determined from Fourier-transform nutational EPR spectroscopy. It is proposed that the center is a ${\mathrm{Ni}}^{\mathrm{-}}$ ion with electronic configuration 3${\mathit{d}}^7$.

We report the development of a novel radio frequency electron‐spin‐resonance spectrometer designed to provide measurements with high molar sensitivity and resolution in vivo. Radio frequency (250 MHz) is chosen to obtain good penetration in animal tissue and large aqueous samples with modest sacrifice of sensitivity. The spectrometer has a lumped component resonator and operates in continuous‐wave mode. The spectrometer is capable of two‐dimensional imaging, and with a modest addition should be capable of three‐dimensional imaging. We demonstrate 3‐mm spatial resolution for DPPH samples. For 10‐ml samples of aqueous nitroxide, we demonstrate sensitivity (normalized to spectral width of 1 G) to 3×10⁻⁸‐M concentrations and spectral resolution of 0.1 G. Spectra from nitroxide spin label injected into a live mouse are shown.

The cytochrome bc ₁ and related complexes are essential energy-conserving components of mitochondrial and bacterial electron transport chains. They orchestrate a complex sequence of electron and proton transfer reactions resulting in the oxidation of quinol, the reduction of a mobile electron carrier, and the translocation of protons across the membrane to store energy in an electrochemical proton gradient. The enzyme can also catalyze substantial rates of superoxide production, with deleterious physiological consequences. Progress on understanding these processes has been hindered by the lack of observable enzymatic intermediates. We report the first direct detection of a semiquinone radical generated by the Q _o site using continuous wave and pulsed EPR spectroscopy. The radical is a ubisemiquinone anion and is sensitive to both specific inhibitors and mutations within the Q _o site as well as O ₂ , suggesting that it is the elusive intermediate responsible for superoxide production. Paramagnetic interactions show that the new semiquinone species is buried in the protein, probably in or near the Q _o site but not strongly interacting with the 2Fe2S cluster. The semiquinone is substoichiometric, even with conditions optimized for its accumulation, consistent with recently proposed models where the semiquinone is destabilized to limit superoxide production. The discovery of this intermediate provides a critical tool to directly probe the elusive chemistry that takes place within the Q _o site.