Pulsed EPR Dipolar Spectroscopy on Spin Pairs with one Highly Anisotropic Spin Center: The Low-Spin Fe^III Case

Pulsed electron paramagnetic resonance (EPR) dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling constants and thus the distance between electron spin centers. Up to now, PDS measurements have been mostly applied to spin centers whose g-anisotropies are moderate and therefore have a negligible effect on the dipolar coupling constants. In contrast, spin centers with large g-anisotropy yield dipolar coupling constants that depend on the g-values. In this case, the usual methods of extracting distances from the raw PDS data cannot be applied. Here, the effect of the g-anisotropy on PDS data is studied in detail on the example of the low-spin Fe³⁺ ion. First, this effect is described theoretically, using the work of Bedilo and Maryasov (Appl. Magn. Reson. 2006, 30, 683–702) as a basis. Then, two known Fe³⁺/nitroxide compounds and one new Fe³⁺/trityl compound were synthesized and PDS measurements were carried out on them using a method called relaxation induced dipolar modulation enhancement (RIDME). Based on the theoretical results, a RIDME data analysis procedure was developed, which facilitated the extraction of the inter-spin distance and the orientation of the inter-spin vector relative to the Fe³⁺ g-tensor frame from the RIDME data. The accuracy of the determined distances and orientations was confirmed by comparison with MD simulations. This method can thus be applied to the highly relevant class of metalloproteins with, for example, low-spin Fe³⁺ ions.