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Ubiquinone-2 (UQ-2) selectively labeled with (13)C (I =(1)/(2)) at either the position 1- or the 4-carbonyl carbon is incorporated into the ubiquinol oxidase bo(3) from Escherichia coli in which the native quinone (UQ-8) has been previously removed. The resulting stabilized anion radical in the high-affinity quinone-binding site (Q(H)(*)(-)) is investigated using multifrequency (9, 34, and 94 GHz) electron paramagnetic resonance (EPR) spectroscopy. The corresponding spectra reveal dramatic differences in (13)C hyperfine couplings indicating a strongly asymmetric spin density distribution over the quinone headgroup. By comparison with previous results on labeled ubisemiquinones in proteins as well as in organic solvents, it is concluded that Q(H)(*)(-) is most probably bound to the protein via a one-sided hydrogen bond or a strongly asymmetric hydrogen-bonding network. This observation is discussed with regard to the function of Q(H) in the enzyme and contrasted with the information available on other protein-bound semiquinone radicals.

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By performing high-resolution electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments on nitrogen atoms encapsulated in C60, the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Symmetry lowering induced by a phase transition in polycrystalline C60 at 258 K can easily be detected by the appearance of a zero-field splitting of axial symmetry. Freezing of cage rotation is observed via the magnetic dipole interaction with 13C nuclei of the carbon shell.