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Abstract

For aromatic organic radicals, pulsed electron-electron double resonance (PELDOR) experiments at high magnetic fields provide information not only about the distance between the paramagnetic species but also about their relative orientation. However, the three-dimensional biradical structure is encoded in a complex pattern of orientation-selected PELDOR traces and the execution of the experiment is generally aggravated by constraints posed by the available hardware and the intrinsically low modulation depth observed. We present a 94 GHz PELDOR experiment performed with a commercial spectrometer and probe heads that permit separation of pump and detection frequencies up to 150 MHz. The setup is employed to examine the orientation selections on a general case of rigid biradicals with non-collinear g axes. The interacting radicals, a tyrosyl radical (Y122·) located in the β2 subunit and an 3-aminotyrosyl radical (NH2Y731·) located in the α2 subunit, are generated by Escherichia coli ribonucleotide reductase with a 3-aminotyrosine (NH2Y) site specifically incorporated into α2 in the presence of cytidine 5′-diphosphate and adenosine 5′-triphosphate. The experimental designs as well as some characteristic features of the observed modulation pattern are discussed.